15MM Tap......Drill Size

[LKeithR]

yup, one of its many redeeming features

There, fixed it for ya. The only problem I have with metric here in Canada is that we didn't go all the way in the 70s. In
order to "fit in" with the States we only adopted parts and now we're stuck with this bastardized mess we have today
that mixes metric and imperial. The only light on the horizon is that my grandchildren are being taught only metric so in
another generation or so we'll finally be able to clean up this mess...

...wehich lead to my writing a computer program in Fortran (THAT long ago)...

I took computer programming in tech school in 1966 and 1967. I used to love programming in Fortran. Haven't had anything
to do with it in many years so I don't know how it has stood the test of time but back in those days it seemed so logical and
simple to use. We ran it on an IBM 1620 and the data and program could only be input with punch cards. I'll always
remember that I had far more problems getting the program onto the cards than I actually did in writing it...

[Yondering]

Why is that? Does converting my final fractions into decimals provide an incorrect answer? Or do you disapprove of my indulging
in some fractional masturbation?

I'm curious too. Paul, are fractions just a problem for you, or do you have an example where that method doesn't work? All the combinations I tried match the tap/drill chart pretty close.

NONE of my machines have fractional dials on them.
I for one are real glad we left all this behind when we adopted metric.

What does a dial on your machine have to do with a tap drill size? None of my machines are fractional either, and they aren't metric.

[danlb]

NONE of my machines have fractional dials on them.
I for one are real glad we left all this behind when we adopted metric.

As I read this I realized that the advantage is not that it's metric. The advantage is that it's DECIMAL. That's the same as all of my imperial machines. My DRO does not measure the X axis in feet and inches and fractions. It measures it in inches and tenths and hundredths and thousandths and ten-thousandths of an inch.

[MattiJ]

Have to agree that Drill size tables look handy if you have to work with three different absurd sizing schemes: fractional inches, gauge numbered drills and letter sizes.
At least bolt threads are what the size implies, unlike pipe threads..

[John Stevenson]

NONE of my machines have fractional dials on them.
I for one are real glad we left all this behind when we adopted metric.

[RichR]

That's not a good way to do it.

Sure.
1/2-13 becomes 1/2 - 1/13 which equals 13/26 - 2/26 which gives you 11/26.
1/4-28 becomes 1/4 - 1/28 which equals 7/28 - 1/28 which gives you 6/28 and reduces to 3/14.
Happy?

Why is that? Does converting my final fractions into decimals provide an incorrect answer? Or do you disapprove of my indulging
in some fractional masturbation?

[John Stevenson]

What sort of standard is that?

?? Simples Metric Fine.

[Lew Hartswick]

:-) Paul , What is this = thing ? Just ;
diameter
Enter
TPI
(the one over x key)
- ( minus key)
the answer is sitting there. :-)
..lew... Long live RPN

[Jaakko Fagerlund]

No, we dont have twist drills in 0,01mm increments, would make no sense since hole tolerance is bigger than that with all the ordinary drills.

Drills and drill sets with 0,1mm increments are common and available from any tooling supplier.
Tapping size drills you can find even in most of the diy-hardware stores.
The sizes you really need are 3.3mm and 4.2mm. In other sizes the nearest (over)size to 0,5mm is close enough for most of the uses. Getting a 0,3mm oversize hole for M8 bolt or bigger is usually acceptable but 3,0 or 3,5mm are poor choices for M4

The usual (useful) drill index has drills from 1 to 10 mm in 0.5 mm steps, including 3.3, 4.2 and 6.8 mm drills for M4, M5 and M8 threads.

The usual size increments are 0.1 mm for drills between 1 to 10 mm, then 0.5 mm up to 25 and then 1 mm increments. Of course you can order whatever you want/need from a tooling supplier, but those are the "normal" increments. For drills below 1 mm the usual increment is 0.05 or 0.02 mm, but that gets to the very special category and fast.

[MattiJ]

BTW, the calculation for a metric thread will yield a mm size that usually has at least one decimal place and often has two or even an infinite number of places. I recently used a 5mm tap. The standard 5mm thread has a 0.8 pitch so the tap drill is 5 - 0.8 = 4.2mm. I don't know about you, but my set of metric drills came in 0.5mm increments so I either have to settle for a 4mm drill, which is the closest in the set, or convert to an English size drill and break out that number-letter-fraction index of English sized drills. That was exactly what I did. A 4.2mm drill = 0.1654". The closest number drill is a #19 at 0.166" and that was what I used. More math and I had to go to the English sized drills to get a close fit. BTW, I purchased those taps in the local ACE hardware and the packages were marked with that same, exact number sized drill. Very few local hardware stores here in the US will stock even whole mm sized metric drills, much less decimal sized ones. I have to wonder if the local stores in England, France, Germany, or any other European countries stock metric drills by 0.01mm size increments.

No, we dont have twist drills in 0,01mm increments, would make no sense since hole tolerance is bigger than that with all the ordinary drills.

Drills and drill sets with 0,1mm increments are common and available from any tooling supplier.
Tapping size drills you can find even in most of the diy-hardware stores.
The sizes you really need are 3.3mm and 4.2mm. In other sizes the nearest (over)size to 0,5mm is close enough for most of the uses. Getting a 0,3mm oversize hole for M8 bolt or bigger is usually acceptable but 3,0 or 3,5mm are poor choices for M4

[Paul Alciatore]

In my old age wisdom, I have found that there are two inexpensive things that I like to keep a number of scattered around the house and shop, in places where I am likely to need them. Calculators and reading glasses. I have at least 10 pairs of glasses and at least 6 calculators, not including the ones in my smart phone, scattered around the house. I did not pay more than $5 or $6 for any of them. The calculators are mostly the scientific models, but I do have a real basic, four banger in a location where I frequently solve problems in a puzzle magazine that I subscribe to. I like it's ability to do successive additions, subtractions, multiplications, and divisions by just pressing the equal key over and over. This is helpful when going through a string of multiples or powers. I have at least two of these calculators in the shop as I want one at each bench. As I complete the shop I will probably add more of them for additional positions. If I am working, eating, or just relaxing there is usually a pair of reading glasses and a calculator within arm's reach.

My point was that the original easy-to-do-in-your-head example given, which involved only 16ths of an inch, was not representative of most of the imperial threads that would be encountered, which are more difficult to figure out than the metric ones, because you need to convert TPI to pitch. A calculator would be handy, as you point out.

Or just use a chart......

[Paul Alciatore]

That's not a good way to do it.

Sure.
1/2-13 becomes 1/2 - 1/13 which equals 13/26 - 2/26 which gives you 11/26.
1/4-28 becomes 1/4 - 1/28 which equals 7/28 - 1/28 which gives you 6/28 and reduces to 3/14.
Happy?

[Paul Alciatore]

Using my sequence of keystrokes on a four banger:

1
/
13
=
-
.5
=

I get -0.423"

To convert to the closest fractional size I simply multiply by 64

X
64
=
-27.076

Which, ignoring the minus sign, means the nearest fractional drill is 27/64"

AND

1
/
28
=
-
.25
=

And I get -0.214"

Again ignoring the minus sign, this is in the range of number size drills so I must look it up in a table or just open my drill index and find the nearest size. Since I have to open the index anyway to drill the hole, that is the easiest way. The nearest size is a #3 which is 0.213". It doesn't get any easier than that. And it will work for any 60 degree Vee thread, standard or as non-standard as you can get.

BTW, the calculation for a metric thread will yield a mm size that usually has at least one decimal place and often has two or even an infinite number of places. I recently used a 5mm tap. The standard 5mm thread has a 0.8 pitch so the tap drill is 5 - 0.8 = 4.2mm. I don't know about you, but my set of metric drills came in 0.5mm increments so I either have to settle for a 4mm drill, which is the closest in the set, or convert to an English size drill and break out that number-letter-fraction index of English sized drills. That was exactly what I did. A 4.2mm drill = 0.1654". The closest number drill is a #19 at 0.166" and that was what I used. More math and I had to go to the English sized drills to get a close fit. BTW, I purchased those taps in the local ACE hardware and the packages were marked with that same, exact number sized drill. Very few local hardware stores here in the US will stock even whole mm sized metric drills, much less decimal sized ones. I have to wonder if the local stores in England, France, Germany, or any other European countries stock metric drills by 0.01mm size increments.

Great! Now do it for a 1/2-13 thread. or 1/4-28.....

[Paul Alciatore]

And that is precisely what my short sequence of keystrokes for a four-banger calculator in post #26 above does. Here it is again, in even simpler form. Each line is a button press or a number that you enter:

1
/
TPI
=
-
MD
=

As I said, it will come out with a minus sign which you just ignore. It don't get any simpler than that. It uses the exact same formula as has been stated for metric threads: MD - Pitch; it just converts the TPI to a linear pitch.

What is the tap drill for a 3/8 - 16 thread? Well a 16 tpi is 1/16 of an inch pitch and 3/8" - 1/16" is 5/16 Well how about that . If you can't do that you probably shouldn't be trusted with any power tools.
...lew...

[danlb]

Having a few basic engineering formulas in my head, like area, pressure, simple angle vectors,
has been key to my career
--Doozer

That is the key. Commit to memory those basic things that you need all the time. Don't try to accurately remember seldom used complex formulas that are critical to the success of your projects. Do remember which formulas are available and how to find and apply them.

When I worked for AT&T as a switching technician they used the same concept.