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DIY Heavy Duty Equatorial Mount

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#1 Matthew Paul

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Posted 17 August 2019 - 10:04 AM

I've been thinking through a million different ways of making a heavy duty equatorial mount. My budget is very limited, though I do have a blacksmith/welding shop and several lathes at my disposal. I've built machinery before, such as a 75lb mechanical power hammer.

My plan from the start was to use tapered roller bearings in the mount, as they can handle radial and thrust loads very well, and the bearing are easily adjustable. Depending on the quality of the bearing, accuracy may or may not be an issue. It's hard to tell without a working unit and an optic attached. 

I've run through many different ways of machining shafts and housings, and all of them wold work, but I have been trying to figure out an easier way. While sitting at my computer in the shop, I looked to my left and realized it had been staring me right in the face. On my mechanical power hammer I use an automotive wheel bearing for the rotating assembly. 

powerhammer.jpg

Stub axle/spindle and hub assemblies for trailers come in a variety of sizes, have tapered shafts, and bearing housings with flanges built in. They are very inexpensive, and can handle an incredible load. The one downside is the weight, but that can be worked around. The load ratings range from 1000 to 5000lb. I already have some new in packaging, rated at 2000lb with 1.75" shafts.

A pair of adjustable plates can be welded to the RA stub axle which would coincide with a thicker single plate on the mount base to allow for fine Alt adjustments on polar alignment, and a tab can be welded to the plates for your Az adjustment. 

The hub can be drilled and tapped for a friction/locking screw to push against the spindle. 

I can hob rudimentary worm gears from CNC plasma cut  1/2"aluminum sheet on my 13in southbend lathe, and then lap them to each other for improved accuracy. 

The worm gear could be centered on the hub using shims or set screws using a dial indicator to eliminate most runout, and the worm gear could be sandwiched between the Dec weldment and the RA hub, 

The worm wheel could be spring loaded in a housing which would engage or disengage with a cammed level, to act as the clutch, in combination with the friction/locking hold screw on the hub for each axis. 

The RA worm, or both axis could be driven by a NEMA23 bipolar stepper motor, controlled by an aurduino uno and driven by the appropriately sized driver for the motor. 

I think that it would be possible to build a mount with a capacity of at least 100lb if not 200lb for well under $1000 said and done, not including time spent. it would be used for visual and planetary photography. 

The main things stopping me from building a larger scope is the lack of wood working tools, and lack of funds for a large equitorial mount. in theory it could be driven on both axis and even auto-guided, if I could gather the knowledge, experience, and need for such a thing. 

The stub axles & hubs cost nothing as I already have them. Steel to fabricate the weldments for each axis is plentiful and would be free to cheep to me. The stepper motors and drivers would be a few hundred dollars for the pair. The worm gears and wheels would only cost me time and a coffee with breakfast for my friend to cut them out on the cnc plasma or mill. So, as it sits the cost would be around $500, allowing for an extra $200 in unexpected expenses and hardware.

The main benefit being that I would not have to machine housings and shafts, just weldments that would attach them and allow for the needed adjustments. The RA shaft could be drilled to allow it to be used as a peep sight for rough polar alignment if need be. 


I think that I might give it a go once I finish up this 6in DK. Just some thoughts that I wanted to get out of my head and possibly get some feedback on. The fabrication aspect is plenty doable for myself. 

-Matt

 


Edited by Matthew Paul, 17 August 2019 - 10:25 AM.

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#2 PrestonE

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Posted 17 August 2019 - 10:39 AM

Hola Matt,

 

It will really depend on what your end use will be...

 

Visual, this will be ok...

 

Photographic, maybe with the Lucky Imaging techniques...

 

But, with a CCD and longer exposures, likely not accurate enough...

Especially the bearings and Worm Gear...

 

Give us a better idea of your end use and we can better make recommendations.

 

Best Regards,

 

Preston


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#3 Matthew Paul

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Posted 17 August 2019 - 10:47 AM

It would just be used for visual and planetary "Lucky Imaging" 
I would not be looking to take 15 minute exposures with a F20+ Cassegrain on a home made mount... just record some video footage of the planets and enjoy the telescope visually. 


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#4 Garyth64

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Posted 17 August 2019 - 10:53 AM

"the lack of wood working tools"

 

With all those metal working tools to make your heavy-duty mount, what do you need wood for? lol.gif



#5 Matthew Paul

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Posted 17 August 2019 - 11:20 AM

"the lack of wood working tools"

 

With all those metal working tools to make your heavy-duty mount, what do you need wood for? lol.gif

That was my point in wanting to fabricate it rather than make it from wood. ;) 


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#6 jtsenghas

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Posted 17 August 2019 - 03:13 PM

Note that Clyde Tombaugh, the discoverer of Pluto, made a telescope mount from the crankshaft of a 1910 Buick! See here


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#7 roscoe

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Posted 18 August 2019 - 09:30 AM

Seems to me that if you can build stuff like that hammer, you'll have no problem with an equatorial mount!!  I think the idea of trailer spindles and bearings is a very good place to start, a whole lot of machining has already been done for you!!  Instead of boring a spindle for a peep-sight, you could just mount a sighting system outboard on one side or the other - could be a tube or just a couple of pins to sight along the tips..... and accuracy in worms and gears is mostly just a matter of time, slow but steady honing will get you pretty close, plenty fine for visual or light astro-photo work.

 

I was going to build a big mount based on farm tractor axles and bearings, and steel plate and pipe, which I have around, being a farmer-type, but then a big Cave Astrola mount found a home here, and..... rebuilding that is easier than starting from a pile of parts......


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#8 Stardust Dave

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Posted 18 August 2019 - 09:43 AM

"Note that Clyde Tombaugh, the discoverer of Pluto, made a telescope mount from the crankshaft of a 1910 Buick! See here."

 

Great video BTW, smile.gif


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#9 Sean Cunneen

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Posted 18 August 2019 - 10:47 AM

I too have a thing for big mounts... My largest had to pass the "Can I carry it to my car" test. Later amended to "Can I carry each piece to my car" and finally ended up with "With a break or two along the way". What I am saying is that being able to get the mount to viewing locations easily is a really integral part of enjoying the fruits of your labor. I found out after more than a few years of work that for me, there is an upper limit on how big is too big. So my new rule is "has to be able to be moved to a better spot by one person in the dark".

Here is my old old build thread;
https://www.cloudyni...-project/page-6

I am envious of your skills!
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#10 PrestonE

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Posted 18 August 2019 - 12:43 PM

Note that Clyde Tombaugh, the discoverer of Pluto, made a telescope mount from the crankshaft of a 1910 Buick! See here

Thanks for posting this JT...

 

I almost purchased his 16inch F10 with the stand many years ago...however, we could not

get an agreement on the timing of dissambling the entire wooden stand and steel structure

 

and, thus the purchase fell through...would have donated it to the George Observatory like the 11inch F16

refractor that I donated.

 

Best Regards,

 

Preston


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#11 geovermont

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Posted 18 August 2019 - 01:50 PM

Old motor vehicle parts are a staple of the heavier mounts in ATM Books 1 and 2. Lots of good ideas there. In particular, R. W. Porter's Chapter on "Fundamentals in the Design of Telescope Mountings; Rigidity" in Book 1 is very good.

 

George Springston


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#12 gotak

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Posted 18 August 2019 - 05:17 PM

You have a friction drive on that hammer.. why not do a friction drive? Might be easier than trying to make worm and wheel.
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#13 RaulTheRat

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Posted 18 August 2019 - 05:54 PM

Cool thread, that's a serious project but it sounds like you have the tools and experience to do it.

Probably you might as well spring for a set of worms though and use OnStep to control it all. If you're going to put all that effort into the heavy engineering parts of the mount, it seems to me it would be better invested in something capable of guiding and long exposures, unless of course you're absolutely uninterested in that.

These comments assume you don't intend this thing to be even remotely portable though. I think you can probably pull off an excellent GEM from the sounds of it, but the constraint of it being a fixed pier mounted thing will help a lot.
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#14 BGRE

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Posted 18 August 2019 - 06:12 PM

4140 machines easily with the right inserts feeds and speeds and is commonly used for shafts. Depending on the insert tip radius etc there may be a minimum cut depth required to ensure a good finish. 

Work hardening is almost never an issue with the right tooling feeds and speeds.

Even turning hardened shafts etc is very easy with the right inserts (preferably ceramic or CBN). However you may want to protect the ways when turning hardened materials particularly if the ways arent hardened.



#15 555aaa

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Posted 18 August 2019 - 11:43 PM

Maybe this will give you some ideas, it only has five machined parts, and those are all manual lathe work. I think that the trailer spindle/stub assy might give you some heartburn unless you can find a convenient location to attach your drive and friction elements and I think the bearings are pretty small actually. You need some sort of friction element to hold the mount still (for example a worm drive & clutch or a tangent arm & locking screw). My build below is clutchless.  But it sounds like you have some clear ideas of what you want to do. Have you also considered using pillow blocks? That is a very straightforward, minimal machining approach. You might also look into using a friction drive as mentioned above or have a look at Don's cable (capstan wire) drive. The engineering on the friction drive is non-trivial.

 

https://www.cloudyni...r-meade-16-acf/

 

Images from that are on my astrobin here https://www.astrobin.com/users/brucev/ - they are the ones that say 'custom offset equatorial'. There are some from my new mount so you have to look at the equipment list.


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#16 Matthew Paul

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Posted 19 August 2019 - 02:44 PM

Note that Clyde Tombaugh, the discoverer of Pluto, made a telescope mount from the crankshaft of a 1910 Buick! See here

I was unaware that he had made a mount like that. Clyde was an interesting person from what i have read about him. Thanks for he link.

 

Seems to me that if you can build stuff like that hammer, you'll have no problem with an equatorial mount!!  I think the idea of trailer spindles and bearings is a very good place to start, a whole lot of machining has already been done for you!!  Instead of boring a spindle for a peep-sight, you could just mount a sighting system outboard on one side or the other - could be a tube or just a couple of pins to sight along the tips..... and accuracy in worms and gears is mostly just a matter of time, slow but steady honing will get you pretty close, plenty fine for visual or light astro-photo work.

 

I was going to build a big mount based on farm tractor axles and bearings, and steel plate and pipe, which I have around, being a farmer-type, but then a big Cave Astrola mount found a home here, and..... rebuilding that is easier than starting from a pile of parts......

I have no real plans to do long exposure work as of now. That was pretty much my though process on the worms, though I have some other ideas now as well. As far as the old cave type of mounts, Ive been keeping an eye out for one of the old giant ones, but have yet to come across one. 
 

 

I too have a thing for big mounts... My largest had to pass the "Can I carry it to my car" test. Later amended to "Can I carry each piece to my car" and finally ended up with "With a break or two along the way". What I am saying is that being able to get the mount to viewing locations easily is a really integral part of enjoying the fruits of your labor. I found out after more than a few years of work that for me, there is an upper limit on how big is too big. So my new rule is "has to be able to be moved to a better spot by one person in the dark".

Here is my old old build thread;
https://www.cloudyni...-project/page-6

I am envious of your skills!

 

That's a neat mount, nice work. 

I lucked into a place where I could leave a pier/tripod and mount set up pretty indefinitely if need be, so I could really make it as heavy as It needed to be. 

 

 

Old motor vehicle parts are a staple of the heavier mounts in ATM Books 1 and 2. Lots of good ideas there. In particular, R. W. Porter's Chapter on "Fundamentals in the Design of Telescope Mountings; Rigidity" in Book 1 is very good.

 

George Springston

I'll have to pick up that series. I've been meaning to but never got around to it. 
 

 

You have a friction drive on that hammer.. why not do a friction drive? Might be easier than trying to make worm and wheel.

The thought had not crossed my mind until you pointed it out. On my belt grinder I use 10" and 12" contact wheels, which are solid aluminum hubs with a 1" thick 2" wide 70-90 duro outer layer. I bet one of those would work well for a friction drive. 
 

 

Cool thread, that's a serious project but it sounds like you have the tools and experience to do it.

Probably you might as well spring for a set of worms though and use OnStep to control it all. If you're going to put all that effort into the heavy engineering parts of the mount, it seems to me it would be better invested in something capable of guiding and long exposures, unless of course you're absolutely uninterested in that.

These comments assume you don't intend this thing to be even remotely portable though. I think you can probably pull off an excellent GEM from the sounds of it, but the constraint of it being a fixed pier mounted thing will help a lot.

Yes, Luckily I would not have to move it every time I wanted to use it. I'll have to look into the OnStep. it looks a bit more complex than I might be able to take on, but reading never hurt anyone. Thank you for the idea. Unfortunately I would not be able to purchase worm gears, my pockets are not that deep. 

 

4140 machines easily with the right inserts feeds and speeds and is commonly used for shafts. Depending on the insert tip radius etc there may be a minimum cut depth required to ensure a good finish. 

Work hardening is almost never an issue with the right tooling feeds and speeds.

Even turning hardened shafts etc is very easy with the right inserts (preferably ceramic or CBN). However you may want to protect the ways when turning hardened materials particularly if the ways arent hardened.

I'm a fan of 1045 personally, though the chromium in the 4140 would probably be better for something that would be left outside.

 

Maybe this will give you some ideas, it only has five machined parts, and those are all manual lathe work. I think that the trailer spindle/stub assy might give you some heartburn unless you can find a convenient location to attach your drive and friction elements and I think the bearings are pretty small actually. You need some sort of friction element to hold the mount still (for example a worm drive & clutch or a tangent arm & locking screw). My build below is clutchless.  But it sounds like you have some clear ideas of what you want to do. Have you also considered using pillow blocks? That is a very straightforward, minimal machining approach. You might also look into using a friction drive as mentioned above or have a look at Don's cable (capstan wire) drive. The engineering on the friction drive is non-trivial.

 

https://www.cloudyni...r-meade-16-acf/

 

Images from that are on my astrobin here https://www.astrobin.com/users/brucev/ - they are the ones that say 'custom offset equatorial'. There are some from my new mount so you have to look at the equipment list.

That's an interesting mount, and from the looks of the photos it works very well! My idea for the drive wheel/worm wheel would be sandwiched between the machined hub face and upper weldment that would bolt to the lugs and be centered on the inner hub. As far as bearing size, the housing is 2 3/4 with a 1 3/8 shaft. On the one that I have anyway.  Though larger would probably be better, I think it should do for what I'm looking to do. If I even make it happen. 



#17 Tom Dugan

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Posted 21 August 2019 - 07:47 AM

A somewhat less expensive, old school, and definitely historically appropriate bearing is babbitt. I've always wanted to give Allyn Thompson's method a try, but instead have a stable of Chinese GEMs.


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#18 Matthew Paul

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Posted 21 August 2019 - 08:44 AM

A somewhat less expensive, old school, and definitely historically appropriate bearing is babbitt. I've always wanted to give Allyn Thompson's method a try, but instead have a stable of Chinese GEMs.

Reading through that book is what got me started on this crazy train. I'd be curious to see what type of capacity a well built Babbitt bearing pipe mount would have at different sizes. The book made no mention of capacity other than increasing the capacity 5 to 10lb by using solid shaft on one part. 



#19 555aaa

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Posted 21 August 2019 - 12:33 PM

Reading through that book is what got me started on this crazy train. I'd be curious to see what type of capacity a well built Babbitt bearing pipe mount would have at different sizes. The book made no mention of capacity other than increasing the capacity 5 to 10lb by using solid shaft on one part. 

If you look at the telescope mounts from the 19th century which are still working, they mostly don't have ball bearings. They have plain bearings and in my experience it's mostly bronze. A 2" bronze oil impregnated bearing from MSC is only about $20. The bearings aren't the capacity limiting element, it is going to be the shaft diameter and the rigidity of the shaft to shaft connection. The shaft stiffness goes up with the fourth power of diameter. So going from 1.5" to 2" diameter is more than a 3x increase in stiffness. 1.5" to 3" diameter change is 16x increase in stiffness for solid shaft. For hollow shafts it goes with the cube of diameter. In my mind, torsional stiffness is as or more important than bending stiffness. The thing with bending stiffness is that it tends to be constant as the mount moves but in torsion, the mount is constantly being turned by the drive, and turned at different rates, so any torsional flex makes itself known. The bearing load rating is irrelevant and we use them way below their rating, but we need relatively large bearings because we are looking for very high rigidity. 


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#20 skywolf856

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Posted 21 August 2019 - 09:49 PM

Check these designs for ideas.

 

http://www.opticcraft.com/


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#21 skywolf856

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Posted 21 August 2019 - 09:50 PM

I like the pillow block method for self alignment and simple construction.


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#22 Matthew Paul

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Posted 25 August 2019 - 10:13 AM

I like the pillow block method for self alignment and simple construction.

That link has some really great information. 

Something interesting that I noticed is that the mounts with the tapered roller bearing pillow blocks hold a substantially larger weight than the traditional bearing mounts. 

They list the tapered roller bearing mount with a 1in shaft and a bearing spacing of 12" as having a capacity of 60lb, while the pipe mount with poured bearings and 2.5in solid shaft as only having a capacity of 52lb. 

Are you aware of how much influence the spacing of the bearings has on the capacity? 


The spindle and hub setup that I was looking at in my first post uses a 1 3/8 shaft at the smallest, with an outer spacing of 3 1/2"
The shorter spacing should help with rigidity as there would be less flexure in the shaft, and a far larger load bearing surface on the hub face than the pillow block setup, while still using the thrust and rotational bearing capacity of the tapered  roller bearing, correct? Plus the ability to preload the bearing of a threaded spindle type tapered roller bearing should reduce play and increase capacity? I'm no mechanical engineer. I spuupsoe I could sit down with the Machinery's Handbook and figure out some of the math.

The tapered roller pillow block looks like a great option, but I looked up the cost per bearing and it's not something that is in my budget, unfortunately. 



#23 Matthew Paul

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Posted 25 August 2019 - 10:15 AM

If you look at the telescope mounts from the 19th century which are still working, they mostly don't have ball bearings. They have plain bearings and in my experience it's mostly bronze. A 2" bronze oil impregnated bearing from MSC is only about $20. The bearings aren't the capacity limiting element, it is going to be the shaft diameter and the rigidity of the shaft to shaft connection. The shaft stiffness goes up with the fourth power of diameter. So going from 1.5" to 2" diameter is more than a 3x increase in stiffness. 1.5" to 3" diameter change is 16x increase in stiffness for solid shaft. For hollow shafts it goes with the cube of diameter. In my mind, torsional stiffness is as or more important than bending stiffness. The thing with bending stiffness is that it tends to be constant as the mount moves but in torsion, the mount is constantly being turned by the drive, and turned at different rates, so any torsional flex makes itself known. The bearing load rating is irrelevant and we use them way below their rating, but we need relatively large bearings because we are looking for very high rigidity. 

This is true. In the mechanical power hammer that I built, I used a hub type double roller bearing for the main portion of the rotating assembly, a pillow block bearing on the pinion where the linkage attaches, and 1"x1.25' oil impregnated bronze phosphor bearings on the six pivot points of the linkage. 


Edited by Matthew Paul, 25 August 2019 - 10:16 AM.


#24 Spectral Joe

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Posted 25 August 2019 - 12:49 PM

The weight ratings for bearings is at the rated rpm, not static.

#25 Matthew Paul

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Posted 25 August 2019 - 04:16 PM

The weight ratings for bearings is at the rated rpm, not static.

You can look up static load for tapered roller bearings. Which are 4-5x the dynamic load rating. 

 

 

IMG_4104.PNG


Edited by Matthew Paul, 25 August 2019 - 04:18 PM.



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