Drill Bits - Need some help and advice

[BukitCase]

SFM is Surface Feet per Minute - example - 1/2" drill bit, 240 rpm - a tooth on the bit travels just over 1.5" for each revolution (formula for circumference of a circle) - X 240 rpm = 360 linear INCHES per minute, divided by 12 = 30 SFM.

Here's a couple of links for more -

Machining Feeds and Speeds Calculator - Engineers Edge Engineering Calculator

Milling: Cutting Speeds & RPM Calculations

Some of that stuff is more for milling cutters but the idea is the same.

My drill press only goes down to 250 rpm and I've used it with up to 1" drill bits and 3" hole saws - the main thing is keeping feed rates up enough so you're getting good chips (drill bits) - also helps a LOT with bigger drills to drill a pilot hole about 1/4", then step up thru 2-3 sizes to the final size.

While annular cutters are great for bigger holes, they are very temperamental about side loading and very few drill presses for under $1000 are rigid enough NOT to break them. At prices from about $25 each up to several hundred$, they hold up a BUNCH longer when used with either a mag drill or a milling machine. Because they don't drill out all the metal but just a "fat hole saw" donut, they are several times as fast as large drill bits.

There are separate speed charts just for annular cutters. The biggest mistake made with these is NOT pushing them - you want a bit of color on the chips or you're not pushing hard enough. Too light a touch and you'll work harden the material and take the temper out of the cutter. $$$

One thing that's a necessity with hole saws is drilling a few holes (1/4" or so) around the inside perimeter of the saw track for filings to escape. Otherwise the saw will clog and you're done.

Another necessity is cooling - either a lube system or a large enough air compressor that you can keep a jet of air on the cutter throughout the cut - either way is messy, but doing neither will cost you a lot more in replaced (or sharpened) bits. NO free lunch here.

HTH... Steve

 

[Shield Arc]

I desperatly need to learn how to sharpen a drill bit.

Here you go.
https://www.youtube.com/watch?v=QbRPPxyw1hw&list=PL_MD_oorfJwFrXFSVwCvr-9t-M7q3tTOQ

 

[bearcreek paul]

Travers tool or any local machine shop supply should have good high speed steel bits. It all depends on what you are willing to spend. Learning how to sharpen bits is a must. It will save you a lot of time and money. The larger bits look for at auctions if you don't need them right now.

 

[KennyG]

I desperatly need to learn how to sharpen a drill bit. I tried once and it was so jacked up.... never tried again.

So stupid me, what is SFM? Can you give me an example of the math you are showing?

Also, hole saws, How do you sharpen them? Do you sharpen them?

Drilling speed. I think my drill press is like 200RPM I feel it might be too fast for larger bits. THoughts?

I don't know of any easy way to sharpen hole saws, but they last really well. For a given size hole, they remove a much smaller amount of metal than drills. I don't have a sophisticated setup but I use them on my drill press with one hand on the feed lever and cutting fluid in the other hand to keep it wet. As someone else mentioned, keep the chips cleaned out.

Most lower priced drill presses are too fast for large drill bits. I know mine is which is why I default to hole saws at the slowest speed.

 

[flusher]

I just bought a set of milwaukee black oxide drill bits and they can cut..

Same here :thumbsup:

 

[woodlandfarms]

Guys thanks for the great info...

But let me press on.... What about step bits and what about fancy coatings on twist drill bits.

 

[Big Barn]

Guys thanks for the great info... But let me press on.... What about step bits and what about fancy coatings on twist drill bits.

I keep one cheap small drill press set up exclusively with a step drill.

Terry

 

[Sodo]

Go to Amazon, search "Blair Equipment 11090N Rotabroach Cutter Kit". Best $50 you can possibly spend on cutting holes in metal up to 1/4". I use these with my 18v drill. You can drill up to 1/2" if you can get both sides, you have to drill a small pilot hole thru (like 3/32) then rota-broach it from both sides.

These Blair cutters make accurate holes, more accurate in roundness than a twist drill (no triangulation). But you don't get a lot of size choices.

They cut fast because you're not turning the entire plug into chips. i use WD-40 for lube it's easy and evaporates all by itself.



- Kit Includes: 5/16, 3/8, 7/16, 1/2, 9/16, 5/8, and 3/4" Cutters, Arbor, Hex Wrench, Center Punch, and Extra Pilot
- Packaged in a plastic carrying case
- Made in the USA

You can get replacement cutters, but they come in "3s".

I also use larger "standard" holesaws (like the ones for wood except "bi-metal") to cut larger holes in steel, if you use oil they stay sharp for a long time. Have had good luck with LENOX, cutting lots and lots of holes, remain sharp as new.

 

[Big Barn]

If you get the Rotabroaches purchase extra arbors so that each cutter has its "own". They can be difficult to change without damaging if you're in a hurry. Plus time consuming. ;( Terry

 

[rgr33]

Shield Arc, thanks for that You tube link, I'm trying to up my game in drill bit sharpening, and that gave me a better idea of how to go about it. Can't quite get the hang of the Drill Doctor.