My old Machinery's Handbook doesn't seem to provide a tap drill size for
M14-1.25 hand tap (modern spark plug size). Maybe I'm losing my marbles, but
I can't find it. Is there a simple formula for determining tap drill size,
given the metric size? TIA, Dave

David Anderson wrote:

Is there a simple formula for determining tap drill size,
given the metric size? TIA, Dave

diameter - pitch = the size to drill
M14 * 1.25 - drill 12.75 table says 12.8

Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

The general formula for finding metric tap drill sizes is: Nominal
diameter - pitch

In the stated case for a M14-1.25 thread that would come out to a metric
drill size of 12.75 mm.

12.75mm is 0.502 in

I assume the general formula gives the tap drill size for nominal 0.75%
thread which is considered practical for a strong thread in average
machineable material.

To test this I converted 14mm and 1.25mm to their inch equivalents and
calculated the inch size of drill for 0.75% thread. It came out to 0.503
in.

Bob Swinney


"David Anderson" wrote in message
news
My old Machinery's Handbook doesn't seem to provide a tap drill size for
M14-1.25 hand tap (modern spark plug size). Maybe I'm losing my marbles,
but I can't find it. Is there a simple formula for determining tap drill
size, given the metric size? TIA, Dave

Robert Swinney wrote:

I assume the general formula gives the tap drill size for nominal 0.75%
thread which is considered practical for a strong thread in average
machineable material.

No!
M14 * 1.25 is a fine pitch. So you would be way off for standard pitch.
Your (and other rules of thump using a simple factor) are simply
useless.
It is always NominalSize - Pitch. No matter wether fine or standard.

Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

http://catalog.starrett.com/catalog/.../2900/2880.pdf


"David Anderson" wrote in message
news
My old Machinery's Handbook doesn't seem to provide a tap drill size for
M14-1.25 hand tap (modern spark plug size). Maybe I'm losing my marbles,
but
I can't find it. Is there a simple formula for determining tap drill
size,
given the metric size? TIA, Dave

Simply:

OD minus pitch

Like:

14 - 1.25 = 12.75mm tap drill (12.8 is more likely though)

This works for inch and metric sizes, although you'll have to convert the
inch TPI into pitch (1/TPI).

Regards,

Robin


"David Anderson" wrote in message
news
My old Machinery's Handbook doesn't seem to provide a tap drill size for
M14-1.25 hand tap (modern spark plug size). Maybe I'm losing my marbles,
but I can't find it. Is there a simple formula for determining tap drill
size, given the metric size? TIA, Dave

Nick,
You'd quibble over a difference of 0.001 inch between the two formula?
AFAIK, neither formula accounts for whether pitch is fine or standard. Can
you say, "Machinist's Handbook"?

Bob Swinney
""Nick Müller"" wrote in message
...
Robert Swinney wrote:

I assume the general formula gives the tap drill size for nominal 0.75%
thread which is considered practical for a strong thread in average
machineable material.

No!
M14 * 1.25 is a fine pitch. So you would be way off for standard pitch.
Your (and other rules of thump using a simple factor) are simply
useless.
It is always NominalSize - Pitch. No matter wether fine or standard.

Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

Sorry Nick for chiding you with less than total accuracy: it should have
been "Machinery's Handbook".

Bob Swinney
"Robert Swinney" wrote in message
...
Nick,
You'd quibble over a difference of 0.001 inch between the two formula?
AFAIK, neither formula accounts for whether pitch is fine or standard.
Can you say, "Machinist's Handbook"?

Bob Swinney
""Nick Müller"" wrote in message
...
Robert Swinney wrote:

I assume the general formula gives the tap drill size for nominal 0.75%
thread which is considered practical for a strong thread in average
machineable material.

No!
M14 * 1.25 is a fine pitch. So you would be way off for standard pitch.
Your (and other rules of thump using a simple factor) are simply
useless.
It is always NominalSize - Pitch. No matter wether fine or standard.

Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

"Chief McGee" wrote in news:t1rNe.263295$x96.130581
@attbi_s72:

http://catalog.starrett.com/catalog/.../2900/2880.pdf

Better yet

http://www.engineershandbook.com/Tab...ictapdrill.htm


--

Dan

Robert Swinney wrote:

You'd quibble over a difference of 0.001 inch between the two formula?

No. Not over 0.001 inch. No use arguing about a hair.

But what's the right drill (according to your formula) for:
(std) means this is the standard pitch,
(fine) is one of the fine pitches.

M3 * 0.5 (std)
M3 * 0.35 (fine)
M6 * 1 (std)
M6 * 0.5 (fine)
M6 * 0.75 (fine)
M10 * 1.5 (std)
M10 * 0.5 (std)

Pick any 7 and please answer with your formula.

Thanks,
Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

Sorry Nick. I guess we have a problem with translation here. Pitch is
distance between the crests of a thread. Pitch is also the reciprocal of
the # of threads per unit measure; whatever that unit may be; inches,
millimeters, etc. AFAIK, there is no specific definition of (std) and
(fine). For the terms to be meaningful in any calclulation they must have a
specific identity re. to the quantity to be calculated. That seems to be
the case in the 7 examples you cited.


There is no "your formula". I merely changed mm to inches in order to use
an "inch" formula I am familiar with and to illustrate the fact it comes out
with only a tiny difference compared to the "metric' formula.

I believe the "metric" formula for tap drill size is: Basic major diameter
minus thread pitch

The "inch" formula I used is: Tap drill diameter = nominal diameter - [
(1.299 x %) / pitch ]

whe Pitch is # of threads
per inch and % is expressed as a decimal

I'll leave proof up to you.

Bob Swinney






""Nick Müller"" wrote in message
...
Robert Swinney wrote:

You'd quibble over a difference of 0.001 inch between the two formula?

No. Not over 0.001 inch. No use arguing about a hair.

But what's the right drill (according to your formula) for:
(std) means this is the standard pitch,
(fine) is one of the fine pitches.

M3 * 0.5 (std)
M3 * 0.35 (fine)
M6 * 1 (std)
M6 * 0.5 (fine)
M6 * 0.75 (fine)
M10 * 1.5 (std)
M10 * 0.5 (std)

Pick any 7 and please answer with your formula.

Thanks,
Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

Robert Swinney wrote:

AFAIK, there is no specific definition of (std) and (fine).

If you leave out the pitch, it is standard. I just quoted the pitch for
(your) convenience. Every (standarized) nominal diameter has a standard
pitch.

Pitch (in metric) is the distance between two (neighboring) crests.


I believe the "metric" formula for tap drill size is: Basic major diameter
minus thread pitch

It is.


The "inch" formula I used is: Tap drill diameter = nominal diameter - [
(1.299 x %) / pitch ]

Ah, in the imperial way messuring the pitch.
Too complicated for me. 8-/

Sorry for any confusion I may have caused.


Nick
--
Motormodelle / Engine Models:
http://www.motor-manufaktur.de
Ellwe 2FB * VTM 87 * DLM-S3a * cubic
more to come ...

D Murphy wrote: ...
http://www.engineershandbook.com/Tab...ictapdrill.htm

Which gives the drill size as diam - pitch (for those subtractively
challenged).

Except for these:
Thread Table Calc
M4.5 x .75 3.70 3.75
M8 x 1.25 6.80 6.75
M9 x 1.25 7.80 7.85
M12 x 1.75 10.20 10.25

M4 x .35 3.60 3.65
M10 x 1.25 8.8 8.75
M14 x 1.25 12.8 12.75

I suppose that the calculated value is not a readily available drill
size, hence it is rounded (some up, some down).

These must be typo's
M8 x .5 7.00 7.50
M8 x 1 7.50 7.00
M16 x 1.5 15.0 14.5
M36 x 3 36.0 33.0 !! Pity the poor guy who blindly follows the
table and uses a 36mm drill for a M36 thread.

Bob (who wonders why he doesn't go to bed instead of verifying tables
that he'll *never* use)

Easy formula. O.D. minus the pitch. Example M6X1 = 5mm Tap Drill. Gives
aprox 75% thread. works with all 60 deg threads metric or english.
Rick R
"Bob Engelhardt" wrote in message
...
D Murphy wrote: ...
http://www.engineershandbook.com/Tab...ictapdrill.htm

Which gives the drill size as diam - pitch (for those subtractively
challenged).

Except for these:
Thread Table Calc
M4.5 x .75 3.70 3.75
M8 x 1.25 6.80 6.75
M9 x 1.25 7.80 7.85
M12 x 1.75 10.20 10.25

M4 x .35 3.60 3.65
M10 x 1.25 8.8 8.75
M14 x 1.25 12.8 12.75

I suppose that the calculated value is not a readily available drill size,
hence it is rounded (some up, some down).

These must be typo's
M8 x .5 7.00 7.50
M8 x 1 7.50 7.00
M16 x 1.5 15.0 14.5
M36 x 3 36.0 33.0 !! Pity the poor guy who blindly follows the
table and uses a 36mm drill for a M36 thread.

Bob (who wonders why he doesn't go to bed instead of verifying tables that
he'll *never* use)