Drilling in steel: chips vs curlies

[DaveO]

Some time back I used a drill press to drill into steel, and got nothing but little chips of metal. Over the weekend I drilled (different project, 3/8" hand drill with a different bit) and got chips while the hole was shallow, but as the hole got deeper, got curly turns. It got to the point that I could tell when the bit was about the punch through when I saw the curlies.

Anyone notice this, or know what causes chips vs. curlies? Do the curly turns indicate more efficient operation, or something?

[geezer]

Saw this the other day,,,good info on that,,,see page 9 on using twist drill explains it all,,,http://www.northerntool.com/downloads/manuals/M0097.pdf

[Rodsmachineshop]

they make hi speed drills and low speed drills i use low speed drills on the drill press last longer if at rite speed for drill . i like Irwin Cobalt HSS Drills they stay sharp longer and resharpen better , I only use cobalt drills ,you can use cobalt drills at low speeds if need like stainless steel drilling good prices on drills http://www.drillbitwarehouse.com/

[NRM]

I think it has to do with speed and feed, in general the softer the material the faster the speed. Here is a link to a calculator http://www.custompartnet.com/calcula...speed-and-feed
I am usually drilling SS and have a sweet spot in each piece. Once you get the cut right you should have a nice long curly as you call it. Hope this helped and if I am wrong someone let me know.

[bobwills]

It seems to me that the sharper better bits make the curls and the dull ones usually just make little flakes because its not cutting and digging as quick taking a thicker layer of metal out of the whole each turn.

[hokiefd]

It seems to me that the sharper better bits make the curls and the dull ones usually just make little flakes because its not cutting and digging as quick taking a thicker layer of metal out of the whole each turn.

This has been my experience as well, but it also depends on the force applied as mentioned by geezer. Not applying enough force will probably results in small chips and no curls.

[Tacoma747]

You know they are cutting good when they grab and break the drill bit.

[zoama]

You know they are cutting good when they grab and break the drill bit.

You know your rpm is too low when they grab and break the drill bit.

[jakeru]

Some time back I used a drill press to drill into steel, and got nothing but little chips of metal. Over the weekend I drilled (different project, 3/8" hand drill with a different bit) and got chips while the hole was shallow, but as the hole got deeper, got curly turns. It got to the point that I could tell when the bit was about the punch through when I saw the curlies.

Anyone notice this, or know what causes chips vs. curlies? Do the curly turns indicate more efficient operation, or something?

"Curlies" like these, Dave?


I'm guessing you were noticing chips when going into steel (and probably had the drill bit chattering as well) during the beginning of the drilling operation, before the lands of your drill bit were engaged with the work.

(The lands are the widest part of the drill bit by the way.)

Before the lands are engaged, the drill bit can chatter (move around sideways as it rotates) and this causes the initial chips. This can happen because the drill bit only has two cutting edges touching the workpiece, and they can dance around sideways as they rotate. Also I'll bet you would would hear it, and if you stopped drilling while it was doing it and looked at the hole, it would not look conical, or round. I wish I knew what it was called, but can't find it right now. But if you're familiar with rotary (wankel) engines, it's a similar idea to how a three-lobed rotor fits into and rotates inside a four-lobed "hole" (that is not a circle.) Except with a drill bit you have a two-lobed rotor (the drill bit with two cutting flutes) fitting into for example, a three-lobed hole.

Once the lands engage, they can begin stabilizing this chatter. Once the chatter is gone, continuous chips (curlies as you call them) can be shaved off. I'm guessing that is what you noticed.

I am still figuring out all the mechanics of drill bits myself. I've been trying sharpening mine with different relief angles to see how it affects the chatter. How large of a hole you pre-drill (pilot hole) can have a big impact as well. One rule of thumb I've read (but not yet totally verified), is that to keep chatter at bay, you don't want to drill a pilot hole that is larger than the web of the drill. (The web is the diameter through the center of the drill by the way, where it's "solid", and when the web is split near the cutting tip, the drill bit is called "split point".)

One thing you can do to get the lands engaging more quickly, by the way, is to increase (make more blunt) the point angle of the drill bit. For example, a 118 degree bit of a given diameter will take a longer diatance before the lands start engaging than a 135 degree bit will. The closer the point angle gets to 180 degrees, the more quickly the lands are going to engage.

By the way, I was reading about some more sophisticated/specialty bits that are designed to make rounder / more accurate holes, which have more than 2 lands. They will have 4 lands or 6 lands. Supposedly having more lands does increase friction though. (Especially for deep holes)

Some more things that may reduce chatter (something I hope to experiment with more) - reduce the relief angle to the minimum necessary. It will keep the bit from trying to "bite" as aggressively. For hard metals, with large bit diameters, as low as 5-8 degrees is recommended. I am experimenting with this on my bit grinds. Supposedly the smaller the diameter of the bit, the more relief angle you need, which makes sense. The larger diameter the lesser the relief angle. It seems to me that reducing this to the minimum that you want would tend to reduce chatter. It does seem to make some difference. I've noticed that a lot of bits are factory sharpened with what seems like a pretty aggressive relief angle - perhaps more aggressive than is necessary. Having a lower relief angle also means more metal behind the cutting edge, supporting it better and making it last longer (less likely to chip off, etc.)

Check out the "typical factory grind" on the right (relatively aggressive relief angle) and my custom/experimental grind (relatively less relief angle) on the left.
Both of these are cobalt 1/2" diameter bits, by the way, both with 135 degree point angles.

Is it just me, or does it seem a little bit crazy how much relief angle they are typically grinding into these bits? It's enough relief angle to shave 1/8"-1/4" wide shaving. Is this much relief angle really good for something?

[DaveO]

Your theory about drill chatter that settles down into properly engaged lands certainly fits with the flakes-to-continuous physical results. Before I saw your note I was thinking about all the variables in the process: drill speed, bit sharpness, bit relief angle (garnered from you post, I'll have to study on that one), bit "angle of attack" to the work surface, pilot hole or not, drilling oil or not, downward pressure on the bit, inadvertent skating of the bit...

Thanks, Jakeru, for your post: a wealth of information, as usual.

[jakeru]

No problem, Dave. Here are some online references with some info (and terminology, pictures, etc) on drill bit geometry and operation, in case you're interested in exploring further:
http://www.neme-s.org/2005%20May%20Meeting/drills.pdf
http://www.free-ed.net/sweethaven/In...um=3&iNum=0404
http://www.losgatosmanufacturing.com...sharpening.pdf
http://www.icscuttingtools.com/the-c...t_geometry.pdf

[martinnman]

Well, I think getting curly turns is common thing while drilling in steel. I also experience this thing already and its nothing to done with efficient work. You can do it with your expertise only.

[Trip59]

Best thing I've come across, go to youtube and search mrpete222. Check out his drill sharpening vids. Unless you go with something like a Darex or other 'professional' sharpening system, you're better off learning to do it by hand. Yes, you'll fk up a few bits a few times, but it's really easy to learn and will beat the pants off of any harbor freight or home depot sharpener.

[Rambozo]

I met this guy at a trade show once and learned more about drill geometry from that conversation than I had in years.
There is no one right grind for a drill and optimizing the point for the job can make a huge difference. At the time I was drilling .093 holes through the middle of semi hardened allen set screws. Drill life was poor with a lot of breakage as the drill broke through into the mess left from the broaching operation. Changing to a multi-facet grind cured everything. Drill life went up several hundred percent, and no more breakage. New drills come with a compromised grind that does nothing well, and should be sharpened to suit the job.


There is a longer version of this article, but I don't see it on the web. This does cover the basics.
http://www.newmantools.com/machines/drillpoint.html

[Brian Ski]

There is no one right grind for a drill and optimizing the point for the job can make a huge difference..... New drills come with a compromised grind that does nothing well, and should be sharpened to suit the job.

Completely agree... I usually wear the bits out pretty fast before I optimize the cut. But I do sharpen them very often.