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Small E/G Filled "Watchmakers Lathe" Design Question
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 was having trouble attaching a PDF and had to instead post a pixelated screenshots of the machine.
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.
Re: Small E/G Filled "Watchmakers Lathe" Design Question
Regarding your worries about the quality of the epoxy you've purchased...
Why don't you reach out to the manufacturer or seller for some specs on the epoxy. They should be able to tell you what the strength of it is.
Also it looks like whatever pics you attached didn't stick around. The same thing happened on my recent build post. You might want to try again so folks can see your build!
There's a lot (maybe too much) info in the epoxy granite mega thread but I did find a post (#364) on this page to be super helpful for a quick intro. It might help with general planning of your project.
https://www.cnczone.com/forums/epoxy...-frame-31.html
Re: Small E/G Filled "Watchmakers Lathe" Design Question
FYI, the underneath structure of the Taig lathe is an aluminum extrusion, but the ways/dovetails on top are steel.
Re: Small E/G Filled "Watchmakers Lathe" Design Question
On the epoxy granite mix question, I recently completed a CNC frame using epoxy granite (and other materials). For an easy and effective option, Home Depot sells 50lb bag of premixed sand and granite stones for around $5. It works well.
On the resin, not all epoxies are the same. There is a huge variation in hardness, stiffness, viscosity, temp resistance, cure time, finish quality etc. If you aren't going to invest in a quality commercial resin, my advice would be to find one with a long cure / working time. This way, you'll be able to apply heat with a hair dryer or heat gun to thin and level it out without causing it to bubble up and cure too fast.
Also, it is worth noting that many epoxy granite builds (including mine) have a steel or aluminum skeleton in the middle. It's a good way of adding strength and saving money. My gantry is made of carbon fiber, epoxy granite and steel and it is incredibly strong and stiff.
Re: Small E/G Filled "Watchmakers Lathe" Design Question
Most people I have read consider that the epoxy and aggregate for damping purpose alone are not critical as the purpose is to refract and reflect and hence diffuse vibration inducing impulses.I would expect little damping from tube filling though as the metals will carry most loads and so there vibrational characteristics will dominate.CI considered to have good damping properties.More damping probably best achieved by adding to more metal, to add stiffness and mass.
Grinding : you should shim out the gaps before clamping or will return to original shape as you suspect.
Hot rolled is considered a better option than crs as less stresses and machines better.Best to do a bit of test machining first on both types.
Many people including taig (for there mill ) weld strip to there tube and then grind the strip.
Its a difficult project with many potential problems to overcome,a real learning experience.Good problem solving.As opposed to wishing you luck.
Re: Small E/G Filled "Watchmakers Lathe" Design Question
Quote:
Originally Posted by
shedbob
Most people I have read consider that the epoxy and aggregate for damping purpose alone are not critical as the purpose is to refract and reflect and hence diffuse vibration inducing impulses.I would expect little damping from tube filling though as the metals will carry most loads and so there vibrational characteristics will dominate.CI considered to have good damping properties.More damping probably best achieved by adding to more metal, to add stiffness and mass.
.
You are neglecting that the whole system contributes to damping, not only the load bearing members. Taking your assertion to an absurd conclusion, an EG filled metal wind chime would make the same sound as the original. But it doesn't. Once you have met your stiffness and strength goals, adding more of the same metal is a diminishing returns proposition. You might have to add so much material that the machine can't be moved any more.
This is a good if somewhat lengthy read: http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf
I agree with everything else you said though. Since you mentioned Taig, a bit of trivia: the Taig lathe bed extrusion is concrete filled...
bob
Re: Small E/G Filled "Watchmakers Lathe" Design Question
Quote:
Originally Posted by
rowbare
You are neglecting that the whole system contributes to damping, not only the load bearing members. Taking your assertion to an absurd conclusion, an EG filled metal wind chime would make the same sound as the original. But it doesn't. Once you have met your stiffness and strength goals, adding more of the same metal is a diminishing returns proposition. You might have to add so much material that the machine can't be moved any more.
This is a good if somewhat lengthy read:
http://www.mech.utah.edu/~bamberg/re...e%20Design.pdf
I agree with everything else you said though. Since you mentioned Taig, a bit of trivia: the Taig lathe bed extrusion is concrete filled...
bob
I agree.
I can state from personal experience that filling a metal tube with epoxy granite changes the frequency and reduces resonance. You can instantly hear the difference. As a general point, large voids increase vibration. That's how musical instruments work. You fill in the hole in a guitar and it loses it's ability to amplify sound.
Epoxy granite has superior damping properties to any metal (or even pure granite) so..... if vibration damping is the goal, more epoxy granite with less metal is better. You always need some metal though as epoxy granite does not hold screw threads very well.
Epoxy granite should not be thought of as a weight saving option over metal. My epoxy granite castings weigh a ton (literally). Once the thickness is adjusted to account for the difference in strength (vs steel or cast iron), there will be little to no weight reduction. I had to buy a shop crane just to move my gantry onto my table...
If the goal is to reduce weight without reducing stiffness, you need to switch to a material with a better strength to weight ratio. E.g. a 1/2" aluminum bar is as stiff (or stiffer) than a 1/4" steel bar but weighs 1/3 less.
BTW, the main purpose of the sand / granite mix is to reduce cost and increase surface hardness. It would cost a fortune to make even a small machine frame with pure epoxy. Even with the addition of 500lb of sand / granite mix plus 50lb of carbon fiber, it still took nearly 15 gallons of resin to fill my machine frame casting.
Re: Small E/G Filled "Watchmakers Lathe" Design Question
Find concrete design course material from civil engineering department. A second year concrete class is very relevant to epoxy granite. This will help understand how to mix and grade the aggragates and to get a compact strong structure.
Good luck. There is alot of papers on epoxy granite and dampening also.
