Centre Drilling

Collated from newsgroup postings.

I have found drilling holes in the centre of a blank to be problematic at best. I find this happens drilling small and large holes. I encounter it especially when I use a drilled hole as a depth gauge on hollow forms. The bit wants to wander off the true centre, and as I get deeper into the blank, the problem magnifies. My last piece I found that the 3/8 hole I drilled was 3/8" off centre at 7" deep. I find this problem using my tailstock and when freehand drilling.

Anyone have a clue about what is happening?

Ordinary drills aren't the best at drilling a true hole in something like wood which has varying densities. .

There are a few things you can try to get better results. Start by drilling a small pilot hole say 1/16" dia and then a couple of intermediate steps before the final size if needed. Also always withdraw the drill bit to keep the flutes clear of chippings, don't try to drill to the full depth in one go. The chippings gather in the flutes preventing the drill from cutting its way properly, especially bad in resinous woods. Put a squirt of WD 40 on the drill or into the first part of the hole just to help things along. A forstner bit usually gives holes of greater accuracy than engineers twist bits, again make sure the chippings are cleared. If you've got a shell type auger for long hole boring that too is quite good for straight deep holes (make sure you withdraw to clear the chips, have I said this already?) .

On the point of withdrawing a long hole auger make sure you have the shell held so that it cradles the chippings in place otherwise as you withdraw it out through your tailstock it deposits chippings into the threads and gums up the works in your tailstock for you.

Finally you might want to try using your spindle gouge to just bore a hole to depth as this again usually runs pretty true so long as you get off to a concentric start with it. You'll know if you haven't as it will shake all over the place, don't try to continue thinking it will get better, it won't. Make a new start by taking a cut across the off centre bit to true it up and try again.

You are using sharp drills aren't you? .

Probably should've asked that first. If they're blunt they're a piece of cake to sharpen on your grinding wheel, they're also a piece of cake to have looking sharp and be totally blunt so if you haven't studied the required angles see if you can find a reference for drill sharpening. Once you know what is required of the drill it's not difficult to reproduce an acceptably sharp tip. There are also drill sharpening jigs available but you don't really need one if you're prepared to look into what is needed in the grinding of the drill and they take an age to set up).

One of the best solutions for this, is a machinists centre drill. It is a short drill point on a heavy drill shank or body. It won't drill very deep, but it will drill straight.

Is your tailstock aligned? If it isn't you'll get a hole that's close when you start but makes the hole larger toward the bottom.

A blunt drill will certainly run off, but there is another possible cause.

Does your lathe have a swivelling headstock, and if so, are the driving centre and tail centre points lined up? If not, this can lead to wrong-directionitis in the drill department. I haven't yet found a drill which is self correcting once it goes off the straight and narrow.

This 'points lining up' test is known as the kiss test, but it is not taken far enough, as it is possible to have the points touching, or 'kissing' and yet still have the Morse tapers themselves out of truth. I check this by using my long hole boring centres Basically I put a counterbore centre in the headstock Morse taper and a hollow centre in the tailstock Morse taper. It is this centre which the auger would pass through during the drilling operation when turning table lamps. I now bring the tailstock up to the headstock, clamp the tailstock onto the lathe bed and wind the handwheel forward until the counterbore pilot starts to go into the hole in the tail centre. About ten thou. of mis-allignment would not be a problem in woodturning circles, but an engineer might raise an eyebrow. If the pilot jams up in the bore as the tail centre is fed onto it, then there is a problem, and this could well lead to drills running off.

I found that back cleaning (turning the gauge upside down and cutting at 2 o'clock on the right side) works for me. Use a fingernail gouge, clean a slight indentation 1/4" deep and the gouge ploughs down the centre.

I don't do anything deeper than 4" or 5" with this technique.

I don't know if you are using twist drills.

Twist drills are notorious for the cutting edge lips not to be of equal length or proper angle to the long axis of the bit. If not the holes drilled will not be straight nor true to size.

Also the work piece, not just the headstock spindle, should be aligned square to the drill bit and the bit's shank should be straight.

Touching the very tip of the bit to the work should cut a dot not a tiny circle.

I found this a very frustrating problem until I discovered that no one seems to be able to produce a Jacobs chuck which is correctly lined up on its morse taper shank.

Check that your tailstock is perfectly aligned and then see if your Jacobs chuck holds the drill in correct alignment. It probably doesn't, so off you go to an engineering shop to have it tweaked!

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