Drill bit holder - Puzzle

[Doug Hobkirk]

This is a mystery to me and I obviously have too much time on my hands. I have spent far more time than is rational. But maybe you also enjoy a mystery.

This drill bit "block" came from a garage clean-out. The block is a piece of hardwood with a metal top, copper I think. I expected it to have a set of holes for number bits and another set for fractional sized bits. But when I examined/tested it, I found there were multiple holes for certain sizes and that not all sizes were there, and they were not entirely positioned by ascending size. Starting at the top row, there are:

• 3 holes at 7/32
• 3 for 13/64
• 6 for 3/16
• 5 for 11/64
• 5 for 5/32
• 6 for 9/64
• 3 for 1/8
• and so on. The above is for the first two rows, and the next 3 rows get VERY small

The final 3 rows are not in sequence. Note the end holes on the bottom 3 rows - the first few holes in the next row are smaller.

There is a sequence of numbers below the bottom row that do not seem to gel with the holes immediately above them - the first two "labels" are "4 32" and "6 32", and both holes are 9/64". The labels continue "8 32", "10 32", "10 24", "12 24", "14 24", and then "14 20" (the 1 is above the 4).

[Eric DeSilva]

I'm assuming you recognize the designations--4/32 on up--refer to bolt sizes, with the number after the slash being the thread count?

[Rod Sheridan]

Doug, those holes marked 6-32 etc. were probably for number and letter drills used to drill holes for tapping for 6-32 machine screws etc.

(The first number indicates screw size, the second is threads per inch).

Regards, Rod.

[daniel lane]

Well, aside from the 4-32, the rest of the labels are standard NC thread sizes. Is it possible that the block held the taps and maybe pilot hole drill bits? There should be an easy to find tap/drill chart somewhere on the interwebs, that may help.


daniel

[Lee Schierer]

I suspect the block was made by a machinist and was made to hold taps and a set of graduated size drill bits. Rather than drill all the holes one bit size larger, the machinist cheated and used a larger bit for several holes.

[Tom Slupek]

Tap Drill, Clearance Drill and Tap hole for each screw size. The block could also been designed to drill close and loose clearance holes for the screw sizes. I have see many of these type of holders over the years.

[Scott T Smith]

Tap Drill, Clearance Drill and Tap hole for each screw size. The block could also been designed to drill close and loose clearance holes for the screw sizes. I have see many of these type of holders over the years.

+1. Not uncommon in some older machine shops.

[Bruce Page]

It looks like a homemade version of a tap & drill index although I've never heard of a 4-32 I looked in my Machinery Handbook and no such creature exists.

Here's a pic of mine.

[Paul Atkins]

It's a decoder block for finding the Lost City of Wood.