Tap/Drill Chart

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Tap/Drill Chart

Post by DianneB » Sun Oct 07, 2018 7:18 am

Does anybody have a chart with tap, drill, and thread data that covers most commonly used model screw and pipe sizes? Where did you get it?

I have various charts hanging in the shop but still run into screws or pipes that aren't on my charts, some British and some oddball!

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Re: Tap/Drill Chart

Post by tornitore45 » Sun Oct 07, 2018 8:08 am


Not a chart but it may help.

The attached spreadsheet is my file. Inserted twice by error because this board gives no hints of whether an attachment operation worked or not.
(154 KiB) Downloaded 123 times
(154 KiB) Downloaded 69 times
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Re: Tap/Drill Chart

Post by Glenn Brooks » Sun Oct 07, 2018 10:58 am

Starrett makes a real nice tap and drill wall poster. Email them and they will mail one for free. Many others on the web have pirated Starretts work and sell them for $10.

Here’s one that might also be printable.

https://www.brokenbolt.com/images/starr ... -drill.pdf

And Scoutingweb.com has the same chart downloadable as a .pdf.

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Re: Tap/Drill Chart

Post by Pipescs » Sun Oct 07, 2018 1:02 pm

There are also a number of Apps you can download for your phone that address this.
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Re: Tap/Drill Chart

Post by Bill Shields » Sun Oct 07, 2018 7:05 pm

take the OD of the thread and subtract 1 pitch for the tap drill diameter

1/4-40 tap drill = .25 - 1/40
7/32 is close enough since I don't have a 1/40th of an inch drill

worked for me for 50+ years
Too many things going on to bother listing them.

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Re: Tap/Drill Chart

Post by ChipsAhoy » Sun Oct 07, 2018 8:12 pm

This is good stuff, thx for starting the thread!

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Re: Tap/Drill Chart

Post by mklotz » Mon Oct 08, 2018 8:56 am

Bill Shields wrote:
Sun Oct 07, 2018 7:05 pm
take the OD of the thread and subtract 1 pitch for the tap drill diameter

1/4-40 tap drill = .25 - 1/40
7/32 is close enough since I don't have a 1/40th of an inch drill

worked for me for 50+ years
Yes, the entire Starrett chart can be replaced by that one, easily remembered equation.

TD = MD - P

TD = tap drill diameter
MD = major diameter of thread
P = thread pitch

With metric thread nomenclature where drills are identified by their diameter, TD directly identifies the required drill. With the inferial system, one must translate TD into fractions/numbers/letters because of the idiotic nomenclature.

The formula as written will produce 75% depth-of-thread (DOT). If you want a different DOT, use this form of the equation...

TD = MD - (DOT/75) * P
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Re: Tap/Drill Chart

Post by Mr Ron » Mon Oct 08, 2018 9:19 am

Bill Shields wrote:
Sun Oct 07, 2018 7:05 pm
take the OD of the thread and subtract 1 pitch for the tap drill diameter

1/4-40 tap drill = .25 - 1/40
7/32 is close enough since I don't have a 1/40th of an inch drill

worked for me for 50+ years
Thanks Bill; What a great tip. At 84, I'm still finding new (old ideas). I checked it out with a few number size machine screws and it comes out pretty close.
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Re: Tap/Drill Chart

Post by dash9 » Mon Oct 08, 2018 9:34 am

As for drill bits, I always used MSC for them I always use Cobalt . I had a few that I needed and of course it always the weekend when you do. I tried Harbor freight for there cobalt set and I was pleasantly surprised that thy worked an all were the exact measured size. The set from MSC about 900.00 https://www.mscdirect.com/product/details/78582558
The one from Harbor freight 104.00 and with 20% off coupon 83 and change. https://www.harborfreight.com/power-too ... 61886.html
It cheaper to buy the whole set when you need 5 or more bits.

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Re: Tap/Drill Chart

Post by Pontiacguy1 » Tue Oct 09, 2018 8:54 am

Be aware that the drills from HF, or other cheap off-shore sets of drills, may not work like you expect them to. Their heat treatment and/or steel used to make the drills is often wildly different, and thus the results are too. I have some drills from HF that would work fine, like you would expect. I have some that will only drill 2 or 3 holes in a 3/8" thick mild steel plate before they got dull. One set I bought, once a drill wore through the supposed TiN coating, was pretty much useless. Spent more time sharpening them than drilling with them. Luckily, it was a small set and I didn't have much in it. Changing to another brand drill, using the same speeds and feeds, and I was able to drill dozens of holes in the same material without it getting dull. One of the reasons they are able to sell them so cheap is because they have almost no quality control on them.

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Re: Tap/Drill Chart

Post by steamingdon » Tue Oct 09, 2018 9:43 am

Pick up A Machinery Handbook ( Machinist H B) All of your questions will be answered. :D :D

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Re: Tap/Drill Chart

Post by Builder01 » Tue Oct 09, 2018 11:08 am

Hi Dianne,

I have a chart that I use for Model Engineer threads. These are threads that are fractional inch sizes that are 1/2" and smaller. The thread pitches are always 32 or 40 t.p.i. The chart is down loadable as a PDF.

(My cone union chart is there too.)

Here's the link to the page: https://supersimplex.yolasite.com/Charts.php


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