Designing DIY CNC Milling Machine

spumco wrote:

It may work, but I wouldn't trust it to remain flat over that distance for multiple uses over a period of time.

Box levels are made from aluminum extrusions, and extrusions have residual stresses from the manufacturing process.  Machining two parallel faces helps eliminate some of the compression forces remaining in the outer skin, but I'd be afraid of thermal movement while handling it or using it on a hot day vs. a cold evening.  Plus the fact that you may need some force to impress a flat surface in to some epoxy and the aluminum would flex more than a steel, cast iron, or granite device.

 

Oh, that's very valid argument. I haven't considered that this will quite a lot of surface and possibly require some significant force. Definitely won't go that route.

I'd like to keep the costs down, since this is purely a hobby but I already built un-square, un-flat and bendy CNC router that's barely usable - and whole goal of this exercise is to make something that's slightly better this time.

Thank you for not letting me make dumb mistakes!

I'll reconsider my options, start looking for real steel straight edge and also call around to some cemetery-stone guys.

If I'll be able to grab something thats, let's say, 1000x300mm - what do you think about using such granite chunk to imprint the shimming for both rails at the same time? I'd feel much better doing both rails at a time, with guarantee that these will be flat (at least as the slab is). Doing each rail separately with straight edge leaves a lot room for error.

One last question, when it comes to granite tombstones - will _any_ shop be good enough? Like, is the granite processing quite universally good enough or should I be looking for someone with some specific equipment? In other words, how to look for a tombstone that's flat enough?

And thank you again for all the words of caution!​​

I think your idea of using a single slab to imprint both rail surfaces at once is excellent.  They would certainly be co-planar, and getting them aligned & parallel would be much simpler from there on.  Just do a test to make sure whatever you use as a mold release will keep you from permanently bonding your casting to the granite.

As far as what tombstone makers can do or what to look for... I've no idea.  But I suspect there are tombstone fabricators as well as re-sellers.  I'd stay away from re-sellers and try to find someone who actually cuts & polishes the things in-house.  Look for someplace dirty - not a flowery funeral home.

Look for somone close, tell them what you're up to and want to achieve, and see if they're interested in something slightly unusual.

Maybe watch a few YT videos on tombstone-fabrication to get an idea of what a shop should look like and what equipment to expect in a full-service outfit?

Another topic to consider - in the video, guy used Diamant 310FL as a shimming compound: www.diamant-polymer.de/en/shop/dwh/dwh-3...egroessen=DWB+250g#0

I assume it was specifically with steel filler to make it expand in similar rate as steel underneath?

However, in my case I won't be shimming steel but cement casting instead. What kind of shimming compound should I look for? Some concrete filling putty?

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Update!

I just got back from neighbour tombstone guy. He had thin (30mm) granite plate cutoffs. While talking to him it dawned on me that instead of casting concrete and then imprinting the shims I could try use the granite plate directly as a reference surface for rails, mold concrete over it and bolt it with through-rods.
This will require slight re-design because ball-screw supports and rails will be mounted at the same level, but that's not a problem.

Only trimming will be necessary when stacking one part on top of another to ensure parallelism between X an Y and perpendicularity between Y and Z, but I think that should give far better chance of success than any imprint (in the end, best case scenario imprint won't be worse than original plate).

So for 3 granite plates I paid less than what it would cost me to buy this aluminium box level
This stuff is heavy! I am starting to worry whether the 3Nm NEMA23 steppers I was planning to use will be strong enough...

Good score!

You've got a lot of work ahead of you, but I think using one of the plates as a built-in reference surface will pay off in the end.

I suspect you won't be happy with NEMA23 motors.  Quick way to check would be to make a rough model in Fusion 360 and set the materials to granite.  That should give you a ballpark weight for each of the moving components.  From there, most of the name-brand ball screw (and servo) manufacturers have on-line calculators (or formula in their catalogs) to determine how much motor power you need to achieve desired accel values.

Yeah, I even tried putting couple of numbers quickly but I'm getting unexpectedly low values. Need to be doing something wrong.

Y axis will be the heaviest with X stacked on top, granite chunk I'm going to use for X weights around 18kg. I tried putting very pessimistic numbers but still getting 1.4Nm top. What am I missing? (see attached PDF)

spumco wrote:

Good score!

 
Not really.
 

 
My mistake, first slab he proposed was nice granite, two other on the same pile were quite dirty. The dimensions were the same, we loaded it into my car. Right now I pulled it out, dusted off and it's marble.

Just want to double check, marble won't do to serve as a baseplate for Y or Z? It sounds spindly, so I doubt it. And even though after I cleaned it it turned out to have some surface defects.
In that the case, I'll probably imprint Y and Z with the granite slab and then make X (longest, ~100cm) axis out of it as a solid. But cement is significantly less dense than concrete, makes little sense to make movable axis the heaviest. That's a bummer. I need to think about it.