Discussion in 'General Engine Discussion' started by cfellows, Jul 16, 2008.
Anybody know the pilot drill size to use for 1/4 - 40 model tapered pipe threads?
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?
Gotta live up to my reputation. Here you go...
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. :-[
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.
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.
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.
Hey Marv!!! I resemble that remark)
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 ;D
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 :big:
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.
http://www.tracytools.com/tapsanddiesmodeleng.htm FOR INDIVIDUAL
http://www.tracytools.com/tapsanddiessets2.htm FOR FULL SETS
I have a question for our British colleagues. Do you ever single-point BA threads?
Seems like it would be difficult. For example, the pitch (mm) of a 6BA is given by 0.9^6 = 0.531441... mm. Even with a metric lathe it seems it would be difficult to obtain, as would be the equivalent 47.79458... tpi on an Imperial lathe.
Just to answer your question.
Never, not even tried, and I don't think I ever would.
I think almost all the older generation of model engineers in the UK would have a set of taps and dies. Plus they are so easily bought here in every length and size, it is just too much trouble to try to cut them. I have thousands in stock, and a quick modification on length and I will have what I want.
I am not into masochism.
Thanks for clearing that up, John. I sort of expected your answer.
It just seems like another confirmation of my picture of the Imperial hodge-podge that a thread form that can't conveniently be cut on either a metric or Imperial lathe would come into common use.
Note that that is not a stab at British engineers, model or otherwise. You know I have a high opinion of both. Lord knows we Yanks have our own complement of unnecessarily confused standards.
This has come a long way from the original post but I would like to add the following point.
Hole sizes and thread type on drawings are not the be all and end all. If you haven't got what it asks for, root around in your motley collections and see if you have something near enough to the size that 1. won't look out of place and 2. it will cause no further problems in the build.
Use what you have and make do. That is what the original designer most probably did when making the plans.
John S has proved it by using 2mm metric on his Halo 3 cylinder instead of 2-56, and you can't tell the difference.
Thanks for the replies. I think #5 is about the right answer. The 1/4 x 40 MTP nipples are 1/4" OD. I found the taps and dies at PM Research, although I spent a king's ransom for them... cost me over $50 for the pair. However, should last me forever!
Marv, I'm 100% with you on the eccentricities of the imperial measurement system.
But is this true?
Or are you extracting the urine?
No, it's just my lame attempt to inject humor. Nevertheless, I'm sure the "reasons" for some of the more egregious Imperial eccentricities aren't a whole lot more logical.
Consider the weight markings on anvils. On older anvils you'll find a triplet of numbers (e.g., a-b-c) stamped or cast into the base. "a" is the number of cwts (hundredweights). "b" is the number of quarters and "c" is the remaining number of pounds.
If you want to know the weight in pounds you have to know that:
A "quarter" is two "stone" and a stone is 14 pounds so a quarter is 28 pounds.
A cwt is four quarters so it's 4*28 = 112 pounds. (Yes, that's right. In the Imperial system a unit called a *hundred"weight weighs 112 pounds, not 100 pounds.)
Thus, to obtain the anvil weight in pounds, you need to compute:
112*a + 28*b + c
Now, consider the fact that no anvil weighs more than a thousand pounds. That means that its weight in pounds can be expressed with only three digits. The three digits, a-b-c, could directly express the weight in pounds. But, no, that would be too easy. Let's express it in some obscure system that requires remembering eccentric relationships and performing unneeded arithmetic to find the desired answer.
Marv: This was the manner used for marking ordinance in the English army and the Royal Navy, certainly into the 19th century, presumably when the foundries had some down-time from government contracts they cast anvils and used the marking system they were familiar with?
That takes me back, I can remember as a kid in school in England doing weight additions and subtractions using 4 columns, cwt, quarters, stones, and pounds. Kept a young brain nimble if nothing else, remembering the relationships and applying them correctly.
I hasten to add I'm reminiscing, not advocating.
I think that back in the dim dark ages a cwt was originally a hundred pounds, but because merchants cut corners and supplied underweight, the cwt was increased to 112 lbs to make sure that you got at least 100 lb. A bit like a baker's dozen.
If that is true (and I have no reason to doubt it), it just supports my argument about the idiocy of the Imperial system that will continue to use a nonsensical system just because it's "traditional". The Imperial "system" isn't a system at all - it's just a hodge-podge collection of measures of convenience and historical artifacts with no attempt to make the collection usable.
It could be fixed but, if you did that, you would end up with something that looked remarkably like the metric system. Perhaps the fundamental units would have different sizes but the relationships among those units and the rules for designation would parallel what is done in SI. And that's the real advantage of metric - it's a coherent system with sensible designation rules adopted to make manipulation simple for the user. The fact that it uses powers-of-ten is convenient but that is not the essence of its utility.
Separate names with a comma.