Machine Threads in Pinblock, was WD-40

[Ron Overs]

Fenton and Michael,

We use machine screws to fix our pin block to the plate. We use 8 mm 
countersunk machine screws and the pin block holes are threaded in 
the maple before we fit the screws. The tapping drill size we use is 
the same as is used for threading metal with the same thread size.

Speaking of tapping drill sizes, I have a short formula which has 
been kicking around in my head for couple decades which I use for 
calculating the tapping size for a thread. The formula is handy 
because it is sometimes more convenient than looking up tapping 
tables. The formula for the minor diameter of a thread is;

	(major diameter in thou) - (1220/tpi)

This is handy when tapping for agraffes for example, when we're 
dealing with an unusual thread that we won't find on a tapping chart. 
For example, lets take an agraffe with a 1/4" diameter and 36 threads 
per inch. The tapping drill size for this thread can be found using 
the following;

	Diameter of major diameter in thou is 250
	Threads per inch is 36

Substituting the above into the formula we get;

	250 - (1220/36)

	250 - 33.88

	216.11

So 216 thou is the minor diameter of this thread. The nearest nominal 
tapping drill will be 7/32", which has a diameter of 218 thou.	The 
tapping drill should always be slightly larger than the actual minor 
diameter of the thread, to allow for better clearance of the swarf.

The same formula can be used for metric threads also, but the major 
diameter in mm needs to be converted to thou by multiplying the 
diameter by 39.37, and the thread pitch in mm needs to be converted 
to threads per inch. To do this divide the metric pitch into 25.4. 
For example, a 0.7 metric pitch, expressed as threads per inch is;

	25.4/0.7

	36.285 threads per inch

For those of your who might be rebuilding grands, and sometimes need 
to use replacement agraffes with a new thread size, when the original 
agraffes are no longer available, these formulae can quite useful.

Ron O.

>Micheal,
>I know some of this thread was meant as a joke, but I'm interested 
>in your comment regarding machine threads in the pin block, if you 
>were serious. I've diffenately used machine treads in maple, but 
>nothing as critical as a pin block. Please comment on thread size, 
>drill size, ect. I assume you might use a smaller drill than 
>recomended for metal. Or maybe I missed the joke.
>Thanks,
>Fenton
>


-- 
OVERS PIANOS - SYDNEY
    Grand Piano Manufacturers
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Web http://overspianos.com.au
mailto:ron at overspianos.com.au
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