Keeping drilled holes aligned

[Mr Ron]

I will be using the mill with a spotting drill to locate the holes, then go back and drill the holes.
When scribing centerlines, how much is allowed for the thickness of the scriber? I'm guessing it would be around ±.003 depending on how sharp the scriber is. When scribing aluminum, I have been using a single edge razor blade which should be accurate to ± .001; getting a prick punch on the intersection is another matter. I can see the DRO machine layout is the only way to go; punching location being a waste of time.

[John Hasler]

Just use the layout for guidance so you don't get mixed up. Touch off from a reference and rely completely on your DRO for positioning. I only use a prick punch to make visual marks or as places to set dividers when I'm going to drill on the mill and need something better than +-.010" (and I don't even have DRO).


I usually prick-punch and then punch when using my drill press, but that's because it has no dials on the XY vise and has quite a bit of runout. The holes are more nearly vertical and it's often more convenient, but other than that it's not much better than a hand drill.

You can also guarantee hole alignment by clamping the parts together and drilling through...

[Lew Hartswick]

<If I use a carbide spot drill, will it stay on location, or will it "skate" on a slippery brass surface?>
It wont "skate" as far as the play in your spindle bearings .
.. .lew...