Design In Progress

[JoeDawg]

I've accepted the fact the I am eventually going to have to build a cnc mill from scratch, not because I need another mill (got a cnc'd taig), but because i need to design and build something else (mechanical engineering student, classes and theories and no application is driving me crazy)

I'll present my general ideas and some questions that i have and would very much appreciate any helpful input. I want something that can remove material at over 2x the rate of my taig with .005 accuracy and virtually no backlash

LM
I have pretty much decided on thk blocks/rails and C7 ballnuts with a zero backlash set-up, hopefully with screw mapping i can get my acuracy down to .001". What are the pros/cons of preloaded vs dual ballnuts?

Rigidity/Frame
I am thinking a steel tubing weldment c-pillar. I considered epoxy granite but it seems much easier to build a square rigid frame out of steel. The rigidity ofsteel is going to help my capabilities more that the vibration reduction of epoxy granite right? Hopefully i can get the rail mounting surfaces ground flat and the mounting holes precisioned with a HAAS here at school. That and precise squared angles gives me a great frame right? I also considered simply filling the tubing with e/g, but the steel will much more thermal expansion that the steel... so i think that's out... what are some other options?

Spindle/Motor
Completely unsure, if i am beefing up everything else i guess i should get a beefier spindle, any suggestions?

I have access to welding supplies and a fixture table, so it shouldn't be to hard to precisely jig up and weld the frame it shouldn't it? My taig is 2.5x3/16 square tubing and i want considerably more rigidity so 4x.25 should do nicely shouldnt it? I will do deflection analysis on my design before i buy materials... I am not oversimplifying the process am i?

I know i need angular contact bearings. I am pretty sure i need doubles (or 2 single, if theres any difference?) But when I look for bearings they are either $10 products that dont mention accuracy or $200 precision grade. There has to be a happy medium. How much should i be paying for bearings? Can i get good bearings for less than $50/axis? If so,where?

I have though more into it but i need to hit the sack for tonight. Any input would be greatly appreciated. Thanks

[ckelloug]

Find a copy of Precision Machine Design by Alexander Slocum at MIT. Actually, a lot of his material is available on the web if you google for it. That book will teach you a lot about what you need to know.

Match ground angular contact bearings are preloaded correctly when they are installed. Unmatched angular contact bearings aren't likely to keep the runout down. A pair of match ground angular contact bearings will cost between $200 and $400 (USD) for the kind of spindle in a bridgeport mill.

If you are truly trying to built an accurate machine, get a set of Timkin-Fafnir, Barden or NSK bearings in the ABEC-7 grade.

Bearings less expensive that these precision angular contact bearings are not likely to be angular contact bearings. I do believe that taig uses deep groove ball bearings which are a lot less expensive than the correct angular contact bearings but have low thrust capabilities and less accuracy. You might be able to get by with abec-3 deep groove bearings at the very worst but spindle runout will not be as good as the spindle of a good machine.

Shooting for an accuracy of .005 is pretty low. You can hold that with a file and eyeball. You can also buy a better machine than that at Harbor Freight.

Several people have had good results filling machines with E/G. Bob Warfield posted about filling his Industrial Hobbies mill column. This is easy to do as the formulation doesn't need to be good, it just needs to have enough filler to keep it from being cost prohibitive.

If you are obsessed with damping and not interested in E/G, get Bamberg's thesis via googling(He's a student of Slocum). He's done damping by placing concrete in a rubber bag inside the machine.

Cast iron is better behaved for machine construction than steel due to improved damping. These a look at dura-bar and versabar. These are continuously cast iron sections which are excellent for this type of thing.

Keep in mind that flexural rigidity isn't necessarily the problem in the taig. It's also likely to have low compressive and tensile rigidity too. Note: It is very hard to get a piece of metal that isn't an inch or two thick truly flat or straight so keep that in mind as you pick materials.

Best of Luck,

Cameron