"Dwelling" sometimes when I'm drilling steel?

[uart]

When I'm drilling steel on a DP, if it's a relatively deep hole and I have to peck it (or withdraw the drill to clear the chips), then when I recommence drilling it often feels like the drill bit just dwells for a while and doesn't start cutting straight away. Even when it's just mild steel and is cutting like the proverbial piece of butter, when I recommence drilling it still sometimes seems to dwell without cutting for a while.

The way it seems to go is that I recommence and apply the same pressure as was previously cutting sweetly, but the drill doesn't progress. So I keep applying more pressure and it dwells for a while and then suddenly starts cutting just as easily as before. Often I'm thinking maybe the drill has dulled, and I'm just about to withdraw it and change or sharpen it, then suddenly it starts cutting again and I realize the drill bit is still fine.

BTW. Although I've noticed this in the past, I'd always thought that it was simply the delay in the chips coming up through the hole making it appear to not be cutting when it actually was. However last time it happened I was carefully watching the depth gauge when this was happening, and it really wasn't moving at all.

So now I'm suspecting that I'm leaving too many chips in the hole and that the drill bit is "riding" on a chip and so not being able to bite in, or something like that.

Does that sound like a reasonable explanation?
Does anyone else notice this happen to them too?

 

[justanengineer]

Yup, happens regularly. If you've got an air line nearby give it a blast of air from a blow gun down the hole to clear it out, make sure you have safety glasses on while doing so bc chips fly.

 

[uart]

Yup, happens regularly. If you've got an air line nearby give it a blast of air from a blow gun down the hole to clear it out

Thanks for the tip.

I don't have an air line, but I do usually keep a pressure can of WD40 with the extension tube handy. So next time I think I'll try putting the tube down the hole and giving it a good squirt and see how that goes.

 

[A_Pmech]

That would be a chip caught under the dead center of the drill.

 

[Techie1961]

It is riding on a chip. If you apply more pressure, it will bite again. On a drill press with feed, it will go through the chip very quickly. You can also try lifting a few times to get a different bite on it.

 

[uart]

That would be a chip caught under the dead center of the drill.

Thanks Andrew. That makes sense, if the chip is spinning too freely in the hole then it just going to spin with the drill so the bit wont cut it. Well that's how I'm picturing it anyway.

 

[Wanna Ride]

Yup, happens regularly. If you've got an air line nearby give it a blast of air from a blow gun down the hole to clear it out, make sure you have safety glasses on while doing so bc chips fly.

^^ Exactly.

 

[ChevyEFI]

Perhaps the hole bottom work hardens a bit and the drill has to break through it when you pause.

 

[dewalt378g]

I don't know how you do your pecking, but I get great results using really short pecks never totally withdrawing the bit. I do about 3 quick pecks depending on when the lube starts to thin out or a hint of vapor, then withdraw completely and re-lube.

 

[404]

You are seeing work hardening. When the drill is allowed to rub, or to remove a very thin (dust like) layer, the steel under the drill will be plastically deformed and will harden.

The work hardening effect can be demonstrated by bending a coat hanger wire back and forth. At first, the wire is soft. As the wire is bent back and forth, it gets harder and harder, eventually it hardens to the point of cracking and breaking.

The solution is to apply the full correct down force on the drill right way, instantly. Only takes a few drill revolutions to harden the metal at the bottom of the drill hole. When drilling stainless steel the work hardening effect is very very strong. A drill bit can be dulled instantly if allowed to rub.

 

[uart]

The solution is to apply the full correct down force on the drill right way, instantly.

I was definitely already doing that.

You are seeing work hardening. When the drill is allowed to rub, or to remove a very thin (dust like) layer, the steel under the drill will be plastically deformed and will harden.

Work hardening was also the first thing I thought about 404, but the more I think about it the more certain I am that this wasn't the case.

For a start I was applying full downward force straight away. More force in fact than it was previously cutting freely at. Also it was a very sharp drill and only mild steel. With the force I was applying it would have cut that steel even in a work hardened state.

Also if it was work hardening it would happen much the same on every peck, given that I was using the same technique each time. However it only happened after several pecks (withdrawing to clear the flutes) and only when the hole was getting fairly deep.

I'm about 99% certain now that it was riding on a chip (or chips), with the chip rotating along with the drill making it difficult to cut.

 

[66354dream]

I always use a little bit of motor oil when i drill, it always makes a huge difference. I keep an oiler near by my drill press just to be ready.

 

[404]

I was definitely already doing that.

Work hardening was also the first thing I thought about 404, but the more I think about it the more certain I am that this wasn't the case.

For a start I was applying full downward force straight away. More force in fact than it was previously cutting freely at. Also it was a very sharp drill and only mild steel. With the force I was applying it would have cut that steel even in a work hardened state.

Also if it was work hardening it would happen much the same on every peck, given that I was using the same technique each time. However it only happened after several pecks (withdrawing to clear the flutes) and only when the hole was getting fairly deep.

I'm about 99% certain now that it was riding on a chip (or chips), with the chip rotating along with the drill making it difficult to cut.

I agree with your analysis.

 

[Burgerkong]

Highly doubt mild steel would work harden to the point where it takes a period of time before it starts cutting again, this is not specially hardened tool steel/exotic stuff.

 

[404]

Highly doubt mild steel would work harden to the point where it takes a period of time before it starts cutting again, this is not specially hardened tool steel/exotic stuff.

Coat hanger wire is soft mild steel. Bend back and forth enough, it will work harden and crack.

 

[CobraRed]

Highly doubt mild steel would work harden to the point where it takes a period of time before it starts cutting again, this is not specially hardened tool steel/exotic stuff.

"Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened."

https://en.wikipedia.org/wiki/Work_hardening

 

[Burgerkong]

Coat hanger wire is soft mild steel. Bend back and forth enough, it will work harden and crack.

That's called metal fatigue not work hardening. You're subjecting it to repeated loading when it was not designed for it. All you're doing is repeatedly stressing the wire at a certain point. Look up what fatigue means.

 

[Burgerkong]

"Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened."

https://en.wikipedia.org/wiki/Work_hardening

Work hardended in that wiki excerpt is referring to the hardening when it was cold worked or formed. It does not mean it will work harden during machining operations.

 

[Fcvapor05]

That's called metal fatigue not work hardening. You're subjecting it to repeated loading when it was not designed for it. All you're doing is repeatedly stressing the wire at a certain point. Look up what fatigue means.

If you do this and pay attention, you'll notice that if you bend the wire in one direction, and then try to bend it back the other direction in the exact same point, it takes more force.

That's work hardening.

Work hardended in that wiki excerpt is referring to the hardening when it was cold worked or formed. It does not mean it will work harden during machining operations.

Work hardening is work hardening. Machining is (most of the time) an operation that occurs below transition temperatures of most metals, meaning that the stress imparted is similar to cold forming via stamping, drawing, whatever.

 

[404]

That's called metal fatigue not work hardening. You're subjecting it to repeated loading when it was not designed for it. All you're doing is repeatedly stressing the wire at a certain point. Look up what fatigue means.

In a happy coincidence, I used to make side money running fatigue tests on notched samples. For this man:

http://www.boston.com/bostonglobe/o...k_mcclintock_90_mit_metallurgist_mountaineer/

Perhaps you knew him. Anyway, the scary thing about fatigue failures are that they happen in the elastic range of the material, where the material springs back to exactly the original shape when the load is removed. No permanent deformation at all is needed for fatigue cracks to start and grow. An internal inclusion in a casting can start a crack, for example the F111 wing spar.

Work hardening happens with plastic deformation, where the material is in a new shape after load is removed.