Shop Doc – Tapped Out

Today’s Machining World Archive: May 2007, Vol.3, Issue 05

Dear Shop Doc,

We can’t accurately control the depth of the tap on our CNC Swiss lathe. The type of component we are making has a blind hole that goes .200” deep. The threads are 6-32 and need to have a fully formed thread of a minimum depth of .185”. The tool is a thread forming tap with a blunt point. We are using a Floating Tap Collet, and we program about 10 percent slower than the tap pitch, stop the spindle, slight dwell, then feed off at 100 percent of the tap pitch. We’ve tried many variations of this method with different spindle speeds, different floating collets and different tool positions, however, we still cannot accurately control the depth. One of the issues of not being able to control the depth is that we “bottom out” frequently and break the tap. Down time, tool cost and scrap material is eating into our profit margin for this job.

Tapped Out

Dear Out,

I can relate to your pain. I know how it feels to quote a job and not be able to manufacture at the production rate you thought you would be able to achieve. Tapping on CNC Swiss Lathes used to be fairly adventurous. It appears you have done your homework in selecting the correct tap style. The problem you are having is caused by the floating collet. It’s assumed that because you are machining on a CNC machine that you have absolute control of all of the dimensions, but when it comes to tapping with floating collets, you don’t have as much control as you think. What is happening is that when the part engages the tap, it is not starting at the same position along the polar axis (C-axis) every time. This is critical to how many threads are produced based on the Z-axis movement. For example, if your part begins to tap at 12 o’clock, which starts to pull the collet because of the 10 percent slower feed, you’ll get one depth, but if it starts 180 degrees later at 6 o’clock, you’ll get a different depth. With floating collets, this is the variable you live with and in most applications it’s ok.

For this particular application I would go to Rigid Tapping. Rigid tapping is something relatively new to CNC Swiss-type machines. Some of older CNC Swiss machines are not capable of this process, but with more manufacturers offering C-axis as a standard option, rigid tapping is a great solution. Rigid tapping has been around for many years on most CNC Lathes & Milling Centers. The programming of it is fairly simple, and you don’t have to worry about using any special collets. Check your machine manual to see if your machine is capable.

David Cogswell
Regional Manager at Gosiger Inc.

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