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Drill Bit Selection

This is a quick and general description and guide of just some of the many different types of drill bits. Check also this super wiki page.

FOR WOOD

-Twist Drill Bits. These are the most common type of drill bit you find at

hardware stores etc. When drilling deep remember to pull up

occasionally to clean out the flutes. Twist drill bits also good for metals.

photo of twist drill bit

photo of spade bits

-Spade Bits. These are available only in sizes from ¼ to 1 ½ in. They

are a flat steel bit with locating center point that peel the hole with the

sharp edges on the bottom of the flat.

Increments of 1/16. Spade bits tend to bore ever slightly oversize holes.

Only for wood and some plastics.

Forstner Bits. A flat bottomed bit with a small brad that centers the cut. Usually in sizes from

¼" to 2", in increments of 1/8". Occasionally you can find them in 1/16th increments and up to 4".

They are precisely made, are more expensive than spade bits, and cut an accurate hole.

Photo of forstner bit

photo brad point drill bit

BRAD POINT BITS They are similar to a regular twist drill bit but they

have a small sharp centered point that helps position the bit and sharp

outer edge points that shave the hole. Only used on wood and some plastics.

Do not confuse with split point bits that can also be used on metal or plastics,

where the 'split point' centers the bit in place.

For Plastics

 

All of the bits used for wood can be used on plastics. On harder plastics and acrylics care must be taken on the amount of pressure (and having a solid backing behind the bit) used so as not to shatter the plastic. Slower speeds work best especially on hard plastics. There are twist bits expressly made for hard plastics that have a sharper angled point (generally 60 degrees rather than 132) and less 'pitch' on the flutes to keep the plastic from riding up the bit.

For Metals

Twist Drill Bits. These are your choices although there are different types categorized by cutting tip angle and metal composition ( i.e.118º is the most common tip angle and HSS hardened steel is the most available composition and good for general use. Unless otherwise mentioned assume the bit has a 118º tip which is really only important if you are sharpening using a jig. Many general bits will state 135 degree split point which works even better on metals.

-Low Carbon Steel. Good for soft woods but even for hardwood are useless. Not for drilling metal as they dull rapidly.

-High Carbon Steel. Marginally better than the above. Good for soft non ferrous only.

-HSS (high speed steel). Made from tool steel and are much more resistant to heat so their tips hold the cutting edge longer. Good for metals and wood.

-Cobalt Steel Alloys. Harder again than HSS so stay sharp longer. Good for stainless steel as well as other metals.

-Tungsten Steel. Extremely hard and very expensive.

 

-COATINGS ON TWIST DRILLS.

Black oxide. Helps protect the bit from oxidizing, nothing else.

Titanium nitride. Hard ceramic material, helps hold edge.

Zirconium nitride. Hard ceramic material, helps hold edge.

The benefit of any coating is gone as soon as it is worn off or

the drill is sharpened which usually doesn't take long at all.

BEST PROCEDURE.

Drill bits get dull from overheating before anything else. It softens the tips and they wear quickly. The best idea is to keep them cool and lubricated.

I keep a cup of water near by and dip the tips frequently to cool them, go slow, and exert moderate pressure.

The friction of drilling heats the tips and at 250 degrees they lose their hardening temper

Doing the above my cheaper black oxide split points last much longer. Cleaner than using a bunch of oil. Just clean the water up

after and lightly oil the surface and hole where there is water.

 

Step Drill Bit (unibit).

A conical shaped bit with steps of increased size stacked on top of each other. Allows you to enlarge a hole to a chosen size with just one bit. Used almost exclusively in sheet metal work.

Photo of step drill bit

Hole saw

Hole Saw. Not really a drill bit but can be used to make holes from ¾

To 6 . A hardened steel barrel with saw teeth along the edges.

The saw is attached to an arbor that is inserted into a drill. Used

On both metal and wood. With VERY careful use can be used on

plastics. 

FOR MASONRY (brick, concrete, stone)

For drilling in brick, stone or masonry use masonry bits. They have a carbide tip that is slightly larger than the shank of the bit. To be effective you really need a 'rotary hammer drill' that pulses as it spins.

masonry drill bit

glass drill bits

FOR GLASS.

Special diamond abrasive bits and special hole saws with diamond abrasive edges are used on glass, glazed ceramics and tiles. You need to go very slowly (so a drill press is super handy), keep the pressure applied low and use water for cooling. You can make a 'dam' of putty to hold a small amount of water around the place to drill. It is very easy to make a mistake and ruin the piece, so be extra careful using low pressure and slow advance.