precision hole drilling ?

[SVanderkolff]

I am noticing that the holes that my drill presses are making are all oversized. I have tried using different bits with no better results. I have the same problem on both presses. Is there something I can check or something I need to do maintenance on in order to stop this. By the way what causes this? Both presses have tapered shanks, if that is relavant.

Thanks
Steve

[RickNJ]

I have had the same experience with my drill press. Usually what I do is drill a little under and then use a reamer to get an exact fit.

[Jamey Saunders]

Steve,

If you have a serious need for precision, I'd suggest investing in a small milling machine. They have bearings designed for side loads. Most of the drill presses (even mill/drill combos) aren't really designed for side loads. As such, the quill usually has some play. Another problem may be the way the drill bits are sharpened. If the point of the bit is ground slightly off center, the hole will end up being larger than the bit. I believe that if you drill undersize and then drill to actual size that your hole will be closer to the correct size. The best way, however is to do as Rick suggested and use a reamer.

[DC KNIVES]

Steve,listen to what they are saying.I assemble precision machinery everyday and thats what our shop does.Drill undersize then ream.But also make sure that everything on your drill press is tightened down or your wasting your time.Dave

[Geno]

Center punch, drill pilot hole, then ream and countersink.
Drill about .005" under the ream size.
Don't ask the ream to cut more than .010", it is a finish cut.

[wrathlord]

Um yes to all of the above,presses will wobble after some time especially if your drilling steel,there are a few that wont,but they are huge and expensive.I use all cobalt bits,center punch,drill under,ream,I use diamond coated tapered ones used in the jewelery industry,they are costly but cut for along time and cut a variety of materials other than metals.It's tricky to get total precision from the home bench,I would drill it the best I could and then find the pins for the holes I can make, some manufacturers see measurements differently,some see a fat measure,some just barly make it so it's on the thin side,so I bet I'd find something somewhere to fit,if not and the holes were that bad I'd replace the drillpress.

[Frank Niro]

I believe the bearings in the quill are badly worn.My drill press has poor bearings that I Must replace very soon and yet with good quality drill bits it still does a good job. I do drill under size and then ream for folder pivot holes but I have done holes drilled for pivots with that size of bit for the pivot and the pivots have fit properly. I sure don't believe you need a milling machine to drill holes. A good drill press and that doesn't mean it has to be an expensive one will do a fine job for you . frank

[Geno]

Grab the chuck(while NOT running) and wiggle it to see if there is play in the bearings, then spin it by hand to see if the bearings run smooth and quiet.

If the press is bad, you won't get good results even using the best of bits.

I buy carbide reamers, and have a variety of sizes.
For 1/8" pins, I start with.125", if that doesn't fit, move up to ,1255", then .126", ect...
For 3/16" pins, start with 187", then .1875", then .188",,,

First check your spindle.

[mike koller]

Already mentioned but worth repeating I think.......measure pins and drill accordingly.

[Frank Niro]

By the way have you checked that the table is at 90 degress to the drill bit? An easy way to go on this this to take a piece of coat hanger or wire and bend it in the shape of a "Z". Place one leg into the spindle and tighten it up. Strech the other leg so it just makes contact with the table. Now turn the spinble by hand. If the "Z" either drags badly or rides high in places, the table is out of alignment. This will make oval holes for sure. Hope this helps to eliminate something. Frank

[navajas]

Steve, If you don,t feel and see any runout on your spindle, the jaws on your chuck may be sprung. Turn the jaws closed all the way enough to bring them together snug and see if all the jaws come down even, then turn on the drill press and look for runout on the jaws. A bad chuck is a very common thing. It can be sprung from over tightening. Hope this helps.
Roland