Small Epoxy Granite Filled "Watchmakers Lathe" Design Question

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.
 

Attachments

  • Lathe Assembly.pdf
    786.2 KB · Views: 95
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.
 
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.
 
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.
 
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.
 
Here's a thread on bentonite in polyester for a turntable base:
http://audiokarma.org/forums/index.php?threads/resin-bentonite-clay-plinth-build.530544/
S523_zps2td9o3fj.jpg
 
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
 
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.
 
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 ?
 
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