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Thinking about trying to build an epoxy granite CNC mill.

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S. Heslop:
A couple weeks ago I was thinking about a use I might have for a 3d printer, but for the sake of space and also making it more useful I figured why not also make it a CNC router. Anyways one thought led to another and i've spent the last few days looking at all the various epoxy granite gantry mills out there. But i've got a few questions I couldn't find answers for.

The big one is 'whats the deal with linear bearings?'. CNC guys love using these over regular slideways, but from what i've read it seems that to survive the cutting forces of a mill you'd need to get the good ones. A quick look at prices made me consider alternatives. My first thought is that it's just the convenience, but then if convenience is the objective then why not just buy a ready made machine to convert to CNC.

The other questions are about epoxy granite. More specifically the shrinkage, stability, and if thermal expansion with changing weather will cooperate with the metal slideways. The main reason epoxy granite is appealing is because the alternative would be welding it and dealing with the stresses from that.

BillTodd:
There is a thread on cnczone, probably the better part of a million pages long by now, but somewhere in the first 1000 pages are many attempts to get an epoxygranite thing that is much stronger than wood. After reading , i think i would just go andbuy a small secondhand /scrap cnc machine to convert.


I repair and occasionally maintain, a good few cnc machines - so if you are going to build one… .

ball guides and screws - I've replaced guides that had  been red with rust and noisy for months while the machine turned out perfectly usable parts - if you can't afford the best available, buy cheap, they'll still work.

thrust bearings and rigid mounts are vital - backlash cannot be fixed in the code.

use the biggest and best servos and motors you can get

use a good spindle  (the last one we had replaced cost just under £6000 exchanged - but you might want to use something cheaper )

don't spare the coolant - cnc does a lot of cutting in a hurry so keeping the swarf clear of the cutter is vital  - buy a mop 'cos it is always on the floor

PK:
Having built a small router from scratch http://caswa.com/cncathome/router.html and converted quite a few machines. I second the advice... If you can buy a manual machine with the machining envelope you want, then it's much easier to convert it than to build from nothing..
PK

S. Heslop:
Thanks for the info, and yeah I think you're right. I've done some more reading, even chipped away at a part of that thread, and found alot of the info to be conflicting and vague but with few success stories. CNC guys love to post charts and numbers that don't seem to mean a whole lot.

I also drew up the general shape of the machine I had in mind to figure out prices and options and I really lowballed the first price estimates. It'd end up costing about as much as a second hand mill in half-decent condition and thats without including the servos/steppers, controllers, and ball screws.

Maybe I aught to scale it back to a router.

PK:
Routers are a bit of a sweet spot for DIY because the large envelope makes them expensive to ship (and hence expensive).

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