Drilling Holes On-Center

[JohnLucas]

I drill some holes then cut pockets for the screw heads and the holes and the pockets are not concentric. Visibly out by about 0.02" on the X axis. Has to be something really dumb and ignorant but can't see what.

Checked backlash and that headstock is running perpendicular to table. I used an edge finder (mounted in 3/8 collet) to zero X to center of a pocket. Next mounter drill chuck with a piece of drill rod - when lowered into the pocket and it is not centered. Checked runout on drill rod. Also rotated spindle by hand and it runs true (within a couple of thou) - just in the wrong place. Seems that anything in a collet is centered but anything in the drill chuck is offset. What I cannot understand is how an alignment problem can run true but offset yet not wobble.

What am I missing!!! Thanks.

[Caprirs]

Sounds like the head is possibly out of tram? In other words, the spindle is not perpendicular with the table. Spindle axis is not parallel to Z axis travel.

Put an indicator in the spindle and sweep the table.

[JohnLucas]

I had already done that - but then moved the head on the dovetail ways to accomodate the length of the drill. So, the head was moving perpendicular to the table but with the tool at an angle.

Thank you very much for the prompt and helpful reply.

[rowbare]

If you are using standard jobber length drills, they can walk a little bit. They are also long enough and flexible enough that they will drill a shallow hole off-centre.

You should be spot drilling first to get your hole started properly. I have a 3/8" one that I mount in the ER collet. This talks a bit about spot drills: Spotting Drill, Centering Drill for Pre Dilling holes on many materials

While most recommend against it, you can get away with using a centre drill for making the pilot holes. The problem with centre drills is that the little tip can break off and cause grief. However they are usually easier to find than spot drills and can do the job till you get the right thing.

Depending on the hole sizes and the size of the end mill you are using, you could also cut your pocket first then plunge the end mill a 10 or 20 thou into the middle to help start the drill right. This isn't a proper substitute for a good spot drill but might be enough to get you going.

As a last resort, you could try mounting the drill directly in an ER collet. That might eliminate enough wobble to get you close.

bob

[JohnLucas]

The head was out of alignment. I was testing by sweeping an indicatior across the table but not checking by running the indicator up and down a true vertical. Just needed to slack the 4 srews and tilt the dovetail support for the head. If anyone else does this take off the motor so less weight. Well, I've learned something so that is great.

Yes, I do use a spot drill and agree that best in a collet and not the chuck. The holes are quite deep so need regular drills but I think jobber drills will go on the Christmas list.

Thanks everyone.