Hand-tapping

[Philip Davies]

Surely you wouldn’t want this done to your taps?
The fact is that I’m unable to exert enough downward pressure to start the thread in these 10mm holes for 1/2” BSW (& the hole, says the tables, is supposed to be 10.5mm) Then I used a taper reamer, no.7, which might suit, though haven’t looked it up.
Still only a half-turn can be done, before moving the clamp, and I cam check each time that it’s square to the material. After 4 goes, the thread has started and little more downward force is needed. Otherwise, blood pressure shoots up!
These are first steps in metal machining, and very totters steps they are, indeed, beginning with T-nuts.

What you see here is a rehearsal in aluminium, and of course the cutter slipped down. Tapping aluminium alloy was hard enough.
You may see that I have got a second tap in the holder. That is because the starting tap has chips in it.
Do you think that this is salvageable?

[BuffaloJohn]

I have found charts that say 10.5mm holes for tapping 1/2"-12 BSW, but I don't think that is correct. The minor diameter is barely 10mm for the minor diameter minimum (minor diameter maximum is 10.57mm) and the pitch diameter is 11.35mm. Even 10.5mm seems way too small. Even tapping a 10.5mm hole would imply potentially 100% thread engagement. That is too high for steel. Normally for steel 50% is workable (75% max for aluminum).

I found another chart that says 65% thread engagement is a 10.9mm hole. https://journeymans-workshop.uk/whitworth.php

Measure the OD of the tap at starting thread and then at full thread and compare it to the minor diameter. That whould help pointing you in the right direction. Remember, the Tap shouldn't be milling the hole way before the first thread.

Interestingly, I found another chart with various tap sizes and it lists both imperial and metric drill sizes and if you check the conversion, for all but 1/2", the drill converts correctly, but for 1/2" is says 0.4465" and 10.5mm but

0.4465"= 11.34mm (which is the pitch diameter)
10.5mm= 0.4134"

So, I would go with the 10.9mm (and maybe a little more)...

[Philip Davies]

Replacement taps being posted tomorrow!

[mklotz]

A formula is always more compact and easier to remember than a chart. This from one of my shop papers, hope it helps...

Tap drill calculation...

TD = tap drill diameter
MD = major diameter of thread
DOT = depth of thread expressed as percentage
TPI = thread pitch expressed as threads-per-inch

TD = MD - DOT / (75*TPI)

Example:

MD =0.5
DOT = 65 %
TPI = 12

TD = 0.5 - 65 /(75* 12)
TD = 0.428 (closest drill = 27/64 = 10.87 mm)

The more simplistic formula...

TD = MD - 1/TPI

makes the assumption that:

DOT / 75= 1

or:

DOT = 75 %

which is actually more than needed in many situations.

Suggested DOTs:

MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60

ISTR that the Whitworth is a 55º thread. My formulae are for a 60º thread but, the difference doesn't seem to matter much. The formula result agrees well with BJ's information.

[BuffaloJohn]

Marv has the calculations!

[Philip Davies]

Thank you for this information, which surely others will find helpful. Turning to pg.1273 of my copy of “Machinery’s Handbook”, I see that DOT is given in 000s of an inch, (0.05335 in the case of 1/2” ,tap drill dia is given as 13/32” - seems a bit tight) The pitch is 0.08333. I take it that, as a percentage, DOT is depth of thread divided by pitch X 100 ? Pardon me, but this isn’t something I have come across before.
From your information, I see that while Whitworth might be suitable for mild steel and even cast iron, it would be less suitable in stainless steel. Is that so?

[mklotz]

Thank you for this information, which surely others will find helpful. Turning to pg.1273 of my copy of “Machinery’s Handbook”, I see that DOT is given in 000s of an inch, (0.05335 in the case of 1/2” ,tap drill dia is given as 13/32” - seems a bit tight) The pitch is 0.08333. I take it that, as a percentage, DOT is depth of thread divided by pitch X 100 ? Pardon me, but this isn’t something I have come across before.
From your information, I see that while Whitworth might be suitable for mild steel and even cast iron, it would be less suitable in stainless steel. Is that so?

13/32 is 1/64 less than the 27/64 I calculated.

Note that thread pitch is to be expressed in units of threads-per-inch. Your "0.08333" is 1/12 and has the units of inches-per-thread. It's the 12 threads-per-inch that you plug into the formula given.

DOT in the formula is not a measurement of a particular thread but rather how far you want the thread of the bolt to penetrate into the threads of the female receiver. You pick a value based on the material to be tapped; it's seldom specified more closely than ±5 %.

A thread's suitability is based more on the pitch and the material to be threaded. Coarse threads for soft materials, e.g. aluminum; finer threads for stronger. Thread form, metric, inferial, antique is less of an issue relative to strength.

[BuffaloJohn]

From your information, I see that while Whitworth might be suitable for mild steel and even cast iron, it would be less suitable in stainless steel. Is that so?

Whitworth is a 55 degree thread angle while Metric and Imperial (SAE) are 60 degree angles. 55 is close enough to 60 and both are triangular threads, so I don't see that it would be unsuitable for stainless steel. Just remember though, while mild steel is 60-65 % thread depth, the recomendation for stainless steel is only 50% thread depth. Also - stainless can be hard on High Speed Steel taps, might need a harder tap and that might be the limiting factor...

[neilbourjaily]

Marv's reply is exactly what I got taught a week ago during the 1 week basic machining course offered in the Summer program at Trinidad (Colo.) State Junior College Gunsmithing School. I assure you, that one week course cannot be compared in multiple multiples to the many YouTube machining videos. There's nothing like a week's worth of focused hours of instruction and doing. I learned to do things Logan's way.

[mklotz]

An aside here...

Note that the one simple, easily memorized formula...

TD = MD - DOT / (75*TPI)

replaces lengthy tables, and, in addition accommodates weird proprietary threads and any other 60º thread form. With a minor change (instead of dividing by TPI, multiply by the pitch as more logically specified in metric threads) it accommodates metric threads. And, of course, it calculates the size hole to bore before single pointing internal threads on the lathe.

All that's required is a user unafraid of learning a bit of math and a handy calculator.

There are many instances in the shop where a little math and a calculator can replace a whole set of tables. I firmly believe that the bulk of Machinery's Handbook could be reduced by 20%. Acres of home shop wall space could be freed by replacing tables with a single sheet of formulae.