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The ISO standard apparently includes the concept of "preferred" sizes and pitches. Sometimes the pitch will be such that it makes for a weaker fastener (or so I was told) and so those are normally skipped. Those sizes are still in the ISO standard, they are just part of a "less desirable" class.
Some of the metric minded folks are convinced that you should never, ever use the size/pitch combinations that are not in the preferred group. Others point out that you can make a valid metric thread using any diameter and any pitch as long as it is measured in mm and meets the standard for the thread form.
Dan
Yep, there is actually three such tabulations, preferred, notsopreferred and oddballsizesthatshouldn'tbeused. Of those you can then have the standard pitch, fine pitch or extra fine pitch. And then the last category is manufacturers "specials" where you mix'n'match different standards or make your own just to force customers to buy (or not) from them only.
Amount of experience is in direct proportion to the value of broken equipment.
No problem. We can make this the the "Metric light bulb" thread too.
I was surprised to find that the good old C7 christmas light bulb has been renamed the E12, and the Edison base is no longer a 1 inch base, it's E26. It seems that ANSI and IEC got together some years back and redefined the dimensions of the Edison bases so that they are "compatible" with the european standards. This worked OK since most light sockets are so sloppy that putting a 1 inch with 5% tolerance into a 26mm socket with 5% tolerance was no problem.
So yes. We have "metric light bulbs" that are simply sloppy renaming of american standards from before world war I. You'll is note the thread pitch on an "e26" is 3.630 mm which is nowhere near any metric pitch. It is, however, exactly 7 TPI.
Sorry. Back to the Tap and drill discussion.
Dan
At the end of the project, there is a profound difference between spare parts and left over parts.
Who needs charts when you can memorize a simple equation that can be calculated on a four function calculator?
TD = MD  0.013*DOT*P
The same equation will work for any thread with a sixty degree thread form.
I agree in concept, BUT... That assumes that I'm confident that I've remembered the formula properly. If I use the wrong constant, then I've screwed up. So I look up the formula. And do the math. And then convert the MM drill size to the sizes that I have in my drill index.
So instead of doing that, I printed out a chart and suck it in the case that holds my taps. I find the thread that I want to use and read the proper drill from the column for "steel (50%)" or "Aluminum (75%)" . Done.
But then again, I do the same thing for lots of shop practices. My lathe has a chart stuck to it with the appropriate RPM for different metals and diameters. My Mill has the same thing for different sized end mills. The drill press has a chart inside the cover that gives RPM for various drill sizes in various materials.
I seldom have to use all the formulas and variables. Which is good, because I can't remember all of them.
Dan
At the end of the project, there is a profound difference between spare parts and left over parts.
Wow..3rd page on tap drill size.
I did have trouble with tapping one thread. 3/8 NPS F..none of the charts were right, i called a tap manufacturer...... What they gave for tap drill size was 105 percent or so ..he finally agreed that must be wrong.. Eventually i figured it out..
Between NPT, NPSF, and NPSC...... Its not all the same.
Btw I need NPSF taps at the time, and got one day service from Taylor taps..
No need to memorize constants. The really simple formula given above, TD = MD  P works for all 60 degree threads and a 75% thread engagement. The problem is the pitch (P) MUST be in the same units as the MD. So it works out real nice with metric threads where the pitch is given as the lead from one thread to the next in mm.
The problem with using that simple formula is with English threads where the pitch is given in threads per inch, not inches. But there is a fast and easy, no constant way to use that formula on almost any calculator to find the tap drill (TD) for them also. Here it is:
Enter a 1
Press division key
Enter the thread pitch in TPI
Enter = key
Enter  key
Enter the MD
Enter = key
The result will be the tap drill diameter. It will also show a negative sign, just IGNORE IT.
Example using 1/420 thread"
1
/
20
=

.25
=
0.2
Ignoring the minus sign, your tap drill is 0.200". The closest standard drills are #8 (0.199") and #7 (0.201"). I believe most tap drill charts will give a #7 drill for this thread size. The hardest part of this process is looking up the fractional or number/letter size of the drill from the diameter expressed as a decimal.
I have a created an Excel sheet that does the tap drill calculation for any Vee thread (not just 60 deg), with any amount of a flat at the crest, any amount of fill at the root, any percent of thread engagement, and any thread, English or metric. I found that this was a real challenge, not for the math, but for the assumptions that are never spoken about. Many people, myself included, would assume that the percentage of thread would be in terms of a sharp vee thread. It is not. If you calculate the tap drill based on a sharp vee thread your tap drill sizes will not match any of the published charts. OK, so it must follow the amounts of crest flat and root fill shown in the Unified thread standards. Wrong again. The problem with that is the root fill percentage that the standard shows is a MAXIMUM value and almost no threads will ever have that much root fill. Yes, you must consider the crest flat and root fill percentages in your calculation, but not the values in the standard. What I had to do was calculate a bunch of tap drill sizes with different percentages for these factors and then see which set of tap drill sizes best matched the published tables. It turned out that the best match was when both the crest flat and the root fill were at 12.5% values which meant that the thread would be 75% as high as a sharp Vee thread. This makes sense if you look at the history of the thing. Older thread form standards used values between 1/8H and 1/10H for these crest flat and root fill values. And the tap drill charts really did not change much when the newer standard took over. Also, if you look closely at any tap, you will see that these values will be close to the 1/8H value. No taps are made to produce that very generous 25% (1/4H) root fill that the standard allows. That is probably there to allow production tools to continue to be used when they wear.
My spreadsheet calculator is not yet ready for prime time. In order to avoid the drill lookup time I am struggling with adding the ability to choose the closest standard drill bit size from a list of English (inch) and metric drills. I want to be able to control those lists to include whatever mix of these drills that a shop will have on hand. Choices would include English only, metric only, mixture of them, and what sizes are available in each. This is proving hard to implement. The calculator will also show the actual percentage of thread engagement with the actual drill bit that you select. Yes, this will be different from the desired percentage that was entered at the beginning of the process because the actual drill will almost never be the actual size that is calculated. This is true for both English and metric sized threads and tap drills.
When I finish this I believe it will be the best, most complete tap drill calculator that is available. And I do intend to make it available at that time.
Marv, your equation uses the constant 0.013 which is a rounded value for 1/75 which actually equals 0.0133333... Your formula would be easier to remember if you just used the 75% value, but in the denominator.
TD = MD  (DOT*P/75)
P/75 reduces it from a 75% thread to a 1% thread and then multiplying by the actual percentage you want brings it up to that level (DOT*P/75).
As I said, easier to remember because the constant 75 is the actual percentage that the formula is based on.
Some of us are comfortable with math, some aren't. If you like your shop papered with cheat sheets, go for it.
A chart will never be complete or, alternatively, if it is it will be too big and unwieldy. An equation covers all cases including "tap drill" sizes for which there are no taps. See if your chart will tell you the size hole to bore for a 2.5  8 thread with 85% DOT.
Paul,
I know how to do the calculation on a 4 banger, thanks very much. I use an RPN which makes it much simpler yet.
The difference between 1/75 and 0.013 isn't going to be enough to matter. Drills don't come in sizes that closely spaced and, besides, they don't drill on size, not to mention that the percentage engagement is going to be a guess at best.
calculated tap drill size = major dia.  (1.08253 x pitch x % full thread desired)
for a 15mm x 1.0mm tap and 75% thread:
15  (1.08253 x 1.0 x .75) = 14.188mm
For 60* unified threads (dimensions in inches):
calculated tap drill size = major dia. ( ( 1.08253 x percent full thread desired) / number of threads per inch)
for a 3/816 tap and 50% thread:
.375((1.08253 x .60) / 16 ) = .334 inch
select a drill nearest the calculated number....or not.
The nominal dia minus the thread pitch works out to about 75% but that will vary w trifle with the actual size of the drilled (bored? reamed?) hole. Remember a drilled hole may be 1% over the nominal dia of the drill. Consider also the 60 degree V internal thread standard crest has a 0.25p flat. This thread crest standard truncation is cranked into the nominal dia minus pitch rule.
Pick a system of calculation and stick with it. Mix them up and you'll either confuse yourself or screw up the job.
I once did deep research into calculating screw thread manufacturing data wehich lead to my writing a computer program in Fortran (THAT long ago) that took thread profile, measurement system, nominal diameter, pitch/TPI, class of fit and number of starts and it gave you about thirty data items from basic pitch diameter to manufacturing thread gage limits. With all that background I still use the old school rule where tap drill size equals nominal diameter minus thread pitch.
Keep it simple.
Last edited by Forrest Addy; 04282017, 05:17 PM.
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