We Are Getting Stainless Steel Chattering.

Marv

Marv

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We getting what I call Chattering (metal is being torn (?)) on our 3/8" Stainless Steel at the same spot every revolution. (See Photo).
Its both on the Threads & Taper cut. WHY?

CNC Machine @ 3500 rpm (we normally use 2500 rpm on our C6 stock w/o any problems).Screw-0829.jpg
 
Maybe slow down the spindle and increase the feed? Also, less radius on the tool bit might help.
 
We can try that. But why did we get that only on the one side of the round stock? I hope that's not a bad spindlebearing or something.
 
Until you try a few things I would not be concerned about spindle bearings. It could also be the particular piece of material, maybe a hard area when is was rolled?
 
Looks like harmonic vibration to me, the give away is the spiral pattern. Could be the tool or holder is vibrating, or a belt is transmitting vibration.
Could be the work itself is vibrating, I assume you are making the point, swapping tools then cutting the thread, then parting. You might have to cut the taper, then move the work out of the collet to cut the threads. That would be a fair bit of stick out.
Maybe just use a different insert and take a lighter cut
 
I am no CNC guy, but I think your speed is high, I wonder if your insert is not sitting in pocket correctly, maybe a chip under it. I have seen chatter like that a couple times and its been in my toolpost or toolholder, usually a chip or a hold down screw clamp that is not functioning correctly.

michael
 
Marv, can I ask a few questions?
  1. Was this piece sticking out of the chuck without tailstock support? It looks like it and it looks like you parted on the threaded end. If so, that might be the problem.
  2. Your thread form looks really poor. Is this a fault of your program, are you not on center, was the threading tool not oriented correctly (compound angle)?
  3. Are you using a carbide turning tool? If so, is it brazed or inserted tip. If the latter, what insert are you using? That might give us a better idea as to whether or not your cutting conditions are off.
  4. What depth of cut are you using? If you are taking light cuts at high speed you may be work hardening the surface and that will definitely cause chatter. What nose radius are you using? If your depth of cut is too small then the nose radius is unsupported and radial forces, while normally small, will increase and may aggravate chatter.
  5. What is your lead angle on your tool? If using an inserted tip tool, is it at the lead angle specified by the maker? For example, if the tool shank should be perpendicular to the work and it is not then the tool can chatter. Insert lead angles are not nearly as forgiving as HSS tools so this needs to be looked at.
  6. What do you mean by C6 stock?
I'm going to guess this is not a problem with your spindle or spindle bearings. Harmonic issues can cause some weird problems but they normally are machine-related (belts, etc), not something that occurs in the same spot on the work.
 
joshua43214 said:
Looks like harmonic vibration to me, the give away is the spiral pattern. Could be the tool or holder is vibrating, ...
Could be the work itself is vibrating, .....
Click to expand...

That's what I was thinking.
 
Mark the chuck and the part with a Sharpie where its chattering, then rotate part 45 degrees in chuck and turn, did the chatter follow the part or stay put on the chuck. This should confirm or eliminate hardness in the stock as the problem. I used to turn cast parts on a huge Mori lathe, we had our own foundry on site. Occasionally castings would come through which had 'chilled' sections that were hard as a rock. Even with flood coolant it was a fireworks show of carbide insert sparks destroying themselves. Typically we had to slow way down and would burn through 4x the inserts as usual turning those hardened castings.

Also how much stock do you have extending into the spindle? Are you using a bar feeder, are you using urethane spindle liners to keep the stock centered?
 
Thanks everyone... All good thoughts... it may take a few days for us to run some more tests based on your ideas. We have some new bar stock on order.
FYI: That part in the past was being made on a Warner & Swasey M-1200 & that was our last left over piece of 3/8 stock. That was our first test on a Haas TL-1 which we are trying to switch all the W&S parts to.
 
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