I have started turning threads on wood using 18, 20 and 24tpi woodturning thread chasers. For the internal threads I would like to be able to put threads into a drilled hole to fit a 6mm threaded spigot.
I'm afraid I have no knowledge of metalwork, could anyone please tell me the tap (or is it die) size that I would need to match each of the
Many thanks for your help
Paul
I was a tool & die guy for several years and do remember that the size for a
1/4-20 thread was a #7 drill. OTOH, you can figure it yourself. Measure the diamter of the shaft of the screw and that would be the size of the hole to drill.
6 mm is just a hair under 1/4", so that takes care of 20 tpi (1/4-20 is a standard inch thread).
For 1/4-18 and 1/4-24, which are non-standard threads, go to MSC Supply:
Here's a link to charts that show the drill sizes for both metric and imperial taps:
A standard M6 x 1.0 tap is 25.4 threads per inch. So your closest chaser for your spigot is 24 tpi. You'll have to make it a little sloppy on the spigot so it will fit the tapped threads which won't match perfectle being 25.4 tpi, and don't make longer than about a half inch or so. After all every inch you'll be off by one thread. Next a good rule of thumb for metric internal thread drill sizes is to subtract thread pitch from the major diameter of the thread. Ex. 6mm
- 1mm = 5mm(.1969) drill. This rule doesn't work so well with english threads.
Some calcs: Internal threads:
Pitch = 1/threads per inch Major diameter = .060 + (.013 * Pitch) Drill size = Major - (1.08253 * Pitch * thread percentage required)
External thread Bolt Minor Diameter = Major - (1.22687 * Pitch)
This may be too much information but it has always worked for me when making threads in metal. You should pick up a copy of the Machinery Handbook. It's expensive but can but very useful for just about everything in the shop.
Mike
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