RequiredOutside Diameter to Cut 5/16 BSF Thread

[Danni BurnsParticipant @danniburns84841]

Hi OM

Please don’t say it was a 1″ die with a standard wrench

[old martParticipant @oldmart]

No, I think it was bigger, but there was a 1/2W one of that size in the box we have at the museum, I went for a longer handled diestock for that thread.

APT sell er 11, 16 and 22 threading inserts singly as well as small quantities of some of the carbide inserts selected for hobbyists, I recommend them highly.

[Howard LewisParticipant @howardlewis46836]

For cutting threads with Taps or Dies, the holders are a body which slides on an arbor with a Morse taper which matches the Tailstock.

In this way, the newly cut thread has only to drag the holder along the arbor, rather than the Tailstock along the bed. (A fine thread is likely to strip when subjected to the forces needed to drag the Tailstock along the lathe bed)

Also, if tapping a blind hole or cutting a thread upto a shoulder, a mandrel handle allows chuck rotation to be stopped as soon as any resistance is felt, preventing broken taps, or stripped threads.

With regard to setting split dies; wherever possible, I cheat by adjusting the Dies so that a commercial bolt gives a nice gentle feel (Unscientific!).

The other advantage of holding a tap in an ER collet, or a drill chuck, (Sliding on the arbor) is that often the tap slips before it breaks.

If the holders are a slightly loose fit on the arbor, the clearance allows the Tap or Die to self align on the workpiece, minimising the risk of a drunken thread.

Howard

[Howard LewisParticipant @howardlewis46836]

FWIW, To calculate by how much the stock material should be undersize compared to the nominal, to prevent crest / root interference..

Whitworth form threads are rounded at the crest by 0.04 of the pitch.

So for a 18 tpi 5/16 BSF, which has a pitch of 0.0556″, the truncation is 0.002224″

Metric threads are truncated by  0.108 of the pitch.

Howard

 

[Clive Brown 1Participant @clivebrown1]

Not really, the nominal diameter of a 5/16″ whitworth form thread is exactly 5/16″. The 0.04p refers to the truncation of the fundamental, (55deg), triangle which is the basis of the thread form.

Any reduction of the nominal diameter is a “metal off” tolerance which is a separate consideration.

[old foolParticipant @old-fool]

Hello all,

All read with interest. For some reason I have a 3BA die with 1.25″ dia o.d. so yes they can come with any size. Like much of my tools and kit no idea where it came from. Or why. I also have a set of WW. dies (1/8 to 1/4) with 2 piece dies. i.e. 2 totally separate halves. Getting them set up can be a path to madness. Sometimes quicker to just buy a new one!

 

Bob

[Danni BurnsParticipant @danniburns84841]

Thank you, OM

and Howard (welcome back) and Clive.

So for 1/2″ BSW external thread, is it 1/2″ or 0.004″ under?

 

 

[Howard LewisParticipant @howardlewis46836]

Hi Danni,

Using the 0.04P quoted in Tubal Cain’s Model Engineer’s Handbook, 1/2 BSW is 12 tpi , so the pitch is 0.0833″, meaning that the truncation should be 0.0033″

So turn to 0.006″ undersize (0.496″)

Howard

[Clive Brown 1]

On Clive Brown 1 Said:

Not really, the nominal diameter of a 5/16″ whitworth form thread is exactly 5/16″. The 0.04p refers to the truncation of the fundamental, (55deg), triangle which is the basis of the thread form.

Any reduction of the nominal diameter is a “metal off” tolerance which is a separate consideration.

Just to endorse Clive’s post … It is worth looking very carefully at this diagram and its accompanying text:

https://en.wikipedia.org/wiki/British_Standard_Whitworth#/media/File:Whitworth_ Thread.svg

https://en.wikipedia.org/wiki/British_Standard_Whitworth#List_of_thread_sizes

MichaelG.

[old martParticipant @oldmart]

The best way to get the finished thread the best size is to fit it to the mating part. Not always possible, of course. Commercial fasteners usually measure under the nominal sizes, but aircraft ones are always very close to size.

When singlepointing a large thread, it is pretty easy to remove it and if necessary fit it back up and get the threads aligned within 0.0005″ for further passes. I have also removed a screw on chuck along with the work and checked the fit without any problems.

[Charles LamontParticipant @charleslamont71117]

It is perfectly simple. A fully formed thread is the nominal diameter. No reduction. Period.

The crest radius on a single point screwcut Whitworth form thread can be produced by a full-form carbide tip, by running a die down, by filing, or, by finishing with a chaser (which is how I usually do it).

Alternatively, you can dispense with the radius altogether, in which case you can reduce the diameter as Howard says. Such a thread does not have the full form, but practically it matters little. I would truncate the crest radius (and use a sharp pointed V tool) if I wanted to be absolutely sure of flank only contact, for example if making a micrometer thread.

The question of rounding the crests or reducing the diameter does not arise with metric threads because the full thread form has flat crests. In practice, commercial rolled metric threads usually do have a crest radius formed within the nominal form, and metric taps may have a crest radius that extends beyond the nominal size, so as to form a root radius in the tapped hole.

 

[Charles Lamont]

On Charles Lamont Said:

It is perfectly simple. A fully formed thread is the nominal diameter. No reduction. Period.

At least until you consider tolerances. For the looser tolerances the upper limit will be slightly below nominal, when I looked into this I concluded that 0,05mm under nominal will be within spec for pretty much any size and class I would encounter.

[duncan webster 1Participant @duncanwebster1]

You can cut whit form threads with flat root, GeorgecThomas gives required tool tip dimensions, although I’ve never grasped how to achieve it without a shadowgraph or similar. Crude people like me do the crest radius with a file.

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