4 TPI Thread: How is this looking to you?

I was cutting this 4 tpi thread. I set the cross slide to zero, and "closed" the compound to it's furthest location (where I usually keep it when I'm not using it) and zeroed it, then I touched onto the area of the work at the depth where the root width was appropriate, re-zeroed the dial, removed the tool from the work, and advanced the cross slide to the full depth of thread. Then I reset THAT to my Zero location. Then I pulled the compound off until the tool would just barely touch the work, and began threading with the compound only, retracting the tool from the compound as needed to clear the thread priorly cut; I repeated this until I was close to where I thought I should be. Here's the problem:

I don't know. Somewhere along the line, I thought the right flank of the thread was looking a little shallow, and I noticed that my HSS cutting tool (which I ground like a CHAMP, might I add) had nudged just a tad off to one side. Maybe 3º. I tapped it into place, and found the lost thread, resetting the dials to the numbers where I was when I noticed the problem. Well, the thread doesn't look right to me, and I was hoping someone could set me straight, or tell me to do it again.

Maybe I should just keep going in with the compound?

This is a hugely complicated part to do, and if I don't have to, I don't want to have to start over from scratch.
I've never cut a 4 TPI thread before, and I've never fed from the compound before, so I'm a bit out of my element here. Please have a look at this for me and tell me if you think this is right.

nope, pretty screwed and not in a good way

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.

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.

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.

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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Yes, you appear to be correct, RJ.

Ouch . Hope you can clean it up .

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.

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.

BtoVin83 said:

nope, pretty screwed and not in a good way

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That joke is just cruel.