Author Topic: Thinking about trying to build an epoxy granite CNC mill.  (Read 560 times)

Offline S. Heslop

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Thinking about trying to build an epoxy granite CNC mill.
« on: August 16, 2017, 11:49:21 AM »
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.

Offline BillTodd

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Re: Thinking about trying to build an epoxy granite CNC mill.
« Reply #1 on: August 17, 2017, 12:03:19 PM »
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


Bill

Offline PK

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Re: Thinking about trying to build an epoxy granite CNC mill.
« Reply #2 on: August 17, 2017, 05:19:19 PM »
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

Offline S. Heslop

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Re: Thinking about trying to build an epoxy granite CNC mill.
« Reply #3 on: August 18, 2017, 02:52:08 AM »
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.

Offline PK

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Re: Thinking about trying to build an epoxy granite CNC mill.
« Reply #4 on: August 18, 2017, 05:44:13 AM »
Routers are a bit of a sweet spot for DIY because the large envelope makes them expensive to ship (and hence expensive).

Offline BillTodd

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Re: Thinking about trying to build an epoxy granite CNC mill.
« Reply #5 on: August 18, 2017, 06:53:57 AM »
A small envelope router/mill would do most of my own cnc jobs. ATM i have a  Taiwanese bridgeport copy with two axis cnc augmentation (SWI prototrak edge) that is so  frustratingly close to all I'd ever need that I'm seriously thinking about adding the third z axis on the back of the mill. (which reminds me about chase up JS's motor-spindle conversion)

Another thought:
if you fabricate in steel, it might be worth packing with filler to add mass and damp vibrations. Consider using used garnet from a waterjet; the stuff will be free from any waterjetter and is amazingly dence (far heavier than sand) - it would need drying and probably binding with a polymer glue
Bill

Offline PK

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Re: Thinking about trying to build an epoxy granite CNC mill.
« Reply #6 on: August 18, 2017, 07:38:53 AM »
:-) I guess you have to find your own balance between 'I just want a cnc machine the easiest/cheapest/fastest way' and 'there's a whole bunch of things I'd really like to experiment with building a cnc machine from scratch'....
Any way is good so long as it's a path you've actually chosen to go down...

I confess I've been tempted to add some epoxy concrete to the head of my 'modified beyond recognition' X3....
PK