Square Thread for Minnie

[Brian AbbottParticipant @brianabbott67793]

Hello again, after more advice.

This will defiantly be a run before you can walk type of question, but I would like to try and cut a square thread on the Minnie break screw.

Is there a formula or something to work out what the depth & pitch should be?.

Thanks for any advice.

Brian

[Brian AbbottParticipant @brianabbott67793]

[Brian HParticipant @brianh50089]

The only one I've ever cut was for a Clayton Waggon and that was 1/4" 2 start! At the same time as cutting the steering column I also cut a silver steel tap so it didn't really need to be a precise size, as long as it fitted (which of course it did).

Brian

Edited By Brian Hutchings on 09/02/2017 14:15:13

The cutter was 1/16" wide and plunged in 1/16" but I don't think it needs to be that deep.

Edited By Brian Hutchings on 09/02/2017 14:18:49

[HOWARDTParticipant @howardt]

3/16 BSW is 24tpi.

According to my old Newnes Engineers Manual it gives the smallest standard Acme pitch as 16tpi. But it also gives these calculations.

width of point of tool = (0.3707 / threads per in) – 0.0052

depth of thread = 1/2pitch + 0.010

flat at top of thread = pitch x 0.3707

flat at root of thread = pitch x 0.3707 – 0.0052

So from those calculations you could make a couple of tools to cut your 24tpi.

[JasonBModerator @jasonb]

Square (not ACME) thread is easy to work out

Assume 24tpi as per the Whit option

width of cutter = 1 / (2xtpi) so 1 / (2×24) = 0.021"

Use this width of cutter to cut the male thread and then stone a thou or two off the side and use the thinner cutter to make your tap from Silver steel. By slightly stoning the tool you get tap teeth that will leave a slight clearance for the male thread.

Depth of cut is the same as the width so 0.021".

When I did my minnie many years ago I did not havce a screw cutting lathe so use 3/16" whit.

One of the reasons I bought my Warco 280 was to be able to screwcut teh 16tpi square thread on teh 2" Fowler and that was the first thread I had ever screwcut so in at the deep end. From that I would say make your tap very long so you can have a very gradual taper or make two shorter sequential ones. My tap in the foreground was really too short.

 

 

 

Edited By JasonB on 09/02/2017 14:48:43

[Anonymous]

Errr, both the title and first post state a square thread!

For a full depth square thread the width of the thread, the width of the gap, and the depth of thread are all the same and equal to half the pitch. So if we assume the thead to be cut is 24tpi then the pitch is ~0.04167", and half that is 0.0208".

A problem with square threads, and one reason why Acme threads became popular, is that the can interfere on the root, crest and flank independently.

When I made my square thread brake shafts I hand ground and honed a HSS toolbit to the correct width, using a micrometer. I then cut taps in silver steel. Square, and Acme taps, usually come in a series with increasing OD, with the last tap cutting full depth. I cut the taps with the full OD and the correct depth of thread. My thread OD was ½" so I made a set of three:

When I cut the brake shaft I made the OD a few thou undersize, and cut slightly deeper than theoretically correct. That should sort out the crest and root interference problems. Once I'd cut the square thread in the nut:

I gradually reduced the width of the thread on the brake shaft until the nut fitted smoothly and without shake:

Andrew

Edit: Bother, too darn slow as usual, I'll just crawl back into the cave and get on with some work

Edited By Andrew Johnston on 09/02/2017 14:53:54

[Brian AbbottParticipant @brianabbott67793]

Thanks Guys,

I never ceases to amaze me the amount of knowledge on this site and the willingness to help.

Thanks again

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