Breaking Small Drill Bits

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one_inch_railroad
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Breaking Small Drill Bits

Post by one_inch_railroad » Mon May 18, 2020 12:59 am

My friend and I have been putting some grab irons on our 1" scale gondola cars. We have been using some number 51 drills to do this. Despite being very careful and choking right up on the drill bit we managed to break all but one of the drill bits we bought. I think we went through 6 drill bits to drill 40 holes. Needless to say, it was a very tedious process. We were just using regular HSS drill bits. We also found there must have been some hard spots in the metal we were drilling through as it took a really long time to drill through the material. They were brand new drill bits from the factory so they should have performed better than they did.

We still have two more gondola's to do and there is no way we want to go through that again. The local shop sells two other kinds of drill bits in the size we need. One is a cobalt alloy and they are about $1.70 per drill. Then they also have a solid carbide drill that is $12 dollars per drill bit. I wouldn't mind spending the money on the solid carbide one if it will do the job and probably won't break. Would we have better luck with the cobalt bits or should I spring for the solid carbide? Or would it not matter either way, would the bits keep breaking no matter what material they are made out of because of their small diameter.

EOsteam
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Re: Breaking Small Drill Bits

Post by EOsteam » Mon May 18, 2020 1:10 am

What RPM’s were they being run at? Handheld, drill press or milling machine?

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BigDumbDinosaur
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Re: Breaking Small Drill Bits

Post by BigDumbDinosaur » Mon May 18, 2020 1:33 am

one_inch_railroad wrote:
Mon May 18, 2020 12:59 am
...We have been using some number 51 drills to do this. Despite being very careful and choking right up on the drill bit we managed to break all but one of the drill bits we bought.
Drills that small need to be run at high RPM in order to cut at a satisfactory rate. Otherwise, you will find yourself pushing too hard on them to hasten the process, leading to breakage. The minimum speed I would recommend is 2500 RPM. I also suggest you use a touch of cutting oil on your drills to prolong their life.

As I was building the body for my F-unit (fabbed from steel), I had to manually drill and tap a whole lot of holes to accept 2-56 machine screws. I have a 1/4" pneumatic drill motor that runs at 3000 RPM, which is what I used to drill the holes. The tapping I did with a 1/4" battery-powered drill motor. By the time I had completed nearly 300 holes I had broken only one drill.

Incidentally, I use HSS drills with black oxide coating.
—————————————————————————————————
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Harold_V
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Re: Breaking Small Drill Bits

Post by Harold_V » Mon May 18, 2020 3:18 am

Heh! For starters, if you've having trouble breaking HSS drill bits, don't even think about carbide, which is akin to drilling with glass at that diameter. It's hard and heat resistant, but fragile beyond belief.

It's not clear how you're drilling, or what material you're trying to drill, but to get an idea of spindle speed, if you were attempting to drill steel, you'd not want to run the drill any slower than 7,000 rpm (which puts you in the neighborhood of 125 sfpm). Drilling by hand (using a drill motor) would complicate things, as they're impossible to hold steady and run way, way, way too slowly. An air drill would be far better suited in this case.

I would recommend against buying cobalt unless you're drilling extremely hard materials. They, by design, have thicker webs, so while they're stronger, they're also harder to use (they require greater pressure in order to drill) because of the thick web.

For small holes like you're trying to drill, if you have to drill them by hand, you'll have much better luck if you'll use a flex shaft, as they provide some serious spindle speed. I use a Foredom.

H
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John Hasler
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Re: Breaking Small Drill Bits

Post by John Hasler » Mon May 18, 2020 7:50 am

I use my Proxxon (like a Dremel but more quality and less marketing) for that sort of thing. Drilling by hand I still break more bits than I like, though.

I once acquired a box of carbide pc board bits in a surplus deal (I do a lot of electronics). I found that they are almost impossible to use without a precision high-speed drill press. You not only need 10,000 RPM, you also need less than a thou of runout.

rkcarguy
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Re: Breaking Small Drill Bits

Post by rkcarguy » Mon May 18, 2020 10:31 am

#51 is a .067" diameter drill, and if you're careful and have quality bits they shouldn't be that hard to work with. You haven't had fun until you're drilling holes for grab irons in HO scale rolling stock lol!
Carbide is too brittle at that diameter. I'd only use those if I was working in a mill with one of those little finger feed micro chucks. I'd try the plain black HSS drills in a split point stub drill. These are all I use provided I don't need a deeper hole. I just use a cordless drill with the clutch set fairly light, and you have to pay attention to when you're about to pop through the metal and lighten up the pressure or you'll break bit after bit.
Also, you mention "choking up on the bit". If they are regular length bits and the flutes are inside the chuck, that's probably part of the problem. Run the stub drill so you are chucked up on the solid shank with the flutes ending just before where the chuck grabs.

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SteveM
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Re: Breaking Small Drill Bits

Post by SteveM » Mon May 18, 2020 1:16 pm

John Hasler wrote:
Mon May 18, 2020 7:50 am
I once acquired a box of carbide pc board bits in a surplus deal (I do a lot of electronics). I found that they are almost impossible to use without a precision high-speed drill press. You not only need 10,000 RPM, you also need less than a thou of runout.
You begin to understand why a Cameron drill press is $895 (and why almost half of that cost is the chuck).

Steve

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SteveM
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Re: Breaking Small Drill Bits

Post by SteveM » Mon May 18, 2020 1:17 pm

Those PC board drills aren't carbide because they are drilling into HARD material. They are carbide because they are drilling into ABRASIVE material. The material itself isn't that hard, it just eats drills for lunch.

Steve

John Hasler
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Re: Breaking Small Drill Bits

Post by John Hasler » Mon May 18, 2020 3:51 pm

SteveM wrote:
Mon May 18, 2020 1:17 pm
Those PC board drills aren't carbide because they are drilling into HARD material. They are carbide because they are drilling into ABRASIVE material. The material itself isn't that hard, it just eats drills for lunch.

Steve
Yes, but without a high-speed drillpress I go through fewer HSS bits drilling pcbs with them until they are dull than I do using carbide ones until they break.

one_inch_railroad
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Re: Breaking Small Drill Bits

Post by one_inch_railroad » Mon May 18, 2020 5:23 pm

Thanks, everyone for all the great replies. I'm glad I didn't waste the money on the carbide bits before I posted the question here. I will buy some more regular HSS bits but this time I will try them in the Dremel with the flex shaft.

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Re: Breaking Small Drill Bits

Post by curtis cutter » Mon May 18, 2020 8:24 pm

Last week I too broke a little bit and it occurred to me that maybe the table was not perpendicular to the quill. I stuck a drill rod in the quill and with a little 6" electronic level saw that the Rockwell drill press (table top unit) was perfectly vertical but my table was 2 degrees tilted down to the right side.

I suspect once it got into the material beyond the tip of the drill it made little difference but the initial start, with no punch to follow, may have been difficult to stay on mark.
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Greg_Lewis
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Re: Breaking Small Drill Bits

Post by Greg_Lewis » Mon May 18, 2020 9:09 pm

Harold_V wrote:
Mon May 18, 2020 3:18 am
...
An air drill would be far better suited in this case.
...
I drilled about 4000 holes in 20 ga. sheet with an air drill and only used about 8 or 10 bits. The air drill really gets them up to speed. I also used a guide to keep the bit on target.
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 30 years of turning perfectly good metal into bits of useless scrap.

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