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Off topic-sort of... Metric system users.

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I have a question for the metric system users.

I am working on a new fZ set of scripts for drilling and tapping holes (Machinist type work.)

The base script will build a mockup of the bolt (for now, I am only doing socket cap head screws, but the script can be built to do others in the future)... No biggie here.  But, I am also going to have the script cut the associated holes in the intersecting materials.  These different holes will be, drill size for tapping, close fit, free fit and counter sink.   All determined by the placement of the screw/bolt.  I am not using the built in screw/bolt generator as I don't need a visual of the threads. I need the information for the hole sizes to cut.

While I have not figured out how to go about doing everything, I am starting with building look-up tables for the sizes.I am tired of having to lookup in a table for the drill sizes, build the boolean tool based on what cut, on and on.  for each and every bolt/screw.

I have everything charted for the imperial system including metric equivalents.  However, I want to make it work for native Metric system users as well.  While as far as I am concerned, the metric system makes far better sense, I am having problems finding index drill bits for tapping metric sized threads.   I can easily find sets that are in .1mm increments.  But I need them in .05 increments or even better for drilling and tapping sizes.     I would even like to get to .025mm increments for machinist level tolerances.  As this is where I have the imperial information lookup tables built.

When I search for such indexes, again, I find .1mm increments.  Of course, since I am searching from within the U.S.  I think my searches are getting generalized.  I am having a difficult time believing metric system machinists track back and use imperial tooling. Especially since the metric system makes more sense to start.

Eventually, I will have the script do the dimensioning and callouts (when permitted by the API.)

As always, I will share my scripts.

Thank you,


¢hris £und



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Thanks everybody!

I think I should clarify a bit more.  I have those tap charts, in fact a lot of my information that I built my look up table came from the Little Machines chart (Santas second submission.)

I probably am getting a bit pedantic here, but I would like my script to eventually be useful for general machining not just bolt/screw sizes.  And I guess I am getting tripped up by differences in precision between the two systems. And, I was hoping to build all of the tables ahead of time so I can just focus on adding features to the code later.

So, to clarify, if you look at the Imperial portion of that chart.  for example: A Number 4-48 uses a drill size for tapping of #42.  and the next size larger bolt of 5-44 uses a #37. (I chose the two fine pitch bolts.) There are 4 whole drill sizes between the two bolt sizes.  (38,39,40,41.)    Most bolt size in imperial has a number of drill bits in between, like the aforementioned. Of course, being imperial, there is no consistency (in that regard, it is a complete mess.) Take a look at the full chart below.  The green highlighted columns are the bolt sizes. The drill bit sizes often (though not consistently) have a step size of .001".  (#s 71,72,73 for example).  or  .0254mm (not shown in this chart)


So, what I am looking for is a "Full" chart of metric drill sizes that has the in-between drill sizes, not just the bolt sized drills (tapping sizes) used in machining.  Like the imperial chart above.

The best metric set I can find* is in .1 mm steps.  or nearly 4 times that the imperial system has. (.003937") But, then again, stepping up by .1mm is pretty crude by machining concerns.   and Stepping by .1 mm doesn't fill the needs for tapping all sizes of metric screw/bolts.  as many of these are in .x5mm values.  Considering the official definition of the inch is actually by metric value (25.4mm). I would assume that the drill sizes of the metric system would be superior. (finer)

I suppose too that all holes could be bored to size. But that is an extra step in the process. A step that has a lot more setup involved.

Thanks again!

¢hris -bit pedantic- £und




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6 hours ago, ¢hris £und said:

I suppose too that all holes could be bored to size. But that is an extra step in the process. A step that has a lot more setup involved.

I think that is your problem! The in-between drills are almost certainly custom made (proprietary) and therefore without published information on the internet. Most of what I find online is standards based, ISO in case of the metric system. I did find, however, some good information on ISO 724 including coarser/finer threads and associated tap sizes at: 


I also found "the Bible", a useful PDF catalogue on everything drill related, which is too large to post here, but the following link will take you there:


Boy! I'm all tapped out (get it?) after all that. I reckon that for an architect I will make a great grease monkey when I grow up!

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I think that is your problem! The in-between drills are almost certainly custom made (proprietary) and therefore without published information on the internet. 

I find that surprising.  Even here state-side where "everybody" fancies themselves as a self reliant cowboy🤪, we have standardized tooling for machining quality precision.  It is a bit of a mess of having 3 notations for gauge sizes (naughts, numbers, letters) and then intermixed fractional sizes.  Still, the precision is there. 

At best, a machine shop would get locked into purchasing all their tooling from 1 company, or possibly a consortium that maintains a proprietary tooling set.  And still, this would have to be published somewhere so that shops would know what to buy.   At worst, each and every machine shop would custom build all their drill bits and gobs and gobs of other tooling.   ???

Think of this, clearance fit, interference fit, transition fits and so much more, each kind of fit would have their specified sizes. On top of drill bits, all the Reaming tooling would have to be custom made too.  Below you will see my 2 sets of reamers.  And these only cover the fractional set.  The large set is from 1/16" through ½" stepped by 1/64". The smaller set is the over and under by 1 thousandth".  And only cover 7 of the fractional sizes.  They also make these reamers in the gauge sets as well as over and under sizes.  I don't own the gauge sets because I do not have enough money.  If each machine shop custom built their own, or even if there were conglomerate level standardizations they would still be so damn costly, I am having a hard time imagining it.  Despite that being an argument from incredulity.   A good gauge drill bit set costs $600 and these are mass produced.  (bottom picture, with a few missing from my set.)

Santa, thank you for those two links.  The first does help me fill in a bit more data and the second will definitely help fill some knowledge gaps.  Tapped out... Yea,

If you or anybody are interested, here is are a great couple of videos about the history of precision.  We can essentially thank the leap of human progress in the last 400 or so years to this one concept.  Personally, I find it fascinating.

I guess for now, I will just build the script to the best level I can.  Cheers,





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That is a bolt alright!  My guess is it is a prop of some sorts.   Imagine the tool it would take to turn something this. But a quick search pulled up this:  Also looks like props.  Straps look a little puny.

 I don't think I will use these in my script.   Though, all you would have to do is add it to the table data and it should work.



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