Vibration due to orientation of tooling

[durableoreo]

I'm facing 4140 bars to length and getting some chatter. It seems to depend on the angle of the tool. When the load is mainly radial, I get chatter. When the load is split between radial and axial, no chatter. Does this indicate that my bearings need more pre-load?

I'm using TPMT-321 tools, the McMaster budget set. As I face across a 2-5/8" bar, if I use a tool that presents the tip of the triangle, raking 45-degrees in the direction of the cut, no chatter. That is, the load is split, half radial (along the tool path) and half axial (into the headstock). I can take a 0.020 cut at 600 RPM, hand feed, with no problems. As the insert dulls, I can feel the increased pressure but still no chatter.

If I use the same insert but the cutting edge is parallel to the axis of the part, I get a lot of relatively high pitch chatter. I recognize the sound from other cuts. I can feel it in the sheet metal around the headstock but not in the carriage. In this mode, the cutting force is perpendicular to the axis of rotation.

When I run at 600 RPM, the bearings warm up after several hours but it is not alarming. If I run at 1000 RPM, the bearings heat up much faster but it's still not too bad. Under 130 oF, I think, because I can hold my hand on the headstock near the bearings. I recently cleaned and lubricated the bearings with Isoflex NPU 15. I adjusted pre-load until axial play was under a 10th with a load of about 20 lbf. I did not fiddle around with it very much. The previous owner of this 9x20 lathe upgraded the bearings with roller bearings. The grease had dried out a but but the bearings looked good so I re-used them, with a good amount of expensive grease worked into them.

I'm open to any advice or suggestions regarding interpretation, tooling, inserts, greasing, bearing pre-load, etc.

 

[GrifterGuru]

Watching...

 

[Alexander McGilton]

So this insert chatters in one direction, turn to cut a length 90 degrees in the other direction and no chatter?

Sounds like the bed to saddle, cross slide or compound slide, have loosened gibs in one direction but not the other.

 

[durableoreo]

So this insert chatters in one direction, turn to cut a length 90 degrees in the other direction and no chatter?

Yes, same insert just turned. In this case, it's triangular, and it's only turned 60 degrees. So the point of the triangle faces the work like this WORK <| That's the no-chatter situation.

Sounds like the bed to saddle, cross slide or compound slide, have loosened gibs in one direction but not the other.

Good point. I measured the deflection with the indicator on the headstock while pushing/pulling on the tool post. There's 0.005 slop if I push/put along the axis of the spindle, about 30 lbf. I'll see if I can tighten it up. But under load, wouldn't it just take up the slack? I guess I don't understand what causes chatter. Some slop is different from a spring in the loop. But I don't know how to measure slop vs spring.

There is quite a bit of lash in the cross-slide lead screw, about 0.010. I'm fairly confident that this is slop, not spring.

 

[Alexander McGilton]

Backlash isn't necessarily the cause of chatter. You might have .005inch of backlash or 0.5 revolution of backlash. As long as you are on the push or pull side of the screw as you advance a cut. There shouldn't be chatter.
Slop in the ways or gibs is a different story. You need to measure this independenty on each axis, place an indicator and stand bridging one dove tail. Rock the exposed parts of the machine, and see if the indicator moves. I'm getting .0005 or less on the Colchester compound slide, perpendicular to its axis of travel. If the gib was loose, or the flatness of the ways were poor. This would be a different situation for chatter to occure.

 

[RaisedByWolves]

I'm facing 4140 bars to length and getting some chatter. It seems to depend on the angle of the tool. When the load is mainly radial, I get chatter. When the load is split between radial and axial, no chatter. Does this indicate that my bearings need more pre-load?

I'm using TPMT-321 tools, the McMaster budget set. As I face across a 2-5/8" bar, if I use a tool that presents the tip of the triangle, raking 45-degrees in the direction of the cut, no chatter. That is, the load is split, half radial (along the tool path) and half axial (into the headstock). I can take a 0.020 cut at 600 RPM, hand feed, with no problems. As the insert dulls, I can feel the increased pressure but still no chatter.

If I use the same insert but the cutting edge is parallel to the axis of the part, I get a lot of relatively high pitch chatter. I recognize the sound from other cuts. I can feel it in the sheet metal around the headstock but not in the carriage. In this mode, the cutting force is perpendicular to the axis of rotation.

When I run at 600 RPM, the bearings warm up after several hours but it is not alarming. If I run at 1000 RPM, the bearings heat up much faster but it's still not too bad. Under 130 oF, I think, because I can hold my hand on the headstock near the bearings. I recently cleaned and lubricated the bearings with Isoflex NPU 15. I adjusted pre-load until axial play was under a 10th with a load of about 20 lbf. I did not fiddle around with it very much. The previous owner of this 9x20 lathe upgraded the bearings with roller bearings. The grease had dried out a but but the bearings looked good so I re-used them, with a good amount of expensive grease worked into them.

I'm open to any advice or suggestions regarding interpretation, tooling, inserts, greasing, bearing pre-load, etc.

Not trying to be a jerk, but you were well into the 4th paragraph before you even mentioned what machine you were using.

Letting us in on the machine and showing a pic of your setup would help us out greatly. It may be something simple like too much stick out of the tooling or material that we could easily give tips on, or it could be something inherently problematic with the brand or size of the machine used.

 

[durableoreo]

Not trying to be a jerk, but you were well into the 4th paragraph before you even mentioned what machine you were using.

Letting us in on the machine and showing a pic of your setup would help us out greatly. It may be something simple like too much stick out of the tooling or material that we could easily give tips on, or it could be something inherently problematic with the brand or size of the machine used.

You're right, some pictures would help.

No chatter


Chatter



Measuring with the carriage locked, I'm getting +/- 0.003 in the direction of the cross slide. Same results if I push on the tool post or on the tailstock.


Maybe it's torsion in the bed?

I had half anticipated this problem and made a weldment to mount the lathe on. The idea is to "bed" the lathe to the weldment using Moglice. The weldment was specifically designed to resist torsional forces and is constructed from 1/4" sheet. Very heavy. Probably overkill.However, the weldment is not quite not finished and I need to finish this job before tearing the shop apart.

 

[Jim F]

Apples and Oranges.....
Different holder is a red flag.

 

[WobblyHand]

Stick out is more with the one that chatters. That being said I usually face with the other handed tool at an angle to the face, like a backslash. Similar to how I'd use HSS.

 

[G-ManBart]

I'm not an expert, but I think your setup is a large part of the problem for starters. Using the front dovetail and having the tools parallel with the spindle isn't ideal. If you look at it, the cross-slide is narrow and long. You're putting leverage across the narrow side, so it's going to lift easier than if you were using the side dovetail which gives the much longer length of the cross-slide to take the load. On a large machine that's much more rigid you could probably get away with it, but on a small machine every bit of rigidity helps.

Why one chatters and the other doesn't is probably because the first setup has less surface contact and naturally has a relief angle built in...really just the very tip touching. The second has more surface contact and very little relief angle so it's taking more total load than the first setup...some on the tip and some on the side.

While I would suggest changing to a conventional setup with the other dovetail, one thing you can try right away is to use the carriage lock before you start a pass and see if that helps anything.