Shop Doc – Threading Issues

Today’s Machining World Archive: May 2006, Vol.2, Issue 05

Dear Shop Doc,

I’ve been tapping parts on Acmes for years, mainly running 12L-14 material. I have a new job, and I have to run titanium. I normally tap the material using a bump cam to start the tap. Now I’m finding that when the tap backs out, I’ve got elongated threads. What could be the problem?

Elongated Threads Torture

Dear Elongated,

You can address this problem in two ways. Rather than using a bump cam, you could use a cam with the exact rise necessary to tap and return on the machine. However, they’re expensive and time consuming. It could take 6-8 weeks for delivery.

Another way to approach this issue is to take the threading hanger, slide and the base off the machine. Then, machine the base and add linear bearing rails to it. After that, attach the slide to the linear bearing rails. Now you should be able to move the slide easily. With just the movement of your little finger, the slide should move back and forth. The friction and build up will be gone. That way, the tap will engage without applying any pressure, and you will get good threads.

Rex Magagnoti
Machinery Sales, Graff-Pinkert

Dear Shop Doc,

In the last few years, we have begun updating our threading attachments using new Logan air clutches to reduce the older electric trip air cylinder shifted mechanical clutches. We experience fewer problems, but we have experienced some problems with the Logan style clutches. One problem that we have had with the Logan style is that they shift so fast it causes a backlash on the tapping chain, which can break the tapping chain. This problem has only occurred when we are running soft steel, and the threading spindle rpm is very high. The high rpm of the threading spindle cause a momentary jerk when shifting from high to low. The tapping chain will fail before the clutches.


Dear Backlash,

Some time ago, we ran into a similar issue. After going through a number of tapping chains on different machines, we came up with a solution. We adjusted the air pressure going to the clutches to what we thought would be the minimum they would need to operate safely. We felt this would allow for a little play, which would help absorb the backlash. Sure enough, it worked, and the tapping  chains stopped breaking. We found this out on a job that already broken three tapping chains in 2 weeks. This is highly unusual by any circumstance, but the rpm was running 1164 rpm in a 1-5/8 RB8, which is fast for that large a machine. The tapping chains stopped breaking, so we must have been correct on our theory.

Wes Szpondowski
Tool Room Leader, Wyandotte Industries,
Wyandotte, MI

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