Thread: Tool diameter accuracy
Jazz, now you're taking the proverbial! The frame doesn't swell at all with heat or moisture. The bed goes up and down in the middle instead!
I'll try chasing backlash and leadscrew float. However, the leadscrew is something like 1.25mm pitch so I'm seeing more than a half-thread backlash if so. I have anti-backlash pairs of delrin nuts on each axis so I'd be surprised if backlash were that bad but I shall check. The nuts are running on commercial stainless studding so while it's a bit smoother than BZP studding, I know that the nuts are going to wear.
Steps per - I did check a while back with a dial gauge (ever tried getting a magnetic base to stick to MDF?) and it was pretty much spot on. Those parameters should be locked in to the software and hardware so I hope they don't drift!
I'll also try a different cutter or two cutting a standard size piece and see what happens.
Measured the shank of the cutter just above the flute. Converting from imperial to metric (only have an imperial mike) that came out at 3.08mm. Plunged a hole straight down and checked size with the shanks of twist drills and that came out between 3.0 and 3.1mm. Pretty conclusive, then.
I was going to measure backlash, leadscrew float, etc., but when I did a quick "shove it and see if it clunks" test, I realised that the whole thing bends so much that it wasn't worth the effort of digging out the dial gauge, etc. As a further demonstration of general bendiness, I changed from two passes of 1.8mm DOC in MDF to a single 3.5mm pass, and the dimensional error changed significantly to the point that the pieces didn't fit any more. The machine works fine for engraving nameplates, or anything involving light cuts where there's no need of great dimensional accuracy, but ask it to machine two components that will slot into each other, and you're on a loser. Those of you who can machine aluminium on a CNC router are allowed to laugh. I can't machine 3mm MDF...
Thanks for comments on the Spitfire - its a mk9 (nicest shape I think!) and has taken 3 years on/off (mostly off) to get to this point.
Have you got any photos of the machine you could post? Might help gauge stiffness.
Are you sure you are not loosing steps - the deeper cut is higher load on the leadscrew and delrin nut. I confirmed my machine was missing steps a while ago by touching the stepper motor body during cutting - I could feel the kick every now and again. I think mine was an extreme case so instead you could also set up a DTI on the tool, gantry, or Y axis plate when it is all at the home position, cut out the shape and then see where the machine homes back to. DTI should be back at zero, otherwise there has been a step or so missed.
What is the control system spec, voltage, drivers, BOB etc?
I won't bother with a picture. It's too embarrassing... The machine is basically the JGRO design, and there are lots of references to it on the 'net.
I'm using (all from Zapp) a 68V power supply driving 3 M752 drivers, fed by a ZP5A-INT BOB. Motors are all SY60-something 3Nm steppers. Given that they are driving relatively fine-pitch leadscrews (1.25mm) at modest speeds (max rapid is about 900mm/min to avoid whip) with 400microsteps, I don't think that the motors, etc, are being overpowered by the cutting forces. 3mm cutter, 1.8mm DOC, in MDF? FWIW, I also use LinuxCNC, which seems to be a minority choice on this forum, with clock rate within the max suggested by PC latency tests. I do seem to get good repeatability; I can cut several sets of those cable chain components and pieces are cleanly cut (as clean as MDF cuts) with good registration of first and second passes, as far as I can see.
Don't build your CNC router from cold-rolled cow dung...
(Indeed, the Spit looks good so far. One of the projects that I shall get round to one day, and one of the drivers for building the router in the first place, was to cut components for a RC glider. That project's still way down the list, though!)
Last edited by Neale; 05-03-2014 at 03:24 PM.
OK, if it's one of these then I guess flex is still a possibility if backlash is low:
The electric spec seems very good, leadscrews are low/medium, and machine is . . . . well, MDF on gas pipe bearings. The next step is up to you - whether to upgrade the machine or stick to engraving / simple jobs.
If you do get around to the glider you'll love cnc cutting out the bits and slotting them togehter. No more fret saw! and everything fits perfectly.
I've had enough frustration and I'm working on the design of the Mk2 which will be welded steel, ballscrews, and profile rails. I'll take the electronics and motors across from the current machine as I reckon that they'll do nicely (and it's why I overspecced them in the first place). In the meantime, I'm doing some interesting projects with some local artists/craftspeople who are more interested in artistic interpretation than accuracy! I spent some time today with a friend who wants me to turn a bunch of photographs of her daughter's head into a full-size 3D version routed into a bundle of wooden sticks. It's the kind of project that no-one doing this kind of thing professionally would even look at
(and yes, my machine looks somewhat like that one. Oh, to have a machine that will take drawings and turn them into fitting components without three false starts while I calibrate out the bend!)
Last edited by Neale; 05-03-2014 at 10:20 PM.
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