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[4]

Another threading help question...

[3]
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cdhknives

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#1
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
 

BtoVin83

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#2
what's the diameter of the threads? on small diameters the nose of the bar gets in the way
 

mikey

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#3
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.
 

cdhknives

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#4
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.
 

mikey

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#5
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.
 

benmychree

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#6
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.
 

BtoVin83

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#7
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.
 

cdhknives

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#9
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...
 

Janderso

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#10
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.
 

mcostello

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#11
If You ever suspect a tool is rubbing and cannot see it, just coat the tool with a permanent marker or layout dye. Makes something seen that is hard to see.
 

markba633csi

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#12
Yes put some Dykem on and see where it rubs
 

westsailpat

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#13
It might be possible to adjust the tool off center in this case down , to get clearance or grind the bar .
 
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#14
Do you do a spring cut when close to size. It does sound like clearance issues. Do you have problems when cutting external threads. If not that helps rule out some of the rigidity problems , with internal threads rigidity is a cause , the bar flexes and skips the cut. Chatter...
 

Bob Korves

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#15
Negative rake for the tool mounting will help the clearance and the chatter. Aluminum cuts very easily, and the pics show little stickout of the bar from the tool holder. I suspect that rubbing is the issue, sometimes rubbing sounds like chatter, deal with that before changing anything else. TiN Coated inserts are not usually recommended for aluminum because of metal sticking. Unless you are threading at high speed, you should not have welding issues with that setup. Personally, I would use a boring bar with a HSS tool for that job, sharp and with plenty of rake, clearance, and relief.
 

cdhknives

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#16
External threads are good...not great...with my ground HSS bits. There are occasional tear outs but nothing like the regular wave pattern I see on these internal cuts. I put that down to the aluminum sticking issue. I have not run the external threading bar that came with this set yet. I do run multiple spring passes, and by the second or third (outside threads) I get foil thin strips coming off...literally feathery cuts. I take that to mean my tools are adequately sharp.:D 17-4PH outside threads are perfect.
 

cdhknives

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#17
Hopefully tomorrow I'll get more machine time...tending the pit doesn't leave much time to make chips.:cool:
 

derf

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#18
You need to learn how to cheat. Fine threads like that in aluminum are easy to do without the aggravation you are facing. Instead of using a single point threading tool, use a tap, in this case a 1/2-28. Set the tap up in the tool holder just like the boring bar, making sure it is true with the spindle, and the cutting edge is on center. You should be able to chase that size in 2-3 passes.
Mark the tap with a sharpie where it has went full depth. Once you hit that mark, crank the cross feed in and disengage the half nut.
 

Guyton

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#19
One thing often overlooked it the tool length. I always keep the tool 'short' don't matter what operation I'm doing and diameter the boring bar, internal threading tool is. Also, relieving the insert helps, just as grinding/forming HS blank. Good Luck.
 
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