I would go with High Speed Steel.

Most commercial users of drills want a drill that will last in production machines.

That is what carbide and Cobalt drills are for.

But modelers are going to break / bend HSS drills long before they get dull.

I have done millions and millions of small holes in circuit boards, but using a huge CNC mills.

-Hudson

On Mon, Jul 11, 2022 at 09:09 AM, Owen Thorne - owen at udel.edu wrote:

So, while choosing high-speed bits for hand-drilling plastic and resin seems counterintuitive, the commonly available carbide bits are specifically NOT recommended, as per above. Does anyone have experience with cobalt or carbon-tipped bits? Or is there another material, tip shape, angle, or coating we should be using for resin or plastic?

Most of the information you quoted is aimed at users who use the drills in machine tools. The material is typically chosen for its wear characteristics... But modelers don't wear drills out, they break them because they cannot keep them lined up with the portion that is buried in the work, which snaps the drill off. Remember, twist drills (the proper name for what you are calling "bits") unlike end mills, are not intended to cut on the side of the tool, so whatever direction the drill starts cutting, the tool has to be held in line, not allowed to bend. This would indicate that the toughest, most flexible drill would have the best chances of survival, and that would be high speed steel. Cobalt steel would be right up there also, but I doubt you will ever find that material in the sizes we modelers need. Solid tungsten carbide (carbide for short) is the exact opposite; extremely hard, but also exceedingly brittle. Carbide tipped drills aren't made in the small sizes, that is indeed a strategy to get hard cutting edges on a tough drill, but there is no commercial need in the sizes we use.

There seems to be a bit of confusion about carbide vs. carbon. Carbide is, as I said, short for tungsten carbide. Carbon is short for high carbon steel, which drills USED to be made of before high speed steel was developed decades ago. Some of the cheap import drills are still high carbon steel. Why? Because it's cheap, its basically high speed steel without all the alloy elements that give HSS its toughness. If you are snapping off drills all the time, they are likely high carbon steel.

As to grind (tip shape) years ago, when I still had eyesight and steady hands, I used to be able to resharpen hobby drills under magnification with a fine stone. In fact, when I snapped all the flutes off the drill, I found I could make an 'emergency' drill by stoning the remaining round rod off at a flat angle, so the result was a flat ovaloid face. These took a LOT more turning, but did get the job done. I wonder if a piece of phosphor bronze wire, like Tichy sells, would work, sharpened like this and driven at slow speed by a battery motor tool? They would certainly be cheap enough.

Dennis Storzek

On Mon, Jul 11, 2022 at 01:28 PM, Dennis Storzek wrote:

I wonder if a piece of phosphor bronze wire, like Tichy sells, would work, sharpened like this and driven at slow speed by a battery motor tool? They would certainly be cheap enough.

I can't remember if I even tried this with PB wire, but I feel it's too soft.  But a steel guitar string?  You bet.  I generally don't even dress the the tip with a stone.  Cutting it with pair of small dikes makes a working tip.  Just need to keep the exposed part to the wire to a minimum.

And yes, slow.  Santa brought me the slowest speed Foredom tool a couple of Christmases back.  This setup works great in resin (and wood), and acceptably in styrene.  I do a lot with 0.008" grabs, so 0.010" wire is a good choice for the drill.
 
--
Dave Parker
Swall Meadows, CA

On Mon, Jul 11, 2022 at 04:14 PM, Dave Parker wrote:

I can't remember if I even tried this with PB wire, but I feel it's too soft.  But a steel guitar string?  You bet.  I generally don't even dress the the tip with a stone.  Cutting it with pair of small dikes makes a working tip.  Just need to keep the exposed part to the wire to a minimum.

And yes, slow.  Santa brought me the slowest speed Foredom tool a couple of Christmases back.  This setup works great in resin (and wood), and acceptably in styrene.  I do a lot with 0.008" grabs, so 0.010" wire is a good choice for the drill.

I donno, Tichy's PB wire is pretty hard, much harder than the DA brass wire. I figured I'd suggest it since it could also be used to make the replacement grabs. But I see guitar strings are available in all the diameters a modeler could need, .008, .010. .012, and .013, so likely a better choice for drilling holes. The best part of not having flutes is there is more material in the drill shank, almost twice as much.

Dennis Storzek

On Tue, Jul 12, 2022 at 10:32 AM, Philip Dove wrote:

At an Nmra convention in Kansas City l attended a clinic on building structures and the person used hypodermic needles for making small holes. The method works in thin plastics and card but the holes are too big for grabs in 3.5 mm scale. 

Actually, the method I described, stoning a diagonal flat on the shank of a broken drill mimics the shape of a hypodermic needle without the hole. It drills soft materials OK, but takes a lot of turns due to cutting via scraping, which is why I suggested driving it in a speed controlled motor tool.

Dennis Storzek

I've used guitar string forever. Cut as described above but also sometimes hammered and stoned to make a spear point if using larger diameter strings. Started out using fine pins but the string/burr drills worked better. The smallest easily available strings are .007" and they work well with .004" grabs. Done the same for simulated NBWs. They flex and cost literal pennies. They're also very, very sharp, so be careful. I mount mine in Excel knife handles. The collets are soft enough to hold them for a time. Eventually they stop holding and the string drill needs to be kinked and flattened slightly to give the collet something to bite.

For handwork larger than No.80 there's a couple of Huot indexes and for the money I think they've stood up well. They break but really only when dropped. Paid $30 and $5 respectively but even at $60+ I think they're good drills. For perspective a single micro cutter from Harvey or MA Ford can run that.

PS Are folks with the shakes using maul sticks when working? Isolating and stabilizing your hands can make a big difference.

Andrew Hutchinson

On Wed, Jul 13, 2022 at 09:00 AM, hockenheim68 wrote:

Are folks with the shakes using maul sticks when working? Isolating and stabilizing your hands can make a big difference.

I had to Google this one, and it's spelled mahl stick (not maul).  Very cool idea that I had never heard of.  Andrew, thanks for the tip.
 
--
Dave Parker
Swall Meadows, CA

I used to get guitar strings free from the music store.  Anytime I went there I'd find drops of strings on the floor from where they changed strings, a lot of it.  Some would look at me funny but most of the time they were more than happy I was picking the stuff up. I would tell them it's for a project.
Fred Swanson

On Wed, Jul 13, 2022 at 09:00 AM, hockenheim68 wrote:

I've used guitar string forever. Cut as described above but also sometimes hammered and stoned to make a spear point if using larger diameter strings.

For reference here is a pic of a spear point used for drilling glass:


This is a carbide tipped tool, the tip is wider than the shank, but doesn't have to be. Also, for use in plastic and resin the elliptical curves aren't needed, as Andrew says, just flatten the end of the steel wire and stone angles to a point.

Dennis Storzek