Carbide Drilling Tolerances
 

Okay, our team is considering using the Dewalt CIM transmission presented by Dr. Joe. http://www.chiefdelphi.com/media/papers/2738

We are having some amount of trouble contacting our EDM sponsor, and I would like to be able to machine the sun gear for the transmission at our school anyways. The diameter of the hole that needs to be machined is .3120, with a possible tolerance of .0005" smaller. The white paper suggests that if an EDM cannot be obtained, to use carbide tools. We are ready to buy, if its not too outrageous, but I can't find a suitable bit.

At http://www.mcmaster.com/, I searched under "solid carbide drills" and got a wide array of drills, however, none quite right. A 7.9 mm drill is .3110" in diameter, .001 to small. Half a thousandth is the tolerance. A 5/16 drill is .3125, or half a thousand bigger than tolerance.

What should I do? Should I buy the bigger drill and hope it still pressfits, or buy the smaller drill and hope that it'll over size correctly? Or something different? Will the bit even last long enough to justify the purchase? And, since the gear is hardened, if I can go with the carbide drill, what type? I was looking at the straight flute, as it was recomended for hardened metals.

Thanks in advance!

Re: Carbide Drilling Tolerances
 

Believe it or not a drill bit does not in fact drill a perfectly round hole anyway. The proper way to get a good round hole with a drill bit is to drill under size and then ream it. Without checking out specific odd drill bit sizes this might be an option.

Re: Carbide Drilling Tolerances
 

Ahh, thanks. Looking again, I can find an undersized reamer from Mcmaster that is .3115" Thats within tolerance, and considering the reamer might go a few thousands big, should be just about perfect.

Re: Carbide Drilling Tolerances
 

MSC has a bunch .3120 reamers of different types including carbide if you have the $$. Enco has some as well and as usual they have better prices but a smaller selection. Enco is part of MSC from what I have been told. I buy a lot of my stuff at home from Enco because I am on a budget. Good stuff at decent prices compared to MSC or McMaster when it comes to machine tool stuff. Their selection isn't as good though.

www.mscdirect.com
www.use-enco.com

Re: Carbide Drilling Tolerances
 

A reamer is definitely a good way to go. Just make sure you use a good lathe that is all in spec and a good chuck that is in spec as well. Mcmaster should have reamers in .0005 increments

Re: Carbide Drilling Tolerances
 

So, the question is, should I buy the .3120 reamer, and be right on tolerance(assuming no tool slop) or go with the .3115 reamer, which is still within tolerance, but would give me a bit of play as far as oversize goes. Does a typical reamer end up making a bigger hole, or a smaller hole due to tool wear?

Re: Carbide Drilling Tolerances
 

Hmm, good question. When you start talking about 1/2 one thousands of an inch you really need to take into account the tolerances of your equipment. I assume you have access to a lathe since you are talking reamers. How good is it? I would be inclined to think that your lathe will introduce at least a little bit of error. A lathe chuck with .0002 or .0003 runout is pretty good in my book. I would think that if you have a decent lathe you should go for the .3115 reamer and with a little added error on the alignment of your lathe and chuck you might be pretty close to spec.

Re: Carbide Drilling Tolerances
 

Thanks. Now, the cost difference between a carbide and a HSS reamer is pretty big -$15 vs. $45- so would a HSS reamer that was only correcting a few thousands be able to tackle a hardened steel? This will only be used a few times a year, so if we can keep it on the cheap, good, if we can't, theres no point in buying junk.

Re: Carbide Drilling Tolerances
 

How hard is this part supposed to be? I know people who machine hardened steels, but they all use carbide tools for everything. I don't think that HSS has the wear resistance to deal with hard metals. You might be able to get one or two holes out of a HSS reamer (if even that) in hard stuff, but I don't think that the low cost will be an advantage this time. If it was me, I think I would splurge on the carbide reamer, it should hopefully last long enough to be used again in the coming years.

Re: Carbide Drilling Tolerances
 

To ream a hardened object, Carbide is your only choice.
However, if you drill it under size, you can just lap the hole to fit. Use fine sandpaper on a dowel, by hand or with a drill.

But, I think more importantly you need to think more about the tolerances you're dealing with. 5 ten-thousandths is really tight, exceptional for most jobs. You're in the realm where the temperature of the workpiece is important, ten degrees warmer and the hole's too big.

If you're trying to get an interference fit, you can be off by a whole lot more than a thousandth, just chill the shaft and heat the gear, press them together and you're done.

There are tables of standard 'fits', how undersize you should make a hole for a certain typr of fit - loose, even, tight, very tight - look that up (or ask a lathe machinist) and machine to that.

Good luck,
Don

Re: Carbide Drilling Tolerances
 

I agree that only carbide tooling should be used.

I am guessing that you want to press fit a .3125" shaft into a hole in the sun gear. Generally you would be fine with a good thousandth of an inch under that, or a .3115" hole. I think if you get a solid carbide reamer that is .3115" and run it really slow through the hardened steel, you'll be fine. Remember also that reamers tend to chatter a lot if you're going fast, and when it chatters you kill the precision and damage the cutting edges. As long as the reamer is running cleanly and not chattering, it should give you really close tolerance, especially with carbide tooling.

Good luck.