Cutting Tool Engineering
April 2012 / Volume 64 / Issue 4

Tips for turning parts on manual lathes

By Tom Lipton

The following are tips for enhancing the operation of a manual lathe.

Machinists can do small work on large lathes but not large work on small lathes. Therefore, buy a lathe a little bigger than you think you might need.

Put target score marks on raw material for fast roughing. Touch off the tool and use a scale to make reference marks so you can reduce measuring time. Rip and tear down close to the lines and then pull out your finer measuring tools.

Use quick-change tool posts. Make sure you have plenty of tool blocks. I hate it when a lack of tooling interferes with productivity.

Always leave a boring bar set up in a tool block. A good starting point for general-purpose work is a ½ " or ⅝" boring bar. When tooling the bar, use indexable inserts if many different people use the machine so they can quickly change inserts, if needed. CCMT or WNMG inserts are a good compromise. You can switch insert geometry easily for different materials. WNMG inserts have six cutting edges, providing good economy. We set ours up with the thought of having one insert optimized for harder materials and one for softer materials.

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All images courtesy of T. Lipton

Put score marks on raw material for fast roughing.

Set up a dedicated turning tool block so it misses the quill on the tailstock when the tool point is on-center. This eliminates repositioning the tool when using the tailstock to support work.

Keep two parting blades set up: one neutral and the other with a couple of degrees left-hand angle.

Keep a long, double-ended, 45° chamfer bit set up all the time at each lathe. It is extremely handy for edge breaks and quick facing. The double end allows you to use it on both axes’ ID and OD.

Never modify someone else’s hand-ground tool left in a tool block. You might as well ask to borrow his toothbrush. Just take it out and leave it on top of the lathe.

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A double-ended, 45° chamfer bit is handy for edge breaks and quick facing.

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Step boring deep bores in two or more steps leaves more room for chip evacuation.

Always leave the lathe in better shape than when you found it. This has the added benefit of highlighting the shop slobs. They stand out in stark relief against a clean background, where they can be properly whipped and chastised.

Use a DOC that is a little larger than the tool nose radius. With inserts, the chipbreakers do not function unless the DOC is larger than the nose radius.

To minimize chatter, apply a tool with positive geometry, a small nose radius and a lead angle near 90°, especially when ID boring. The tool tip should be on-center or a few thousandths of an inch high.

Step-bore deep bores in two or more steps. This leaves more room for chip evacuation, and you can use a boring bar that fills the bore more completely.

Carbide and heavy-metal boring bar shanks are much more rigid for deep holes. You can buy the heavy-metal material and make your own custom boring bars. The longer bar in the photo on page 29 showing two boring bars is a rigid heavy-metal one called “No-Chat.” It can be machined into any configuration and lives up to its reduced-chatter advertising. The shorter bar in the photo has old, broken ¼ " carbide endmills for tool bits. Every shop has an endless supply of broken ¼ " tools that can be reused in this boring bar.

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The shorter boring bar uses broken ¼ " endmill inserts for tool bits, and the longer bar is made of a heavy-metal alloy to reduce chatter.

When you have chatter, try increasing the feed rate before you slow everything down. Another trick is to move the boring bar in the holder a fraction of an inch in either direction. Sometimes this small change in the resonant frequency can reduce or eliminate chatter.

Always try to increase cutting speeds and feed rates. If you never push the envelope, how do you know where the limits are? A 20 percent increase in feed returns a greater reduction in part cost than a 50 percent increase in tool life.

Test new tools once in a while. Lots of smart people are working on some effective new designs. Besides, it’s fun to test a salesman’s tools at full throttle. 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. 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., New York. The publisher can be reached by calling (888) 528-7852 or visiting www.industrialpress.com. By indicating the code CTE-2012 when ordering, CTE readers will receive a 20 percent discount off the book’s list price of $44.95.
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