Drill Bit Question

[mattd860 162]

My bandsaw has a main shaft size of 0.7882 inches. I need to drill out the center of a new pulley to match that dimension. I need the pulley to fit that shaft perfectly.

However - the only two drill bits that come close are 25/32" (or 0.78125") and 51/64" (or 0.796875"). From past experience, drilled holes can be slightly larger than the drill bit for various reasons so I'm leaning towards getting the 25/32" bit. I'm afraid if I get the 61/64" bit then the pulley won't sit snug to the shaft. But I'm also concerned that the hole from the 25/32" bit will come out tight and true which will prevent the pulley from even fitting on the shaft.

So what should I do?

[massey 118]

That comes out to 2.002028 cm. look for a metric drill bit or reamer.

[KC9KAS 4,741]

If it is that critical, I would take it to a machine shop and have it cut on a lathe.....perfectlly.

[SousaKerry 501]

Personally I would ream it to 20mm but I do have access to a full machine shop. but if you drill it to 25/32 it would probobly work however you may have to do some sanding on the shaft or the bore to get it to fit without too much force. .006" is a bit much for a press fit even heating the pulley would be tough and it would never come off.

[can whlvr 993]

if it were me i would drill it small and use my dremel to file it a bit more,i buy diamond burrs to sharpen chainsaws,just go slowly and you could get it

[Don1977 604]

20 mm is .78740 did you measure the shaft or the hole in the original pulley.

[sorekiwi 761]

My bandsaw has a main shaft size of 0.7882 inches. I need to drill out the center of a new pulley to match that dimension. I need the pulley to fit that shaft perfectly.

If you "need the pulley to fit the shaft perfectly" then you need to take the pulley and the shaft to a machineshop and have the pulley bored to fit the shaft perfectly.

[mattd860 162]

My bandsaw has a main shaft size of 0.7882 inches. I need to drill out the center of a new pulley to match that dimension. I need the pulley to fit that shaft perfectly.

If you "need the pulley to fit the shaft perfectly" then you need to take the pulley and the shaft to a machineshop and have the pulley bored to fit the shaft perfectly.

Am I going overboard by requiring the pulley to fit "perfectly"? The shaft is keyed and the pulley has TWO set screws..... so if the pulley bore was not perfect and slightly oversize - will I experience any negative results???

[sorekiwi 761]

Am I going overboard by requiring the pulley to fit "perfectly"? The shaft is keyed and the pulley has TWO set screws..... so if the pulley bore was not perfect and slightly oversize - will I experience any negative results???

I guess it depends on what your definition of "fits perfectly" is. I guess I was trying to say that you are very unlikely to get a properly sized, concentric and straight hole by drilling it with a drill bit.

I dont know exactly what this pulley is doing and I dont know what setup your bandsaw has, so I'm guessing here.

If you end up with a bore that is not concentric you may end up with a belt that gets tight in one spot and loose in another. Depending on how that belt is tensioned it may or may not be a big deal.

If the hole ends up a little big the pulley may run out of true when you tighten the set-screws. It may also have a tendency to come loose on the shaft, much the same as a wheel horse hub that is all flogged out will continue to come loose.

You might also end up with a hole that is a little crooked in the pulley, this will cause tension to change on the belt and the belt to rub unevenly on the side of the pulley.

All of these issues might be enough to induce vibration in the machine which will affect the quality of the cut.

My feeling is that a 20mm hole is a pretty large hole to be doing with a drill bit, its not going to be much fun. Its gonna be hard to drill slow enough and the thing is going to want to bite in as you do it.

If it were mine I'd get someone to bore it in the lathe.

[Paul D. 26]

I would take it to a machine shop (Or in my case to work with me) Even if you had the exact size drill bit you needed chances of you being able to keep it centered are slim and then things get out of whack . Even though I have the tools available to me at work to do it in a drill press I would still opt for the lathe as I know that I am centered then. If taking it to a machine shop is just not an option then I would go with the 25/32 cross my fingers and sand it out to fit. I would avoid sanding on the shaft though.

[mattd860 162]

I guess I should have mentioned before that the pulley already has a 3/4 inch hole. Does that make it easier to drill a larger/centered hole with a drill bit?

[Paul D. 26]

If you have a drill press that could lock things into place then maybe. If you do then start the drill in reverse to center it in the hole and then run it slow. But if you are planning on free handing it with a handheld drill I fear it will grab and screw itself into the hole or chatter. Either way there's a real chance of ruining your pulley. I'm not trying to be a downer, I've just ruined a couple things myself trying to take shortcuts.

[DoctorHfuhruhurr 137]

and don't forget a mouth guard

[Paul D. 26]

:text-yeahthat:

[KC9KAS 4,741]

How are you going to get the key-way back in to the pulley if you drill it???

[mattd860 162]

How are you going to get the key-way back in to the pulley if you drill it???

The pulley has two set screws so I'm not worried about the key-way if it doesn't fit.

[6wheeler 603]

I am with PaulD on this. Take it to a machine shop. In my experience, I cannot drill that straight. Even on my DP which does lock everything very tight. I have a very good friend of mine that is a machinist for Toro and has a home shop full of almost every type of machine that he uses at work. His DP is digital and extremely accurate and, He is what I call anal about measurements. When I bring stuff to him like this, it goes on the lathe or his bridgeport mill. I don't know how the machinists on here feel, but he always tells me " It came with a keyway for a reason, It WILL leave here with one!" If you want it to work and last as long as the old one did? Get it done right. Seems like we never have time or money to do it right. But, we always have time and money to do it AGAIN......I've learned that lesson the hard way. Good Luck Pat

[Iggy68 28]

The key is very important if it is on the drive pulley of the saw. The pulley will slip eventually and the set screws will end up ruining the shaft, then you have real problems. I would have it done at a shop to match original specs. You will be glad you did in the long run. Plus your blade will run more true. I know on my band saw it gets a little fussy at times when the band blade gets a little worn, couldn't imagine what it would be like with a not so perfect pulley. Good luck in whatever you decide to do!