Erik,
right tomorrow morning I'll copy another table (fine threads) for you, 
for today I have had sufficient wine for not attempting (I found a 30 
year old bottle this afternoon in our rarely used basement room).

In case you need the size still tonight: the formula: diameter minus 
pitch is always correct for metric threads, also for fine threads as M12 
x 1,5, as long as they are 60 degree, als Andy stated. I admit that they 
get easier to cut and breakage is less likely as the prefabricated bore 
is larger, with the limitation of bore diameter equal to outer thread 
diameter.

Good night
Peter

Am 25.01.2016 16:07, schrieb Erik Christiansen:
> On 25.01.16 14:28, Peter Blodow wrote:
>> I have run a machine shop with up to twenty workers for 36 years and I
>> never heard about percentage of engagement etc.
> Peter,
>
> On the 10.2mm hole for M12x1.75, given in the table you posted
> yesterday, engagement is 84% - very strong, but hard on taps.
>
> Industry standards are necessary, and we rely on bought bolts
> conforming to them when we tweak % engagement in the tapped hole to
> receive them.
>
> Please have a look at: https://en.wikipedia.org/wiki/Screw_thread
> under the heading "Thread depth". You will discover that nefarious
> tapping practices are institutionalised beyond anyone's ability to
> eradicate. ;-)
>
> The text at the OSG calculator, cited upthread:
> http://www.osgtool.com/Technical.asp?tid=1&id=1
> is also relevant. To wit:
>
> »
> To minimize tapping problems and lengthen tool life, use the largest
> drill possible to produce a minor diameter that will result in the
> lowest percentage of full thread consistent with adequate strength. A
> minor diameter that provides a 55% to 65% thread is sufficient for most
> requirements, but in some cases a higher percentage of thread may be
> necessary to conform with the minor diameter limits of the thread class
> specified.
> «
>
> Seriously, once we buy an ISO bolt, we can use it as circumstances
> permit. In industry, you just need an engineer to sign off on that.
>
> Would you like to post the drill size your tables specify for M12x1.5,
> and we'll check the engagement on that? (The table from yesterday
> included only M12x1.75, so does not help Tom, or allow comparison with
> our preferred hole size. )
>
> Erik
>
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