Cutting Tool Engineering
January 2014 / Volume 66 / Issue 1

Programming a CNC lathe

By Tom Lipton

A typical CNC lathe program is quite short compared to some of the mammoth CAM-generated programs for milling machines. Here are some tips and tricks to programming a CNC lathe.

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All images courtesy T. Lipton

Use a full-scale drawing to check tool clearances with the actual tools you plan on applying.

  • Because I operate lathes without CAM software, I have set up my program library into families of parts. I use a couple of generic starting templates to get the machine into a basic configuration, then start adding program elements and events.
  • Using multiple windows in the text editor, I can copy and paste from one program window to the next. Once you have an inventory of working, tested programs in a few different workpiece materials, it becomes easier to leverage this work against another job. In the low-volume job shop world, anything to speed up the non-part-making portion of a process is worth trying.
  • Copious notes within the program help speed up the identification of specific elements that you may want to reuse in another program. This is a good thing to do during the lag time while running parts. You can develop several generic program scenarios that frequently repeat and just need some quick editing to run.
  • Use a full-scale drawing to check tool clearances with the actual tools you plan on applying. When in doubt, check it out. The bottom of a deep bore is not the place to find out the tool didn’t have enough clearance.
  • In many cases, it’s nice to have parts come off a lathe complete. However, this is not always possible or practical. One useful trick is to cut multiple diameters and recesses into soft jaws so you can flip a part after a pause and finish a part in one cycle. There’s nothing like having a machine set up to run complete parts when the customer calls and increases the quantity. Programming and setting up to complete parts in one fixturing allows you to leverage the initial setup and reduce setup time on repeat jobs.
  • It’s nice to complete a part from one side of the workpiece. One thing that prevents doing this easily is the final little chamfer or edge break on the ID of a bore that you are parting off into. You can use a threading tool modified to 45° to prechamfer the ID where the parting tool will break through. You can also profile the chamfer with a standard internal grooving tool. If you choose that route, be sure to pregroove straight in a few thousandths of an inch deeper than the chamfer before profiling with a regular grooving tool. It can completely eliminate a secondary operation and the associated handling and price of an extra tool change.
  • Delicate parts that shouldn’t go through the chip conveyor can be caught using a magnetic catcher snapped onto the parting station. Be sure to include an M01 or M00 full stop in the program to remove the parts before too many collect and cause a problem.
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    If you don’t have a parts catcher on your lathe, catch parts with a catch cup, such as this one with a telescoping handle.

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    Delicate parts that you don’t want going through the chip conveyor can be caught using a magnetic catcher snapped onto the parting station.

  • If you don’t have a parts catcher on your lathe, use constant cutting speed when parting right up to the point where the piece almost detaches, then switch to a slower constant speed for the final parting off. This prevents flinging the part against the turret or enclosure and damaging it.
  • If you really need to catch the part, use an M01 or M00 right before the part separates and use a catch cup on a stick—the one I made has a telescoping handle (see photo on page 23). Be sure to turn the coolant off in the program so you don’t take a bath when you fire it back up, but don’t leave too much to part off with the coolant turned off. Leave just enough to give you time to sneak your catch cup into position.
  • Keep a simple, easy-to-edit program in the control for boring soft jaws or any other repetitive operation your shop encounters. This is a common setup event for a CNC lathe. If your control supports parametric programming, so much the better. A soft jaw program might be as simple as changing a couple of values at the beginning of the program and touching off the Z-axis. CTE

About the Author: Tom Lipton is a career metalworker who has worked at various job shops that produce parts for the consumer product development, laboratory equipment, medical services and custom machinery design industries. He has received six U.S. patents and lives in Alamo, Calif. For more information, visit his blog at oxtool.blogspot.com and video channel at www.youtube.com/user/oxtoolco. Lipton’s column is adapted from information in his book “Metalworking Sink or Swim: Tips and Tricks for Machinists, Welders, and Fabricators,” published by Industrial Press Inc., South Norwalk, Conn. The publisher can be reached by calling (888) 528-7852 or visiting www.industrialpress.com. By indicating the code CTE-2014 when ordering, CTE readers will receive a 20 percent discount off the book’s list price of $44.95.

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