Multiple Start Threads on a CNC Lathe

Fri, Jun 4, 2010

FAQs, G-Code, Threading and Tapping

Dear Answer Man,

Is it possible to cut multiple start threads on an SL-30? If so, how?

Carter Marcy

Dear Carter:

Yes, you can cut multiple start threads on an SL-30 – and on every other Haas lathe. Just change the start point of each threading cycle. For example, to cut a two-start 1”-8 thread:

G00 X1.2 Z0.5

G76 X0.8446 Z-1.25 K0.071 D0.022 F0.250

G00 X1.2 W0.125

G76 X0.8446 Z-1.25 K0.071 D0.022 F0.250

(1st thread Z start)

(2nd thread Z start)

(Note: 0.250″ lead ÷ 2 starts = 0.125″ Z shift from original start point)

Here’s something else to think about: Watch your feedrate! ( says it so well, we’re quoting them almost verbatim.) While the machine’s maximum feedrate is more than adequate for most machining applications, occasionally it can be a limiting factor. When threading on a turning center, for example, it is possible to unwittingly exceed the maximum feedrate. Say a turning center has a max feedrate of 300 ipm (with 600 ipm rapids). If you’re machining fine threads in large diameters, you are not likely to exceed the max feedrate. A 3.0″-16 thread machined at 300 sfm, for example, would require a feedrate of 23.88 (3.82 x 300 [sfm] ÷ 3.0 [dia] x 0.0625 [pitch]). This relatively slow feedrate is allowable on all current CNC turning centers.

On the other hand, when machining coarse threads at high speeds, you do need to worry about maximum feedrate. And multiple-start threads present the most problems, since the lead of the thread (not the crest-to-crest pitch) determines the feedrate. Say you’re machining a four-start 1.25″-4 thread having a lead of 1″ (0.25″ crest to crest) at 600 sfm. Now, the required feedrate becomes 1,833.6 ipm (3.82 x 600 [sfm] ÷ 1.25 [diameter] x 1.0 [lead]). This exceeds the capabilities of even the fastest CNC turning centers.

Sincerely, Answer Man

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