On Saturday, I made a piece of scrap metal. I did a stupid, and then could not recover.

Then I proceeded to think my way out of my issue and into an incorrect answer.

Today I did the job over again, making different mistakes.

I’ve had more crashes making this backplate than any other task I’ve done.

The safest way to cut threads to a hard shoulder is to not cut towards the hard shoulder, cut away from it.

This means that you need to run the lathe in reverse. You have to position your cutter upside down or on the back of the side of the work.

Both options require a “left hand threading tool.” Don’t worry about it. Left hand and right hand just indicate which side of a tool the cutting edge is on.

Working in reverse, cutting an internal thread from a hard shoulder is simple. You pull the tool towards you, away from the work. You move the tool to the left to a hard stop. Your tool is now in the correct position along the axis. You then dial in the cut. This is the amount of material you are going to remove.

The final step is to engage the half nut. When you do this, the cartage starts to move away from the shoulder at the pitch required. When the cutter leaves the work, you disengage the half nut, rinse, and repeat until you have cut your threads correctly.

Chances of a crash? Almost none.

With my lathe, this can be an issue. The chuck is just screwed on. This means that it will just unscrew if the cutting force is high, and you are turning the chuck in reverse. You can make light cuts, but you have to watch, and you have to be careful.

This means I don’t do it. I don’t have the right tools to do it. That has been corrected.

I will be able to cut in reverse shortly.

If you are not cutting to away from a hard shoulder, you need to have a relief groove for the threading tool to enter. Without that, there can be issues with thread form at the end of the cut.

I have external grooving tools, I didn’t have an internal grooving tool. That has been corrected.

Because I didn’t have the left hand boring bar threading tool, and I don’t have that grooving tool, I decided to just do a through hole and thread that.

Because I turned my cross slide, all of my clearances are different. Even when I checked for clearances, I still messed up. I had the cross slide touch the work, no big deal but stupid. I had the 7/8 drill touch the jaws. About the only touch that didn’t happen was the tooling.

I got the threads cut. I got the registration cut. I then went to measure and double-check the size of the registration boss for the chuck.

That is 0.53 tall and 2.26 in diameter. The 2.26 will be cut to fit the chuck. The hole was bored to 1.75. It was then threaded 8 TPI. This means it has a major diameter of 1.87 or so. The size matching hole in the chuck back is measured at 2.17 this gives me 0.29 inches. Divide that in half, and we have 0.145 wall size, instead of the 0.59 that we would have done if I had used the proper design.

Regardless, I now have a collet check on a backplate that fits my lathe. The world is better.

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By awa

One thought on “It is the little things that drive me bonkers”
  1. I understand the issue of the chuck unscrewing from the spindle; my lathe could do that as well. I haven’t made cuts heavy enough for that to be an issue (it’s a pretty large machine). What did you do to cure the problem? Some locking system on the chuck to spindle connection?

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