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Old 07-16-2008, 04:35 PM   #1
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Default Model Tapered Pipe Thread Tap Drills

Anybody know the pilot drill size to use for 1/4 - 40 model tapered pipe threads?

Thx...
Chuck



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Old 07-16-2008, 05:09 PM   #2
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Default Re: Model Tapered Pipe Thread Tap Drills

Chuck,

Did some searching on the net and couldn't find an answer. They call for 7/16" drill for a 1/4-18. I would use a piece of material and drill a 7/16 hole and then tap with the 40 thread tap to see if it works or not.

I'm sure somebody has that info right at their finger tips. Right Marv?

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Old 07-16-2008, 06:13 PM   #3
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Default Re: Model Tapered Pipe Thread Tap Drills

Gotta live up to my reputation. Here you go...

1/8-56 40
5/32-48 30
3/16-40 23
1/4-40 5
5/16-27 F

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Old 07-16-2008, 07:06 PM   #4
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Default Re: Model Tapered Pipe Thread Tap Drills

Wow, #5 drill Boy was my guess way off, 7/16" drill.

I'd of been able to throw the tap through the hole from the other side of the room. That's a big difference from .205" to .4375".

Ofcourse I was using the regular pipe thread size from the chart for a 1/4-18 for a close "guesstimate. :P

Thanks Marv.

Bernd

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Old 07-16-2008, 08:59 PM   #5
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Default Re: Model Tapered Pipe Thread Tap Drills

Bernd,

Pipe is sized by the ID so a 1/4-18 pipe has an OD of 0.54. Hence a tap drill of 7/16 (or 29/64 as my chart says) makes dimensional sense.

However, the Imperial system of measurement has a fundamental dictum - Consistency in measurement definitions is to be avoided at all costs.

Someone correct me if I'm wrong but I believe that the "1/4" in the 1/4-40 MTP designation refers to the major diameter of the thread and not the ID of the associated pipe. Viewed from this perspective, a #5 tapdrill makes a lot more sense.

Another Imperial dictum reads - The user should not be able to derive any useful dimensional data from the designation. Numbered screws, sheet and wire gages and number/letter drills adhere religiously to this dictum with the predictable result that the sale of tacky little shirt-pocket charts remains a viable business niche.

The Brits who invented MTP threads missed a golden opportunity to introduce more confusion into the Imperial system. They could have labeled the MTP series with Greek letters, thus further obfuscating what the thread designator denoted. Had they further made the pitch an irrational number, confusion perfection would have been achieved. A "gamma-2.718281828045" designator would have represented a new high in content-free designation.

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Old 07-16-2008, 10:28 PM   #6
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Default Re: Model Tapered Pipe Thread Tap Drills

Shame on you Marv, pulling all our threads to bits.

I am sure if your lot had invented them, they too would have come up with their own methods to protect their trade secrets. What does 2-56 and 4-40 mean? I can understand the pitch, but not the first bit.

But we did come up with a fantastic set of threads specifically for model engineering, and it seems that you lot haven't picked up on it much.

M.E. threads consist of just two pitches 32TPI and 40TPI over the whole range. For going into softer materials where a course and stronger thread is required 32TPI, and 40TPI for harder materials. This is the standard that is used on most of our smaller boiler fittings and engines. It is also very good for making your own precision instruments, as you know the standard imperial micrometer uses 40TPI threads, giving 25 thou forwards movement for each revolution.

32TPI run from 5/32" to 1/2"
40TPI run from 1/8" to 1/2"

Under these sizes we tend to use the very baffling B.A. system.

Here is a link that gives you all the drilling sizes that are needed, from tight to slack fit, plus a load of other threads as well, in fact a real load of very useful model engineering thread data. Plus if you root around on the site you will see full sets of drill conversion charts.

http://modratec.com/mud_me.php

John

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Old 07-16-2008, 11:31 PM   #7
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Default Re: Model Tapered Pipe Thread Tap Drills

Sorry about that, Wallace, but it had to be said.

I won't deny that we Yanks made our own stellar contributions to the confusion that makes the Imperial system such a worthless hodge-podge. (Although we learned our skills at obfuscation from you lot.)

Sometime ago there was a long discussion about Imperial vs. metric. In that go-around I made the point that powers-of-ten, while nice, was not the major advantage of metric. Metric is a *system* where, besides the different standards, the whole process of designation has been rationalized for the better. This is just one example of what I was talking about.

Length designations should be open-ended (no #5 or, worse, 0000 screw designations) and should easily convey information about the actual length. Satisfy both of those and you end up with the metric designation, major diameter - pitch expressed as length. Calling it a #5 screw because it was made with the fifth machine the factory bought may be ok within the factory, but it's idiocy to allow that designation to find its way into general usage. Same argument with gage numbers that reflect how often the material went through the rolling/drawing mills.

-------------

I think you probably already know that the relation for our numbered screws is:

major diameter = 0.060 + 0.013 * N

where N = screw number.

I can do the conversion mentally but I shouldn't need to. Mathophobes end up with another one of those tacky pocket-sized charts. There is a similar mathematical relationship for BA (which I've published here before) but it's even farther beyond the typical math skills of products of our laughable education system.

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Old 07-16-2008, 11:51 PM   #8
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Default Re: Model Tapered Pipe Thread Tap Drills

Quote:
Originally Posted by mklotz
but it's even farther beyond the typical math skills of products of our laughable education system.

Hey Marv!!! I resemble that remark)
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Old 07-17-2008, 12:32 AM   #9
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Default Re: Model Tapered Pipe Thread Tap Drills

The "metric versus imperial" argument has been running in Model Engineer magazine as far back as you'd want to go.The reasons for or against can make some quite entertaining reading,as few of the comments are based on logic.
Personally,being Dutch,I was brought up on the metric system,and had to learn the imperial system when I came to New Zealand.The most confusing to me was actually weights.Ton,ok,CWT(hundredweight!surely a metric measurement),stone(what the devil does THAT relate to?).Pounds and ounces make some sort of sense,I guess.
And when NZ went metric,i had to go back to what I knew best
And then we get "model" threads.I took me a long time to work out what 4-40 stood for.I became familiar with these when taking up RC modeling.I try to avoid BA if possible,although when a nice boxed set of BA tackle came up at a good price on a local auction site,I bought it
ME's 32 and 40 does appear quite useful,as metric fine is virtually unheard of here.I just wish the taps and dies weren't so darned expensive.I can buy 4 metric taps for the price of 1 ME one!
Hence I only use ME if there's absolutely no alternative.

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Old 07-17-2008, 01:46 AM   #10
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Default Re: Model Tapered Pipe Thread Tap Drills

Bentprop

http://www.tracytools.com/tapsanddiesmodeleng.htm FOR INDIVIDUAL

http://www.tracytools.com/tapsanddiessets2.htm FOR FULL SETS


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