vmc intermittant rigid tapping problem

[ethanbrush]

I am baffled on this one. We are tapping 3/8 -16 in a plastic material, 5/16 drill, tap held in collet, speed is slow, around 60 rpm. It will do a dozen or so holes fine but sometimes it messes up and strips out the threads. I have been watching it like a hawk and although it is hard to see because of the chips, it does look like the spindle/feed rate gets out of wack on the messed up hole. Also when I first start, it usually will mess up one of the first ones, but then do another dozen or so fine. If it is the case that the feed, speed ratio gets out of wack, why would that be happening so intermittently? Any other ideas? could it be that Im going way to slow and the machine isnt good at tracking the spindle rpm down there?

Thanks.

[Neal]

1. Mechanical slop in the system
2. Marginal calibration of the R/tap gain and ramp
3. Parts not tight in holding device
4. Electrical noise in the system
5. Power variation from either internal or external source.

Neal

[ethanbrush]

ok thanks Neal, some things to look at. I thought about the gain and ramp but dont think that is it since it works perfectly or really bad. We recently had a problem where the bolts holding the z axis thrust bearing were loose but once we tightened that up the backlash was down to a few tenths. I am leaning toward that it is an intermittant miscommunication/error/noise between the spindle and z axis feed ratios. Where is the rigid tapping capability physically held in the machine? Is it all software?

[alex gitzel]

you should go to fadalcnc.com and download the maint. manual there is a troubleshooting flow chart #23 that goes through all the checks to do.you may need to get a tech to come in and tune the drive,ramp,and gain with the testing tool if the mechanical stuff checks out ok.

[dblox]

Hi,

I had a very similar problem on my machine, which I thought might have been due to a few of the issues already posted - bad synconization, or slop in the spindle belt (my machine is almost 10 years old, not a Fadal though).

I even bought expensive floating holders to see if I could correct the issue.

I was stripping out M4 tapped holes in extruded aluminium - seemingly random like the issue you describe.

In the end, my problem was chips binding in the tap while retracting, and stripping the threads, often breaking the tap.

I corrected this by using a gun tap (spiral point) instead of spiral flute or straight flute, and using kero as lube. Seeing as you are tapping a plastic material, despite the low RPM, I can imagine heat & welding could still be an issue.

[ethanbrush]

right I was thinking that perhaps it was chips binding up/clogging and sometimes things would just work out where it would cause enough stress/resistance to damage the threads. However watching the tap, it seems that the sync is off and starts stripping it out within the first few threads even.

[alex gitzel]

Z axis backlash is the most common issue look in the manual to set backlash if it is more than .001 out it adjustment that may be your issue.otherwise check the spindle motor mounts if the rubber is all sagged out of the sides they should be replaced also check for belt slip.if the machine was just moved it drive may need to be zero balanced.

[alex gitzel]

Originally Posted by ethanbrush ok thanks Neal, some things to look at. I thought about the gain and ramp but dont think that is it since it works perfectly or really bad. We recently had a problem where the bolts holding the z axis thrust bearing were loose but once we tightened that up the backlash was down to a few tenths. I am leaning toward that it is an intermittant miscommunication/error/noise between the spindle and z axis feed ratios. Where is the rigid tapping capability physically held in the machine? Is it all software?

the software is on a chip on the spindle card.

[R&Rmachine]

speed up the tapping speed (about 300 rpm)