I think a lot of it is, a good quality drill. The flute sides are not worn, plenty of lubrication, The bit is sharpened correctly. One problem I was warned about & have noticed to be true is that once you start increasing the size of the hole. Which by the way I have been doing on some 316 today. The work starts to heat up. If that happens I do & would stop until the work cools down. The work does some weird things with temperature. I watched Blondiehacks (youtube) drilling deep holes, & do as she said which was to go steady & back out often to clear the flutes.

Steve.

P.S I'm only a beginner so I am sure someone will be along to advise you better.

This is the problem with the internet. It's full of self-appointed, self-taught "experts" full of dubious information — be it on climate change, coronavirus or drilling holes.

Anyone who has actually taken a training course (back when such things were available in engineering craftwork) would have been instructed to drill about an 8mm pilot hole then drill your 16mm hole in one go. No faffing about with 2mm steps encouraging the corners of the cutting edges to dig in etc.

"invariably, somewhere above 12mm, the bit snags and something gives way – the work in the lathe chuck, the bit in the chuck or the taper in the tailstock. "

You don't say what lathe you are using but it sounds like possibly it is a mini lathe or other small lathe that is being overloaded by the larger drill bits? Job should never spin in the chuck unless something is wrong with the chuck, or it is hopelessly overloaded by too large a diameter for jaws to grip firmly etc. Morse taper spinning may just need a good clean out and tap the taper home with a brass hammer/bar to seat the taper firmly. And holding bits larger than half inch in a chuck is always a gamble. It's beyond what the chuck is designed for. (But I have used a 1" drill with 1/2" reduced shank in a chuck mounted on an MT1 taper in the old Drummond with success so it can be done.)

Edited By Hopper on 13/02/2020 01:31:13

Think about the metal cutting process being undertaken. The centre of any drill is cutting at zero speed – while at the circumference, the cutting speed is maximum. It makes sense to drill a pilot hole (small diameter, high speed) before using a very large bit at lower rotational speed.

Larger drill bits may not cut too well at the centre unless sharpened precisely.

Only go up in steps that the machine can tolerate. That means that a 6mm drill as a pilot could be followed by a 16mm as the cutting surface will be the difference between the two areas. Trying to go to 26mm as the next size up is likely impossible on a small machine.

28mm^2 for a 6mm pilot – easy for the machine.

175mm^2 for the jump from 6mm to 16mm

330mm^2 for the jump from 16mm to 26mm

Some machines may not cope well with the jump from 6mm to 16mm in one go, as that requires over six times the power used to cut the pilot. Clearly, to increase the hole from 16 to 26 mm (in one jump) would require 18 times the power used for the pilot! That is one reason why larger holes are bored.

For a mini-lathe, 6mm, 12mm and 16mm might be a sensible progression. I’m sure the advice is not ‘invariably’ 2mm steps, even on the net. Any linear diameter increase is mathematically flawed (area of a circle is proportional to the diameter squared) and that would indicate the first hole should be 2mm (an increase from 0mm to 2mm as the first step) – patently just absolute rubbish.

The best way to avoid the risk of a less than perfect drill is to get a smaller boring bar to start with. They don't have to be expensive.

**LINK**

Provided you have the power to drive the drill at something close to the recommended speed and rigidity to keep it straight best technique is indeed to start with a pilot hole a shade bigger than the centre point then switch straight to the big drill.

Getting a good start can be an issue with big drills on small, inevitably less stiff machines. Until the drill has a decent cone a significant fraction of the diameter to work in it may well vibrate, wobble and generally complain about the job. Especially if its not as sharp as you'd ideally like. Cure is to use a smaller drill or two in succession to create a wider starting cone of the correct angle. Just create a cone, not a hole.

I suspect that the 2 mm step thing comes from someone trying to cope with poorly hand sharpened drills with off centre points. If the cut isn't too wide the effect of concentricity errors is minimised and the drill rapidly becomes stabilised by the side of the hole. Clearly if the cut step is too big then the off centre cutting cone will generate enough side force to wobble the drill causing enlarging the hole. Its a fearful bodge but most of us don't have facilities to sharpen larger drills well and accurately.

As has been said a sharp drill cutting the main hole in one pass at the right speed and feed actually runs cooler than nibbling the hole out. When things are running correctly heat should go more into the chip than the work or tool. Book speeds and feeds will surprise most folks. Scary fast really. For the likes of us half should be fine but not too much lower or the chips misbehave.

Concerning sharpening. I have just got my Clarkson drill sharpener attachment up and running after far too long on the "get round tuit somewhen" list. Its impressive. As usual proper professional tools are leaps and bounds above the usual offerings at a amateur friendly prices. I can, with some concentration, get reasonable results on a swing jig or Plasplugs mulit-sharp but the Clarkson puts a super sharp edge on with impressively casual effortlessness. Big slow down is getting hand sharpened drills back properly concentric. Surprised me how many of my good looking larger ones were hand sharpened and not quite right.

That thing could put a point on a (small) rolling pin. Max size it will take is only 3/4" tho' so now I have to come up with something equivalent for the bigger ones. My collection goes out to 1.5".

Clive

Edited By Clive Foster on 13/02/2020 10:18:19

A very effective way of opening up a pilot hole is to use a core drill with three or more flutes.

In default, the pilot hole can be opened up to fit the larger drill with a boring bar even if this is too big to enter the target hole to any depth.

Have you considered that 180 rpm is too low. Recommended rpm from various sources for 16mm HSS into steel is about 500 rpm. This is a maximum speed but since I don't know if you are controlling rpm with pulleys, gearbox or electronic speed control it may be important. Electronic speed control causes a drop in power as the rpm is turned down.

Martin C

Well, not trying to court controversy but I use Centre Drills to start holes and will most likely after all this time continue to do so

However, to be clear, I do NOT drill a "centre" with them when doing so…

I seat the drill right back in the chuck and get the tailstock body as close to the work as possible – this then gives a very short and rigid set-up (as much as my lathes will provide at least). The centre drill is then used to make a deep 'dimple' (more than a centre punch but less than a hole!). The slender front piece is not pushed into the work and no 'cone' is produced. However this dimple is quite enough to give the next drill a good start without wandering.

I do the same in my drilling machines, although this work is usually already marked out (which it generally isn't in the lathe). Usual caveats – my work is mostly small in nature and I don't [normally] 'drill' in my mills – not having drilling quills on them as yet.

It may not be modern practice but this works for me..

Regards,

IanT