What causes this behavior, and how to correct it? PM 728 VT

[koenbro]

I was milling some mild steel for the t-nut of my QCTP when I noticed a certain skipping noise. so I grabbed several short videos that i spliced together a 20-second video to show it. See for example at 2 seconds, 10s, 13s (this is the most pronounced, the whole spindle seems to jerk quite prominently), and 19s.

Is this only chatter, or is there something wring with the spindle? Thank you all for your help.

 

[mmcmdl]

You are climb cutting and the table is jumping . Mill conventional .

 

[koenbro]

Thank you, will change the milling direction to do conventional. Seems like my mill doesn't tolerate climb in steel. As you see in the video, I took very light, 0.02" passes.
Is the table jumping a measure of the x axis double nut having backlash or what?
Also I am concerned about the spindle deflecting like at 13seconds.

 

[woodchucker]

you also can tighten your gib lock up, so that it drags more. I do this, as I like climb milling if I can. so I create more drag so it can't overcome the pull. You can only do that with lighter cuts, it will not work with heavier cuts.

 

[kenjesse]

No expert here but I don't think your dead on about the table moving into the backlash. Same is likely happening at the 13 second mark rather than a deflection of the spindle. If I had to bet I'd put my money on the backlash of the X axis being to the left in the vid at 13 seconds.

For better table lock function I've always added a small piece of drill rod cut to match the dovetail angle between the locking screw and the gib. Quick and easy mode that makes locking the table more efficient, easier and does less damage to the gib IMHO.

 

[parshal]

For better table lock function I've always added a small piece of drill rod cut to match the dovetail angle between the locking screw and the gib. Quick and easy mode that makes locking the table more efficient, easier and does less damage to the gib IMHO.

Got a picture of that?

 

[kenjesse]

The reasoning I think is that by having the flat end that the screw can push against with the angle of the other end matching the dovetail angle when you tighten the lock the force is in a straight line. Without the mode the end of the screw wants to ride up the angle of the dovetail redirecting the direction of the locking force. Simple to do and worth the minimal effort. Best if you can harden them but not strictly needed. I've also seen the use of a ball bearing but I think the method above works best for any dovetail locking screw.

 

[7milesup]

@koenbro As others have mentioned, the table may be moving during those cuts. I usually reserve a climb cut for the last finish pass, but it is very shallow, maybe a thou.
The other scenario that might be playing out is that the cutters on the end mill are not able to clean themselves out before contacting the material again. In other words, the swarf is following the cutter around and actually getting re-cut.

 

[koenbro]

Very helpful replies, thank you all!

Will try conventional milling next, and reserve climb for the finish pass (although it does seem to work with aluminum). I never lock the table as I am using CNC, so can't use that trick; my quill is always locked maybe try it there.

 

[B2]

The 2 and 13 second events certainly look different! It is hard to say from this video. I would be disappointed to see that on my machine as well. My machine is less than perfect and has considerable backlash, but I do not see this jumping while cutting either way. Catching debris under the cutter can cause some cutting problems. Lubricant can help. I have a CNC machine and there would be no way to lock down the the position and still cut a circle, arc, corner turn etc. So tighten your gib up until you just start to create backlash. Looser and the way parts can move about. Any tighter and backlash starts to be a problem. This is the correct gib tightness anyway to prevent twisting (maintaining 90 degrees between x and y). This jumping should not be the bearings, if they are that loose you have more to worry about. If your spindle head or table is light weight or the structure is not strong then maybe the mill "bends" when it hits a strange spot of cutting. Or maybe the spindle motor takes on a sudden load and slows a bit. A thought, one way to check on weigth is to add weight to the table or to the head so that its inertial/momentum is larger. Lastly, you have a quill. Make sure that it is locked down. Some quills have considerable looseness and so the spindle can effectively move about. It can be bad enough on some machines that drillling holes are not located where they are suppose to be. When I want to use the quill I tighten it up to where it is a bit difficult to drive. I never leave it unlocked while moving in x or y directions.