Another threading help question...

cdhknives

cdhknives

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Apr 12, 2013
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I am getting a chatter pattern in my internal threads. This is a brand new threading bar:

http://www.precisionmatthews.com/shop/12-internalexternal-threading-tool-set/

The threads have the classic chatter pattern, constant wavy pattern in the sides of the threads. 2024-t3 aluminum tube. Internal 28 tpi threads. I notice the support under the threading insert appears to be rubbing. Maybe it is oversize and needs to be ground down a bit? I have tried tap magic for aluminum and dark threading oil for cutting oils. I have sped up and slowed down. I have adjusted height. I have put a steady rest right at the end of my tube to support it. I'm out of ideas...suggestions?20180526_171306.jpg

20180526_171325.jpg
 
what's the diameter of the threads? on small diameters the nose of the bar gets in the way
 
Are you boring from the inside out with the tool in the back side of the part? I assume so; otherwise the tool is mounted upside down. If boring from the rear, tool height needs to be about 0.005 -0.010" below center height to handle tangential cutting forces. Depth of cut needs to be at least 1/2 the nose radius to reduce chatter. Might try messing with these things and see how it goes.
 
Yes, I cut inside threads upside down so the muscle memory works the same way as outside threads.

It also pushed REALLY hard when I try to wallow out a relief groove before threading. I assume that is the support under the insert making contact.

I have tried raising and lowering the bit off the centerline and that doesn't seem to change anything.

ID is about .85" Supposedly this kit is good down to 1/2" but I am starting to doubt that. My hand ground (from a 3/8" square HSS blank) inside threading tool never had this problem.
 
Did I understand correctly that you used the threading insert to cut a relief groove? If so, maybe not the right tool for that. Did you use the same insert to try to thread after forcing the groove? If so, the tip may not be very sharp anymore. Hard to know what's going on but threading in aluminum is usually pretty simple.

A 1/2" steel boring bar will easily do a 0.850" ID provided the bore is less than 2" deep.
 
Certainly no need to use carbide tools to thread aluminum. I use Bokum HSS tools for threading like that, the are form relieved, so you only sharpen them on top.
 
If I'm reading this correctly that's 3/8 ic insert, no way will it do a 1/2 hole. At .85 it's probably pretty tight. I've had to rotate the bar to gain end clearance which makes it even more negative rake on the insert. Given enough room that bar should make nice threads.
 
I bought the set because my home ground HSS tools keep getting aluminum buildup sticking to the tip. I was hoping the coated tools would stop this. All in all the HSS problems are much less at this point...
 
Watching with interest, I have never cut internal threads on aluminum, I was considering a similar cutting method.
Let us know what ended up solving the chatter.
 
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