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Hi Brian, What Mark said and get you primary turntable up to at least 10% plus of the mirrors thickness before you start...and flatten it with the machine...like at least 2+ inches thick...

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I may be taking a bit simpler approach then Preston describes, but the goal is still the same - to get to a flat turntable with no runout (running true).

One way is to grind the table flat, whatever it's made of, another is to start with thick aluminum plate that's machine finished flat.

If you don't want to adjust that surface (I don't!) you still need to be sure that the table runs true to the shaft support or you will generate astigmatism from the induced wedge. It only takes a few thou side to side to do this.

On my last refit of the main machine, since I knew the main support pulley wasn't entirely true to the shaft rotation , I used the two pieces of Corian countertop to accomplish the truing, and planned it out this way:

I put 3 machine bolts (3/8 or 5/16) tapped into the bottom piece and bearing on the top piece, with additional bolts tapped into the top and serving to hold position and add structural integrity. Than when I was ready I trued it up roughly using the 3 bolts and a laser to line it up. Took the other bolts out and filled the gap with epoxy, but it back together, and while the epoxy was still setting did a dead-on alignment of the new top with a flat part rotating on it and a laser bouncing off and onto the wall beyond. Let it cure and that was all she wrote, worked (and works) like a charm.

Best,
Mark

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It's a wonder John Dobson ever made a decent mirror..by simply sitting on a bench and hand working it. About as inaccurate as one can get(you would think)...but he did it.

All my tables are flat and run true...but the equasion breaker is the pad benieth the mirror and on top of the table. That permits the mirror to *float* and shift(Compress) under the weight of the tool. This would induce that *few thousands* you commented about.

I tend not to be so worried about such things...as long as your strokes are as random as possible and *astig* is kept at bay and you have produced a nice sphere. If the mirrored surface isn't exactly parallel to the back of the mirror....no problem. That is why you use a many point contact cell that is collimatable. All those aspects are designed to align the front of the mirror(the important part) in the right direction.

Rob(My pathetic view any way)

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

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The more logical thing would be to put the mirror right on top of the steel plate.

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Right now I am wrangling with the idea of should I let the pin arm push "down" on the tool such that the arm "floats" like a record turntable or do I fix the arm so no weight is transferred down onto the tool and just put weight on top of the tool if needed.

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I wrangle with the pad idea also. It would logically seem to make the problem worse with the mirror/tool compressing it randomly. The more logical thing would be to put the mirror right on top of the steel plate.

I have used carpet in the past with success and I must trust the wisdom of far more experienced mirror makers than me.

Right now I am wrangling with the idea of should I let the pin arm push "down" on the tool such that the arm "floats" like a record turntable or do I fix the arm so no weight is transferred down onto the tool and just put weight on top of the tool if needed.

In the past I had the pin pushing down on the tool, with success but it was a much thicker mirror.

Right now, considering the advice of others, I am thinking I should fix the pin so that it doesn't push down on the tool.

Thanks...

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First...Like Mark said..if your mirror is not flat on its back it will rest on the 3 highest points...like a tripod...so then the carpet backer.

However...like you've pointed out...if the care is taken to gind the back *FLAT*(Like everyone here keeps pointing to)//then there will be no tripoding of the mirror on the flat steel table. IN this case...logic dictates you would be correct. Flat table..against flat mirror back...is as perfect a contact as one could wish for.

Far to often we perform out of routine...then out of logic.

My tables are flat...but I still use the carpet backing to aid in contact and to aid in retention.

Because my machines *stroke* the mirror...my tables also have adjustible stops.

My modified Draper machine uses poker pins(quils) and I set them in the tool bib and then unlock the pin so it can move within its sleave. I then can add weight to the pin so I can *Press* the tool as it works. The over arm(control arm) strokes freely without weight upon it.

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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I've made mirrors up to 12.5" the way Dobson teaches - because I took his class a long time back! And for hand work it works fine, of course. The key there is consistent rotation of both mirror and tool, and a certain amount of inevitable (but designed) randomness in both the strokes and the rotation. So yeah, I hear ya about that.

Machine polishing, especially for thin mirrors, is a different creature.

The flotation pad won't fix a table that's off from true, nor wedge in a blank, nor an off-center generation. Any and all of these things will cause astigmatism in the mirror. Compression in the flotation pad averages out and won't cause it.

Which just means that if you're working with machines you don't want a mirror where the surface isn't parallel to the back of the mirror, as it won't get a good sphere, and it will show astigmatism. I've had to go back to the generation stage on occasion to fix defects that caused that.

Best,
Mark

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OK..you're gonna really need to help me understand something here.

Your comments concerning *hand grinding* makes it sound as if the mirror knows what kind of grinding is taking place upon it. *Consistant* rotation and stroke is key on a machine as well...and if you use a carpet pad you have induced the *designed* inevitable randomness.

I agree...an off true table or poor off center curve generation will create astigmatizm. This is true with a machine or by hand grinding.

If one is mindful...and they know the off set generation error...they can polish on a machine noting that error and still create a nice sphere. (That's wishful thinking)

Traditional grinding on the back of a steel drum while stroking and rotating and revolving around it is in of itself the greatest potential for causing astigmatizm in a glass blank. But still the randomness of all aspects creates a perfect sphere.... Magine that.

This is why it is quite important that you make sure your blank is dead on center on your hogging table.....

Fun fun fun..Oh...How can you create a *wedge* situation when you're rotating the mirror during generation? Or grinding for that matter?

Also...I was under the impression..*wedge* is typically a lens description.


Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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The more logical thing would be to put the mirror right on top of the steel plate.

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So, do you feel lucky? Also see previous comment.

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Right now I am wrangling with the idea of should I let the pin arm push "down" on the tool such that the arm "floats" like a record turntable or do I fix the arm so no weight is transferred down onto the tool and just put weight on top of the tool if needed.

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The pin (quill) just locates the tool in my work. All the weight that's applied comes from the tool and weight added directly to the back of the tool. This way the forces on the tool are spread out so it doesn't change shape. But I use a lot of force in grinding and polishing out, not much in figuring.

Best,
Mark

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You use lots of force while grinding?
Can you explain? How much actually?

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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...if the care is taken to gind the back *FLAT*(Like everyone here keeps pointing to)//then there will be no tripoding of the mirror on the flat steel table. .... Flat table..against flat mirror back...is as perfect a contact as one could wish for.

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Rob, sadly this isn't true, though it's a very small amount, the contact is far from as perfect as one could wish for. You probably know well enough what Josephson (?) blocks are, and that they actually stick together through molecular force. Neither the mirror back nor the table are ever going to be that flat on a micro/macro level.

The whole point of the flotation is that, no matter how flat you have the table or the mirror back, they are only guaranteed to make physical contact in 3 points, unless one or the other deforms. And it isn't going to be the table.

So the mirror sags onto the table when you're talking about wavelengths of light (kind of like a pitch lap would if you laid it face down). And every time you pick it up and put it down again, it's going to take on a different shape, and when you polish that and then pick it up off the table, you don't have a good sphere (or whatever), you have the shape of the table (in reverse) + the shape polished onto the mirror. You need the flotation to fill the gaps evenly.

Note that I'm talking about polishing, not about grinding. I doubt that it makes much difference in grinding, but I use the flotation for both, so when I'm done with grinding I just hose the table off and it's ready to go for polishing, no adjustments.

There's always two sides of course - on my generator I put the blank straight on the flat table and the diamond cutter just chews away at it with what I suspect is hundreds of lbs of pressure (sometimes). But since that blank is cleated down like heck and can't go anywhere, and I want the same rough curve ground in, it comes out even all around (like a planer would), to within the tolerance needed (a few thou on the edge thickness).

Every step along the way from generating to figuring the tolerances you need to hold the parts to get smaller, that's about the size of it.

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Your comments concerning *hand grinding* makes it sound as if the mirror knows what kind of grinding is taking place upon it. *Consistant* rotation and stroke is key on a machine as well...and if you use a carpet pad you have induced the *designed* inevitable randomness.

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John would know more about the first part of it...

You don't need consistent (or any) rotation of the mirror on the table on a machine, if all of the following is true:

• Table is really flat
• Table is really trued
• Workpiece is centered, or, if not restrained, centered on average
• Workpiece is on good flotation
  

Some people do practice regular rotation of the substrate on the table, even swearing by it, but it's only useful if one of the things in the list isn't true. If you do it, I think it has to be consistent and indexed physically, and you have to give equal time to all the different positions over the course of any work session, and it can't be done just randomly unless you're consistently lucky.

But you don't need randomness there (from grinding thorough polishing out), you just need predictability.

If you're using spin grinding/polishing (fixed quill position) you need the tool to be held at the neutral spot (holds the ROC constant) and you need a faceting pattern on the polisher that doesn't imprint on the mirror.

If you're using strokes (SFAIK!) you need a pattern that doesn't repeat (rotation speed and stroke speed should be incommensurate, ie, not divide evenly into one another), and the average position of the stroke work needs to hold the ROC constant.

When you get to figuring an argument can be made for randomness. It will help to disguise machine artifacts, SFAIK. But I figure everything completely by hand, and I'm not trying to be random when doing it, though I'm not trying not to be random either - there are inevitable small variations. I don't think they are part of the essential process.

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off center curve generation

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But still the randomness of all aspects creates a perfect sphere....

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Not often and only for small mirrors, anyway. The odds of getting a perfect sphere by simple chance are tiny in larger sizes. But I did once pull a 1/6th wave paraboloid off the polisher by "chance." Not quite good enough to use, though I had to give the polisher big props for making it automatically.

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How can you create a *wedge* situation when you're rotating the mirror during generation? Or grinding for that matter?

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You use lots of force while grinding?
Can you explain? How much actually?

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It averages about 1/2 lb per square inch added to the tool weight, but since the tools are tile/glass embedded in Hydrostone they aren't 100% grinding surfaces. So I'm guessing anywhere from 1/2-1 lb per square inch of grinding surface, like 25 lbs for an 8.5" tool, or 50 lbs for a 12" tool (as for a 20" mirror). These are both grinding and polishing out loads. Figuring is about half that much, 10 lbs on a 7" polisher, for example. Or less, depending.

If you want the explanation that will take a lot longer. It's just what works best for me.

Best,
Mark

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...if the care is taken to gind the back *FLAT*(Like everyone here keeps pointing to)//then there will be no tripoding of the mirror on the flat steel table. .... Flat table..against flat mirror back...is as perfect a contact as one could wish for.

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Rob, sadly this isn't true, though it's a very small amount, the contact is far from as perfect as one could wish for. You probably know well enough what Josephson (?) blocks are, and that they actually stick together through molecular force. Neither the mirror back nor the table are ever going to be that flat on a micro/macro level.

The whole point of the flotation is that, no matter how flat you have the table or the mirror back, they are only guaranteed to make physical contact in 3 points, unless one or the other deforms. And it isn't going to be the table.

So the mirror sags onto the table when you're talking about wavelengths of light (kind of like a pitch lap would if you laid it face down). And every time you pick it up and put it down again, it's going to take on a different shape, and when you polish that and then pick it up off the table, you don't have a good sphere (or whatever), you have the shape of the table (in reverse) + the shape polished onto the mirror. You need the flotation to fill the gaps evenly.

Note that I'm talking about polishing, not about grinding. I doubt that it makes much difference in grinding, but I use the flotation for both, so when I'm done with grinding I just hose the table off and it's ready to go for polishing, no adjustments.

There's always two sides of course - on my generator I put the blank straight on the flat table and the diamond cutter just chews away at it with what I suspect is hundreds of lbs of pressure (sometimes). But since that blank is cleated down like heck and can't go anywhere, and I want the same rough curve ground in, it comes out even all around (like a planer would), to within the tolerance needed (a few thou on the edge thickness).

Every step along the way from generating to figuring the tolerances you need to hold the parts to get smaller, that's about the size of it.

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Your comments concerning *hand grinding* makes it sound as if the mirror knows what kind of grinding is taking place upon it. *Consistant* rotation and stroke is key on a machine as well...and if you use a carpet pad you have induced the *designed* inevitable randomness.

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John would know more about the first part of it...

You don't need consistent (or any) rotation of the mirror on the table on a machine, if all of the following is true:

• Table is really flat
  
• Table is really trued
  
• Workpiece is centered, or, if not restrained, centered on average
  
• Workpiece is on good flotation
  

Some people do practice regular rotation of the substrate on the table, even swearing by it, but it's only useful if one of the things in the list isn't true. If you do it, I think it has to be consistent and indexed physically, and you have to give equal time to all the different positions over the course of any work session, and it can't be done just randomly unless you're consistently lucky.

But you don't need randomness there (from grinding thorough polishing out), you just need predictability.

If you're using spin grinding/polishing (fixed quill position) you need the tool to be held at the neutral spot (holds the ROC constant) and you need a faceting pattern on the polisher that doesn't imprint on the mirror.

If you're using strokes (SFAIK!) you need a pattern that doesn't repeat (rotation speed and stroke speed should be incommensurate, ie, not divide evenly into one another), and the average position of the stroke work needs to hold the ROC constant.

When you get to figuring an argument can be made for randomness. It will help to disguise machine artifacts, SFAIK. But I figure everything completely by hand, and I'm not trying to be random when doing it, though I'm not trying not to be random either - there are inevitable small variations. I don't think they are part of the essential process.

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off center curve generation

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I just put that in the list, it's hard to do considering that you're always going to line the blank up in the generator somehow, to within a fractional mm, and that's all you need. Rough grinding centers the curve automatically...

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But still the randomness of all aspects creates a perfect sphere....

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Not often and only for small mirrors, anyway. The odds of getting a perfect sphere by simple chance are tiny in larger sizes. But I did once pull a 1/6th wave paraboloid off the polisher by "chance." Not quite good enough to use, though I had to give the polisher big props for making it automatically.

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How can you create a *wedge* situation when you're rotating the mirror during generation? Or grinding for that matter?

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Huh? The table rotates on my generator with the blank locked down on it. I added an extension table to it once and didn't check it carefully enough - it turned out a few astigmatic generations before it was caught. Now it has a really flat 5/8" aluminum table running on 3 big casters, it's scary how accurate it is.

Hogging out by machine takes an even amount off all around, so if there was a lot of wedge in the blank when you start, there's still some left when you're done. It shouldn't cause it though, rotation or not.

Grinding a mirror with wedge, or generating a blank with wedge if you don't clean it up - both produce a shape that when polished by machine shows astigmatism, as the polisher hits the high zones harder. You can polish it by hand without any issues though.

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You use lots of force while grinding?
Can you explain? How much actually?

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It averages about 1/2 lb per square inch added to the tool weight, but since the tools are tile/glass embedded in Hydrostone they aren't 100% grinding surfaces. So I'm guessing anywhere from 1/2-1 lb per square inch of grinding surface, like 25 lbs for an 8.5" tool, or 50 lbs for a 12" tool (as for a 20" mirror). These are both grinding and polishing out loads. Figuring is about half that much, 10 lbs on a 7" polisher, for example. Or less, depending.

If you want the explanation that will take a lot longer. It's just what works best for me.

Best,
Mark

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Mark..thanks for commenting extensively to my points.

I find we have much to agree upon and our practices are near mirror images of each other.

I use less weight on the tool then you...but as you mentioned that is generally a personal preference...or simply a *practice*.

I'm currently hogging out a new 13" with 10lbs of weight added to the already 25lb over arm. This works good for me.

Got 2 13" mirrors on the grinding machine as well and their tools are pretty heavy...made from stainless pot lids for form and filled with Die Stone and covered with pennies.

Your posts are extremely helpful and directing.....can you tell me when you perform your first test on a new mirror...and do you *flash* polish it or do you simply wait till you polish it to see the sphere and mirror surface?

Rob(always modifying my facts)

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

Post Extras:

Ah ha - mirror images...

Thanks for the nice comments, Rob!

Regarding the amount of weight - you can actually use more and achieve even more consistent grinds in less time. I settle on the amount I'm willing to throw about. But right now I'm cleaning up all my grinding weights and coating them with PlastiDip from Ace. Epoxy just doesn't seal them well enough, the way they get banged about and then rust is a continuing problem.

Here's the reason for using a lot of weight (caution - slogging ahead):

When the glass fractures under a piece of grit that the tool that has brought to bear, the energetics can be analyzed, at least roughly. The glass fracture always proceeds in the direction of releasing the most energy. Under a low compressive load the fracture can go in any direction, along the surface, or into the body - there's little to guide it, or stop it, until it runs out of oomph (another name for energy).

But under a higher compressive load, fracturing into the body of the glass is energetically unfavorable compared to along the surface. I think the compressive force preloads the body of the glass in bulk, increasing the energy required to create a deep fracture. So more fractures go to planing or glancing, and less to making deep fractures. This results in a smoother grind with fewer deep "pits" to take out in the remaining stages.

You can demonstrate it pretty easily by doing the same grind with low force and with high force, and then taking a loupe to inspect the results. It's really quite dramatic. After examining the results I went with the higher weights every time. But it stresses the need for good support to get the full advantage.

I may try using pennies when I run out of these little (1.5") quartz rounds I've been using. But there's an endless supply of the rounds, and so far they haven't cost anything. Here's a picture of a current grinding tool, about 10" size (the stuff at the bottom of the tiles is frit from the ovens they come out of, as scrap ends). It could have a little denser packing but it seems to work fine (for several 14.7" mirrors now). I wanted to keep the tiles from contacting on their sides as it creates gaps in the Hydrostone where grit can get stuck, but overdid it a bit. Anyway, these tools go straight through from 60 or 120 SiC to 12 micron AlOx without ever leaving the mirror (OMG! That can't possibly work!).

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can you tell me when you perform your first test on a new mirror...and do you *flash* polish it or do you simply wait till you polish it to see the sphere and mirror surface?

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More heresy on the way.

I don't even bother to look at it until I expect it's pretty close to being ready to figure. I would do the initial polish out, as long as it takes until there's no laser spot and no pits evident with a random loupe inspection near the edge. Then it goes maybe 8 hours more, depending, just to be sure. That'll be about 24 hours on the machine for quartz.

After that it goes into the prep mode. Before this the figure on the glass could be almost anything, but often it's considerably overcorrected. It doesn't matter as the next stage takes care of anything. I do the spin-polishing edge prep I've described before, at the "sweet spot" (about 15% overhang on a 70-80% lap), with a succession of short duration sessions. I prep the AccuLap with a stainless wire brush under hot water and immediately hot press for excellent contact. Facets should be minimal at this stage. It goes on the machine for about 30 minutes. Repeat several times, reducing the duration gradually. After a few of these I'll look at it on the bench and continue as needed until the edge comes clean. It doesn't have to be a sphere, it just has to have a smooth conic on the parabolic side of a sphere, with an edge that has no issues.

Then I'm checking it for astig as well as smoothness, prior to figuring.

Best,
Mark

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Ah ha - mirror images...

Thanks for the nice comments, Rob!

Regarding the amount of weight - you can actually use more and achieve even more consistent grinds in less time. I settle on the amount I'm willing to throw about. But right now I'm cleaning up all my grinding weights and coating them with PlastiDip from Ace. Epoxy just doesn't seal them well enough, the way they get banged about and then rust is a continuing problem.

Here's the reason for using a lot of weight (caution - slogging ahead):

When the glass fractures under a piece of grit that the tool that has brought to bear, the energetics can be analyzed, at least roughly. The glass fracture always proceeds in the direction of releasing the most energy. Under a low compressive load the fracture can go in any direction, along the surface, or into the body - there's little to guide it, or stop it, until it runs out of oomph (another name for energy).

But under a higher compressive load, fracturing into the body of the glass is energetically unfavorable compared to along the surface. I think the compressive force preloads the body of the glass in bulk, increasing the energy required to create a deep fracture. So more fractures go to planing or glancing, and less to making deep fractures. This results in a smoother grind with fewer deep "pits" to take out in the remaining stages.

You can demonstrate it pretty easily by doing the same grind with low force and with high force, and then taking a loupe to inspect the results. It's really quite dramatic. After examining the results I went with the higher weights every time. But it stresses the need for good support to get the full advantage.

I may try using pennies when I run out of these little (1.5") quartz rounds I've been using. But there's an endless supply of the rounds, and so far they haven't cost anything. Here's a picture of a current grinding tool, about 10" size (the stuff at the bottom of the tiles is frit from the ovens they come out of, as scrap ends). It could have a little denser packing but it seems to work fine (for several 14.7" mirrors now). I wanted to keep the tiles from contacting on their sides as it creates gaps in the Hydrostone where grit can get stuck, but overdid it a bit. Anyway, these tools go straight through from 60 or 120 SiC to 12 micron AlOx without ever leaving the mirror (OMG! That can't possibly work!).

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can you tell me when you perform your first test on a new mirror...and do you *flash* polish it or do you simply wait till you polish it to see the sphere and mirror surface?

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More heresy on the way.

I don't even bother to look at it until I expect it's pretty close to being ready to figure. I would do the initial polish out, as long as it takes until there's no laser spot and no pits evident with a random loupe inspection near the edge. Then it goes maybe 8 hours more, depending, just to be sure. That'll be about 24 hours on the machine for quartz.

After that it goes into the prep mode. Before this the figure on the glass could be almost anything, but often it's considerably overcorrected. It doesn't matter as the next stage takes care of anything. I do the spin-polishing edge prep I've described before, at the "sweet spot" (about 15% overhang on a 70-80% lap), with a succession of short duration sessions. I prep the AccuLap with a stainless wire brush under hot water and immediately hot press for excellent contact. Facets should be minimal at this stage. It goes on the machine for about 30 minutes. Repeat several times, reducing the duration gradually. After a few of these I'll look at it on the bench and continue as needed until the edge comes clean. It doesn't have to be a sphere, it just has to have a smooth conic on the parabolic side of a sphere, with an edge that has no issues.

Then I'm checking it for astig as well as smoothness, prior to figuring.

Best,
Mark

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I'm pushing more weight for sure. The over arms on my new machine are quite heavy and the addition of about 10lbs will be nice. I currently use about 7lbs additional.
I've found that the pennies epoxy on well and leave a nice spooth surface between them....not creating pockets for grit to hide. I cover the entire tool surface with a nice thin coat of epoxy...then press pennies down firmly(face down). I make sure the same face is up on each penny(probably means nothing). The epoxy seals the tool surface and secures the pennies as well....nice.

I bet you could do the same with those glass tiles....

I might make a tool with them just to see the grinding difference.

One thing I have noticed with pennies is that when you are close to reaching final fine grinding....the penny itself aids in the finish if you lower your water content in the slury...it almost causes the pennies to shine like little mirrors themselves. The last 8" I did fine ground down so nicely it was almost unreal looking.

That edge stage you perform with the 15% overhang...you use that to pull in the edge? Do you end up pushing the edge almost turning it? Or does the tool pull all zones together in that short session without creating any further issues?

I like your tools. What type of bib do you create for the quill pin to press into?

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

Post Extras:

Epoxy on grinding tools is too much trouble for me. I'll grind through 1/4" of the tool on multiple mirrors before the thicker quartz tiles get too thin to use.

I can cast a Hydrostone tool and be ready to use it in half an hour, and there's never an issue with adhesion or contamination unless there's bubbles in the mixture or it's too thick to get into all the voids. But if that does happen I just fill them with candle wax and proceed. One of the keys I think is rinsing the edge clean between grits (while it's spinning), but never taking the tool off the mirror until it's finished. Never give contamination a chance or it'll take it.

What's an "unreal" finish in fine grinding mean?

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That edge stage you perform with the 15% overhang...you use that to pull in the edge? Do you end up pushing the edge almost turning it? Or does the tool pull all zones together in that short session without creating any further issues?

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It brings a sphere all the way to the edge, or a smooth conic at least. If you're not in the right spot it can raise the edge, but at the correct overhang it won't, only bring it clean. It's not one short session, it's a series of them having perfect contact and only lasting a short while so that the action is consistent. There aren't any zone towards the outside with a smooth conic, but inside where the edge hits you can have a narrow zone that goes away in figuring. Here's a picture of what the edge typically looks like with the KE well inside of focus and more than half-way across the mirror (see Texereau for a full description of what this is showing ).

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What type of bib do you create for the quill pin to press into?

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Epoxy on grinding tools is too much trouble for me. I'll grind through 1/4" of the tool on multiple mirrors before the thicker quartz tiles get too thin to use.

I can cast a Hydrostone tool and be ready to use it in half an hour, and there's never an issue with adhesion or contamination unless there's bubbles in the mixture or it's too thick to get into all the voids. But if that does happen I just fill them with candle wax and proceed. One of the keys I think is rinsing the edge clean between grits (while it's spinning), but never taking the tool off the mirror until it's finished. Never give contamination a chance or it'll take it.

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What's an "unreal" finish in fine grinding mean?

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It is an emotional expression..nothing to do with the physical process.......

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What type of bib do you create for the quill pin to press into?

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Pipe plug, with the threads down (and the rounded bowl up). Cheap, recycles almost indefinitely. Downside is they rust. The quills are stainless, and it wears forever on the cast iron (with a bit of oil in there to make it quiet).

Best,
Mark

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I do the same thing..however.I think the cast iron ones are a bit big myself...so I use smaller brass ones. Same principal/method..... I also have many steal ones that are actually used on steal tools. From back in the day.

Thanks so much for your kind descriptions and examples.

Rob

--------------------
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Brass or steel, it needs to have threads to get a grip on the Hydrostone. I like the lifetime of stainless on cast iron, that and the price is right.

Generally on grind out (to 12 micron AlOx), once the blank is dry, if it was polished on the back I can easily see through the ground surface. Probably not unreal, but cool to see.

Best,
Mark

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Nifty...hey..some questions. Do you place your glass tiles on the pregenerated surface...then pour your hydostone on them with a dam agound the blank?

I noticed your tiles are well imbeded...without much extrusion of the tiles. I know this isn't a problem cuz the hydrostone ebrades much faster and doesn't effect the process.

Also....doesn't your hydrostone ebrade under contant water immersion without it being sealed with epoxy or like material?

And finally...what micron size of AlOx do you finish fine grinding with prior to polishing?

Sorry for the barrage of questions.

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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Brass or steel, it needs to have threads to get a grip on the Hydrostone. I like the lifetime of stainless on cast iron, that and the price is right.

Generally on grind out (to 12 micron AlOx), once the blank is dry, if it was polished on the back I can easily see through the ground surface. Probably not unreal, but cool to see.

Best,
Mark

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Nifty...hey..some questions. Do you place your glass tiles on the pregenerated surface...then pour your hydostone on them with a dam agound the blank?

I noticed your tiles are well imbeded...without much extrusion of the tiles. I know this isn't a problem cuz the hydrostone ebrades much faster and doesn't effect the process.

Also....doesn't your hydrostone ebrade under contant water immersion without it being sealed with epoxy or like material?

And finally...what micron size of AlOx do you finish fine grinding with prior to polishing?

Sorry for the barrage of questions.

Rob

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Nifty...hey..some questions. Do you place your glass tiles on the pregenerated surface...then pour your hydostone on them with a dam agound the blank?

I noticed your tiles are well imbeded...without much extrusion of the tiles. I know this isn't a problem cuz the hydrostone ebrades much faster and doesn't effect the process.

Also....doesn't your hydrostone ebrade under contant water immersion without it being sealed with epoxy or like material?

And finally...what micron size of AlOx do you finish fine grinding with prior to polishing?

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Questions are fine so long as the answers are easy.

I put plastic over the generated blank and then I duct-tape a plastic dam to the front of it. Place the tiles and the pivot (pipe plug), mix and pour the Hydrostone, let it set, remove the dam and bevel the edges with a Surform, that's all there is to it.

Does Hydrostone abrade (or change in any way) with water immersion? It shrinks a bit when dry. If you use it wet it doesn't care about water much, so no. Though if you put some under a dripping faucet overnight it will wear a hole in it.

12 micron AlOx, only. Anything finer doesn't gain enough to be worthwhile, in my experience.

Best,
Mark

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Nifty...hey..some questions. Do you place your glass tiles on the pregenerated surface...then pour your hydostone on them with a dam agound the blank?

I noticed your tiles are well imbeded...without much extrusion of the tiles. I know this isn't a problem cuz the hydrostone ebrades much faster and doesn't effect the process.

Also....doesn't your hydrostone ebrade under contant water immersion without it being sealed with epoxy or like material?

And finally...what micron size of AlOx do you finish fine grinding with prior to polishing?

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Questions are fine so long as the answers are easy.

You're a smart guy Mark..these answeres are easy for you...

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I put plastic over the generated blank and then I duct-tape a plastic dam to the front of it. Place the tiles and the pivot (pipe plug), mix and pour the Hydrostone, let it set, remove the dam and bevel the edges with a Surform, that's all there is to it.

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Again...my same technique... Cept when I make subdiameter tools....

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Does Hydrostone abrade (or change in any way) with water immersion? It shrinks a bit when dry. If you use it wet it doesn't care about water much, so no. Though if you put some under a dripping faucet overnight it will wear a hole in it.

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Attrition is the subject I'm talking about. Even the hydrostone will *desolve* under normal watery conditions.

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12 micron AlOx, only. Anything finer doesn't gain enough to be worthwhile, in my experience.

Best, Mark

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12micron...good. That's what I was thinking.

Thanks for the great answeres.

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--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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The Hydrostone may suffer attrition but the time scale is too long to make any difference when used for grinding - the tool wears out long before that can matter.

Ask about how polishing works exactly and see what I say.

Best,
Mark

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The reason why I asked about hydrostone attrition...or attrition for any jypsum product is as you grind the jypsum erodes away from around the glass tile. Both sides and even the back can be effected. I know this because I have worked extensively with these products for over 30 years...and am quite aqauinted with this issue.

I was wondering if you personally had noticed any defects in tile retention during the course of the grind.

One reason I epoxy my pennies to the die stone...to eliminate the contact error that can occure. Since tiles are held in by mechanical means.

Sorry if I have degressed.

I was planning on getting to the polishing aspect of things soon.

Question: Do you use the grinding tool when making your polishing tool? Meaning...do you pour your pitch directly upon your warn out tiles. Since it has formed to the curve of the mirror?

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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No, it's not a problem in practice that I've ever seen. Mechanical abrasion predominates by far. The only thing with the quartz tiles that can be a problem is lack of adhesion to the back on smoother ones, they have to be roughened up to get a grip. This also happens with tile tiles as they thin out.

Anyway, it doesn't make any real difference if the tile is a bit loose so long as it's surrounded by the matrix, if you don't remove the tool from the workpiece, because it's almost always under some pressure and doesn't have anywhere to go.

Polishers are all separate parts in my work, but you can make a tool into a polisher after sealing it up well. But Hydrostone isn't dimensionally stable enough to make a precision polisher - for that I use glass, cement, or aluminum.

Best,
Mark

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Indeed I can see where tool attrition isn't an issue with you...because you seldom remove the tool from the mirror while grinding(Not sure how that is possible)..but if it works for you.

The properties of hydrostone are not as precise and the material is not as demanding as dental die stone. Die stone has to be accurate to within hundreds of thousands of an inch. You can't create a PFM crown coping on a material that can't retain its original form when poured from a polypropaleine impression. Dies are steam cleaned....wetted...rewetted....and it must retain structual accuracy or the crown coping will fail to properly seat on the crown prep. Generally you have to either be a Dental office or Dental Lab to purchase high quality die stones.

It is good you have made hydrostone work well in your operation.

My die stone polishing tools are quite thick...over 3~4".

However I do use glass tools as well...and generally reuse tools after reworking the the lap

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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How is it possible to not remove the tool from the mirror while grinding???

You just leave it on. There's no good reason to take it off. The only place you need to rinse off is the bottom edge of the tool where grit hangs on. Think about it - if you leave the machine running with a water drip long past the time the grit dies, what do you have? No grit...

Hydrostone isn't really strong enough to make a good polisher, I mean it flexes too much when made the necessary thickness for a grinding tool. Dental die stone might work better once sealed with thin hot pitch or epoxy. And if 3-4" thick - well sure, go for it. That's a lot of stone!

Good questions, Rob.

Mark

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How is it possible to not remove the tool from the mirror while grinding???

You just leave it on. There's no good reason to take it off. The only place you need to rinse off is the bottom edge of the tool where grit hangs on. Think about it - if you leave the machine running with a water drip long past the time the grit dies, what do you have? No grit...

Hydrostone isn't really strong enough to make a good polisher, I mean it flexes too much when made the necessary thickness for a grinding tool. Dental die stone might work better once sealed with thin hot pitch or epoxy. And if 3-4" thick - well sure, go for it. That's a lot of stone!

Good questions, Rob.

Mark

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Interesting evaluation. I can assume that running water(dripping) it will indeed remove much of the used grit...but assuming that some hidden scratch maker lies unseen between tile and the hydrostone matrix..it could in all practicality be a real issue. I prefer to remove the tool and completely wash and inspect the tool for grit residue. But I have no critizism of your technique if it works for you. It makes sence on one hand...but caution directs me to take that extra step and remove the tool for inspection.

One last question....If you depend on the hydrostone to be adequately stable for use as a grinding tool...why does it fail the test for polishing in your assessment.....A polishing tool as everchanging pitch on it..which is far more unstable in that regard.

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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I've read that a lot of mirror makers use it (and/or dental plaster) for their pitch laps.

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Interesting evaluation. I can assume that running water(dripping) it will indeed remove much of the used grit...but assuming that some hidden scratch maker lies unseen between tile and the hydrostone matrix..it could in all practicality be a real issue.

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Nah, it's just superstition. I used to believe it too, until I proved it wrong by using the technique I've described exclusively for at least the last several years with no scratches. However, if you take the tool off for inspection, then you have to clean it up very carefully, or you'll introduce residue from earlier stages.

Important point! See the earlier pic - I never channel the tool either! If you believe in channeling the tool you do have to take it all apart before every grit change, clean it all up, and hope that you didn't miss any.

Anyway, it's not the dripping water, it's the action of the tool on the mirror that breaks down all the grit underneath it, leaving nothing to scratch. You just change sizes whenever you want - typically I only rinse it every couple of sizes. And in the new computer controlled grinder I'm building I won't rinse it at all. Just start it up, and let it run through the entire sequence all by its lonesome. Come back 4-5 hours later and pull the mirror off and put it on to polish.

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One last question....If you depend on the hydrostone to be adequately stable for use as a grinding tool...why does it fail the test for polishing in your assessment....

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Grinding tools aren't expected to work at optical tolerances is why. I said something quite a ways back that describes the progression of tolerance needed in this process: "Every step along the way from generating to figuring the tolerances you need to hold the parts to get smaller..."

Think about it this way - the pitch flows slowly on the tool, but if the tool itself changes shape and/or flexes, the optical surface of the pitch is moving around without control. That's why the substrate for the polisher should be dimensionally stable. There's lots of things that won't work very well, and I'm sure I've tried most of them at one time or another.

Best,
Mark

Edited by mark cowan (09/16/09 11:33 PM)

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Grinding tools aren't expected to work at optical tolerances is why. I said something quite a ways back that describes the progression of tolerance needed in this process: "Every step along the way from generating to figuring the tolerances you need to hold the parts to get smaller..."

Think about it this way - the pitch flows slowly on the tool, but if the tool itself changes shape and/or flexes, the optical surface of the pitch is moving around without control. That's why the substrate for the polisher should be dimensionally stable. There's lots of things that won't work very well, and I'm sure I've tried most of them at one time or another

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Mark...I see perfectly now. Not that I didn't earlier...but your explanation proved to be the factor that illuminated my understanding.

Each step must become more optically accurate then the previous...this is easily seen in hogging to grinding to fine grinding to polish...to figure.

I agree completely.

Looking forward to your computer driven grinding setup.

Thanks for a very lively and educational exchange.

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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Glad to help.

I'm looking forward to the computer controlled grinder as well!

Best,
Mark

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Your help is in essence validation.
This coupled with quality technique results in a quality product. Since I've only been at this for a couple of years..I'm pleased with my progress. I just have to get past making stupid mistakes and not properly protecting my work. From everything...sometimes even myself....

Thanks

Rob

--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.

http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.

www.vimeo.com/6014031

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"...The plate is centered with 3 ball bearing races inside a machined ring attached to the plate."
Can't visualize this one. What are the races used for? If ball bearings were in them, what would turn the table?

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