Need Help! Correct drills for trapezoidal taps?

# Thread: Correct drills for trapezoidal taps?

1. ## Correct drills for trapezoidal taps?

I recently purchased a 10x2 trapezoidal tap to make some new nuts for the leadscrews in my proxxon KT150 table, and have realised I've no idea what the correct drill size is to make the holes.

A good bit of googling tells me lots about metric thread forms, and a reasonable amount about acme threads but nothing of use for trapezoidal threads. Could someone help me out here?

• If I've missed an obvious web page, I'd be grateful if you could tell me where it is!
• Is acme similar enough to trapezoidal that I could just use the rules for acme threads with appropriate measurements?
• Would it have just been easier to drill a whole bunch of holes of decreasing size until I found one which fitted the leadscrews snugly when tapped?

Thanks in advance!

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2. Originally Posted by serriadh
I recently purchased a 10x2 trapezoidal tap to make some new nuts for the leadscrews in my proxxon KT150 table, and have realised I've no idea what the correct drill size is to make the holes.

A good bit of googling tells me lots about metric thread forms, and a reasonable amount about acme threads but nothing of use for trapezoidal threads. Could someone help me out here?

• If I've missed an obvious web page, I'd be grateful if you could tell me where it is!
• Is acme similar enough to trapezoidal that I could just use the rules for acme threads with appropriate measurements?
• Would it have just been easier to drill a whole bunch of holes of decreasing size until I found one which fitted the leadscrews snugly when tapped?

Thanks in advance!
According to DCSwiss its 8.2mm

However Maryland say

"Formula for tap drill (round to closest available drill size): D1+(0.05*p)
D1 = minimum internal thread minor diameter, p = pitch"

= 8 +.05*2 = 8.1mm

both claim a 7H fit...

BTW was that a LH or RH tap and where in UK are you?

3. According to the formulas found in ISO 2901, the minor diameter should be 8.000 - 8.236 for a 7H fit.

4. Excellent stuff. Cheers guys, you've been a great help.

5. Originally Posted by irving2008
BTW was that a LH or RH tap and where in UK are you?
Its a right-hand tap. It was about 20 quid from Marchant Dice. I'm not entirely sure why it costs a fraction of the price of trapezoidal taps from elsewhere; I expect I'll report back at some point when I find out

I'm Cambridge based, as it happens.

6. Originally Posted by serriadh
Its a right-hand tap. It was about 20 quid from Marchant Dice. I'm not entirely sure why it costs a fraction of the price of trapezoidal taps from elsewhere; I expect I'll report back at some point when I find out

I'm Cambridge based, as it happens.
Ah OK, yes I'd like to know how good the MD ones are - I want to replace the leadscrews in my lathe with metric LH 10x2... if it had been a LH one I might have negotiated a loan... I've made a RH one for delrin from a bit of scrap TR10x2 and might do the same for a LH one for brass with some TG10x2 and some case hardening power - MD want £31 for a LH one

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