The ruler trick would be the easiest.

Don't punch the mark. Put the work in your vice slightly proud of the top and a centre or spotting drill in your drill chuck. Balance a 6" rule on the work and without the motor running bring the drill bit down so it pinches the rule which will likely tip one way or the other, move the vice until the rule is sitting level and clamp the vice to the mill table when it is.

Now you can centre/spot drill followed by your tapping drill and finally the tap which can be guided in a loosly tightened chuck to keep it true to teh hole.

Easier done than said

For a quick and dirty centre within a couple of thou I sometimes use the edge of a 12" file to file a small flat say 2mm wide on the top of the work mounted in the vice. The file must be kept parallel to the table or you will be off, the longer file makes it easier to judge. Once marked I centre punch in the centre of the flat and drill.

I guess if the file was mounted in a jig that keeps it parallel it would be more accurate

You can also mount an endmill in the chuck and mill the small flat with that by just skimming the top surface of the work as with the file. If the end mill is smaller than your drill there will be no tell tale mark after the hole is drilled.

I know the above methods will raise a few eyebrows among the purists, they do get the job done quickly… it depends on the job you are doing and knowing the limits you need to work to.

Regards
John

Another quick way if the diameter of the item you are drilling is small enough to fit into the drill chuck is to mount up a piece of it in the chuck and then clamp the vise onto the other end of it then clamp the vise down to the table . The centre of the spindle is now on the centre line of the piece you want to drill, if your vise has those vee grooves (usually one horizontal and one vertical) in the fixed jaw and you are going to use it to hold the job make sure you use the vertical one to do the set up otherwise the piece will end up being indexed to one side by the same depth of the vee groove , if the vise only has the grooves in the moving jaw then it won't matter . I have seen somewhere a tool that goes in the chuck and has a vee piece in the end that is meant to locate on the piece of bar you are drilling but it would be useless for very small diameters as the vee had a radius in the guts of it .

Ian

Andy, I use a V block, and to centre it I use a rod that fits in the drill chuck with a face about 1" diameter, this pressed down in the V then the V block is squared to the table(vertical mill).

Ian S C

Yes Chris…unless the part isn't parallel over the holding section

Apart from what has already been suggested.

In the Mill/Drill, I use one of the centering devices, (a 90 degree V, with a small pointer, pivoted on a spindle held in a drill chuck) followed by a Centre Driil.

Also, I have the redundant centre square from a scrap combination set, which has been drilled/reamed and carrying a hardened silver steel centre punch, which is used to dot the centre of the round bar.

If you are dubious about using a centre drill straight away on the round surface, after centering, mill a small flat, and then centre the work piece, as above.

If all else fails MEASURE, either using cigarette papers, held on with oil/grease on each side with a drill/end mill/slot drill ; or use a wobbler or edge finder, or zero a clock on the fixed vice jaw, and move the table half the diameter of the bar.

DON'T forget to take out/allow for, the backlash when noting the scale readings and to allow for the diameter of the cutter/wobbler/edgefinder probe!

Howard