Unless the shoulder is not much more than the 1/4 thread then using a 55deg DCMT insert will mean it hits the shoulder. A 55 deg threading insert will get quite a lot closer. But you could probably get closer still with an HSS one if the “point” were kept to the edge of the tool as much as possible.

Would have thought in his tool post, may need packing below and even above if screws are not long enough, that’s how I do it with 3mm and 4mm.

Some people make a dedicated holder that then goes into the toolpost.

Are you sure it is a 55° (whitworth) thread, most threads are 60° (Metric, UNC, etc).

Make a jig to hold the tool at the required 55° or 60°. That makes (re)grinding an easy job and you don’t grind away a lot of the tool so it can last a hobby life time.

For special thread forms, i put a square bar in the tool post and drill a 4 mm hole in it (drill in the chuck). That way, the hole is on center and aligned. Then I glue (soft solder) a piece of round 4 mm HSS stock in that hole. Finally I grind the required cutting edges.

If you put a grinding disk in the chuck, you can set the angle using the tool post. Shim the tip of the cutter a bit for a little clearance.

On 14 April 2024 at 10:02 Peter Simpson 3 Said:

It is definitely 36TPI the OD is 0.248″ I assume it is imperial 55′ as the part was made by BSA in the late 1940’s  Tracy tools do W/form taps and dies in 1/4″ 36TPI but do not show LH versions.

BSA guns or BSA motorcycles? If the latter, they used a mix of BSW, BSF and CEI, ie Cycle Engineers Institute thread, which is 60 degrees. But usually only comes in 20 or 26 TPI. But thread standards are only for fasteners. Special parts etc used all kinds of oddball threads according to the draughtsman’s whim. In practice, commercially made threads generally have enough clearance that 60 and 55 degree parts will screw together, so it is not critical.

You might try using the more usual 1/4″ (6mm) HSS blanks to make your screwcutting tool. It will allow more stick-out without flexing than 4mm. You just grind the V point over to one side so you can get up close to a shoulder. Also, a hand crank for the lathe spindle makes screwcutting up to a shoulder a doddle.

One thing to note about inserts, should you in future go that route, is that standard inserts are for RH threads only. LH thread requires the flank clearance on the insert to be on the other side to clear the LH helix.

Just to support JA’s microscope comment:

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… and the reference gauges that were supplied to the major British manufacturers were produced by the Whitworth company.

MichaelG.

My latest Tracy Tools catalogue is dated 2020, and on page 3 they offer 1/4 x 36 in BSCY, UNS and Whitform (Specials) but there is not the usual mention of LH carrying a 200% premium. It is there on page 4.

Have you tired contacting them, to see of they can provide a Die that you want?

If your chucl is screw on without safety dogs, beware that it doesn’t try ro unscew as you use the Die.

Alternatively, you will have to start the die and then continue rotating the Die by hand with the chuck staionary.

If you are screw cutting, You won’t have that problem, but the tumbler reverse will be moving the Saddle away from the chuck, and, the shoulder on the job.

If not, then you are going to be looking for the changewheels for 36 tpi, and making up a tool, either to hold an inseret, or grinding one.

Howard