Problems using a die for threading

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rronald

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I'm trying to make a differential screw. This is a screw that has two threads so that the amount of movement is very small (the net effect is that the total forward movement is the difference between the two thread pitches. The concept is described on this web site:

link

I thought the best way to make the screw would be to start with a M5 socket head bolt. The one I had handy was a 60mm long Class 12.9 alloy steel black oxide finish. Since this was only threaded on the last 25mm, I wanted to do three operations.
  1. Extend the M5 threaded area another 15 mm or so (using a die).
  2. Turn the last 20 mm of the bolt to remove the 5mm thread and leave a 4mm section.
  3. Use a die to create M4 threads on the turned area, about 20 mm in length.
To hold the piece, I used a 5mm ER40 collet. The head of the bolt is on the inside of the collet and about 45mm sticks out.

I constructed a die holder. Even machined the mt3 tape successfully. I was easily able to remove the existing M5 threads (operation #2) with no problem. I was totally unable to thread using dies for operation #1 and #3.
  • No matter how tight I cranked the ER40 collet, the bolt moved during the threading attempts (turned relative to the chuck and slipped deeper into the chuck). Any guidance on how to hold a bolt like this would be appreciated. And it's possible that the hold pressure would be sufficient if the dies were working.
  • The dies seemed to require inordinate force. I could barely get the M5 die to turn when going over the store-bought bolt M5 section. When I reached the unthreaded part, the die would get a few turns and then nothing could move it. In either direction. Actually had to remove the collet and cut the bolt to get my collet back. The piece stuck in the die was not removable. I was turning the right direction. The die was M5 and the bolt was M5. I did have the chamfered end of the die pointed towards the bolt to start.
Before losing the M5 die, I had tried using a hand taping setup against the store-bought M5 thread and didn't get more than 2 or 3 threads down before it got too hard. My conclusion (hope) is that it was just a terrible die. Or that class 12.9 bolts are too tough/hard to tap with a die.

I can't single-point cut the M4 threads with the gears I got with the lathe (no .7 pitch available). I plan to do the electronic leadscrew someday, but only after installing a DRO.

The die was from Harbor Freight. However, I've used taps from the same set for quite a while and they aren't that bad.

I have a new and hopefully better die (HSS and it has the adjuster) on the way.

If worse comes to worse, I have a CNC mill that I built. This differential screw adjuster doesn't need a ton of strength , so I could threadmill all the threads I need in aluminum. But, my lathe is new and I don't want to give up on it!

Comments and advice would be appreciated!
 
Those threads are rolled, and somewhat hard, that is why the die is hard to turn.
 
And, likely HF dies are a long ways from even moderate quality.
 
And likely the HF dies are not even on size. Been there, done that. My HF die cut "sloppy" threads, meaning the pitch diameter was less than it should have been. The nut rattled around on the male threads.

If you need to cut threads with a die make sure you use a decent quality die made of HSS (high speed steel).

I made a differential screw M6/M5 pair for adjusting my tail stock on my mini-lathe. The diff screw made it a lot easier to dial in the tail stock position than using the original 1mm pitch thread. I single pointed the threads on the lathe. The coarse adjust is 1mm/turn, the fine adjust due to the differential screw is (1mm - 0.8mm) = 0.2mm/turn. Made the brass knob, the steel bracket and captive nut in the bracket, as well as the nut you see in the rounded slot in the cast iron base.
PXL_20210524_154322131.jpg
 
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It probably doesn't help that you picked a socket head cap screw. Class 12.9 is 180 ksi tensile and in the Rc 40 hardness range. That poor HF die is being asked to do an awful lot, not to mention it's hard to get a lot of grip on a diameter that small. Single-pointing is really the way to do this.
 
rronald said:
My conclusion (hope) is that it was just a terrible die. Or that class 12.9 bolts are too tough/hard to tap with a die.
Click to expand...

You called it, on both counts. You can find dies that will cut that reasonably. It's still quite hard as bolts go, it will take some effort. In small sizes like this, it'll still be a gamble as to the quality of the result.

rronald said:
The die was from Harbor Freight. However, I've used taps from the same set for quite a while and they aren't that bad.
Click to expand...

Are you sure it's from the same set? I haven't seen a tap or a die from Harbor Freight that I would call plausible for making threads. Cleaning existing threads? Sure. Possibly. If you're lucky enough that they felt like making them kinda sorta on size a little bit. Usually they're just off the wall in how much "tolerance" they have built in, regardless of the ability to cut. I'll defend Harbor Freight for some of their hidden gems sometimes, but their taps and dies I just make up enough excuses for. Of course, with that said, if you did find a couple (or more) functional pieces in a set that's typically pretty unacceptable. Take it where you get it and get what you can out of 'em... But if you got a whole set of these that were of equal quality and tolerances I'd be really surprised.
 
WobblyHand said:
And likely the HF dies are not even on size. Been there, done that. My HF die cut "sloppy" threads, meaning the pitch diameter was less than it should have been. The nut rattled around on the male threads.

If you need to cut threads with a die make sure you use a decent quality die made of HSS (high speed steel).

I made a differential screw M6/M5 pair for adjusting my tail stock on my mini-lathe. The diff screw made it a lot easier to dial in the tail stock position than using the original 1mm pitch thread. I single pointed the threads on the lathe. The coarse adjust is 1mm/turn, the fine adjust due to the differential screw is (1mm - 0.8mm) = 0.2mm/turn. Made the brass knob, the steel bracket and captive nut in the bracket, as well as the nut you see in the rounded slot in the cast iron base.
View attachment 465845
Click to expand...
Cool setup! Brass knob is rotated for fine adjust and the hex nut above the bracket is rotated for course adjust?

I might switch over to M6/M5. Pretty sure I have the gears to single point those threads. Guess I need to see if I can grind a tool for threading.
 
Jake M said:
You called it, on both counts. You can find dies that will cut that reasonably. It's still quite hard as bolts go, it will take some effort. In small sizes like this, it'll still be a gamble as to the quality of the result.



Are you sure it's from the same set? I haven't seen a tap or a die from Harbor Freight that I would call plausible for making threads. Cleaning existing threads? Sure. Possibly. If you're lucky enough that they felt like making them kinda sorta on size a little bit. Usually they're just off the wall in how much "tolerance" they have built in, regardless of the ability to cut. I'll defend Harbor Freight for some of their hidden gems sometimes, but their taps and dies I just make up enough excuses for. Of course, with that said, if you did find a couple (or more) functional pieces in a set that's typically pretty unacceptable. Take it where you get it and get what you can out of 'em... But if you got a whole set of these that were of equal quality and tolerances I'd be really surprised.
Click to expand...
I've been using that set occasionally for many years. Almost always using the taps. If I needed to tap many holes for a project, I'd always order additional taps as backups.....but I've generally not seen the harbor freight taps fail or cut unacceptable threads. Lately, I've switched over to thread milling...which is very satisfying. I did an 18" by 24" fixture plate for my CNC mill and had to thread about 600 M6 holes (1/2" deep), which completed in about 90 minutes if I remember right.

It's starting to look pretty obvious that the 12.9 bolts were not the best place to start my lathe threading journey :)
 
bolts are always hard to extend the thread on. They're tough, a little bit harder than usual and difficult to hold onto.
 
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