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chatter with form tool at low speed

pcd

Cast Iron
Joined
Mar 25, 2024
Location
MA, USA
I use form tools for putting on radii with an engine lathe. With a R.25" I get terrible chatter at low speeds even in brass, but the results look ok at 1,800 rpm. It still chatters at 1,800 rpm, but the divots are shallow enough and wide enough that they're basically invisible. Chatter is about the same on brass and steel.

The chatter seems to be at a particular frequency like from a resonance. I say this because if I halve the spindle speed then the distance between the marks is also about halved. I have tried a bunch of things to change it like tightening compound gib until compound is locked, tightening X gib until X is locked and feeding with Z, locking Z by clamping carriage to bed and feeding with X, applying force to workpiece with tailstock, tightening spindle thrust bearings, and adding a T-land to the form tool. None of these things change the chatter in any way.

I am aware that I could cut a radius with other methods that involve less engagement.

What should my expectations be? Can your lathe fully engage an R.25 cutter without chatter?
 

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Use tapping paste or heavy oil. Chamfer first to rough. Feed fast and don't dwell, as in feed fast to a depth on the handwheel and back out immediately. I would use a carriage stop and Z this enables you to lock the compound and X.
 
Use tapping paste or heavy oil. Chamfer first to rough. Feed fast and don't dwell, as in feed fast to a depth on the handwheel and back out immediately. I would use a carriage stop and Z this enables you to lock the compound and X.
Tried that just now: chamfer to rough, X locked with gib, feed fast with no dwell to a hard stop, heavy oil.

Zero difference. Chatter starts as soon as full engagement is reached.

Above you mentioned a 1" square HSS tool and a rigid machine. How would I judge whether my machine is "rigid" other than saying that if there's chatter it's not rigid?
 
I don't use these tools, so I don't know, that's why I'm asking. Looks like you're cutting with the face of the cutter also. Is this face a properly relieved cutting edge?
1714001006423.png
 
Is this face a properly relieved cutting edge?
I like you're thinking. Had never though about that edge before since I don't use it. However, it is in fact relieved and the chatter is just as bad even if that edge isn't cutting. Like if I stop a few thou shy of the tool radius being tangent to the face of the part.
 
Have you tried finding your finish diameter in X, locking the cross slide, and then doing your feeding in Z instead? Just for curiosity's sake, it seems the bar would be stiffer if you're putting the cutting forces into it longitudinally rather than radially.
 
When using big form cutters, or even a heavy countersink, I start feeding into the work and shut off the spindle and let it coast down. Right before it stops back out really fast. That was a hot tip on a Hardinge HC chucker we had, it took too long to change the spindle speed so you’d kick it into neutral and let it coast down, back out, engage the belt again and repeat. On a big lathe the spindle can go slow so it’s easy to get a low enough speed.

If you can’t cut without chatter at the lowest speed and you’re sure the cutter isn’t rubbing - you either need more lathe or less engagement. Make up two tools and cut it in two halves etc
 
Just a thought: Your bit may be too sharp. Try lightly honing the edge to dull it out.
I've had this same problem on form type tools before. Dulled it out and chatter went away.
 
Have you tried finding your finish diameter in X, locking the cross slide, and then doing your feeding in Z instead? Just for curiosity's sake, it seems the bar would be stiffer if you're putting the cutting forces into it longitudinally rather than radially.
Yes I've tried that. Doesn't make a lick of difference.

Have also tried locking Z and feeding X.

Doesn't seem like the bar stiffness comes into it. I can have the stock buried in jaws like in the attached photo and the chatter is the same.
 

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When using big form cutters, or even a heavy countersink, I start feeding into the work and shut off the spindle and let it coast down.
That's a clever way to get some torque at low speed on a light lathe. I'll have to keep that trick in mind.

I'm glad you asked about this. I just tried and if I make the cut at 40 rpm that's slow enough that I can retract the cutter right after the work make a full revolution and then there's no dwelling. Chatter only gets bad if I let it dwell.

If you can’t cut without chatter at the lowest speed and you’re sure the cutter isn’t rubbing
I'm pretty sure the cutter isn't rubbing. It looks form relieved. Got it from McMaster-Carr and looks to be made by Clapp & Haney.

- you either need more lathe
I already feel pretty silly about how small the part is for the size of lathe.

Can your lathe take this cut without chattering at a reasonable spindle speed?
 
Just a thought: Your bit may be too sharp. Try lightly honing the edge to dull it out.
I've had this same problem on form type tools before. Dulled it out and chatter went away.
I honed the edge with a diamond rat tail until it wouldn't shave my fingernail anymore. No difference.

Can your lathe take this cut without chattering?
 
Stupid, stupid, stupid question here....

But you've confirmed the tool is on center, right?

Sometimes it's the simple stuff....
 
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