35/64 Twist Drill

I spent some time this afternoon sharpening some fractional drills that belonged to my late father. I think that they date back before WWII when the family company (now defunct) was involved in architectural ironwork

I was VERY surprised to find in the box TWO drills marked "35/64 in".

There were NOT multiple drills in any of the other sizes larger than 1/2 in

I have NOT been able to find any threads for which 35/64 is a tapping drill

Can anybody suggest any specific uses that a 35/64 drill might be used for, please?

MC

Just part of a standard drill set that went up in 64th increments. Possibly used as a loose clearance drill for half inch bolts, or to drill the hole before using a 9/16 reamer.

EDIT: Could have been a clearance hole for half inch hot rivets used in architectural ironwork in those days too.

Edited By Hopper on 06/02/2023 00:45:58

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

Thank you both for taking the time to respond.

There are no other 64th increment drills; others include 5/8 and 9/16

From discussions with my late father (before he died over 40 years ago), I believe they tapped holes on site for bought bolts rather than hot rivets. There are a lot of Whitworth taps up to 1"!

My Zeus Tables are the Metric edition but a tapping drill for 5/8 BSF seems very likely. I'll see if there are any 5/8 BSF taps in my inheritance

With best wishes and thanks again.

MC

.

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

Zeus:

My "Metric Revision" from 1995 gives the BSF data as well as some metric. What, I wonder, did they substitute for BSF? No doubt BSW and others went out as well.

There are other uses for drills than tapping hole sizes.

I have now been able to check and my largest BSF tap is 1"

I know that one can use drills for other than for making holes that will be tapped - but 35/64 is a size that I would NOT jump to use rather than say, 9/16.

I also inherited a lot of drills smaller than 1/2" and 13mm - it was fun trying to sort them out and I was fortunate in being able to buy metal Drill Indices for Metric, fractional letter and number sizes. I now have complete sets in 0.5mm up to 13mm. 0.1mm up to 6mm, 1/16 increments up to 1/2" and a weird assortment of letter and number sizes.

When I received all these drills, I tried to classify them but the shank of far to many had been mangled by use in a drill with a loose check. So I visited a Hardware shop and asked for a Driil Gauge. They must have seen me coming because the assistant sold me a gauge for Letter Drills - obviously "old stock".

MC

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

Might I suggest that maybe your father didn’t have a use for them - otherwise they may not have still been there?

I doubt there is much call for such small increment drills - unless buying a set of ascending drills in 0.1mm increments. I have a set of drills from tiny numbered drills right up to half an inch. Most of the set might still be here when I am gone, because they don’t get used very often.

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.

Look through your older books folks,and particularly US ones.
35/64" regularly comes up as the tapping size for 5/8" BSF, in books where 14mm isn't suggested.

As examples;
My early Machinery's handbook 6th edition suggests it, though my 20th edition says 14mm
Myford Series 7 Manual by Ian Bradley mentions both 35/64" & 14mm as alternatives, like my Zeus tables.
The Amateur's Lathe, by Sparey only mentions 35/64"

Metal Turning Lathes by E.T.Westbury interestingly shows different;
So does my Speeds & Feeds Pocket Book ~ Hayward-Tyler & Co. July 1942
They list the tapping sizes for 5/8" BSW as 35/64" and BSF as 9/16"

Bill

I don't know what sort of Ferrous metal was used pre-war so that could affect recommended tapping drills. I know the firm built fire escapes. Also Father seemed to be proud of the gates built for The People's Palace by the firm.

MC

Machine shops and toolrooms I worked in had a tool crib that kept all drills in increments of 1/64" all the way up sometimes to 2" diameter. Common practice in large industrial shops.

Common use would be for drilling holes to be reamed to the next size above. 1/64" was standard undersize to drill for a reamed hole, regardless of diameter, except for the really tiny stuff where you would use number and letter drills to get a bit closer before reaming.

I gained the impression that the work was done on site rather than pre-fabricated in a factory. In those circumstances, nobody would want to be loaded down with unnecessary drills or other tools.

I suggest that Fire escapes do NOT involve "precision engineering" and that reaming would be unnecessary.

MC

I too have a 35/64" drill. (Amongst many other drills)

I purchased it as I wanted 14mm holes and was offered the 35/64" drill a lot cheaper than 14mm. 35/64" is 0.1mm (0.004"  ) diameter undersize, and in the real world would effectively meet my spec. & do what I wanted. (Yea, yea, I know - I'm an animal!)

However, maybe the answer to your query is something equally prosaic.

Regards, Phil

Edited By Weary on 07/02/2023 08:41:43

Edited By Weary on 07/02/2023 08:44:52

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

Your comment appears to be directed at somebody else since I am no longer young. I retired from the university nearly 20 years ago.

However, I have a lot of tools that I inherited from my late Father and I derive a lot of please from restoring and using them.

During the early lockdown, I replaced all the slotted grub screws in his Die Stocks (some of which he appeared to have made himself) with socket screws, for example.

I'm very reluctant to buy a 19mm drill when I have one marked 3/4 in.

MC

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 .

You have to remember too that tapping drill sizes are only approximate. Charts specify the nearest common size to give approximately 85 per cent thread depth to make sure there is sufficient clearance for the bolt to screw into the tapped thread, so plenty of room to move either way to suit the nearest fraction 64th drill. Metric suggestions came about only in more recent years when metric drills became more common. Back in the mid-20th century and earlier in UK and the rest of the empire, metric drills would not have been commonly available. If you get stuck, especially in the larger sizes like 5/8" BSW etc you can often go one 64th bigger or smaller and still get away with it.

I have a fairly recent Tracy Tools tapping drill size chart taped to the wall next to my lathe. Even for BSF/W and UNF/C etc it gives the drill sizes in metric in 0.1mm increments. But notes at the bottom that the nearest fractional sized drill is acceptable. So it is not an exact science, despite the implications to the contrary of today's charts listing tapping drills to within 0.1mm.