Practical? Question regarding drill chuck for knee mill

Hi folks,
I have a 2hp Bridgeport knee mill and have two drill chucks at present. One Chuck is a keyless with up to 3/8” capacity mounted on a 1/2” arbor and the other is a Jacobs 16N for up to 5/8” mounted to an R8 arbor. Often times I mount the big chuck to do smaller jobs like power taping M8 or M6 holes. Using the keyless Chuck isn’t great as it won’t always reverse drive the taps.

I also often use stub length drills around 12mm and have to crank the knee to get my R8 mounted chuck in the spindle. I’m lazy... I acquired a nice used 14N chuck with a 3JT taper and was thinking of mounting it to a straight shank adapter to lessen the time needed changing setup.

Is a 1/2” arbor enough or should I go up to 3/4”? 3/4” seems more better but is also longer. I appreciate any input.
Rick
Asheville NC

Nothing says you have to leave the 3/4" arbor longer, but there is not much need most times for a 3/4" arbor on a 1/2" chuck. If you're using a large drill with reduced shank you may want to go through the work to put in your R8 arbor, but for most work in that chuck 1/2" shank is fine.

I have a 1/2" keyless chuck on a 1/2" arbor for the same convenience you're after. I can quickly change out to 1/2" indicators holders, end mills, edge finders, etc.

That makes sense. The mass of the chuck it’s self looks “funny” on the 1/2” arbor but my 1/2” collet is the most used so that’s good. I’ll cut the arbor down to not give any “excess” engagement.
Thank you for your reply!

FWIW I have a chuck , drills , taps and end mills mounted in 3/4" (19mm ) arbours . I leave the same collet in place for most work , only swapping out if I need the extra accuracy or capacity of a specific collet.

My boring and facing head has a precision fitted arbour and I've a few large hss cutters hard-turned down to 3/4

I pinched the idea from DanGelbart from youtube and it really speed up the work I do.

On most of my drill chucks the arbor is the same size as the maximum capacity of the chuck. i.e. a 1/2" capacity chuck has a 1/2" arbor, a 5/8 capacity chuck has a 5/8" arbor, and a 3/4" capacity chuck has a 3/4" arbor.

I had a short pipe wrench and replaced the std jaws with aluminum pads, could snug my Albrecht chuck say 15% or so tighter than hand tight, would tap 1/2-13 in mild steel, reverse out nicely, never hurt the chuck a bit. jmho, ymmv

A twist drill bit has flutes that eat away from the cross sectional area of the bit. So a 3/4" bit may only have the cross sectional area of the 1/2" shank anyway. OK, a larger shank will have more meat at a larger diameter, but just how much does it need. And the fluted part of the drill bit will be weakest across the openings.

It is just a seat-of-the-pants estimate, but I suspect that the 1/2" shank is just as strong as a 3/4" bit and perhaps even a 1" one.

Anyone have an old 1" bit with a 1/2" shank that they are willing to test to failure? I will bet even money on the fluted part breaking first. Or has anyone ever broken a S&D 1" bit? Where did it break?

projectnut said:

On most of my drill chucks the arbor is the same size as the maximum capacity of the chuck. i.e. a 1/2" capacity chuck has a 1/2" arbor, a 5/8 capacity chuck has a 5/8" arbor, and a 3/4" capacity chuck has a 3/4" arbor.

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