threading problem

[bman356]

We are trying to machine a 3/4 ght(garden hose thread 11.5tpi) female thread on a daewoo puma 240 with a fanuc 21-I control. The material is aluminum. The program runs fine and the minor is good but our threads are ugly. The root ot the thread looks good and has no vibration but the crest has chips ripped out of it. There is a nice flat on the crest and our test piece fits in fine. I have tried all of the normal variables like rpm, bigger/smaller first cuts, entrance angle (from streight to 29, to 30) but we still end up with the same result. I have tried 2 different tools and had no luck with either of them so i don't think that is the problem. My only guess is that because we are starting about .07 from the start of the thread the tool doesn't have time to accelerate before it cuts. We can't start any farther away because we are inside a groove.
Any suggestions on how to clean this thread up would be great. Thanks
Brad

[Geof]

You say aluminum but presumably it is an alloy; what alloy and what temper? Some alloys no matter what the temper have a tendency to rip like you describe when the cutting speed is slow and even alloys that are normally good for machining will rip if they are very low temper.

Use a positive rake cutter and a very rich coolant mix and you may be able to improve it.

[Delw]

Is your bar supported in the back? I assume you are running a length of rod, if there is no support to stabilize the bar in the back you will have issues.
On Alum pipe or coarse threads I always run a .002 finish pass then a dry pass.

your fine on the machine and the time to acc.
I believe its more of a tooling/set up issue than a machine problem.

what type of tooling are you using a 1"square tool with insert? are you on center

The only other time I seen this if everything is correct (tooling/program)) is the head alignment to the tool changer. one part of the part you cut good the other part your dragging the insert cause the tool is out of alignment or the bar in the back was not supported and it was wobbling and transfering to the front.

oh yeah one other thing, do you have the CORRECT insert for the thread? that thread pitch needs lots of clearance so the angle doesnt rub on the back side of the insert.