4 TPI Thread: How is this looking to you?

T Bredehoft said:
Resetting zero on the assumed depth of cut is a theoretical solution. In practice we cut until some solution is found, either with a thread mike, thread wires or a nut screwed on. There are too many pitfalls around theoretical depth

Oops, I missed the lower portion of the image, showing the buttress thread effect.
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I was planning on checking progress before getting to full depth. I agree, "theoretical" limits are just a place to start.

RJSakowski said:
It looks like your compound angle is set wrong. The angle should be 29.5º (more or less) to the cross slide feed. Some compound dials measure the angle between the spindle axis and the compound. If that is the case for your lathe, you should be setting the angle 60.5º.
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I'm using an HLV-H, and I think you may be right. It looks like I set it on 30 when I needed "60".

Bob Korves said:
Depending on how much depth of cut is remaining, you might be able to save it. I cannot tell by the pics how far you have already cut. You will lose maximum diameter while getting the flank angles correct. If that puts the threads under the major diameter, then you will have to start over. If you think you can save it, make sure you are moving toward a position where the current O.D. will get cut down as little as possible while getting the geometry correct.
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tcarrington said:
In addition to the compound angle - maybe the tool orientation with the work piece axis of rotation.

There are some really fine you tube how to videos in this regard. 4 tpi is a "fast" thread. Having a very low spindle speed would be a great help.
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I'm doing this at 125 RPM. Do you have a video recommendation?
RJSakowski said:
Some further explanation. If you ignore the compound and just advance the feed with the cross slide, you will cut both flanks of the thread equally. The cutting tool is a form tool and the shape of the of the thread mirrors the form.

On the other hand, if the compound is set at exactly 30º, the tool will travel along the previously cut surface and all cutting with the exception of a small amount at the tip of the tool will occur on the left flank. If the compound is set at an angle greater than 30º, the tool will not engage the previously cut portion of the right flank as it is advancing at a greater angle to the work. The cut at the right flank will create a small step or saw tooth. This step is visible in your photos.
The compound angle is set to slightly less than 30º to avoid any step caused by mis-setting the angle to more than 30º and to provide a slight shearing cut to dress the right f.lank. Although a smaller angle will do the same, cutting both flanks equally or close to equally has a tendency to cause chatter, especially on coarser threads.
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I tried this before, with an Acme thread, and feeding directly in, but the machine just couldn't handle that much material removal. Even at .001 on diameter DOCs, feeding straight in was just too much for the machine. Chips would pack in there, and it would just stall.
 
And, if you don't have/use one, you might want to get/use a "fishtail" to assist with making sure the cutter is properly aligned to the workpiece (also helps with making sure cutter has the right overall angle if you are hand-grinding it from hss)

you can google for "lathe fishtail" for it, if you are unfamiliar with it.
 
Almost looks like a buttress thread... Make sure you use a lathe 60 degree threading tool to get the cutting tool square to the work...1547638886101.png
 
I agree with double checking the angle you have your compound set at. Also, another thing to watch on very coarse threads is make sure your threading tool has adequate clearance on the leading side. Your tool is moving rapidly down your ways when cutting coarse threads and you will need to make sure your tool is not rubbing on the leading side clearance. Sometimes I'll grind a secondary clearance angle on the leading side when cutting coarse threads.

Ted
 
dave_r_1 said:
And, if you don't have/use one, you might want to get/use a "fishtail" to assist with making sure the cutter is properly aligned to the workpiece (also helps with making sure cutter has the right overall angle if you are hand-grinding it from hss)

you can google for "lathe fishtail" for it, if you are unfamiliar with it.
Click to expand...
Oh, I've got a fishtail. The tool was where it was supposed to be when I started the cut...
 
Technical Ted said:
I agree with double checking the angle you have your compound set at. Also, another thing to watch on very coarse threads is make sure your threading tool has adequate clearance on the leading side. Your tool is moving rapidly down your ways when cutting coarse threads and you will need to make sure your tool is not rubbing on the leading side clearance. Sometimes I'll grind a secondary clearance angle on the leading side when cutting coarse threads.

Ted
Click to expand...
Yeah, I went out there last night after reading these messages and couldn't sleep. Very embarrassing. The tool has 10º of side clearance on the front flank, which more than clears the helix angle of 2ºish.
 
Now you may be able to fix this part if there was a lot of time put into it.... You can cut turn the threaded part down just below the minor diameter of the thread and then bore the ID of another piece of stock to get a heated interference fit of about .0005" -.001" and then cut the threads again in the new stock you pressed on...
 
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RJ nailed it. It appears that you're compound is set at ~30° off the spindle axis, as opposed to 29-30° off perpendicular to the spindle axis as it should be if you thread using the compound to advance the tool. Those tiny steps in the right half of the thread are a dead giveaway. Not to mention the slope.

Tom
 
SmokeWalker - MrPete222 is a standard for beginning shop how to.
This is another -
It has a lot of detail wrt threads of all types (ACME, pipe, M and UNF) which might be useful or boring, depending on where you are.
at about 25:00 he shows what I was talking about with the fish tail thread gauge.
His video will let you see about where / how the compound should be.
The gauge is also very good for checking cutting tool geometry.
I think RJSakowski has it right in your case. Getting the compound turned the right way and proper angle has been an issue for many an external threader.
 
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