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What Do People Want In A CNC?

Posted by Trakyan 
Re: What Do People Want In A CNC?
March 01, 2018 12:15PM
Well, there are several things that I could would do with a CNC router/mill which have varying levels of "good enough."

Doing projects like speaker boxes, so long as straight lines are straight, and right angles are 90 degrees, 1mm of tolerance is good enough.

Doing things like making parts for a new 3D printer, or the next generation CNC, it might not be, though probably I'd use it to make parts that are more tolerant of inaccuracy in order to make better.

There are some really good DIY CNC designs out there, many of which aren't that expensive to build, but require that you are able to get some reasonably high precision parts cut, better than what I can do with a table saw and drill press.

So I guess what I'm looking for isn't so much a "good enough" machine, but more of a "good enough" design that can be made with the tools that I have. I'm relatively fortunate in that I have a fairly complete set of woodworking tools. Between that and a 3D printer, I would think that there ought to be something that could be done in order to make a CNC router accurate enough to get started on its next generation. To date, I haven't found the design that will do that for me, even if I accept that the precision and durability of the machine might only be such that its only real job will be making parts for its more durable and accurate replacement.

I figure that I'm probably going to want about 600X1200mm workable area from the end result. I was planning on using Home Depot threaded rod, since it's actually cheaper than belts, though it has a low potential feed rate. This also means that they act as torque multipliers, meaning that less powerful motors can be utilized. As a result of the cheap and easy access to threaded rod, plus the lower torque requirements for the motor, I think that the end result will be both more accurate, and less expensive.

MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts
Re: What Do People Want In A CNC?
March 02, 2018 07:12PM
Since I'm designing a machine similar to the MPCNC, the limiting factor for precision is the rods/tubes you use. That and the rigidity of the plastic, but that's fairly easy to account for by making oversized parts. My design is fully parametric and constrained using only a handful of variables, so it's really easy to upscale the bulk factor as I prototype.

I personally wouldn't worry about the low feedrate of threaded rods, on low cost machines it's rarely the speed of your axis motors that's the limiting factor. I'd be more worried about backlash and bent rods.

But you seem to get my point. This will be a machine for things that don't require high precision. Where +-1mm is plenty. And to be honest, when working with woods that's plenty accurate. I'll see f I can squeeze out sub mm precision but that'll be once I have the machine built and can see what needs tweaking. My design will use a similar sort of layout as the MPCNC, but with a different drive system since I don't like how the axis can rack or skew when the motors go out of sync. I find that the thing I hear most people mention when they say hobby machines aren't rigid enough is that the XZ assembly is weak to torsion about the X axis. The crossing rails of the MPCNC seem like they would brace against this type of torsion better. Plus it looks downright cool.

EDIT: The two factors I see as limiting precision are the straightness of the rails (for obvious reasons) and rigidity. Anything else like orthogonality, parallelism and the likes can all be adjusted out by the operator if a way to adjust these things is included in the design. Because of this I can see something like the MPCNC being plenty good enough for hobby/home use as long as the plastic parts are oversized enough and the tubes used are straight enough.
Re: What Do People Want In A CNC?
March 03, 2018 10:22PM
+-1mm tolerance?? Is that a joke? You can hold better tolerances with a handsawspinning smiley sticking its tongue out
Re: What Do People Want In A CNC?
March 04, 2018 02:47AM
Doing it with a handsaw becomes tedious quickly, especially with complex shapes. Right now I'm just trying to avoid making promises I can't keep and +-1mm is a conservative estimate.
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