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.

[Kiwi BlokeParticipant @kiwibloke62605]

I second the suggestions of a tapered, bent, or otherwise defective drill bit or rogue material. However, in spite of it being common practice, and what the OP says he’s done for years (as most of us have), I’d suggest that progressively ‘opening up’ drilled holes in small diametrical increments is poor practice. I think it has become popular with home machinists because of machine power and rigidity limitations.

The practice risks over-loading the drill bit’s corners, and also makes it difficult for the bit to start cutting evenly, thereby tending to produce an oversized hole, or at least its first few mm. Because the total cutting force is low, it’s difficult not to push the drill in with an excessive axial feed. Much better, I believe, is to drill a pilot hole with a bit of about the same diameter as the final bit’s chisel edge, and then go straight to the final size. However, this might be beyond the home shop’s machine capabilities. If so, keep the number of steps to a minimum.

This is contentious, so I’ll await the brickbats and naysayers’ comments…

[DiogenesParticipant @diogenes]

It’s never worth persevering with a drill that doesn’t cut properly from very first contact and the sooner it’s withdrawn the more chance there is of successfully recovering the workpiece (and sometimes the bit).

As to your question, the answer probably lies in how much confidence you have in the supplier of;

A) the drill(s)

B) the material

 

 

 

 

 

[derek hall 1Participant @derekhall1]

I agree with kiwi, (no brickbats coming from me!) his suggested technique is more or less what I was taught as an apprentice many, many years ago. You have to give the drill some work to do.

Maybe first drill a pilot hole of say 5mm, then use your 10.5mm drill.

On my Myford I have frequently used the biggest drill I have which is 1 1/8 ins, to open up holes, then I bore the hole to finish to size. I have no problems drilling holes this size using backgear and coolent, I do get some belt slip though, but I am using link belting (another topic)…

I think the problem is either the material being drilled, or the drills.

[Speedy Builder5Participant @speedybuilder5]

Chuck the bar away and start with a different piece ??

[Neil LickfoldParticipant @neillickfold44316]

With .25mm per side, you are at a reamer size. And should be at reamer rpm or slower. At 90rpm or so, it will re drill fine, but will need a fair bit of lube as it will get hot. Making the outer of the drill with a 45 chamfer and it will behave like a reamer.

Normally would sharpen the drill like a chisel point style, and drill in directly. If you do a pilot hole, make it about the diameter of the core thickness of the drill, so 3-5mm. If the pilot is too large, it will generate more heat, and wear the edge of the drill at the diameter of the existing hole.

Neil

[Andrew CrowParticipant @andrewcrow91475]

<p style=”text-align: left;”>I would agree with what has been said above, drills need to do some work. However, if you need to remove a small amount in steel with a drill use neat cutting oil as a lubricant. Cutting with soluble oil tends to make the drill cut very tight to size and can make it bind.</p>
Andy

[Andrew CrowParticipant @andrewcrow91475]

Just to add, if the original hole wasn’t straight, the second drill will try to follow it and may not flex like the first one, also causing it to bind.

[Martin KyteParticipant @martinkyte99762]

If the tips of the drill used to open up are blunt or rounded or if the amount being taken out is small the drill will have a tendency to close up on the helix. This will cause the flanks of the flutes to rub and heat up possibly with the drill heating more than the workpiece. Eventually everything seizes up.

If you want to do an experiment, take a 1/4” drill and stone the corners to a small radius. Drill a hole with a reasonable feed and then see if the tail of the drill will enter the hole. Generally it won’t and drill tails are  undersize by a small amount. It’s worse if you open up a hole.

Of course if you actually want to produce a slightly undersized hole this effect becomes a help rather than a snag.

regards Martin

[noel shelleyParticipant @noelshelley55608]

Pilot hole from the other end at say 6mm, punch out the broken bit and start again ! If you can ? As others have said, duff material, a blunt drill bit and poor technique ! Pilot at 3/16″ or 1/4″ and then open out to size with a sharp bit. If one must step drill then do it in steps of 1/4″ or more, NOT 20thou.

As true model engineers we all try to use a donkey to do the job when a good shire is what we need !  Noel.

[Clive FosterParticipant @clivefoster55965]

Unless the second drill is perfectly aligned the sharp tips will be wiped out almost instantly by the loads applied pulling the new drill into line with the hole.

If you absolutely must use a drill to take out small amounts to bring a hole up to size the only marginally safe way is to bore the end of the hole out to size first going deep enough to give the drill plenty of support. Two or, better, three times diameter. Lubricate well and start up with the drill inside the bore having pulled the lathe over by hand a few times to ensure all is in line.

Very much an inspector meticulous technique.

It’s possible to get away with just drilling out in a stout, well aligned, lathe if the drill is sharp. I’ve done it. But you can never be sure if things are going to behave. Even if you do get away with it teh drill will almost certainly need re-sharpening.

Clive

[Howard LewisParticipant @howardlewis46836]

+1 for what Clive says.

If in doubt slow down speed and feed. Especially if the material not free cutting mild steel

Never a good technoque to expect a big drill to do all the work.

I have come across drills that have been ground with a negative clearance (Rarely, thankfully) Resharpening cures the problem

I prefer to open up gradually from a small, pilot, to minimise load on machine and drill

If it is beyond or near maximum size drill, hopefully there will be a large enough hole to admit a boring bar, to produce a straight hole.

On long holes, it pays to withdraw regularly to clear swarf from the flutes. Seizure is like;ly of you don’t.

If the going seems hard, is the drill sharp, and with equal length lips?

Don’t let the drill rub, but most certainly, don’t over feed and force it. (that can shorten drill life very greatly.

On steel, lubrication rarely does any harm

HTH

Howard

[gerry maddenParticipant @gerrymadden53711]

Thanks for all your thoughts chaps. I think the mystery is solved.

Last night I did a careful inspection of my existing large drills and my new/unused suspect ones. Attempting to check for taper is not such an easy task as the true cylindrical land is small, and, being helical, its easy to go ‘over-centre’ or ‘under-centre’ on the micrometer anvils. However with perseverance I was able to convince myself that there was no more taper on the suspect drills, than on my others. (There is a little of course due to wear.  At the tip region the diameter was typically of 10 to 20um undersized (approx. half to 1 thou”).

I then gave the cutting edges a good inspection under the microscope and again everything looked ‘normal’. The only slight concern I had was that the new drills probably had less relief angle than my others. That though should really only affect cutting and wear rates, not cause seizures.

Finally I had a look at the material. When I was doing the turning ops, I did think the material was harder than the EN1A I’d used just a week before but just put that down to my imagination or worn cutting tips. However a look at the cut surfaces of the bar in question, and that of my previous EN1A, showed a significant difference in texture. Perhaps that was just down to a difference in cutting conditions ?

A scratch test was inconclusive so I decided to try doing a rough Brinnell Test. I put a small bearing-ball between the two bar ends and impacted them together. I did this twice to make sure. The results are unequivocal as you can see here….

The the suspect bar, on the right is significantly harder than that of my previous EN1A bar.

Just to underline the fact that the material is not right, today I put an 11mm reamer though the badly drilled hole in my work piece to clean it up. I and then attempted to tap it M12. Despite the hole now being very oversize, I was unable to put more than 10 taper-tap threads in it. So this stuff is not just hard, work hardening too!

I will contact the supplier tomorrow.

Gerry

 

[Michael GilliganParticipant @michaelgilligan61133]

An excellent qualitative test, Gerry … Well-done, Sir !

MichaelG.

[Howard LewisParticipant @howardlewis46836]

I like the Brinell test method to compare hardness.

And it worked, telling the difference between the materials.

Well done!

Howard

[Howard Lewis]

On Howard Lewis Said:

I like the Brinell test method to compare hardness.

And it worked, telling the difference between the materials.

Well done!

Howard

Me too.  A spark test on a grinder might reveal even more.

Dave

[Simon CollierParticipant @simoncollier74340]

I recently bought some drill sets from Temu, accepting that I might be throwing away the money. They arrived. Worse than I thought possible. No or negative relief. Today I wanted a 5/8 drill so I tried it as an experiment to open out a 13 mm hole. Made a chattering mess. I do not grind drills but thought I’d give it a go on my CBN wheel. First attempt only one flute was cutting. Second attempt, two lovely ribbons streaming out of the hole. This is a 9/16 to 1” reduced shank set so big enough to sharpen but the small sets I got are a throw out. No more Temu for me.

[bernard towersParticipant @bernardtowers37738]

Thats how they survive by the amount of stuff we throw away and have to buy again. If its cheap that should tell you something

[not done it yetParticipant @notdoneityet]

First was Rolson, for tools, many years ago (a few more tool suppliers in recent years).  Then bang good, several years ago (MT2 not quite MT2).  More recently, vevor (but I could fix the items to be good enough for my use)  Temu added to the list very recently (I’ve no intention of even trying them, if possible!).

Anyone ‘black-listed’ other suppliers/manufacturers?  Buying cheap often means buying twice.

[gerry maddenParticipant @gerrymadden53711]

Update – the supplier has agreed to send a new bar of EN1A free of charge. A few days of ‘production’ lost but a material issue is a first for me so cant complain too much.

Gerry

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