Expected tolerance for a hobbyist machine (example: SainSmart Genmitsu 3018 PROVer)

[broke-enthusiast]

I'm in need of a CNC for a single job - cutting slots out of 2mm thick polycarbonate pieces. The height of the pieces is around 100mm, so I will need to increase the Z axis height by around 70mm to be able to cut these holes (the geometry of the piece dictates this necessity), which I hope to achieve by printing/milling a custom bracket (if anyone has a better idea please let me know. I've designed for CNCing before, but never done it myself).

The piece itself has a wall thickness of 2mm however (it is hollow), and the slot only needs to be 2mm deep. The dimensions of the slot are around 3mm by 7mm. And there are several slots of the same size that need to be cut within about a 50mm x 10mm area. Ideally I would be able to make a jig to hold multiple pieces at the same time, as I will be cutting at least hundreds and possibly thousands of these.

So, with or without this modification, is anyone aware of what kind of tolerances I should be able to expect out of this machine? Ideally I'd like to get close to +/-0.1mm on all axes, within about a 100mm travel distance minimum on the X and Y axes. Z axis doesn't really matter. 0.2mm tolerance is pushing it a bit but it's workable. The actual accuracy doesn't matter, just the repeatability once I get everything lined up.

I'll be getting the pre-assembled machine for QC purposes and making sure that the machine is able to achieve the best tolerances possible without taking up too much time - is that the right move?

And if there's an ideal machine that's no more than a grand or two, that does the above, I would really appreciate any recommendations.

Thanks in advance!

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[awerby]

How many of these things do you need? Is it really worthwhile to get a machine just to do these slots? CNC routers, especially at the cheap end, don't have a lot of Z height. Modifying one to be higher will cause a loss of rigidity, which isn't going to be overabundant in the first place. Wouldn't it make more sense to find someone else who's got the right sort of machine and get them to do this for you? A grand should buy you a lot of slots.

[joeavaerage]

Hi,
tolerance is not really the right question. I would guess that even hobby machines could do 0.05mm repeatably......until you start cutting!

The problem is that the machine flexes, all machines do, but small hobby machines are notorious for it. If you cut very slowly and with light cuts
you would probably get 0.1mm or better.....but how long would it take? A bigger more rigid machine will do the same job faster but with vastly less flexure and
therefore more accuracy.

A second thing you may not have considered is flush cooling. Polycarbonate and acrylic are actually quite hard to cut, it's not that they are hard but
the chips get hot and weld themselves back together or worse weld themselves to the tool. All plastics tend to do this but polycarbonate and acrylic are amongst
the worst for it.

I ALWAYS use flood cooling when I do plastics, all plastics, but polycarbonates and acrylic especially need good flood cooling and chip removal or you will
get chips welding to the tool and wreck the job.

If you want to use flood cooling you'll need some sort of enclosure or you'll spend all day cleaning up the mess. Been there, done that!

Craig

[broke-enthusiast]

> How many of these things do you need? Is it really worthwhile to get a machine just to do these slots? CNC routers, especially at the cheap end, don't
> have a lot of Z height. Modifying one to be higher will cause a loss of rigidity, which isn't going to be overabundant in the first place. Wouldn't it make
> more sense to find someone else who's got the right sort of machine and get them to do this for you? A grand should buy you a lot of slots.

Thanks for the reply. I am going to be cutting at least a thousand, most likely 4-5 thousand pieces total. I would definitely like to outsource this, and I have been for the past year, however it's hard to find someone reliable local to me and I'm quite sick of dealing with the issue. I figured it may be cheaper and less headache to do it myself.

[broke-enthusiast]

Hi,
tolerance is not really the right question. I would guess that even hobby machines could do 0.05mm repeatably......until you start cutting!

The problem is that the machine flexes, all machines do, but small hobby machines are notorious for it. If you cut very slowly and with light cuts
you would probably get 0.1mm or better.....but how long would it take? A bigger more rigid machine will do the same job faster but with vastly less flexure and
therefore more accuracy.

A second thing you may not have considered is flush cooling. Polycarbonate and acrylic are actually quite hard to cut, it's not that they are hard but
the chips get hot and weld themselves back together or worse weld themselves to the tool. All plastics tend to do this but polycarbonate and acrylic are amongst
the worst for it.

I ALWAYS use flood cooling when I do plastics, all plastics, but polycarbonates and acrylic especially need good flood cooling and chip removal or you will
get chips welding to the tool and wreck the job.

If you want to use flood cooling you'll need some sort of enclosure or you'll spend all day cleaning up the mess. Been there, done that!

Craig

Thanks for the reply Craig, really valuable info! I think I'll outsource this job for the first couple hundred, get a jig made, see how it goes, and if the flood cooling will be necessary. I'm hoping the hole is small enough so that the piece won't heat up to quickly. Only one way to find out I guess!