Single point threading

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Mr Ron
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Single point threading

Post by Mr Ron » Thu May 03, 2018 10:16 am

I keep hearing the lathe compound should be set at 29.5° for threading. Why is this so? It seems it doesn't matter where the compound is set as long as you are just using the cross feed for advancement. I know the threading bit will be cutting both sides of the thread, but it would still work and by setting at 29.5°, you would be advancing the threading bit to cut only one side of the thread profile, so does it really matter all that much?
Mr.Ron from South Mississippi

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GlennW
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Re: Single point threading

Post by GlennW » Thu May 03, 2018 10:20 am

If you are advancing with the cross slide it really makes no difference where the compound slide is set, as long as it is locked down.

If advancing with the compound slide the slide needs to be set slightly less than the half angle of the thread being cut, so for a 60 degree thread, around 29 degrees from perpendicular to the thread axis with it advancing toward the direction of carriage travel.
Glenn

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John Evans
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Re: Single point threading

Post by John Evans » Thu May 03, 2018 11:22 am

Try cutting a course thread in 1018 steel on a light flexi lathe with just the cross slide !! You will see why using the compound works a lot better,as you are only making one chip rather than 2. 2-3000 lb lathe in good tight shape ,use the cross will be faster . The 29* usually gives a better finish also.
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DavidF
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Re: Single point threading

Post by DavidF » Thu May 03, 2018 11:37 am

I get better a finish with the cross slide set at 30' rather than 29'. Try it both ways on some test scrap and see what works best for you...

Harold_V
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Re: Single point threading

Post by Harold_V » Thu May 03, 2018 4:03 pm

There's a reason why 29° (or 29½°) is chosen. First, when feeding with the cross slide, there's a serious conflict with chip flow, as it's generated on both sides of the threading tool, causing the chip to flow towards the center of the tool. That works just fine for light passes, but when roughing the thread, not so much. The increased pressure often leads to tip failure, which creates serious problems when one approaches the target pitch diameter. The tool tends to float on the failed end, so light cuts become impossible. For that reason, regardless of the rigidity of one's lathe, feeding by compound, at the preferred angle, is highly recommended. I go so far as to remove and sharpen my threading tool to ensure it is capable of fine feeds when sizing the thread. All depends on the application, of course.

There's risk in choosing 30°. If the marks on the lathe are not perfectly oriented, you may feed the tool beyond the half angle of the thread. When that happens, the trailing flank is generated with a series of steps. For critical applications, that isn't desirable (nor would it be acceptable, as it no longer conforms to proper thread form). It is for this reason that my choice of compound rest angle setting is 29° instead of 29½°), as it assures that I feed at less than half angle of the thread.

Lastly, when feeding the tool near the half angle, it is constantly fed on the trailing flank, albeit only very slightly. That has the advantage of keeping the flank clean of chip rubs.

To improve the overall appearance of threads, my choice is to plunge cut the last few thou, taking very light passes so chip flow isn't an issue. With a sharp tool, the chip, depending on the material being threaded, will form a perfect V and come off easily. It's a good indicator of how sharp your tool is, and assures the thread generated is of proper form (assuming the tool is). It goes without saying, proper lubrication is always applied (acid brush is adequate).

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Harold_V
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Re: Single point threading

Post by Harold_V » Thu May 03, 2018 4:07 pm

DavidF wrote:
Thu May 03, 2018 11:37 am
I get better a finish with the cross slide set at 30' rather than 29'. Try it both ways on some test scrap and see what works best for you...
It would be rather unusual for a cross slide to be adjustable.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Magicniner
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Re: Single point threading

Post by Magicniner » Thu May 03, 2018 4:42 pm

With a DRO depth of cut is set with the cross slide, you can then choose your linear offset with the top slide parallel to the bed ;-)

DavidF
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Re: Single point threading

Post by DavidF » Thu May 03, 2018 10:37 pm

Harold_V wrote:
Thu May 03, 2018 4:07 pm
DavidF wrote:
Thu May 03, 2018 11:37 am
I get better a finish with the cross slide set at 30' rather than 29'. Try it both ways on some test scrap and see what works best for you...
It would be rather unusual for a cross slide to be adjustable.

H
Yea Yea.... I'm fairly certain that the OP understood the point I was alluding to... But just in case the poor fellow was sitting there trying to figure out how to set his cross slide at 29 or 30' its a good thing you pointed out my error :wink:

Harold_V
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Re: Single point threading

Post by Harold_V » Fri May 04, 2018 1:41 am

DavidF wrote:
Thu May 03, 2018 10:37 pm
Harold_V wrote:
Thu May 03, 2018 4:07 pm
DavidF wrote:
Thu May 03, 2018 11:37 am
I get better a finish with the cross slide set at 30' rather than 29'. Try it both ways on some test scrap and see what works best for you...
It would be rather unusual for a cross slide to be adjustable.

H
Yea Yea.... I'm fairly certain that the OP understood the point I was alluding to... But just in case the poor fellow was sitting there trying to figure out how to set his cross slide at 29 or 30' its a good thing you pointed out my error :wink:
You're welcome. :lol:
We have a wide and varied readership on this board. Some of the folks come here to learn---so anything posted that isn't correct can lead to less than desirable convictions--that one sets the cross slide, not the compound rest. It may be obvious to you, and most readers, but it may not be obvious to those who hope to learn proper procedures and operations. For that reason, when I am able (I don't claim to know everthing), I will correct anything that is posted that isn't proper. Nothing personal.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Mr Ron
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Re: Single point threading

Post by Mr Ron » Fri May 04, 2018 9:38 am

Thanks Harold; as usual you make good sense when answering my questions. I have worked with machine tools most of my life and although not completely proficient in any of them, at 83, I'm still learning and hope to keep doing so.
Mr.Ron from South Mississippi

DavidF
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Re: Single point threading

Post by DavidF » Fri May 04, 2018 9:38 am

Harold_V wrote:
Fri May 04, 2018 1:41 am
DavidF wrote:
Thu May 03, 2018 10:37 pm
Harold_V wrote:
Thu May 03, 2018 4:07 pm

It would be rather unusual for a cross slide to be adjustable.

H
Yea Yea.... I'm fairly certain that the OP understood the point I was alluding to... But just in case the poor fellow was sitting there trying to figure out how to set his cross slide at 29 or 30' its a good thing you pointed out my error :wink:
You're welcome. :lol:
We have a wide and varied readership on this board. Some of the folks come here to learn---so anything posted that isn't correct can lead to less than desirable convictions--that one sets the cross slide, not the compound rest. It may be obvious to you, and most readers, but it may not be obvious to those who hope to learn proper procedures and operations. For that reason, when I am able (I don't claim to know everthing), I will correct anything that is posted that isn't proper. Nothing personal.

H
No offense taken. Although now I am envisioning Homer Simpson sitting in front of a lathe, reading my post, and trying to make the necessary adjustments......

Conrad_R_Hoffman
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Re: Single point threading

Post by Conrad_R_Hoffman » Mon May 07, 2018 12:33 pm

The tool should move almost parallel to the trailing side of the thread, but just a bit less, so it shaves that side too. If it were a bit more, you'd cut steps in the trailing side. Different lathes sometimes put the zero degree mark in different places, leading to confusion. The rule I use is point the compound at your belly button. It should read zero or ninety. From whichever it is, turn the thing counter-clockwise 29.5 degrees.

As an example, a Hardinge HLV will show a compound angle of 90 degrees when pointed at the belly button. It counts down in both directions from that point. So, you start turning counter-clockwise from 90, 80 degrees, 70 degrees, 60.5 degrees, and lock it. That's 29.5 degrees in the normal scheme of things.
Conrad

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