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

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.

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

Seem to remember a series of drilling/tapping charts published in MEW a bit back. Can’t remember which issues though.

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 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.