Center drill body diameter, old vs. new

[Andy FitzGibbon]

Does anyone know when center drill body diameters went to normal sixteenth sizing, as opposed to the oddball 64th and tenth sizing of the older ones? Or why they did such oddball sizes in the first place?

Years ago, a fellow here was selling the remaining NOS inventory of Huron Machine & Tool of Yale, MI (I seem to recall it was his father's business). I bought this set of center drill arbors, and they are the letter sizes of D (15/64"), E (.302”), and F (7/16”). I'm not sure when they were made... maybe 50s, judging by the included brochure.



Here are pages from ca. 1920 Morse and Cleveland catalogs. Cleveland used the same letter system and sizing as the Huron arbors, while Morse used numbers and had some slightly different sizing.




Just a curiosity I'd never thought about before. Thanks for any information.

 

[L Vanice]

There are people who use the term Slocomb drill for combination countersinks. I think that name indicates that J. T. Slocomb invented the drill and I expect he was the one who selected the sizes. He was an interesting man and there is much more info here:

John Tibbets Slocomb

I am sure a name that most machinists are familiar with, but just in case a few of you might not have ever heard his name before, this is his story. For the most part I thought I found his date of birth in New Brunswick, Canada as most of the research points to July 21,1864; that is till I had...

www.garagejournal.com

The Catalog 18, p. 64-65 says they started "perfecting" the combination countersink in 1891.

https://ia800704.us.archive.org/12/items/JTSlocombCoCatalogNo18/JT%20Slocomb%20Co%20Catalog%20No%2018.pdf

As for the shank sizes, it may remain a mystery, but odd sizes in various old tools are certainly not at all uncommon. I do recall that I have a number of long, like 6 inch, combination countersinks with 3/8" shanks, so that was a stock size back around 1975 when I first got them. A few decades ago, I ran across a few 2MT or 3MT combination countersink drivers and tried them briefly. I discovered they had slip fits for the drill shanks and did the driving with an internal feature that caught on the 60 degree cutting edges. I soon found that the drivers chipped that cutting edge, making one end of the drills useless, so I stopped using them.

Larry

 

[Andy FitzGibbon]

Great information Larry, thank you for posting that. These Huron center drill arbors have a set screw to retain the drill... can't imagine who though that driving from the cutting edge, as yours did, was a good idea.

I used to have a Nikken live center that had a quick change "cap" that fit over the center point and held a center drill in a small collet. It worked well, but required the user to cut his center drills in half.