A question about thread cutting with a tap:

[rolgiati]

I need to cut a 3/8" x 20TPI thread, to take a bolt that will engage about 20 mm length.

I have calculated (Ø - pitch, both in mm) that I should drill the hole at 2.255 mm Ø.

Problem is I live in Darkest Paraguay, where number drills, and metric drills in 0.1 mm increments are unknown, all I can get, or have, are metric drills in half-mm increments.

Would you recommend I try to tap in an 8mm Ø hole, or an 8.5 mm Ø one ?

PS My Cowells90 lathe is purely metric, no hope of cutting the thread with it...

[ettingtonliam]

Sorry, but there is a mistake in your calculations. No way is tapping size for 3/8" x 20 TPI 2.225mm. I'm assuming that this is 3/8" BSF, for which my chart gives the tapping size as 8.3 mm, or 21/64".

Unless you are really cunning, and can grind an 8mm drill to cut oversize (its possible, I'm always managing to grind drills that cut oversize ), then no. don't use an 8mm drill, its far too small. It will have to be an 8.5mm one, but drill a pilot hole, say 6mm, first, so the 8.5mm won't cut over size. You don't say what material you are drilling/tapping into.

Good luck with your task.

[jem]

You could try drilling an 8 mm hole and hope that it is oversize, and if not then drill it out very carefully to 8.5mm perhaps leaving a bit of the hole at 8mm and see hoe it goes.

best of luck

Jem

[SteveW]

Rolgiati,

If you can afford to destroy a drill the next size up from the tapping size and you have either a bench grinder perhaps if desperate a mate with an angle grinder ... to help.

Put your embryo drill bit in a an electric drill and sort-of carefully reduce its size for the first 2.5cms against a bench grinder. Only the cutting end needs to be accurate.

Alternatively you could grind a small drill bit off centre enough to rouge out the correct size for your tap, maybe better a bit too big than a bit too small. As suggested above use a near size pilot hole to follow. Maybe use enough pressure for the offset drill to follow the pilot hole centre rather than self- centring itself.

Another tip would be to tap through a wooden block with a clearance hole for the tap as a guide to keep it all square.

Drill a practice hole first.

[weary]

20 TPI = 1.27mm thread pitch.
Are you sure that your lathe won't cut 1.25mm thread pitch which may-well be 'close-enough' with a bit of easing/aggression ? ? ?
After-all the thread pitch is only going to be 0.4mm 'out' over 20mm
Maybe only thread half or a third of the depth and open rest to clearance may suit?
Depends of course on your usage and requirements.

(Yea, yea; I know that I'm an 'animal' with no finesse - no microns ever at risk with my 'engineering'!)

Phil

[millman]

Somewhere on this forum is a thread about screwcutting on a Cowells and it gives gear trains for all the BA threads plus ME threads of 40 and 32 tpi, if it can cut 40 tpi it can do 20, try a search, if I remember correctly Jo was a contributor I think she was interested in the BA sizes.

[weary]

Excellent memory 'millman'!

The thread referred-to is here.

Posting by phil1001 on Feb 2nd 2012 at 9:00PM is relevant posting. Click on associated attachment to access pdf.
Attachment shows that closest you can get is 19.84tpi (1.28mm pitch) using 32T on Spindle drive and 25T on Leadscrew.
25T & 20T respectively give you 20.32tpi (1.25mm pitch)

Phil

[andyhigham]

Grind up a thin boring tool
Drill the hole with the nearest size drill you have
Bore the hole to the tapping size
Tap the hole
Pour a drink and feel smug

[Jim]

Along with the other suggestions I'd also suggest you use a thread taping paste on the tap to help it cut.

Jim

[David]

A small endmill can make a good emergency boring bar. If you have a 6mm one it would work after you've drilled the hole 8mm. Put one of the endmill teeth pointing out to the side of the hole nearest you, obv.

[chrisb]

Jul 8, 2023 7:43:52 GMT David said:

A small endmill can make a good emergency boring bar. If you have a 6mm one it would work after you've drilled the hole 8mm. Put one of the endmill teeth pointing out to the side of the hole nearest you, obv.

Funny, I watched one of Mr Criapin’s Youtube videos yesterday and he did the same thing.