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

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

Andy

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

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

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