Cutting metric threads with an inch leadscrew

**mklotz** · Sep 5, 2015, 04:55 PM

When single-pointing metric threads on a lathe with an Inferial leadscrew the inherited wisdom is to never disengage the half-nuts. However, this makes threading to a shoulder or a runout groove difficult at best. Thankfully, there is a technique that allows one to disengage the half nuts without losing registration on the thread. It involves the use of the threading dial but not in conventional manner.

1. After setup (including checking the pitch) start the first pass by engaging the half-nuts on a selected number on the threading dial. (Here we'll assume you engage on '1'.)

2. When you reach the runout groove/shoulder disengage the half-nuts and turn off the lathe motor. Back the tool out but do not move the carriage. You will note that the thread dial no longer points to '1' because the motor inertia continued to turn the lead screw after you turned the motor off.

3. Start the lathe motor in the reverse direction. The '1' on the thread dial will begin to move back towards the index mark. When it reaches the index reengage the half-nuts. You're now back in sync with your thread.

4. Let the leadscrew drive the carriage to the right until the tool clears the workpiece. Stop the lathe. Put on the next cut increment and start the lathe in the forward direction to make the next cut on the thread.

**Paul Alciatore** · Sep 9, 2015, 06:33 AM

Marv, I think you should mention that it is important to turn the motor off quickly because if it is on long enough for the threading dial to go past the selected index point (1 in your example) then you will not be synchronized if you re-engage at the first time it comes around while running in reverse. You have to back up the same amount that it went forward while disengaged.

**mklotz** · Sep 9, 2015, 07:47 AM

Originally Posted by Paul Alciatore

Marv, I think you should mention that it is important to turn the motor off quickly because if it is on long enough for the threading dial to go past the selected index point (1 in your example) then you will not be synchronized if you re-engage at the first time it comes around while running in reverse. You have to back up the same amount that it went forward while disengaged.

You're absolutely correct, Paul (don't I know you from the HSM forum?) I do it quickly without thinking so I neglected to mention it but threading novices should be reminded. Good catch.

**Paul Alciatore** · Sep 10, 2015, 10:13 PM

Yep Marv, same Paul Alciatore as over there. Not many around with that name. Somehow I wound up on the e-mail list from here so I decided to join. Looks like you are quite active here.

Originally Posted by mklotz

You're absolutely correct, Paul (don't I know you from the HSM forum?) I do it quickly without thinking so I neglected to mention it but threading novices should be reminded. Good catch.

**mklotz** · Sep 11, 2015, 11:04 AM

Originally Posted by Paul Alciatore

Not many around with that name.

Yeah, I know all about distinctive and unusual names, believe me. Grade school was hell. :-)

**kbalch** · Sep 11, 2015, 11:22 AM

Originally Posted by mklotz

Yeah, I know all about distinctive and unusual names, believe me. Grade school was hell. :-)

I wouldn't like to redo grade school with my last name, either…

Ken

**mcostello** · Sep 11, 2015, 12:14 PM

Last name Costello, and the kids back then knew what that meant.

**kbalch** · Sep 11, 2015, 01:08 PM

I watched every Saturday morning! Can I get an autograph?

Ken

**mklotz** · Sep 11, 2015, 02:07 PM

Who's on first? I still wonder how long it took Lou to memorize that entire routine.

**kbalch** · Sep 11, 2015, 03:09 PM

A classic! Somewhere along the way, I remember seeing a version of this routine filmed during a radio performance when they were both much younger.

It was great to see this again - thanks for that!

Ken