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Thursday, November 22, 2012

Threads and their pitfalls...

Thread cutting can sometimes be a bit tricky. It's not terribly difficult, but there are things to watch out for. Every now and then I get a bit ahead of myself and screw up a thread (nice pun, eh?).
This is one of the main stems for the Norris adjusters I've been working on. Notice the right end of the threads, they are a bit jagged. That can be caused by a couple different factors.

A nice brand new die, adjustable...which is important.
So, what problems occur while cutting male threads? Well, here are the easiest mistakes to make and how to (hopefully) fix them:

Off center: while cutting a thread, it is easier than you think to have the die start cutting off of center. It is a common misconception that dies are self-centering. The easiest way to avoid this is by using a die-stock in a lathe. If you don't have a lathe, you need a good touch to keep the die running straight while you steer the die holder. Another cause of off-center threads is a chipped tooth (or two or three) in the die. Dull or chipped teeth will cause uneven cutting.

Jagged threads: When you run a die down the shaft, you must remember to back it off and break the chip. For every 1/2 or full turn, back the die off far enough to break the chips. This will help to make smoother threads. Lubricant is also important to avoid rough threads. Adjustable dies can help as well. If you open up the die (by tightening the adjustment screw) you will be taking a smaller bite into the piece. On days when I have my brain turned on, I like to open the die up before I start. It's easier to adjust the die and run it down the piece a couple of times than to fight against it and make crumby threads.

Die won't start: If you have an adjustable die, open it up. If you have a non-adjustable die, check to make sure it is sharp. Or...throw out the die and buy and adjustable one!

Don't forget that different materials act differently. Cutting threads on aluminum and say, 304 stainless is not the same chore. The techniques are mostly the same. But you will want different lubricants. Aluminum can be sticky, using a thin lubricant helps to stop aluminum from sticking. I like a more viscous fluid for something like stainless. Aluminum is soft, so it will probably feel as if everything is going just fine. This is the point at which most of us will forget about backing off the die to break the chip. Since it produces less resistance, it will be easier to get out of true and make a crooked thread.
Sharp cutting tools are always important. A dull die will cut bad threads no matter what. Sharp enough to cut aluminum may not be sharp enough to cut stainless. Inspect your tools for sharpness!
I mentioned adjustable dies above, besides being helpful for avoiding bad threads they have another bonus. By starting with the die wide open and working your way down, you can adjust the fit of the thread. Since you're starting with a wide open die, you can work your way down to the perfect fit. Also, by adjusting the bite, you are putting less stress on the cutting edges and the tool will last longer.

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