[badassdevil]

I have just got my home made cnc up and runing everything is fine on it except small holes ie 10 mm dia arnt coming out round does .Also every now and then the z loses its 0 and goes deeper then it should does any one have any ideas on what is causing either of these problems

[badassdevil]

hey guys when doing smaller diameter holes the come out oval any one have any ideas why

[ger21]

For the small holes, it's probably flex in the machine. Try cutting them slower, much slower.

As for the Z, try lowering the acceleration by half.

[MattTheNoob]

You need to help us out a little more with your problem. Drilling? Boring? Lathe? Plasma Table? Shooting it with a .45?

If it is the latter, you are getting what is called keyholing because the bullet is not stabilized. Somehow I don't think that is what you are making holes with.

[badassdevil]

sorry matt is a home made router using mach 3 software

[jsheerin]

If Gerry's suggestions don't work, tell us what controller you're using, motors, screws or other type of drive system, power supply details, motor speed and acceleration settings in mach3, materials you're cutting, speeds, feeds and depths of cut, cutting tool types, and also posting pics of the machine would help.

[badassdevil]

i am using 3/8 chains for the x and the y and a 16mm trap screw foe the x with 2.5 drivers and motors from routout cnc i am mainly cutting hardwoods but also cut 3mm plastic for guitar scratch plates i use tungsten carbide 3 fluted supermills the holes in question where 10mm in plastic using a 2 flute 2mm endmill at 2000mm/min

[robe_uk]

sounds like you have backlash or flex on one of the axis, the picture below shows backlash in the x axis. You say you only see it in smaller holes, are larger hole round? or is it a case the problem may not be as noticeable in larger holes.
Double check for backlash.

Rob

[badassdevil]

there is no noticable backlash by the way guys i should have pointed out in my 1st post i am a cnc programer / machinest by trade so when building my machine it was built to a high standard but it is possible that its flexing when changing direction

[michaelthomas]

If you have a chain drive, it can measure very low backlash without a load present.......but display quite a bit of backlash under a load. Its pretty much impossible to get a chain drive system tight enough to have no backlash without inducing an unacceptable amount of drive friction.

Gerry is probably correct on your Z troubles. You are losing steps on retract. You can add some sort of counterbalance via springs, or gas cylinder if your already down to a pretty low acceleration.

I dabbled with a chain drive a while back, but it just wasn't very accurate. The inaccuracies compound with travel length and chain size, too. May not be your problem, though.

[Markwind]

I would agree, the chain drive is probably the trouble,


here is some information somewhere down this page there is a small animation and a good description of what is going on,
see this page

[louieatienza]

Maybe it's your bit flexing? If you can, try hogging out the waste leaving about .2-.4mm or so, then use a profile toolpath, leading in and out from the center, at a slow rate. Might have to use a small end mill to get a lead in/out path...

[PaulRowntree]

I have seen identical problems with my XY drives, which are #25 chains. Doing air-cuts, the accuracy (and speed!) are great, with 0.005" backlash, about 0.002" reproducibility. When cutting, the error goes up to ~0.040", which is no good at all. Tightening the chain doesn't help much as it turns out. You can see the problem just by looking at the bit position as you push the chain sideways. On a 4' length, you can easily push the chain 0.25" off axis, and this translates to about 0.04" at the bit. I don't think I can beat this, and am now going to lead screws on X & Y.

Sorry that I can't provide happier news.
pr

[badassdevil]

i am thinking switching to rack and pinion do you think adding a belt reduction to the z to get more torque would help on the retract

[ger21]

do you think adding a belt reduction to the z to get more torque would help on the retract

It may make it worse, as it will require higher rpm's, where the motor has less torque. Using a screw with a higher lead might be a better option.

Or, use the belt to slow the motor down will do the same thing. What's the opposite of belt reduction called?

If the Z motor has a dual shaft, the first thing I'd try is making a damper.
http://www.cnczone.com/forums/steppe...tml#post256639

Those drives are similar to the Xylotex I use, and adding dampers more than doubled the performance.

[michaelthomas]

Yes.....that would surely help. What size is your motor and how much weight is it moving. Light oil should help....and keeping the acceleration low.

I had that problem with mine when using the Dumpster CNC delrin anti-backlash nuts. They were just terribly tight. It would loosen up eventually, but about .003 of backlash came with it. I think it may have had to do with not having the recommended acme screw. I don't know. I bought mine through a vendor who was reselling them, and I think he was sourcing the screw from someplace cheaper. I know lots of people find them to be smooth and long wearing, but my experience was poor. I think they (Dumpster) recommended the 4130 screws from McMaster Carr.

I would really like to know what went wrong on my end, but I have moved on to ball screws and will never look back. If you have the machinery to turn the ends down, thread them, and make some simple mounts, the Rotons are very cheap and turn effortlessly. I use a double nut and and some wave washers and I have near zero backlash. I'm sure there is some there, but I can't detect any with a .001 indicator.

[michaelthomas]

I addition to what Gerry said......I used a hockey puck as a cheap stepper damper and it was an amazing difference.....although I didn't notice an improvement it torque, I did gain about 4x the rpm. It allowed me to get through the 400 rpm range without stalling.

[PaulRowntree]

Michael, to use the puck damper, do you just drill a mounting hole close to centre and secure it on the shaft?
Cheers!
pr

[michaelthomas]

Yes.....I used a 15/64" bit or a 6mm and just drilled as deep as I needed. It will fit on a 1/4 shaft pretty snug. It's actually snug enough that I can move my axis' about by turning the puck.

This particular gain (4x rpm) was with my Keling driver setup.

With Gecko 203v's, my motors would already spin up to 1600rpm.....and they do it more quickly since they don't have to accelerate the puck, too.

What I'm trying to say is......if you have drivers with mid range damping like the Gecko's......you may not see any improvement.

[michaelthomas]

One of my friends that lives in Kentucky attempted to give this mod a try, but had to end up mail ordering hockey pucks. I guess hockey isn't real popular in Kentucky.....you probably may have some locally, or even in your basement.