Picking up an existing screw thread for repairs

[Andrew Moyes 1Participant @andrewmoyes1]

Joe Pieczynski recently posted this video **LINK** which shows how to pick up an internal thread. With the half nuts engaged, he inserts an internal threading tool tip part way into the thread, slides the tool back and forth between the flanks and finds the mid point using a dial indicator against the back end of the tool. He then locks the tool in its specially made holder.

I liked the basic idea and it got me wondering if it could be done quickly, without the need for a special holder, using a DRO. Indeed, it can. What is more, using a DRO enables it to work equally well with external or internal threading tools of any form. Here’s how…

1. Set the compound slide parallel to the lathe bed. Set the gear train to cut the appropriate thread.

2. Retract the tool to clear the thread. Engage the half nuts, observing the correct thread dial indicator position as per normal practice. Run the lathe under power and stop the motor, with the half nuts still engaged, part way along the thread. At this point I give the chuck an extra turn by hand to ensure that all the backlash in the gears and half nuts is taken up in the correct direction.

3. From this point on, do not turn the chuck. Set the DRO X-axis to zero. This memorises the carriage position relative to the leadscrew. The half nuts can now be disengaged.

4. Move the tool slightly to the left using the carriage handwheel and enter the tool tip part way into the thread. Bump the tool right and left against the flanks of the thread using the carriage handwheel, leaving sufficient sideways movement for the following step.

5. Advance the compound slide (turning the handle clockwise) while moving the carriage back and forth until the DRO X-axis reads equal amounts either side of zero. In other words the + and – numbers are equal. I work to the nearest thou and ignore tenths of a thou. The tool is now centred. Lock the compound slide in that position.

6. Advance the tool fully into the thread and set the Y-axis to zero.

You can now proceed to recut the thread by dropping the half nuts onto the leadscrew in any position permitted by the thread dial indicator.

The method also works with metric threads on an imperial lathe (and vice versa) BUT in this case the carriage must be returned to X=0.0000” to engage the half nuts and they must remain closed until screwcutting is finished, again as per normal practice.

[Andrew Moyes 1Participant @andrewmoyes1]

[Dave SParticipant @daves59043]

No need to use a DRO on external threads.

Engage half nuts as though cutting whatever thread it was.

advance the correctly ground cutter using the cross slide, and line it up with the topslide as it engages the thread.

I've done this many times to pickup an existing thread.

Even works with metric on an imperial lathe.

Cant say I've needed to do it on an internal thread – where the visibility of the tool is some what less.

Dave

[DC31kParticipant @dc31k]

Posted by Dave S on 17/05/2023 12:35:39:

Can't say I've needed to do it on an internal thread – where the visibility of the tool is some what less.

I agree on the DRO being over the top.

In some respects an internal thread is easier to pick up than an external one because the shank of an internal thread cutting bar is parallel to the axis of the thread. You just loosen the bar in its holder and slide it fore and aft, gently manipulating the cross slide outwards until the tool tip centres itself in the thread groove. This allows those who thread with an angled compound to continue to sleep at night.

Iscar make an external threading tool that presents the shank parallel to the thread axis, which allows the same method to be used for external threads. They call it type G, for gang tooled lathes. It is also available from overseas, with a suffix B.

https://www.iscar.com/eCatalog/Item.aspx?cat=3800002&fnum=357&mapp=TH&GFSTYP=M&srch=1

https://www.aliexpress.com/item/32924338528.html

[HOWARDTParticipant @howardt]

Advancing the compound also allows you to cut multi start threads so no magic really.

[John ReeseParticipant @johnreese12848]

To pick up fine threads, it helps to have good lighting and a magnifier.

I normally keep my to slide set at 29.5*. It is no problem to get the tool aligned with the thread using the cross slide and to slide. Just remember the half nuts need to be engaged, and the carriage preloaded opposite the direction of travel. That takes care of the backlash in the lead screw.

[egaParticipant @ega]

Posted by John Reese on 21/05/2023 00:51:21:

To pick up fine threads, it helps to have good lighting and a magnifier.

…

And to place a sheet of white material under the work.

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