Single Depth of Thread for 26 TPI

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Thread man

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Maybe the easiest way to find the thread depth on any Whitworth profile thread (55º) is simply to multiply the pitch (in decimals) by 0.640327 (0.64 should be accurate enough for most ☺) The 0.64 applies to threads that have the correct radii.

The correct radius is 0.137329 times pitch. Personally I'd use 0.14.

The thread depth for H is 0.96 times pitch.
 

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Thread man

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Probably most Americans don't think about it but the most commonly used thread profile (in the world) for pipe threads is the 55º Whitworth.

BSPP is now "G" and BSPT can be R, Rc or Rp depending on whether it's external or internal. The "new" designations are ISO.
 
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Thread man

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I'm wondering if anyone wants similar information re 60º threads? Metric and ISO Inch (UNC etc.) have identical profiles except for a very small difference in the radii.

While I'm at it ☺ US pipe threads (NPT & NPS)) only have 5 different pitches. Whitworth profile pipe threads (taper and straight) only have 4 different pitches. Typisk (?) but the only pitch the two types have in common is 14 TPI.
 
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A small side remark that is probably obvious to many: when cutting a thread with a pitch that can divide the pitch of the lead-screw in a whole number you can disengage and engage the halfnut at will and still stay in pitch. For example with a 4 mm pitch leadscrew you can do this with treads of the pitches 4 mm, 2 mm, 1 mm, 0,8 mm and 0,4 mm. The benefit is that you can use the stops to dis-engage and thus cut at a much higher cutting speed resulting in a nicer finish.
 

Ken I

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A small side remark that is probably obvious to many: when cutting a thread with a pitch that can divide the pitch of the lead-screw in a whole number you can disengage and engage the halfnut at will and still stay in pitch. For example with a 4 mm pitch leadscrew you can do this with treads of the pitches 4 mm, 2 mm, 1 mm, 0,8 mm and 0,4 mm. The benefit is that you can use the stops to dis-engage and thus cut at a much higher cutting speed resulting in a nicer finish.
I gave up on using that idea or the indicator dial years ago simply because I've had the half nuts "ride" improperly closed and therefore out of pitch - it only has to happen once and your job is destroyed. I have had it happen on a number of different lathes including a wonderful Graziano but much more prevalent on hobbyist machinery.

My method is slightly more time consuming but is guaranteed.

I screw in reverse with the tool upside-down - that way I can cut at whatever speed I like without any danger of crashing into the bottom of a blind hole or a shoulder. No accuracy is required for where you stop. You never disengage the half-nuts until the job is complete.

This is a particularly useful method for screwcutting blind holes.

I then retract the tool and go forward slowly - turning the last bit to the shoulder or bore bottom (undercut) by hand - hand turn the chuck backwards to remove lash - set tool to next depth cut - (check rotation is set correctly) - hit start and go like the clappers away from the work.

Almost foolproof - unless you hit start with the incorrect rotation - which as you can imagine, I have actually done.

Regards, Ken
 
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Hello Ken, I hear and feel the crunch! With my Schaublin 102 VM off about 70 years old when I set the endstop on the halfnut relief the carriage will come to a standstill after 2,5 mm. So with a blind hole I turn a relief groove at the inner shoulder of 2 mm wide and then very carefully set the stop for the tool also 2 mm before the start of the groove. After simulating outside the bore I pray and engage. So far all went well. But maybe I should switch to your method before the hideous crunch gets me!
 
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