Shop Doc – Loose Ends

Today’s Machining World Archive: August 2006 Vol.2, Issue 08

Dear Shop Doc,

I have a medium sized turret type 2-axis CNC lathe. I followed the tips regarding the laydown threading systems from last month, and my tool life increased a great deal by using the proper adjustable anvil. However, when using this style of threading system, I still seem to struggle with the smaller diameter threads.

The No-Go Gage we use goes on 2-3 turns when cutting threads under .500 in diameter. These parts are made of low carbon steel (like 1018), and I try to follow the recommended SFM provided by the tool manufacturer. With the coatings typically seen on these tools, it seems the SFM range can be very high, thus resulting in high RPMs. It seems that if we do slow down the RPM, the No-Go Gage doesn’t go on as far, but the tool life isn’t as good.

We have tried several thread cycle options such as “infeed” variations, depth of cut, etc., but it doesn’t seem to change the results.

I’ll take thread quality over tool life for now, but is there something I can do to get great results for both?

Loose Ends

Dear Loose Ends,

When you said your lathe was a medium sized turret type CNC, that was a great clue to your problem. When you mentioned smaller diameters, it was another great clue. I think you are starting too close to the thread starting point in your Z axis. When you do that, the turret can’t synchronize quickly enough with the spindle.

It’s not uncommon for turrets to travel at 300 IPM while threading. To synchronize that much weight with the spindle is difficult at those feed rates. Until the turret and spindle synchronize, the form of the thread may not be correct.

Here’s what you should do. Keep the RPM range the tool’s manufacturer recommends. Then, do the following equation: ((1/TPI) * RPM) / 400 = Start Point in Z.

The result of the equation equals the distance in front of the thread starting point where I suggest you start the Z axis in your threading cycle. By positioning the tool far enough in front of the thread for the spindle and turret to synchronize (calculated from the RPM to achieve the proper SFM for cutting the specific material with the specific grade of carbide), I’m confident you will achieve great thread quality as well as improved tool life.

Good luck!

Jim Rowe
Application Specialist/Medical Accounts
Mahar Tool Supply, Warsaw, IN.

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