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Cutting fine internal threads on thin tube

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compact8

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#1
I am trying to cut 0.5mm pitch 60 degree internal threads on a thin-wall 6061 T6 aluminium tube ( 40mm outer and 38mm inner diameter ) . This is the first time I did it and multiple problems were encountered :

1) The tube is not perfectly round. The dial reading swings between 0 - 0.04 mm when measuring the outer surface , 0 - 0.12 mm for the inner surface
2) The tube wall is not uniformly thick as indicated by the dial readings above
3) The tube wall flexes upon cutting.

Because of these issues, the thread produced is really messy. The tube was supported by a fixed steady in the process. Can anyone lend me some help pls ? Thanks in advance.
 

T Bredehoft

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#2
Are you usilng a tap? or a threading tool? (I can't imagine a tap that size) I occasionally work on 6061 8mm tube, outside threads, about 1mm, .041 inches pitch. I chase them about 3/4 deep then run a die on them with light oil to lubricate the cutting. If I only chase them it distorts the tube badly near the end, If I use the die only it twists the tube as soon as it catches.
 

Rooster

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#3
Greetings, perhaps you could try boring the i.d., not much wall thickness to work with though.
 

compact8

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Are you usilng a tap? or a threading tool? (I can't imagine a tap that size) I occasionally work on 6061 8mm tube, outside threads, about 1mm, .041 inches pitch. I chase them about 3/4 deep then run a die on them with light oil to lubricate the cutting. If I only chase them it distorts the tube badly near the end, If I use the die only it twists the tube as soon as it catches.
I am using a lathe with single-point thread cutting tool.
 

BtoVin83

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#5
You have multiple issues to overcome in order to produce a decent thread. the ID is out of round and the OD is out of round plus the steady rest is transferring the out of round to the ID. The tube is too thin to cut a steady rest spot and will heat up rapidly when spinning in the rest. First step is make a sacrificial donut for the lack of a better term, to shrink on the tube OD not too tight or it will distort the tube, just enough to keep it from getting loose.
Position it where you would like to support the tube with the steady rest.
Now face off the end of the tube from inside to the outside with the tube, not the donut supported in the rest. It is best to run upside down while doing work in the steady rest as you can leave the top jaw loose to reduce friction heat.
Support the tube with a live center and back off the steady rest jaws.
Turn the donut to a smooth finish, now you have a round OD as close the true center of the ID.
Support the tube with the rest running on the donut you just turned.
Skim cut the inside just to round up the ID so you wont have high spots on the thread crest.
Now you can cut the threads, as always it's best to run upside down when performing work in the steady rest.
When finished remove the donut.
 

compact8

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#6
Thanks for all the replies. Seems that there is no simple solutions. Will try out the advice given. Thanks again.
 

BtoVin83

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#8
It's not hard to do just a lot of steps to make it come out. That's the difference between a machinist and somebody who owns a lathe. These are things I have learned in over thirty years in the business.
 
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