Concentricity of Drill Rod in 3 Jaw Chuck

Have you physically checked the drill rod? I've seen some that had section profiles like you might see with DOM tubing, with a dip inward from the OD and a bulge outward to either side. That's to say looking at the end of the stock.

michael

Is the axis of the chuck aligned with the axis of the spindle? Are the chuck jaws clean and in good shape? Or could the rod be slightly bent?

Imagine a perfectly round, perfectly straight rod rotating around the axis of its length in free space so that if you were to run an indicator along its length it would give a zero reading end to end.

Now imagine that the rod has been tilted slightly off of its axis. At the point where the rod intersects its axis of rotation the indicator will give a zero reading, but as it moves away from that point the error reading will increase.

It could be that something is causing the (presumed straight) rod to not be aligned with the axis of the spindle. There could be dirt or burs in the chuck's jaws or maybe the same in between the chuck and it's mounting plate.

Diameter, yes.

What this spec is really saying is that at *any one point* on the rod, the rod should be round to within +/- 0.0005. In other words, the rod is pretty round. But the rod's diameter can vary as much as +/- 0.001 along its length, and not necessarily a linear taper either.

Why would you expect that? If the chuck jaws are barrelled, or if you just put the rod in slightly cocked, the error should increase the further out from the chuck you measure.

The error should be in direct proportion to the distance from the chuck, though. So doubling the distance should double the error (+/- the concentricity spec). You're seeing 6 times the error for a doubling of distance. So either you're making a mistake in how you're measuring it, or the rod is bent. Roll the rod on a surface plate to see if it is bent. If you can't see light under it at any point of its rotation, then take another look at your measurement technique.

Gary

Hi Steve,

Gary addressed the questions on the drill rod, but I think that you are expecting way too much from a three jaw chuck. They simply do not routinely run as closely as you'd like, especially if they've seen much service. If a three jaw has been stressed, especially tightly chucking pieces at the tips of the jaws, they're usually sprung to some degree. It's far more common to find them in that condition than not, especially on small chucks.

If you'd like to check how yours is, I strongly suggest you use a drill blank, not a piece of drill rod. There's a world of difference between the two, in spite of the fact that they could have possibly come from a common source. Drill blanks are heat treated and ground to size after heat treat. You'll find they are far more likely to be straight (and round) than drill rod stock is, and won't have any distorted sections of surface on which you might chuck, giving a false reading of the condition of your chuck. In essence, it's hard to detect a half thou error with something that has an inherent error of a thou. How do you know what you're actually measuring?

When you chuck up a length of something (drill blank, or long dowel pin is best) that is very straight, at first tighten your three jaw very loosely, enough for it to just barely grab the item, which should be inserted deep enough for it to reach the back of the jaws. At that point, take a look at the front of the jaws, where you'd expect the chuck to be making contact. You might use a feeler gage, or a piece of cellophane from a pack of cigarettes, which is only a thou in thickness. My money says they are floating, not touching. As you tighten the chuck tighter they may or may not make contact, but it's the rearward contact that is giving you the readings you describe. The pin is not being held straight, and is somewhat free to float about.

The solution is to grind the hard jaws after proper loading, or forget that and use soft jaws for critical turning, leaving the hard jaws for everyday use, where the small amount of deflection may or may not be a problem. On small chucks the sprung jaws will often give problems when taking hard cuts, though, so you'll have to make some decisions on how much you can live with.

Good luck, let us know what you learn about your chuck.

Harold

This does not answer the question but may be worth considering. If there is a truck engine repair shop in your area, drop by and see if they will give you a piston wrist pin from one of the big diesel engines. You'll have a larger test bar (1.5" O.D. or so and 4" long?) that was ground to some very high standards for round and parallel. And it will be hard enough that the odds of damaging it in use are very remote.

The guy I got mine from handed me a couple right out of the trash can. One showed some color changes from use but nothing registers on a dial indicator as you slide across the changes.

The ends of the pins are not finish ground. They are square but may not be flat (slightly cupped IIRC). I wanted to have one of these center drilled but never found a way to get a hole drilled in one.

I found I could stand one of mine on its end on my surface plate and use it for a vertical reference (sort of like a cylindrical square). Very handy to have around.

Big engine shops can have some interesting metal scrap laying around. Big chunks of aluminum (pistons), steel (flywheels), and what not. Most of the stuff has some quality to it and may be destined for the scrap yard.

Excellent suggestions, Jack.

Harold