What milling machine are you using and how old is it?

This is purely theoretical as I'm a youngun but did the drill "rub" more at full quill extension (quill or machine column misaligned slightly? – I'm just stabbing in the dark here)

– you said the stock was squared, can you quantify it somehow (surface plate and precision angle – I don't know how its done?)

Len Mason in his book "Using the Small Lathe" recommends drilling from both ends. At least then the two ends are correct, and I suppose if one is slightly out in the middle, the second drill might help to straighten it up by removing/reducing the step – even if you can't see it.

Peter G. Shaw

I would say drilling a hole that small through 80mm of commercial mild steel and only running out by 0.020'' pretty good going, if you wanted a completely parallel small diameter hole through that thickness of material you should consider spark erosion. Which reminds me of the yarn an ex Rolls Royce machinist told me. Apparently in the early days spark erosion a Japanese engineering company sent Rolls a thick chunk of steel with a minute diameter hole, completely parallel, right through it as a demonstration of Japanese engineering prowess. The story goes that Rolls Royce sent it back tapped!

I heard that story slightly differently, a 0.001" hole which was sent back with a tube inside it.

Bob

Two thing strike me, when starting the holes did the drill (s) grab?. Did you peck at the holes? , clearing the swarf every 2 seconds or so.

It would have been very instructive to have a look at how the holes progressed. Pity its very difficult to replace the workpiece in exactly the same place.

One thing that happens if the centre of the drill is not exactly on line with its centre of rotation, i.e. it rode of to the side into the centre pop mark, the drill will drill but will be "bent" and exerting more pressure will bend it more, so the hole would then tend to curve away from its nominal centre. So one interesting thing to do is to see if the hole is curved.

Most likely its the rough old piece of metal with soft/hard spots that pulled the drill of axis.

Frank

Returning to pgk pgk's question:

I would consider 0,5mm error across 80mm pretty good. The first drill was 4,5mm which gives a depth to bore ratio of close to 18:1. In my eyes anything over 10:1 is in the realms of gun barrel boring and should be treated with respect and care.

A drill is rather flexible, the flutes mean that there is a lot of missing metal from a hypothetical circular rod. If one cutting face is doing most of the work,either by poor sharpening of a bit of swarf, the drill will bend towards that face. Once the drill has deviated from its intended path it will continue on its own sweet way and nothing will bring it back on course. Put a larger drill down the hole and all it will do is to follow the hole. To prevent this happening you need a rigid drilling set-up (drill, milling machine or lathe), the best quality drills one can afford and clear the hole of swarf very frequently particularly if the hole is deep. If you see the drill wobbling as it drills you have lost control of the hole. In some materials, particularly plastics, drills can wander alarmingly.

To correct a wandering hole, a D bit might work (at least it is stiff) but it will need frequent clearing of swarf. However, if possible, start boring the hole out. Recently I tried to drill a 5/8" diameter holes through 4" of gun metal casting. The ends had been faced and I drilled from both ends starting, after using a centre drill, with something like a 3/16" drill. By the time I had used a 3/8" drill one hole had wandered by an 1/8". A potential disaster was averted by using a small boring tool. It took time and there was still a blemish on the bore.

Obviously there are ways of producing holes without these problems – casting, EDM and laser drilling come to mind – but these are generally outside our "capabilities".

JA

I use a 7mm diameter pilot point drill for this sort of thing. It has a small split point extension about 3 mm dia on the centre of a square ended 7mm drill rather like a slot drill.

Dewalt Pilot Point Drill Bits – a ToolGuyd Favorite

The pilot point locates the centre but the whole drill is much more rigid. I suspect that if one follows the approach of using a succession of drills any wander on the smallest won't be corrected by the larger ones. These pilot point drills used to be sold by Black and Decker, I got a set branded ELU, and bought a miscellaneous selection of new news on eBay. I think they are excellent. Another approach could be to drill the hole with a 7mm slot drill from the start as deep as possible then finish with a long 7mm ordinary drill.

As far as I can understand, badly ground or blunt drill, might (will) drill an oversized hole but because the "error", say one long cutting edge , is rotating, there is nothing to make it zoom off to one side.

I feel its the very first cut that starts the cone of the cut that is critical. when you put a drill through a pilot hole, one cutting edge of the drill must cut first, even if its due to vibration of the chuck. So you have started to develop the cutting cone but its .0001 off to the side. So as you pile on the feed, the drill as it unsupported will bend and the hole will also be bent.

I like the look of those Dewalt pilot drills, they have the major diameter definitely tied to the pilot! . I have seen on the American "practicalmachinist" website, people say that the pilot should be just enough to clear the web, i.e. just 3mm for a 7mm hole. This would help with my idea of the initial cut, as closer to the drill centre, the stiffer the drill would appear to be.

Frank