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Small Epoxy Granite Filled "Watchmakers Lathe" Design Question

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shermama96

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I'm a second year mechanical engineering student, and as a summer project I've designed and plan to build a small lathe from bar stock. I am doing this a learning experience and plan to validate all the design aspects with hand calculations and FEA/Modal analysis. The entire machine will only be 15 inches in length. I plan to start machining the bed next week I figured I would try and resolve some questions regarding the bed material and the E/G fill. I attached a PDF which shows an incomplete model, but should be enough for me to get started.

The bed is to be constructed from a 2"x 2" square thick walled tube. A piece of cast iron Durabar will be bonded to the tubing with machine epoxy and countersunk machine screws will be used to hold it in place. The flat head screws will pass into the center of the tubing and act as anchor points for the E/G. I'll then machine a dovetail into the CI bar and surface grind the profile.

Bed material:
- I was using other small machines as a reference, and noticed the Taig lathe was made from an aluminum extrusion and also filled with E/G. For the longest time I planned to use aluminum for the bed, and now I can't seem the understand why, considering steel will have a significantly higher young's modulus and damping coefficient. The only fighting chance extruded aluminum has is it has a much tighter tolerance on straightness and twist from the distributor, and I'm worried grinding the CRS tubing straight will be difficult as it flatten out of the magnet and then return to it's original bowed state.

E/G Mix:
-I have read several threads on this site about different epoxy granite mixes. Most are either for bulking up very large machines, or casting machines totally out of E/G. Because my machine is so small and the cutting forces won't be as extreme as most of the machines on here, I was hoping the follow a post by Taz on this site where he sieves locally sourced decomposed granite to obtain different grain sizes. Does anyone have a rule of thumb on aggregate sizes and ratios to use for a slightly more simplistic E/G fill for such a tight cavity.

-The inside dimension of the tubing is going to be around 1.5" square. Since the machine will only be 15 inches long, and only 8-10% of the volume taken up will be epoxy, I was hoping to avoid buying the expensive West Systems epoxy, most of which will go unused. I have some cheap general epoxy resin from ebay that I planned to use, but I am worried the performance of the E/G will suffer dramatically due to the poor quality of epoxy. At the same time I'm assuming the epoxy acts purely as a binder and might not have such a huge effect. I was hoping someone could speak on the effects on the mechanical properties of the E/G they might have done tests on that didn't use the west systems and maybe used the generic cheapo stuff.

Anyway, any help would be greatly appreciated.

Thank you in advance.
 

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ACHiPo

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#2
Cool project! Look forward to following it.

I made a turntable for playing vinyl LPs (another application where precision and vibration damping is important). I investigated using polyester resin filled with bentonite (cat litter), which reportedly has fantastic vibration damping properties. I ultimately decided to use a constrained layer + glulam construction for ease of fabrication. It turned out well.
 

Dave Paine

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Interesting project.

If you want the weight and stiffness of granite, consider using solid granite and attach to the base or tubing. You should have companies which make granite counters in your area. They always have scraps/offcuts/broken pieces of granite in their dumpsters. Normally they are happy to let someone pick out some of these, since they have to pay to get this hauled off.

I do a lot of wood turning and often mix epoxy with other solid items for embellishment. A recent example to illustrate.

Bowl_Norway_maple_completed_top_view_9025.jpg

A challenge for me has been getting air bubbles out of the epoxy. The cheap epoxies I have used are fine to spread over a surface to glue one piece to another. They are too viscous for the resin to flow evenly between granite particles.

I have used System Three resin, this is not as viscous but I still had too many voids from air bubbles which did not get to the surface, or from the resin not getting between particles.

I now use a resin by Smooth-On called EpoxAcast 690. This is much thinner than the System Three resins. Also has a long pot life, 5 hours. It does a good job getting between the particles and releasing air bubbles.

Smooth-On EpoxAcast 690 resin

Resins are not cheap. The 690 resin is mixed by weight not volume. 100 parts of A to 30 parts of B by weight. You need a decent digital scale. I now find I prefer the accuracy of mixing by weight.

The 690 resin will run into the particles slowly. It takes at least 24 hours before it is cured. If you need to sand any surface it must be fully cured or it does not sand and will gum up any abrasive.

You also need to seal the bottom and sides or it will run out slowly as it cures. You may need to make a form from some plywood. Just seal the joints. Masking tape and hot glue work for me.

I hope you post your build progress.
 

shermama96

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I plan to take photos of every step along the way so I'll be sure to update a forum page.

It's also worth noting that I'll be doing this at my university so I'll have access to almost any equipment I would need, including a vibrating table for removing air from the granite mixture.
 

MSBriggs

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Also considering putting the mixture into a vacuum, such as a lab bell jar, if you access to one large enough. Whatever plan you select, it's probably worth doing a test cast of the mixture first.
 

f350ca

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Neet project.
If the cast bar for the bed is thick enough I'd consider blind drilling and tapping from the bottom, then bring the fasteners up from the base. The bolt heads and countersinks will trap dirt and spread it under your carriage.
The bed design is the same as a Hardinge HLV, they use a longer carriage, more wear area and longer gibs to guide the carriage.
Steel and cast iron have SLIGHTLY different thermal expansion, probably not an issue but might cause a slight warp as temps swing, be most evident when machining.
Why not use durabar for the base too.
Keep us up to date.

Greg
 

shermama96

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Wow I like the attention this is getting. I also plan to make available all the prints when I'm done. I doubt anyone will ever actually make the thing though, but figured it couldn't hurt.

In regard to the tailstock, there definitely will be one, I'm just playing with ideas in my head for both clamping and head stock tail stock alignment. One issue I am having is finding a 0 Morse Taper "socket". I don't have the small tooling to turn and ream a 0 ]Morse Taper and I've been trying to think of a solution to maybe buy a stock part used on another lathe and then integrate it into the overall design. I really would like to keep the Morse Taper so I can use standard tooling and not have to custom make arbors for every chuck or center I'd like to use down the road.

For the screws I figured I'd leave the head slightly proud before grinding, fill the allen drive with JB weld and then grind all the fasteners flush with with the dovetail to avoid chip and oil build up. I think I might leaning towards your idea now though. I know on the Hardinge lathes the dovetail is removable. Since the E/G is going to ruin that possibility by casting around the fasteners, I figured I'd add a layer of epoxy between it and the square tubing to help dampen vibrations.

Initially I planned to use a big piece of square stock for the bed, but it started becoming expensive. I would need a piece as wide as the dovetail which is 3 inches. A 3x3 piece of cast iron bar 15 inches long was several hundred dollars which didn't even include shipping, where as a steel or aluminum square tube is 20-30 and the materials for EG will be hopefully 30 dollars max if I can run with the cheap epoxy. I also planned on bonding and fastening the entire lathe to a slab of granite if need be, still haven't decided.
 

westsailpat

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Hats off to you @shermama96 , I like your choice of career path . Your observations about the E/G fill I find very interesting because it reminded of when Warner & Swasey was experimenting with welded up bed frames and then filling them with a type of concrete . They proved that their lathe could produce a finish that could negate grinding because of the fact there was no vibration . I would love to see a picture of your lathe when it's done , Are you planing on a thru hole spindle ?
 

hman

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Couple thoughts ...

(1) As far as Taig using aluminum extrusions, I'd strongly surmise that it's because a (custom) extrusion is a net or near net shape. So they can use it easily. Yes, it's "ringy" (my non-sophiosticated hillbilly engineering terminology). So they fill it with the E/G.

(2) I've filled the column of my Harbor Freight 44991 mini-mill with E/G. Just for fun, I decided to add some "long distance" stiffness by including 1" drywall screws and some ~½" 4-40 screws in the mixture (along with glass beads and aquarium gravel). Didn't do any sophisticated testing of the final result, but it does seem to have helped with rigidity and vibration damping. I used West Systems epoxy with the slow hardener, in hopes that the long cure time would allow air bubbles to escape. When mixing it , I stirred slowly, trying to reduce introduction of bubbles. Once poured, I used a vibratory engraver (with an extension) to agitate the mix. A bunch of bubbles did float to the surface before the epoxy cured. And I was pleasantly surprised by how well the epoxy wetted out the filler materials.
kHPIM2076.jpg
 

shermama96

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Hats off to you @shermama96 , I like your choice of career path . Your observations about the E/G fill I find very interesting because it reminded of when Warner & Swasey was experimenting with welded up bed frames and then filling them with a type of concrete . They proved that their lathe could produce a finish that could negate grinding because of the fact there was no vibration . I would love to see a picture of your lathe when it's done , Are you planing on a thru hole spindle ?
For the spindle I plan to use an ER-16 collet chuck that I’ll cylindically grind to the proper bearing fit.

The physics dept at my school has an ancient wire edm designed for burning out deep holes. Instead of annealing the through hardened collect chuck to drill its center out only to re-harden it, I’ll burn out the center of the collect chuck to get my through spindle.
 

JohnG

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I've seen similar construction used in industry for large, high precision machine tools--8" x 8" x 1/2" wall steel tube 20' long for a base with laser interferometers to guide the cutting head. The machine was going to grind and polish steel rolls used to make super calendared paper. Here's what I remember.

The steel was all normalized before fabrication. The top block which was machined to be the precision way was also normalized steel and was continuously welded at both bottom corners to the base tube. The difference between fabricating a composite beam like this with intermittent fasteners and continuous adhesion is night and day. The machine screws you plan on using will do nothing to improve the bending moment of this beam but will provide some shock resistance which epoxy will lack. West System epoxy gets my vote for adhesion to steel, but this isn't my specialty. I always just had base fabrications welded.

I'm not sure about filling the core. You may already have looked at the construction of One Way lathes which seem similar to me. They rely on the rigidity of a large steel tube, and that simple formula worked well for the machine builders I worked with when I built tools for industry.
 

shermama96

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I've seen similar construction used in industry for large, high precision machine tools--8" x 8" x 1/2" wall steel tube 20' long for a base with laser interferometers to guide the cutting head. The machine was going to grind and polish steel rolls used to make super calendared paper. Here's what I remember.

The steel was all normalized before fabrication. The top block which was machined to be the precision way was also normalized steel and was continuously welded at both bottom corners to the base tube. The difference between fabricating a composite beam like this with intermittent fasteners and continuous adhesion is night and day. The machine screws you plan on using will do nothing to improve the bending moment of this beam but will provide some shock resistance which epoxy will lack. West System epoxy gets my vote for adhesion to steel, but this isn't my specialty. I always just had base fabrications welded.

I'm not sure about filling the core. You may already have looked at the construction of One Way lathes which seem similar to me. They rely on the rigidity of a large steel tube, and that simple formula worked well for the machine builders I worked with when I built tools for industry.
I was hoping to braze or weld the composite beam together but as you said a normalizing cycle would be essential afterwards, especially with CRS. I don’t have an oven even close to large enough to run normalizing cycles though. The machine epoxy and fasteners seemed to be my next best move.
 

whitmore

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I'm a second year mechanical engineering student, and as a summer project I've designed and plan to build a small lathe .
-I have read several threads on this site about different epoxy granite mixes... I have some cheap general epoxy resin from ebay that I planned to use, but I am worried the performance of the E/G will suffer dramatically due to the poor quality of epoxy..

That oughtn't be a major concern. If you expect vibration to be damped, it must be absorbed (not reflected) by the
bed, so a material that is soft and low density (the epoxy) is less effective than something with similar density to steel and
granite. If it were my project, I'd consider plaster of paris or hydraulic cement (both are intended to set up without
any volume change). Do you plan to make any test samples? Even just a phonograph cartridge and dropping a BB
on a sample can tell you a lot by how it rings or thuds.

Most of what you pay for in an adhesive is not the quality that matters for this application. Crack resistance, because
the whole thing is under pressure while it cures, and has a steel skin, is one of the irrelevancies. You don't require
tensile strength.
 

JohnG

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OK. Makes sense. I can see that the hollow steel tube gives you a high moment of inertia, but I've also found hot rolled steel tube to be unstable for small, precision machined pieces. It's full of internal stresses that relieve themselves anytime you cut it. If you do use it, orient the weld in the least important face, most likely the bottom, and cut it as little as possible. I'd mill the top face flat, drill & tap for the mounting screws, grind it flat, then not do anything else to the tube, and count myself lucky to get one true reference surface. My antivirus software is blocking your PDF, so I can't see if you've already thought of this. On the other hand, you might just start with a 2" x 2" cold rolled bar, particularly if you need to work off more than one face.
 

Asm109

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I'm worried grinding the CRS tubing straight will be difficult as it flatten out of the magnet and then return to it's original bowed state.

Thank you in advance.
That is why you lay the tube on the chuck and then place shims under the middle of the tube so it does not distort when the magnet is engaged.
Shim it, clamp it, take a light pass. Release the chuck, flip the part, re shim, cut the second side.
Release Flip back to first side and take another cut. Rinse, Repeat until it is as straight and parallel as you require.
 

shermama96

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OK. Makes sense. I can see that the hollow steel tube gives you a high moment of inertia, but I've also found hot rolled steel tube to be unstable for small, precision machined pieces. It's full of internal stresses that relieve themselves anytime you cut it. If you do use it, orient the weld in the least important face, most likely the bottom, and cut it as little as possible. I'd mill the top face flat, drill & tap for the mounting screws, grind it flat, then not do anything else to the tube, and count myself lucky to get one true reference surface. My antivirus software is blocking your PDF, so I can't see if you've already thought of this. On the other hand, you might just start with a 2" x 2" cold rolled bar, particularly if you need to work off more than one face.
This is the issue I'm running into now, I can't seem to source steel tubing that would be ideal for this application.

The hot-rolled steel has large corner radii that would compromise some of the parts functionality, its wall thickness is non-uniform, it lacks straightness in every sense, and its composition is usually less than perfect.

On the other hand I can't even find thick walled CRS square tubing. Even if I could, I'm worried about the dimensional stability of the cold rolled steel, although it only needs several small holes drill in it. Since the foundation for this machine's accuracy is the bed, I'm worried even thermal cycling in the winter could induce some slight movement. I don't have the facilities to stress relieve the steel before grinding either so welding sections up is out.

I don't want to make the bed from solid solid steel either because I'd like to get the vibration damping properties for the epoxy granite and I don't want to make it out of solid cast iron because of cost. This is also supposed to be a unique learning project so I wanted to try some unique and less conventional methods to make the thing instead of just milling it from solid.


If anyone has any ideas I would love to hear them. You've all been very helpful so far.
 

whitmore

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This is the issue I'm running into now, I can't seem to source steel tubing that would be ideal for this application.

The hot-rolled steel has large corner radii that would compromise some of the parts functionality, its wall thickness is non-uniform, it lacks straightness in every sense, and its composition is usually less than perfect.
Purity of composition is no problem (it's gonna be packed with rocks, you know), and if the project requires corners that aren't
in the structural tube, get some corner-like stock and weld it on. Normalizing stressed material (and welds) is basically the
same thing, just a few slow ramps up and down in temperature. An earthen pit and ten pounds of charcoal would
be useful. Dry ice, too.

Would two C-sections welded to a square, or a couple of ell-sections welded on as rails, be an improvement?

Straightening and sizing afterward is always going to be a problem, probably there's a surface-plate in your future. Or,
maybe you can mount the bed between centers and rough-machine it well enough on a mill? At least you could
get things parallel that way.
 

shermama96

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I was wondering if anyone has experience drilling granite on a millling machine. Can it be done with specialized tooling without the hammering action of a rotary hammer?

I’m thinking about using solid granite for the machine base and need to drill some holes for the dovetail and for the rack gear mounting fasteners. I’d be drilling relatively shallow holes for 8-32 and 4-40 threaded inserts.
 

DAT510

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#21
Cut the faucet holes in our counter tops with a diamond hole saw and water from a squeeze bottle. I'm not sure you would want the grit from the granite on your mill. Could raise havoc with the bed ways if any gets in there.
 

shermama96

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Cut the faucet holes in our counter tops with a diamond hole saw and water from a squeeze bottle. I'm not sure you would want the grit from the granite on your mill. Could raise havoc with the bed ways if any gets in there.
I was planning on using water as coolant with a diamond core drill and vacuum the grit right as it comes off.
 

francist

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I've drilled marble a number of times on a drill press, works fine. i realize marble is a much softer than granite, but the technique was the same. I don't think I would do it on a milling machine. As well intentioned as the idea sounds, vacuuming it off or trying to collect it at source will likely not be as effective as you envision. If done wet, the droplets sling all over the place and before you know it the machine is coated. Similarly dry, even with close proximity collection there is likely to be "leakage". At least this has been my experience with it.

-frank
 

whitmore

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I was planning on using water as coolant with a diamond core drill and vacuum the grit right as it comes off.
That might work, but if the granite is big enough, you won't need coolant (it's a heat sink). Just go slow, and work the vacuum
nozzle around the cut. I'd prefer a drill press (easier to clean after). Better still, an ultrasound drill (it just pecks up-and-down
very fast).

A 'core drill' is like a holesaw? So it's gonna be a wide through hole?
 

shermama96

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That might work, but if the granite is big enough, you won't need coolant (it's a heat sink). Just go slow, and work the vacuum
nozzle around the cut. I'd prefer a drill press (easier to clean after). Better still, an ultrasound drill (it just pecks up-and-down
very fast).

A 'core drill' is like a holesaw? So it's gonna be a wide through hole?

I saw some pretty tiny core drills online. I figured I’d be able to hit the slug left in a blind hole with a punch at an angle to knock em off but I just know that won’t work out as nicely as I think it will
 

westerner

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#26
My goodness, what an amazing wealth of experience in such varied areas of expertise! It never ceases to amaze me, how many VERY sharp people hang out here.
That said, my hillbilly mind got snagged on this concept
including a vibrating table for removing air from the granite mixture.
In my experience with concrete, as in slabs on grade, formed stem wall/foundations, or grout into block, the vibration applied is CRITICAL to the strength of the final product. This strength relates to tensile, or compressive. How it relates to the OP issue, I do not know. Sorry for the intrusion. Please, carry on.:encourage:
 

shermama96

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Not sure if anyone cares but I'm almost finished with the bed and ways figured I'd share some photos. I went with a steel bed and welded some reinforcements. IMG_4161.JPG
Here's the bed as of today. I had the steel stress relieved twice, a very nice local shop hooked me up free of charge.
The cast iron is fastened on bonded to steel bed with Devcon metallized epoxy.

IMG_4152.JPG
A shot of the roughed dovetail. The profile was milled after bonding and was left oversized ~.002 and will be ground to final size tomorrow on a sine mag chuck.


IMG_4130.JPG
The epoxy granite setting up on a vibratory table.

IMG_4052.JPG
Cleaning up the feet after the second, post weld stress relieving. You can see some of the welded reinforcements. It was the first time I'd picked up a mig gun so try not to laugh too hard.

Capture.PNG
Working on the motor mount and tensioner this week.
 

westsailpat

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Looking good Shermama96 , keep up the good work !
 

ACHiPo

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Nice! How deep is the granite epoxy? How many layers did you cast it in? How much did it tweak the cast iron bed?
 
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