Strange drilling situation

[gerry maddenParticipant @gerrymadden53711]

I decided to put an M12 (std. coarse 1.75 pitch) thread in the end of a piece of EN1A bar using the lathe recently. I drilled it to 10mm as that was the largest near-size drill that I had. The ‘recommended’ drilling diameter for M12 is of course 10.2, but as most drills make oversize holes I thought I would get away with the 10mm drill. However it became clear that the taper tap just wasn’t going engage in the hole I’d made so I decided to order and 10.5 drill to make it a little larger. (It was a long thread, so I always like to keep the tap in the tailstock chuck loosely fitted in the tailstock for good initial alignment.)

After the 10.5 drill arrived I popped it in the tailstock chuck and expected to be able to wind it in with ease to open out the hole. This ‘simple’ op. didn’t go at all well. The drill initially cut, but as it went further in, it got noisy and began to seize.  A couple of times the chuck spun in the MT2 socket and when I fixed that problem, as I continued to try deepen the hole, the lathe was on the point of stalling. The work piece was well over 100C despite my cutting speed only being about 200 RPM. Eventually I gave up as I decided there must be something wrong with this brand new drill. It was a bit of a battle to remove it from the workpiece but that didn’t surprise me after the problems I’d had.

Since the hole was a bit of a mess now I decided to compromise again on thread depth and just drill the thing out with my unused 11mm drill. However as I started the process, the same issues occurred. Lots of heat and noise. The lathe was stalling again so I stopped and moved the belts over to slow-speed mode. I thinking that perhaps a faster wind-in speed was what was needed, I wound faster on head-stock. Well this wasn’t an improvement at all. The drill seized the chuck yet again and with all the belts squeeling like stuck pigs I hit the emergency stop ! I removed the VERY work-piece from the chuck with the drill stuck in it. In trying to extract the drill, I broke it!!

I’m totally mystified. I have been drilling and opening out holes on my lathe for 20 years and have never encountered any difficulties like before. In the past, when I have been intending to bore large holes I always tend to run through with drills up to 16mm in small steps and its always been a piece of cake. So what on earth was different this time ?

One thought I have is that the ‘new’ 10.5 drill, and my off-the-shelf 11mm unused drill, came from the same source within a few months of each other. They both ‘look’ completely normal. But could there be something wrong with the spiral edge perhaps ?

Anyone have any thoughts on this strange state of affairs ?

Gerry

 

[VicParticipant @vic]

Someone recently gave me two 16mm drill bits. Both of them looked pretty sharp. I tried to open up a hole with one the other day and it was very reluctant to cut so I gave up. I then tried the other bit and it cut just fine. Obviously something wrong with the geometry on the first one. Both bits look to have machine made cutting edges?

[Clive Brown 1Participant @clivebrown1]

That behaviour seems what might be expected from a steel that work-hardens, not EN1A. Are you sure of the spec.

[JohnFParticipant @johnf59703]

Have you checked the drill diameter with a micrometer both at the tip and further along the length to ensure it is not tapered ?  It should be size at the tip and smaller at the end of the spiral by a small amount –a thou or so.

I have seen this on some “cheapo” imported unbranded drills when a chap I know had a similar problem.

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