Slippage of Y axis - heavy cut?

[bevinp]

I suffered a slippage of the Y axis during a pocketing operation and would like opinions on whether the cut was heavy enough to be the cause.

Early in the operation the Y axis slipped about 10mm during a full width cut. I did not notice immediately because of the flood coolant, but I decided to let the operation finish because the remaining cuts were only half diameter. After the operation, I MDI-ed to X0Y0 and confirmed the Y as about 10mm out of position; the X and Z were OK.

I then Referenced the machine and again MDI-ed to X0Y0 and the tool moved to the correct Part zero. I believe this confirms that something caused the Y axis to loose reference to Part zero. The controller thought the tool had correctly moved as per instructions, but Y axis had failed to move correctly, causing all subsequent Y movement to be displaced by the amount of slippage.

I emailed Tormach for advice and received a list of possible causes such as Y axis jamming, software or hardware intermittent failures, and a loose stepper motor coupling. I have checked the oiling and security of the Y gib and they are OK. While the computer may be the cause I believe loose coupling is more likely and will be able to examine that next week. I believe it is the couplng because of the significan vibration during the full diameter cuts.

However, I am not sure the cut was excessive - cutting 1020 mild steel with a new 3/8" solid carbide 2 flute endmill (Maritool), 0.280" axial depth of cut, full diameter slotting at 1650rpm, 6.3 in/min ~0.002" per tooth), with strong flood coolant to flush the chips. The workpiece was 3" square by 5" long and very securely held full depth, full length in a 5" vice.

Two years ago, I tightened the gibs after they had bedded-in during the previous year since new. I will be checking those again at the same time I check all couplings.

Any opinions would be appreciated.
Bevin

[bobeson]

I suffered a slippage of the Y axis during a pocketing operation and would like opinions on whether the cut was heavy enough to be the cause.
...
I emailed Tormach for advice and received a list of possible causes such as Y axis jamming, software or hardware intermittent failures, and a loose stepper motor coupling.
...
.
Bevin

I had problems with loose axis motor couplings, and would suggest that is the first place to investigate. I don't think the cut parameters you listed should be causing an appropriately tensioned coupling to slip. My recommendation is to tighten them up as much as you think the mechanism will take without failure, and hopefully not need to worry about them again. Your gibs will need continuing attention and adjustment as part of regular machine care, as will the lube system.

[MetalShavings]

Greetings Bevin:

Before I ask my question I need to pre-empt it by stating that as of right now I have no Hands-On experience with any CNC machine. I have only book learning at this point so any thing I say or ask carries absolutely no weight.

Here's my question; In your post you state that you're milling 1020 Mild Steel. You also state that your RPM is 1650. Is this 1650 RPM in effect when your tool enters your metal stock/part?

I was under the impression that when your end mill or tool initially enters your part, the RPM should be approximately 30 percent of the RPM as your stated 1650 RPM. That would be about 495 RPM if my math is right.

Wether this is correct or not I don't really know. As I stated, this is only book-learned information. I don't even know if it applies here or not.

If it does apply, might this mean that there's a possibility that the initial jolt (however slight) of your tool spinning at the full 1650 RPM would be enough to effect you motor couplings?

If it happened once or twice it may not have any effect at all but, over time, might it have a cumulative effect that would cause your motor couplings to be effected?

I know that you're the one seeking help in trouble shooting the things you've described but, being a newbie to CNC and a future Tormach owner, your answer to my inquiry may be a big help to me in the future.

Thanks.

MetalShavings

[Amir Mughal]

tell me your machine name/model number and controller name/model number
best regards,
Amir

[philbur]

MEPro comes up with 3.2 ipm and 1,700 rpm for the other parameters you quote. So your load per tooth at 0.002" is double MEPro. did you calc fo a 4 flute by mistake.

Phil

However, I am not sure the cut was excessive - cutting 1020 mild steel with a new 3/8" solid carbide 2 flute endmill (Maritool), 0.280" axial depth of cut, full diameter slotting at 1650rpm, 6.3 in/min ~0.002" per tooth)

[flyinchips]

GWizard says 2043 RPMs with a feed of 6.9 IPM. It also says that this will require 1/2 HP to make the cut. I would say you were in the right ball park and there is some other problem, like the Y axis coupling. Happy and productive hunting.

[bevinp]

I did use GWizard but backed off the rpm while keeping the feed per tooth about the same. I did think it was a little aggressive. But as flyinchips said, GWizard's power calculation was moderate. I am using the 1100 which has 1.5HP.

Phil, yes I was calculating on 2 flutes, but I now question whether I should have used climb cutting. This could have been the cause of the vibration, since most of the cutting was half diameter radially.

metalshavings, the problem occurred during the horizontal movement, not the plunge, which in this case was a spiral plunge at 50% of horizontal feed; the percentage can be easily set in SprutCAM and probably any other CAM program.

Now my plan is to check and tighten all stepper couplings (thanks for you input bobeson), check for out of limits gib play on all axes, and if no fault found, do a few trial straight cuts working up to the .002" per tooth (practise using Feed Override first).

Bevin

[Attila Bertok]

Bevin,

I had a strange Y axis slip (around 8mm) a little while ago.
This was the reason:
The area between the table and the machine column filled up with swarf and I did a cut almost as long as the Y axis, around 260mm. Basically the table coudn't compress the accumulated swarf anymore and slipped.
First I thought the clutch slipped, but the screws were already very tight. Then I tried again, slipped again. After cleaning, no problem. It could be your problem. Compacted wet swarf just not that compressible!

I tried out of curiosity to hold onto the clutch with a pair of wisegrips and I could hold it quite easily while the stepper kept skipping the steps and Mach counting up happily. I can tell you, those steppers are not that mighty. I just mention this because people tight the crap out of the clutch srews, but it would jump way before a tightened clutch would slip.
All the strenght is coming from the mechanical advantage gained by the ballscrew. This was the only time I would have liked a feedback, like a servo system!

So keep your Y axis rear bellows clean, it can only help.

I had some strange axis (X, Y, Z) jumping before and it always happens when I have multi tool programs with toolchange more than once in the program. I couldn't figure this out yet.

I hope it helps

Regards
Attila

[bevinp]

Hi Attila,
That is an interesting post. I was surprised that you could hold the stepper motor so easily. But when I had a closer look at the specs of the steppers on the Tormach, yes the "holding torque" is only 4.5 Nm (about 3.3 pound.foot).

I don't think chips on the Y axis cover was my problem because I moved the table beyond the point at which the slippage occurred without any problem. But it is something to remember.

I have checked all axes gib security, all axes lateral play, looked for galling on the X and Z stepper shafts and the tightness of their couplings. I hope to get to the Y axis motor withi the next few days. I am also considering checking for endplay on ball screws.

Bevin

[bevinp]

I had some strange axis (X, Y, Z) jumping before and it always happens when I have multi tool programs with toolchange more than once in the program. I couldn't figure this out yet.

Attila,
I recently had some very sharp and short rapid tool paths generated by SprutCAM 7 Tormach told me there were a few bugs in the Tormach postprocessor I was using (1.2)and that I should upgrade to 1.4, which I did and problem solved.

The rapid tool paths were related to G2 and G3 circular movements and caused the machine to really thump. Perhaps that may be your problem.

Bevin