Telescope Reviews: Diamond tool hogging problem

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cnchillstar
sage

Reged: 12/29/08

Diamond tool hogging problem new

#5291498 - 06/27/12 12:17 PM Attachment (34 downloads)

Hi,tonight I grind a mirror with diamond tool, but fail to hog it, after one and a half hour, the mirror is just plate(even no 0.001mm change with a spherometer check).Here is the situation:
Machine grind,tool on top, 9" mirror with a 6" diamond tool.
Mirror shaft 60rpm, tool stroke 10rpm.
Overhang is 1" to keep the tool cross mirror center(with small overhang, mirror center will remail 1 squre centimeter area where can not be ground)

The diamond tool has a working ring where consist of some 1 square centimeter surface)

I don't know what is the problem, and how can hogging happen because the diamond tool will never change its form(plate work squares).

There is Mr.Jones' diamond tool article link: http://www.atmsite.org/contrib/Jones/Diamond/diamond.html

please help to figure out the cause, thanks!

Post Extras:

cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5291503 - 06/27/12 12:18 PM Attachment (27 downloads)

this is the tool

Post Extras:

Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5291523 - 06/27/12 12:35 PM

do you have any weight on the arm?

from my experience, the more weight the deeper I can go...for the diamond tool. I was hogging out for a while and couldn't go past an f5 till I added more weight.

rough grinding

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cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: Pinbout]
#5291545 - 06/27/12 12:50 PM

30 pounds, there is fast abrasion(noise and blank gets thiner),but just keeping plate...
the tool is too big?
or wrong diamond tool type?

thanks for reply

Post Extras:

Al8236
Pooh-Bah

Reged: 08/18/10

Loc: 48.9173N 122.1390W

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5291555 - 06/27/12 12:54 PM

Do you have the mirror on a sine table? you need a slight angle between the tool and mirror for it to hog out the center.

Post Extras:

cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: Al8236]
#5291565 - 06/27/12 12:59 PM

?? Cup diamond tool will need a sin table, for a ring tool I think a rotate table is OK, just like Mr.Jones said in his article.

Post Extras:

Pinbout
Post Laureate

Reged: 02/22/10

Loc: Montclair

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5291574 - 06/27/12 01:02 PM

here's my diamond tool that I use for my MOM.

grinding a flat

when generating a curve I move it like the rough grind vid.

when I look at your tool it is a ring the teeth look wierd

I would find it hard to judge the amount of swing...

but these cup ring tools work really well. I sawed off the stem and used alum tape to tape a plywood disc with a hole in it for the arm pin.

Post Extras:

Al8236
Pooh-Bah

Reged: 08/18/10

Loc: 48.9173N 122.1390W

Re: Diamond tool hogging problem new

[Re: Pinbout]
#5291607 - 06/27/12 01:26 PM

In Jones article He has a tilt to the spindle.

Quote:

In a curve generating machine the glass is held on a rotating spindle and a diamond tool on a top spindle is tilted at angle that will generate the required radius. The edge of the tool crosses the rotation axis of the glass part making a smooth spherical surface. A water based coolant is used aid this process. This machine is rather beefy like a milling machine. However you don't need such an expensive machine to do generating especially on concave curves. Many amateurs make grinding and polishing machines and these can work for generating as well.
Quote:

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Paul Drufva
super member

Reged: 06/26/11

Loc: Connecticut

Re: Diamond tool hogging problem new

[Re: Al8236]
#5291642 - 06/27/12 01:46 PM

I have used ring tools with carbo, same principal. Tool should be 1/2 of mirror dia. Your tool may be too large. Try no overhang. If the stroke is too long the mirror will go convex, you seem to have hit the sweet spot for a flat!

Post Extras:

Ed Jones
Carpal Tunnel

Reged: 04/06/04

Loc: Sin-sin-atti

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5291643 - 06/27/12 01:46 PM

cnchillstar,
Yes your tool is too big. The overhang will negate any deepening it might do. Try running a diamond tool about half this size with a stroke taking one edge over center and one to the edge. A small tool may make it a bit aspheric but run this large tool only long enough to keep it spherical.

One other thing. Your spindle point is very high on your tool. This aggrivates the problem too. The spindle point should always be placed as low as possible.

Edited by Ed Jones (06/27/12 01:49 PM)

Post Extras:

cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: Ed Jones]
#5292111 - 06/27/12 07:42 PM

Thanks for every reply. I will try a smaller tool today.
Mr.Jones, is the spindle point issue very critical? I found the joint bearing is convenient to use,it is bad to know it is a problem.

Post Extras:

Ed Jones
Carpal Tunnel

Reged: 04/06/04

Loc: Sin-sin-atti

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5292126 - 06/27/12 08:00 PM

I don't recommend it, I would never run it that way. It might run just fine but then I've seen a tool fly off the machine due to spindle point height.

Post Extras:

rwiederrich
Goldfinger

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5293104 - 06/28/12 12:07 PM Attachment (13 downloads)

Quote:

Thanks for every reply. I will try a smaller tool today.
Mr.Jones, is the spindle point issue very critical? I found the joint bearing is convenient to use,it is bad to know it is a problem.

I agree..the quill tip is a bit high..this makes the tool want to flip laterally when it gets deeper within the sagitta.

On my Porter machine, I use a diamons blade with a machined flange for the quill. The quill must be flexible enough to rotate within its seat...to allow the tool to flex as it goes across the sagittal plane. Also...you tool is too big for the blank(as was mentioned) I have 9", 7" 4" blades for varied sizes of mirror blanks I'm going to hog. In the picture is a 7" blade hogging out an 18 blank. I took my time on this one(I was doing it for a friend), it took several days to reach the f/4 sagittal depth. I have a water drip system to keep the mirror wet and cool and the blade works. I also use about 2~4 lbs of weight added to the over arm.

Good luck with your changes.

Ron

Post Extras:

rwiederrich
Goldfinger

Reged: 11/17/05

Loc: Always Dark skies of Belfair W...

Re: Diamond tool hogging problem [Re: cnchillstar]
#5293115 - 06/28/12 12:13 PM

OH..one other note...to shorten the quil point for your tool it's an easy job...simply umbolt the wooden piece from the aluminum tool top...trim off the corners. then reinsert it from the bottom so the quill seat comes up through the tool base..re-bolt using same holes. This hides the wood portion inside the tool base and lowers your quill seat by half....see? Much more stable.

Rob

Post Extras:

cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: rwiederrich]
#5296056 - 06/30/12 09:15 AM Attachment (13 downloads)

I fail to generate the curve again today. This time it is a 12cm tool(mirror is 21cm).For grinding the center, there must be a 15mm overhang. After 2 hours' working, again no even 0.001mm change with spherometer reading(I must be a secondary grinding genius.

)
I will change a 8cm tool,and worry if there was someone ever succeed with same type tool of mine?

Another question, what cushion do you use? The picture shows my rubber cushion, but maybe it is hard and not very even, the right side is a thick cushion which was sell as grinding rag,but it is thick, the mirror will tilt on it when grinding.

Cushion must be very critical for astigmatism free, please give me some advice, thanks a lot!

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Paul Drufva
super member

Reged: 06/26/11

Loc: Connecticut

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5296071 - 06/30/12 09:28 AM

It's the back and forth movement of the tool that does the hogging. you should switch the turntable speed with the stroke speed , slow turntable speed to 10-20 rpm. fast stroke speed= 60 rpm. Layers of wet newspaper works well for a cushion, cheap too.

Post Extras:

cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: Paul Drufva]
#5296284 - 06/30/12 12:17 PM

Ron,what is your speed for turntable and stroke? I remember a article said it is OK that slow stroke and fast turntable.

Post Extras:

Paul Drufva
super member

Reged: 06/26/11

Loc: Connecticut

Re: Diamond tool hogging problem new

[Re: cnchillstar]
#5296490 - 06/30/12 02:54 PM

I work with a turntable and hog with a ring tool ,pushing it by hand. turntable is 20 rpm and 60 to 100 strokes a minute. If you can't slow the turntable, try working by hand for 10-15 minutes, pushing the tool rapidly, at least as fast as the turntable, also apply weight, don't be afraid to bear down HARD, applying as much pressure as you can, even your whole body weight. Try the 8 cm. tool also. Check out Gordon waite's web site, he has good videos of this and much more, http://www.waiteresearch.com . At the bottom of the page you will find a video Hogging the curve on a 20" quartz mirror. Hope this helps, and good luck.

Post Extras:

Paul Drufva
super member

Reged: 06/26/11

Loc: Connecticut

Re: Diamond tool hogging problem new

[Re: Paul Drufva]
#5296913 - 06/30/12 08:29 PM

One other thought, I have a tool similar to yours. I use it for grinding the backs of mirrors flat. I have never tried to hog out with it though. The tool I have, which is diamond, like yours, is made for surfacing granite and marble counter tops flat. If the tool does not wear, and stays flat, so will the mirror. I would suggest making another tool, you will need one anyway.

Post Extras:

cnchillstar
sage

Reged: 12/29/08

Re: Diamond tool hogging problem new

[Re: Paul Drufva]
#5296953 - 06/30/12 09:02 PM Attachment (12 downloads)

Paul,thanks for help.
I bought two tool like the picture,and will try them again.
is you ring tool same with this one?
About machine speed, you give me a link in another thread which say M-o-M type machine can work with slow stroke when hogging.
http://www.mirror-o-matic.com/Mirror-Operations-5-2-5.html

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