According to my Zeus tables, the recommended BSI tapping size for 5/8" BSF is
14mm (0.5512" ), with an alternative of 35/64" (0.5469" )

Bill

Edited By peak4 on 06/02/2023 01:39:12

Nice to have such useful family heirlooms. Much handier than overstuffed chairs and Wedgewood china.

Sounds like there is quite a history to them.

Edited By Hopper on 06/02/2023 12:08:56

There are other uses for drills than tapping hole sizes.

Mine are Metric version as well, but maybe an earlier one.
Since your query is about 35/64", this forum thread might be interesting
https://www.model-engineer.co.uk/forums/postings.asp?th=135532&p=1

Bill

IMHO it is for drilling holes ready for a 9/16" reamer, as has been said.

I sought out and purchased all the morse taper drills 1/64" less than the big, major inch increments, for just that purpose. And there were lots of them on second hand tool tables at shows so industry used to keep them.

Don't expect to find obvious logic in Imperial Drill sizes, because they either pre-date or are early examples of standardisation.

Imperial Letter and Number Series drills date back to when manufacturers and trades all had different proprietary Gauge sizes, creating a need for apparently odd sized drills. Gauge sizes seem to be have been determined by the needs of early wire making machines. These progressively swage wire down to create a series of useful diameters, and did so before micrometers existed. Practical rather than scientific technology, and haphazard because disorganised.

Industry has progressively moved away from Gauge systems towards simplified systems based on measurement since Whitworth, Whitney and part interchangeability. ¹⁄₆₄" drill sets were a step in that direction: a compromise that reduced the need to stock Letter and Number drills. Another example was precision machinists dumping fractions in favour of thou.

The Imperial system began as a complete muddle and many sacred cows were slaughtered getting to the much simplified system we know. Still far from rational though. May not matter much for basic practical work, but it annoys design engineers by complicating the maths. Scientists find Imperial positively harmful, because it's internal inconsistencies confuse natural relationships.

Go metric young man! Not perfect, but it meets practical, engineering and scientific needs with a single rational system. With metric there's no need to worry about Letter, Number and 64th drill-sets because drills are available in 0.1mm increments.

Dave

If your drills are pre war then you may find that some could be carbon steel rather than HSS easy check is when grinding them HSS gives off reddish colour sparks ,carbon drills give off white bright sparks. Carbon steel drills are not much use nowadays cutting speed is far too low and the temper easily lost,if I find any in a job lot they go in the bin.I have a full set of fraction drills from 3/64 to 1 inch theres nothing odd or unusual about them, the larger sizes are very useful for roughing out holes when turning,and as every one now has gone to metric a lot of my drills paricularly the larger ones have been gifts,or bargains found at auto jumbles/tool dealers and many were new and if I get multiples i can have one of them permantly backed off for brass and cast iron, 3 very common "odd"sizes found are 47/64 , 53/64 and 61/64 which were commonly used for tapping drills for British standard pipe threads.Some other good buys have been long series drills ,they are very useful .

Zeus charts often give the "ideal" tapping size, but also list the likely nearst stock size, whether Numer or Letter .

For many purposes, a slightly larger tapping drill, although it reduces % engagement will not prevent a safe clamp load being applied.

Most of us are not into jobs that need fasteners to be tightened into yield, anymore.

Howard

Edited By Howard Lewis on 07/02/2023 11:50:15

you are all just over thinking it. that drill is just right for drilling a 35/64th holes