A straighter hole?

I had need to drill through 80mm hot rolled that had been squared (as in no mill scale). I finished late last night and haven't doube-checked but I think it came out 0.5mm off center exit compared to start.

Now that doesn't matter on this applcation but I was curious as to how one might improve on that. This hole was located by DRO then a fine centre drill and cobalt split points full depth of their jobber lengths 4.5mm, 6.&. 7mm.

the mill had been retrammed shortly before to less than 0.01mm over a 220mm span and i was using a moderate priced drill chuck in the mill.

Would it have been better to bore from each end? Would it have been better to make the pilot hole with a fine endmill? Would it have been better to use collets instead of a drill chuck - or all of the above?

If I was looking for a good answer I would search on gun sites

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?)

.

I would carefully measure the size and shape of that material before worrying about the machine or the drills.

MichaelG.

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

Chester superlux mill bought earlier his year...not the best mill in the world I'm sure..give it a hard time and there's nasty rumbles from the head bearings and vibrations. The head moves on these rather than the knee/table.

I'd squared the stock myself after freshly tramming the mill..and tramming the vice both for travel and checking for 'perpendicularity' to the head. Quill extension was just that needed to bore the depth. As for how square the stock was.. well the end in the mill measured with a mike exactly the same across both left and right of the stock and the edge was checked with a machinists square as square in the vice ...but all those methods will have limitations.. and front to back there isn't a reference on the mill vice apart from trusting the fixed jaw. A square on the corners has limitations too.

Perhaps it's expecting too much of a drill bit in an average chuck to drill perfectly vertical over that distance anyway since it works out as 0.36 degs deflection and in reality i doubt I had the combination of all factors better than that.

So it has to be a case of finding methds to drill to make up for the inaccuracies of the operator..and drilling from both ends and using collets to hold the drill bits might be a way forward.

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!

You could use a micrometer or calliper to check that the length of all four faces is the same (near the corners, not from the centre of the face) and then measure the diagonals. If these are the same as well, the bar is square. The beauty of this method is that you don't need any kind of calibrated equipment, or to trust that a square is truly square, even a friction-joint calliper is enough to check if two dimensions are the same.

I will confess to an unfair bias against squares due to the sheer number of complaints forwarded to me where customers, mostly "engineers", tried setting out a greenhouse base using a 6" square rather than simply following the instructions and measuring the diagonals. Even a piece of string would be more accurate than a square on a 12 x 8 greenhouse.

Brian

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

Bob

Do it the opposite way. Drill the hole through the centre then mill the sides true to the hole then mill the ends square.

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

If you have a tube it is easy to die it down to a smaller tube. A well known party trick like turning a tube inside out.

JA

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

To continue the discussion.

It wasn't center popped...just straight in with a cenre drill by DRO then the different drill sizes to their jobbing lengths - the 4.5mm split point only goes about 1/3 the depth. the 6mm further..But yes it was pecked to clear swarf as were the follow on drills. The final 7mm split point cobalt was a new drill and went through the pilot holes and the remaining stock like butter - I did clear swarf as it went but it would have happily chomped straight through.

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.

http://toolguyd.com/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.

..or a short then long series centre cutting endmill for the pilot...?

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

I take your point. Perhaps the use of a drill chuck will aso introduce more potetial slop to he start of that pilot compared to a collet - the chuck body itself having some inherent 'slop' and being further from the quil than an ER32 collet (in my case) would be... the greater rigidity of the system the better the hole start..