Cutting Square Threads (Fitting a Barrel to a Rolling Block)

[Vernonia MW]

A while back, a customer brought in an old, roached out Rolling Block action and barrel. He wanted a new barrel fit to the old action. This immediately presented a problem, as the tenon threads were of the square variety. I had never cut square threads, but I had always been told that they were a challenge.

The threads measured 1.055" x 12 TPI. I reached out to my tool rep @huntpotter, and asked if there were any inserts available that would cut square threads. He was unable to track down any off the shelf options. I hate grinding HSS with a passion, so I started to dig a little deeper. For 12 TPI square threads, the tool should be .0417" wide (1 / 12 TPI / 2 = thread width/depth). I managed to find this grooving tool on Ebay:

https://www.ebay.com/itm/226348672143
KTKFR1212JX-12 Tool Body
TKF12R100-S 1mm Wide Inserts

The 1mm (.0397") wide inserts were a close match to the .0417" that I needed. The next problem was that the tool needed to be angled to match the lead angle of the threads. To find this angle, I used this online calculator:

Lead Angle Calculator | Evolvent Design

The thread lead angle calculator calculates the lead angle for a screw or bolt based on either the threads per inch, thread pitch, or lead. While English screws typically use the number of threads per inch, metric screws use pitch, which is the distance between two threads. Ultimately these are...

evolventdesign.com

For 1.055" x 12 TPI threads, the lead angle should be 1.44°. I set up the tool body in the Bridgeport, using 1° and .5° angle blocks stacked under the toolmakers vise. I then machined the top and the bottom surfaces of the tool body, such that the cross section was now a slight parallelogram. When loaded in the lathe tool holder, the tool would now be angled as required to cut the threads.

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[Vernonia MW]

I got the barrel blank loaded up into the old Leblond engine lathe. I forgot how much I enjoy running this machine. The Prototrak is certainly efficient, but the Leblond is a rigid torque monster.

I cut the thread major diameter a bit under 1.055" to try and match the old rotted out barrel. I also added a thread relief to make threading a bit easier. The modified threading tool worked beautifully. I was taking .005" per pass at 80RPM. I could probably have gone faster, but the chips were looking great, so I left good enough alone. I had the compound set to 0° so that I could offset the tool by .002" to make up for the narrower width. The receiver threaded on smoothly after getting to my final thread depth.

The extract cuts and recess for the breech will be added by the customer. Hopefully I get to see it when it is all done. It should be a fun rifle.

 

[huntpotter]

Very nice work.

 

[old greg]

nice work! Thats fun to cut a weird thread like that and i appreciate getting to see your approach.

It also makes me feel lazy with a wire edm at work when i cut form tools into broke endmills or whatever haha.

 

[red442joe]

Thank you for sharing!

Joe

 

[dangerranger60]

Congrats on your first square thread! DR

 

[Steel Head]

Love the look of the older square threads.