measuring thread depth

[david lockwoodParticipant @davidlockwood10028]

hello every one,I have just roughed out two lead screws for a jacobs gear hobber 3/8 BSF. I should probably have bought a die when I bought the tap but I forgot, any way I have a set of wire pins for mesuring thread depth, can any one tel me how to use them?

I have measured accross the tap with some pins and got a measurment of .4 inch, I could just cut untill I get the same measurment on the work but how much clearance I will need?

It might be worth mentioning that the tap is oversize by 8 thou accross both sides when measured directley without the pins, I was expecting it to be less then 3/8 inch because of the rounding of the thread crest but it isn't

any thoughts on any of this any one?

[david lockwoodParticipant @davidlockwood10028]

[David LittlewoodParticipant @davidlittlewood51847]

David,

Can't help with the pins, but the tap dimension is not unusual. Many thread forms have a clearance in the root of the female thread in order to avoid the risk of binding. To do this requires that the tap is slightly over nominal size. This is particularly pronounced on ISO metric threads; I don't have details of the BSF thread form to hand, but it doesn't surprise me that the tap is over nominal.

David

Edited By David Littlewood on 13/06/2013 00:13:07

[Michael GilliganParticipant @michaelgilligan61133]

Posted by david lockwood on 12/06/2013 22:47:12:

I have a set of wire pins for mesuring thread depth, can any one tel me how to use them?

.

David,

There is a pretty good description here

MichaelG.

[AndyfParticipant @andyf]

NB The Little Machine Shop calculations cover 60° threads (US and metric) whereas BSF is 55°.

Andy

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Andyf on 13/06/2013 08:02:06:

NB The Little Machine Shop calculations cover 60° threads (US and metric) whereas BSF is 55°.

Andy

.

Very true [and clearly stated in the document]

This, from NPL, gives much more information, but the Little Machine Shop notes are a valuable introduction to the method.

MichaelG.

[LambtonParticipant @lambton]

David,

According to the Zeus chart the depth of thread for 3/8" BSF is 0.0320". This is what you should have aimed for when screw cutting the thread. My late Father taught me to always finish of a screw-cut thread with a die to produce the correct thread profile and size.

The simplest way out of your situation is to buy a die as this will produce the result you require.

Eric

[Speedy Builder5Participant @speedybuilder5]

Use a thread chaser, or make one from an old 3/8 BSF Tap?

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Far easier to make the mating nut first and just make feed screw to fit .

There is no guarantee that your tap is within limits anyway by your description so making screw to match nut as actually made with tap is a better bet than trying to measure anything .

When trying screw in nut be sure to round corners of cut thread to avoid false impression of fit .

Just for interest :

Measuring smaller threads with wires is somewhat problematic anyway . It can certainly be done but a sensitive feel and a bit of practice are needed . Very rarely done nowadays since better methods of gauging screws are available in industry . In fact many CNC set ups are so accurate that you can trust the machine to get exact right size and profile every time and only sample verification testing is needed .

MikeW

[Ian S CParticipant @iansc]

The rounded tops and bottoms of Whitworth thread forms is the main reason I use UNF/UNC for most of my imperial threads, but a flat top on a BSF thread would do no harm. Ian S C

[david lockwoodParticipant @davidlockwood10028]

thank you every one for your help i will get on with the job

[Anonymous]

An old (1943) copy of Machinery's Handbook states that a BSF nut should be 0.002" oversize. The same copy gives the full diameter of a 3/8" BSF nut as 0.3770" to 0.3860", ie, 0.002" to 0.009" oversize. So it would appear that the OP's tap is just in tolerance. Presumably it will get slightly smaller with regrinds. Earlier I measured a 1/4" BSF spiral flute tap and a normal 5/16" bottom tap, both unused, and both are 0.004" oversize.

Regards,

Andrew

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Taps have specified tolerances for different purposes . These tolerances are similar to but not the same as for machined shafts and reamers .

The basic controlling dimension for a tap is the pitch diameter – other dimensions like OD just follow on .

Thus a tap could be dimensioned for precision fit , intermediate fit or loose fit on a nominal size screw .

There is more than one tolerance schedule in use . ANSI and ISO schedules are most common .

Better quality taps are properly marked with a size and tolerance eg M12 – 6H .

Bit of a problem in that some makers use their own codes and some only produce general purpose taps and omit the tolerance altogether .

Taps for making hex nuts for fixings are usually loose fit hence David's max size tap .

For precision work like feed screw nuts a close tolerance tap would be preferable .

When dimensioning screws and nuts for feed screws and other precision work it is usual to specify a close tolerance male screw and select the tolerance in the female nut to get desired running clearance .

The above is just for interest – David already has a tap and since he is doing a one off job the make to fit option is still the easiest .

MikeW

Edited By MICHAEL WILLIAMS on 14/06/2013 09:42:54

Edited By MICHAEL WILLIAMS on 14/06/2013 09:57:41

[Ian S CParticipant @iansc]

I'v got a 1/4" UNC +.012" tap, we used to use them for fitting over size exhaust studs on the Continental aero engines, next stage was to drill out and fit Heli-Coils and standard studs, about 3rd rebuild.

Ian S C

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