As is often the case with data in Machinery’s Handbook, the specific information one is trying to find is difficult to locate. After many minutes of scanning I found the tolerances of a class 6H internal metric thread M12 1.5 thread: Minor Diameter - Min:10.376 Max:10.676. So, this says that as long as the Minor Dia is no bigger than 10.676 I am in spec (of 6H class)…And it says… In the section on tap drill sizes for metric coarse threads:
"When relatively soft material is being tapped, there is a tendency for the metal to be squeezed down towards the root of the tap thread, and in such instances, the minor diame- ter of the tapped hole may become smaller than the diameter of the drill employed. Users may wish to choose different tapping drill sizes to overcome this problem or for special purposes, and reference can be made to the pages mentioned above to obtain the minor diameter limits for internal pitch series threads." That says to me that if I use a 10.7mm drill bit as Erik has suggested that when finished my (soft) brass may indeed be within 6H spec (10.676) after all. But that 10.7 will give me some relief on the required torque. I couldn’t find a table for anything other than metric coarse and MH appears to punt: "Reference should be made to this standard BS 1157:1975 (1998) for recommended tap- ping hole sizes for other types of British Standard screw threads and pipe threads.” -Tom > On Jan 25, 2016, at 6:34 PM, John Figie <zephyr9...@gmail.com> wrote: > > Why not reference the Machineries Handbook? > > https://www.google.com/search?q=diyhpl.us+%E2%80%BA+papers2+%E2%80%BA+27_Thread_09A&oq=diyhpl.us+%E2%80%BA+papers2+%E2%80%BA+27_Thread_09A&aqs=chrome..69i57.1787j0j4&client=ms-android-att-us&sourceid=chrome-mobile&ie=UTF-8 > On Jan 25, 2016 4:33 PM, "Peter Blodow" <p.blo...@dreki.de> wrote: > >> 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 >>> >>> >> ------------------------------------------------------------------------------ >>> Site24x7 APM Insight: Get Deep Visibility into Application Performance >>> APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month >>> Monitor end-to-end web transactions and take corrective actions now >>> Troubleshoot faster and improve end-user experience. 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