O.K. I know this may be a little mundane for the Advanced Forum, but could one of you engineer guys explain to me a little about the physics of drilling metal.

After drilling through some relatively thick stock yesterday I began to wonder why the drill performs better with lubrication, (I used WD 40). Adding lubricant to the drilling process intuitively seems like it would slow, not speed up the process, even though emperically I found that adding lubricant in fact increases the ability of the drill to cut through the metal. I guess I'm thinking that the more friction and heat the better, but I'm sure one of you theoretical types will explain to me why this isn't the case.

 

The short answer is that cutting fluid reduces heat caused by friction and lowers friction and surface tension, which increases swarf dispersion. The goal is the exact opposite of "the more friction and heat the better", which quickly ruins cutting tools.

There are specific cutting speeds recommended for various materials which produce the best results and extend cutting tool life. Tool speed and feed rate is determined by the material being machined and the diameter (or blade surface area) of the cutting tool.

You can cut faster with a 2-flute milling bit than a 6-flute milling bit, for example, but you'll get a smoother finish from the 6-flute bit. Likewise, a large hole saw requires a very slow rotational speed to reduce heat build-up from the large surface area represented by so many teeth in contact with the work surface at one time.

 

When properly used a drill bit cuts two continuous chips, much like peeling an apple in one long "chip". The cutting action is much like a lathe tool turning the outside of a part. Another analogy would be plowing a field. It's a similar action. It's not a wearing away or abrading of the material...well it's not supposed to be!

Unfortunately, too many people don't let/make the drill cut a chip. Rather they let it rub without cutting. That genetrates lots of heat, very few chips and usually a frustrated operator. In non-magnetic stainless (among other metals) it also work hardens the material which makes drilling the hole much more difficult. We've all been there, right?

The correct method is to choose an rpm appropriate to the material and drill diameter and "feed" it into the metal at a rate which makes a chip. Less rpm and more feed rate is preferred to more rpm and less feed rate. This is especially so when drilling by hand with a portable drill or even a hand fed drill press. If the chips become too small and break up, you probably need more feed. Push harder! There are exceptions to the rule, of course.

The heat generated is from the power it consumes peeling off the metal chip. There is friction and heat generated from moving the metal. A lubricant at the point(s) of cut can lessen some of the friction and in enough volume, remove the heat. This is a lot like how engine oil works.

WD40 helps keep the chips from sticking to or loading up the drill bit especially in aluminum. If the drill flutes get stuck full of aluminum chips, penetration of the drill virtually stops. A heavier cutting oil or even motor oil works better in steel. Here lubrication is more important than preventing stuck chips like in aluminum.

If you are having trouble penetrating, slow down and push harder. That works in other things in nature too, I guess.

 

 

I know this might be a stupid question, but can anyone give me the proper way to drill through metals and even drilling out broken bolts etc.

Growing up i drilled thin metals and woods occasionally with fixing things and small projects and was always taught to spin to push hard and just keep the drill straight. If the drill moved on the surface, i would use a punch to produce a divot in the surface to start the hole.

Since starting working on car, I have broken a few rusty bolts and even over tightend a few things as well as had to drill things. I seem counter productive since these are harder metals that require more drilling. I almost broke or dulled all of my drill bits (3 sets of fairly nice ones) (So now i have learned to apply less pressure and let the drill move forward at its own space. Is there anything else i should use? Ive seen people use cutting lubricant but I never used it, is wd40 good enough to use?

Ive gotten frustrated, i had to drill a hole and tap it to put a screw in but i ended up breaking a drillbit and it got lodged inside the hole and i couldnt seem to do enough damage to the back of the drillbit to get it out with the screw extractor.

Do i need to get different kinds of bits (been using 2 sets of dewalts and another bosch set or something). Or do i need to do anything else special? I read oldsstrokers info and that seems helpful ill try to apply it next time.

 

Jim, thanks for posting that link. I deal with swarf all day. We just call it chips. I learned something new today

 

as for drilling out bolts invest in a good center punch and a center drill. I assunme you ise easy outs as well

 

Broken off bolts - Penetrant and heat. Slowly work it back and forth, not just one direction.
Broken off bolts below the surface - Penetrant, drill center out, extractor and heat if necessary.

Common denominator is HEAT.

As for drilling, I like colbalt bits. Like oldsstroker said, slow down and push. I use Tap-magic for steel's, and wd-40 for aluminum. Work your way up to final size with drill bits, that can make it easier.

 

As far as getting out broken bolts, I haven't found any I haven't gotten out with these-
http://www.grainger.com/Grainger/wwg...537944&ccitem=

I usually take a punch and hammer and tap on the bolt first. Then center punch the center of the broken bolt and drill a small hole..then you can use those extractors with ease..use heat as stated if you have a hard time.

Im with machinistone on the cobalt drill bits...They may be more expensive but they are worth it in the long run..they can drill through all your tough metals also(stainless)

Cody

 

Yep - I use the same product, except it's a kit of 25 from snap on, they work great.

 

I see lots of manufacturers making drill bits with "pilot points" now. I had some, and they work good on wood, but they seem to suck for drilling metal. Once the pilot point gets in there, the rest of the blade rides on a lip around the edge of the hole and doesn't cut in as fast as a normal bit would.

 

It's best to start with a centering drill as was mentioned. They are very stiff and hard and you can walk the drill to the center of the broken bolt if you need to and get your hole started straight before starting in with the regular drill. The only thing bad about them is that you need a good quality chuck with a key to tighten it or the drill bit might spin in the chuck and since it is double-ended it can drill backwards into your drill chuck.