Tap drill chart...with correct figures please !

Is there an accurate tap drill chart on the internet ? I have been comparing a few websites and many give different answers, especially when listing the metric sizes.

I am mainly interested in 1/8-40, 3/16-40 and 1/4-40.

Does anybody still make a steel rule with this information printed on the back ?

Hi I have been away from this hoby for a number of years, life getting in the way, however I remember the workshop mag used to send out Data sheets , I am sure this would have been covered . I think I still have some ( never had full set) I will look and see.

Wondering if these data sheets could be reproduced like the free drawings coming on the site, what do you think Neil

Partly in Dutch, the last colum is the drill diameter, first link at the end of the page brings you te Metricfine threads. Niko.

http://www.werktuigbouw.nl/calculators/metric-iso.htm

I'll e-mail you the one I have always used Brian but for now

1/8 x 40 tapping drill 2.4mm

3/16x40 tapping drill 3.9mm

1/4 x 40 tapping drill 5.5mm

If you only have fractional imperial drils then a good get you out of trouble on teh 40tpi series is to take 1/32" off the nominal dia.

Tracy and Myford(RDG) sell large charts

One of the things that make differences between charts is the % engagement. The suggested tapping drill may be for 60% or 80% ie be bigger than the absolute minimum given by the core diameter of the bolt.. This also may be changed depending on the material to make it easier though weaker and to allow for a certain amount of extrusion of the material into the gap thus left at the core of the tap.
Even then quality of fit will depend on whether the bolt was also made undersize.

So there isn't a specific 'correct' answer.

There have been a few threads on the chart topic over the year.

Brian

The following figures are from the "Model Engineer's Handbook" written by "Tubal Cain"

1/8" x 40 tpi - tapping drill 2.65mm (no 37)

3/16" x 40 tpi - tapping drill 4.2mm (no 19)

1/4" x 40 tpi - tapping drill 5.8mm (no 1)

From the text by the table those sizes are all based on 65% thread engagement.

For the 1/8" size the Zeus tables give - BSI recommended 2.55 mm (dec 0.1004", no 39),

BSI alternative 2.50mm (dec 0.0994", no 40).

The size that Jason gives above, especially the one for 1/8", sound a bit tight - heading for broken tap territory, mind you I gather that Jason has quite a bit of experience tapping holes.

Hi Brian,

The problem is that the drill size depends on what % thread depth you want. 60-75% is common.

For 40 tpi the full thread depth is 0.016" so for 100% thread depth your would use a drill twice that amount smaller. 75% thread engagement it would be 2*0.016*0.75 or 0.024" or 0.62mm.

For 32 tpi, a drill 1/64" smaller gives agood tapping size, but is only 50% engagement for 40tpi, even without the drill working slightly oversize.

Tubal Cain suggests the following metric sizes for 40 tpi: 1/8" 2.65mm, 3/16" 4.2mm, 1/4" 5.8mm.

In my experience this tends to be a little too big a hole and I always use one drill size up from his suggestions (I generally use number drills)

Jim,

There's merit in that, although the tables have appeared both as posters and tables in the mag several times over the years. I do have some tables that might suit, but they may need a fair bit of resetting.

Neil

The Workshop Practice Series "Drills, taps and Dies" (Tubal Cain) is usually quoted as the "Bible" in this respect for model engineers. Its reference tables cover a range of thread depths for just about any thread system you can think of.

On top of that the whole book is a darned good read (one of the few reference-type books that I read from cover-to-cover).

Well, that explains the discrepancies. I have settled on the figures that Jason has given as they seem to be the numbers that come up the most often when searching the internet.

Yes, I also noted the ''take off 1/32'' rule. I have both metric and imperial drill sets so that is the easy way to remember the correct size.

Something it is worth publishing every few years - we all need it, and it's especially useful to beginners who don't have the back issues.

I have an excellent tap drill/thread equivalents chart by an anonymous author that gives 75% and 50% engagement drill sizes.

The only reason I haven't published it, is that the pdf format is proving impossible to turn into a spreadsheet for resetting. I have asked the author for an editable version we can use in the magazine as a the next 'cut out and keep', but if he can't supply I'll put it up as the pdf.

Best of all, the royalties will go to REMAP.

Neil

I use the tapping drill sizes quoted by Tubal Cain in the Workshop Practice series book no 12: Drills, taps and Dies. I usually use 60% engagement. There are many threads debating how much engagement to use, but the consensus seems to be that 60% is plenty for most home shop uses.

John

I use the Tubal Cain sizes as well. With the smaller sizes though you do have to be careful and remember that a twist drill often cuts oversize on the first cut. So drill a tad smaller first and then follow up with the correct size.

Russell.