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Left-hand Screwcutting Woes

Greetings

Has anyone had success in screwcutting left-hand threads on a Myford S7B where both flanks of the thread come out looking good? I've gone through several feet of leaded steel, trying to cut a 7/16" x 20 tpi,

60 degree vee thread. I'm cutting under power, from left to right, back-geared low speed, tool facing the rear of the lathe and ground for left hand cutting. I assume this is correct for left hand threads. However, while the leading flank always come out looking great, the trailing flank is torn and pitted with occasional chatter marks. Any comments?
Reply to
Pat Dworzan
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Have you got the top slide set at 29 1/2 degrees pointing towards the right-rear? This will allow almost all of the cutting to be on the left hand flank and just a polishing action on the right hand flank. it will also stop the saddle from "hunting" back and forth due to slack in the lead screw.

Mark Rand RTFM

Reply to
Mark Rand

I think you will find it to be 27 1/2 degrees. Don't forget when direct screwcutting you need to take more than the indicated thread depth on the compound slide. I always take at least 2 spring cuts if not more.

Cheers Dave

Reply to
Dave Jones

I spoke too soon!. Upon closer examination, the condition is still there, but now on the leading flank as well as the trailing flank of the thread.

Reply to
Pat Dworzan

Sorry, I have just re-read the original and agree with you, I must have had Whitworth on my mind. Same principles otherwise.

Cheers Dave

Reply to
Dave Jones

That was the slack to which I was referring. But there is also the possibility of the saddle pivoting around slightly if the gibs are not snugged up. This is likely to be more of a problem on Myfords using both sides of the front way to guide the saddle rather than the front of the front way and the rear of the rear way (you don't say which yours is). A good dose of way oil can help to damp out the vibration here.

Since yours is a S7 I assume that it is very unlikely to be the headstock mandrel thrust bearing.

Is it worth trying a travelling steady?

Mark Rand RTFM

Reply to
Mark Rand

The saddle and gibs appear to be adjusted correctly. I just completed another test. This time I cut a normal, right-hand thread, 3/8 x 20 tpi x 2" long, and used the mandrel handle--no power. The work was gripped in the three chuck with the free end supported by a dead center. I used a freshly ground tool bit set exactly on center; topslide at 29 degrees. Infed at first was .004, reducing to .001. Plently of thread cutting oil slathered on the work, including me :( The leading flank turned out good, but the trailing flank looks like hell. I do notice that the amount of metal removed varies with the marks on the indicator, even though I take a clean up pass every now and then, followed by another limited to .001. Is this indicative of a problem with the half nut?

Reply to
Pat Dworzan

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