Please teach me about drill bits

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JHZR2

JHZR2

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Hi,

Ive searched here and on the interweb, but have never found a good, concise explanation of drill bits.

Long story short, I need to drill a hole in some 3/4" thick pieces of wood. The hole would be about 3/4" in diameter.

Ive always used "standard" drill bits for various things, but there are other kinds. A 3/4" drill bit is quite expensive.

I noted that there are spade bits, and then augers and "super spades" which are like three spades with an auger behind it. There are also fancier bits for making really nice holes.

So my questions are:
-when do you use a spade bit versus a regular bit?
-will a spade bit ruin the wood on the side where it comes out due to ripping the wood?
-Does a more brittle standard bit, like a cobalt bit make any difference if drilling something soft like a new 2x4? The chart at HD said cobalt bits were not recommended for wood, just metal.
-Why would you use an auger over a spade, or just a longer regular bit?
-When would you use a hole saw versus a big spade bit for wood?

I can see that there is a benefit for the augers and spades for getting them centered perfectly, vs standard bits which can walk a bit. However Ive never really had an issue if I made a small indent with an awl and/or made a smaller initial hole. If placement isnt a huge issue, does any of this really matter? Are spade bit just a far cheaper way to make holes once the holes get larger tan 1/2" which is the end of available and cost effective bits?

Thanks very much!
 
Spades are cheap and work quickly but can leave a ragged hole. The trick is to switch to the other side of the workpiece as soon as the pilot breaks through. Spade bits don't eject the chips so if it's a deep hole it can get bogged down with chips
Auger bits are self-feeding and self-cleaning so if you have to drill a lot of holes or deep holes the auger is much more pleasant to use.

As you mention, a regular twist drill bit is fine in wood up to about 1/2", when the hole is larger it is easier to drill with a spade or auger.
 
Twist bits are designed for use on metal, but everybody uses them for wood also.

Spade bits are a poor man's way to drill a large hole in wood without spending the money on a better designed bit which gives much better performance. The best trick with an spade bit if you don't want tear-out on the back side is simply to lay your board on another board, then drill the hole. The second board will act as a backer board and prevent tear-out.

Cobalt bits work just fine in wood, the cobalt is a just a coating on a twist bit, unless they've changed them recently.
 
Originally Posted By: hate2work

Twist bits are designed for use on metal, but everybody uses them for wood also.

Spade bits are a poor man's way to drill a large hole in wood without spending the money on a better designed bit which gives much better performance. The best trick with an spade bit if you don't want tear-out on the back side is simply to lay your board on another board, then drill the hole. The second board will act as a backer board and prevent tear-out.



+1 on using a backer board with a spade bit. That sounds like your best solution for a 1 time job.

If you only expect to be drilling a few 3/4 inch holes in wood and won't need a 3/4 inch bit in the future, get a spade bit.

If you are going to be drilling a bunch of holes in wood, get an augerbit.

If you think it's worth while to to have an all purpose drill bit in your tool box and have a 1/2 inch drill chuck on your drill, get a standard 3/4 inch metal drilling bit with a shank that will fit your drill. If your biggest drill has a 3/8 chuck, don't bother getting a 3/4 inch metal bit.

AFAIK, a Cobalt bit is Cobalt steel all the way through unless it's a fake cobalt drill. No harm using it on wood but no reason to buy one unless you need super hard drill bit and have a drill to take advantage of it's properties.
 
A spade/paddle bit will make a hole fast, but will blow-out on the back. Use a backer board and the hole will be cleaner. DeWalt and Bosch make decent bits that cut a fairly clean hole. Cheaper ones dull quickly.

Auger bits are great for hogging through 2x4's (think electricians and plumbers). For a 3/4 hole, you need a pretty hefty drill with a side handle for control. If the bit binds, it can hurt.

A good twist bit with a brad point will work for most woods, but they can be expensive for just a few jobs.

I do quite a bit of hobby woodworking and anything above 1/2", I tend to use a hole-saw. It leaves a clean cut on both sides. The trick is to let the pilot bit just poke through on the first cut, then flip the wood over, and put the pilot bit in the mark and finish the cut. A piece of blue painter's tape on both sides keeps the chip out to a minimum.

Just my .02.

Dave
 
On related topic, why does Drill Doctor cost so much money? The whole contraption should be selling for $19.95 or less but they go for like multi-hundred dollars.

- Vikas
 
The cleanest holes in wood are drilled with a Forstner bit using a drill press, and a sacrificial board clamped under the main board to the drill press's table.

I like running the bit in reverse until it scores the outermost layer of wood. No tear out.

I've found I can sharpen a drill bit better by hand with an angle grinder than a drill doctor. As long as I use a pilot starter hole with a factory drill bit which will not walk, my angle grinder sharpened bits out pigtail the drill doctored bits every time.
 
I was always under impression that drills were consumables and reasonably cheap to throw away. It is like buying a $200 spark plugs cleaning machine to clean regular plugs. Oh, and that spark plug cleaning machine can not be used to clean platinum or iridium plugs.
 
Originally Posted By: Vikas
I was always under impression that drills were consumables and reasonably cheap to throw away. It is like buying a $200 spark plugs cleaning machine to clean regular plugs. Oh, and that spark plug cleaning machine can not be used to clean platinum or iridium plugs.


You haven't bought many good quality American made bits lately have you? They are not cheap and you NEVER toss them when they get a little dull. Do you throw away your knives when they get dull or do you sharpen them? At a cost of $4.00 to over $20.00 each, I can't afford to NOT sharpen them. It only takes a few minutes and the bits stay sharp a lot longer than the imported bits that I have tried.
 
Originally Posted By: Vikas
I was always under impression that drills were consumables and reasonably cheap to throw away. It is like buying a $200 spark plugs cleaning machine to clean regular plugs. Oh, and that spark plug cleaning machine can not be used to clean platinum or iridium plugs.


Cheap drill bits are extremely expensive to throw away if you break one off in a hard to access place you need to get it removed from.
shocked.gif


And, if you do use cheap bits, if you resharpen them properly, they will work better than when they were new.
 
Originally Posted By: hate2work
Cobalt bits work just fine in wood, the cobalt is a just a coating on a twist bit, unless they've changed them recently.

Originally Posted By: XS650
AFAIK, a Cobalt bit is Cobalt steel all the way through unless it's a fake cobalt drill. No harm using it on wood but no reason to buy one unless you need super hard drill bit and have a drill to take advantage of it's properties.

Please allow me to chime in...

A cobalt drill bit is entirely made of cobalt (typically M35 or M42), rather than the high speed steel substrate that standard drill bits are made of. HSS is typically M2 material. Perhaps you were thinking of the gold coating seen on some drill bits, which is titanium nitride (TiN)? Cobalt drills are a straw color as opposed to silver for uncoated hss, so maybe that's where the confusion came from?

Most standard HSS drills are fine for mild steel (and wood for that matter, in most applications), but they usually have a standard 118º point that may walk on hard surfaces like metals, unless you've located the hole with a pilot or used a spot drill first. Most cobalt drills have a 135° split point that helps prevent walking, plus they have the added benefit of a higher hot hardness making them suitable for more difficult materials like stainless steels.

Just be aware of the tradeoff between HSS's toughness vs. cobalt's hardness, which you may understand when comparing them to tires in a very general sense. A hard tire will last a long time but won't grip well. A soft tire will grip well but won't last long. A hard tool will last long but will be more brittle. A tough tool can take abuse but will wear faster. For most applications a standard HSS drill bit works just fine.

Hope this helps, and no I'm not a metallurgist or an expert on the subject, but I've sold cutting tools so I have a bit of experience.

[/end thread derail]

OP:
A couple links below with pictures that may be helpful. Note, some auger and spade bits have a spur that helps with breakthrough, although another piece of wood as a backer is a good idea as mentioned by others.
http://www.magbit.com/tech_info_auger.php
http://www.bamanufacturing.com/page_40.htm
 
was a machinist at Boeing for 25 years. and i tend to get picky sometimes. i bought some cheep drill bits, found some to be as much as .002-.003 undersize. for must work that will not be trouble. but for you talking about making aircraft tools. one will need to go a realy good reamer.
 
Originally Posted By: morris
was a machinist at Boeing for 25 years. and i tend to get picky sometimes. i bought some cheep drill bits, found some to be as much as .002-.003 undersize. for must work that will not be trouble. but for you talking about making aircraft tools. one will need to go a realy good reamer.

Congrats on being a machinist, these days most are machine operators not true machinists.

For metals, there are few drills that are finishing tools. As you're aware, they're primarily used for roughing and the hole is usually either bored or reamed later if necessary.

However, OP is working with wood, so I doubt a reamer is necessary.
 
If your like me and use drill bits for metal in a drill press, and really like to force it through, cobalt is the way to go. TIN bit are horrible in my experience in a drill press and just chip the point off. HSS dulls very quickly when heated up. Cobalt is just right.
 
TiN is a coating on HSS bits for cooler operation in machining operations. No reason to buy TiN for home use unless they are cheaper than standard HSS. Inexpensive bits, i.e. harbor freight, seem to come TiN coated.
 
I agree, but if they're poorly coated and/or poor quality bits to begin with, it may not even be worth it for TiN.

Warning: Long winded response below...
coffee2.gif


A TiN coating provides two main benefits. Hardness, bringing longer tool life, and it's friction reducing properties which results in cooler operation as stated. I think the hardness for home DIY use may actually diminish it's value rather than enhance it. Although, your mileage may vary.

The reason, is that the benefits of hardness become it's liability when toughness is required. A hand held drill needs a tough drill bit, period. While a drill press helps improve the rigidity of the operation with improved workholding, I'm not sure TiN will help all that much. The real problem may be with surface footage and feed rate.

For example, in 1018 low carbon steel, a standard ¼ HSS jobber drill is recommended to be run at approx. 100 surface feet per minute (SFM), which converts to about 1,500 RPM, with a feed rate of about .006 per revolution, or 9 inches per minute (IPM). For a machining center, those speeds and feeds can be met easily and accurately, but for the home DIY'er, not so much. And the smaller the diameter of the drill, the faster you have to spin it to meet the SFM requirements. Tough tools are required when being run slower than they were intended, and when the feed rates are inconsistent like home use. In a machine shop, higher performance tools can be employed for improved cutting data.

In short, for home use, you can't go wrong with good quality standard HSS drill bits. If you can take advantage of cobalt in your specific application, fine. But for the average Joe, you don't need to. And keep them sharp no matter what you use. The increased tool pressure required to push a dull drill bit, will increase it's susceptability to breakage.

BTW, I'm not trying to come across as a know-it-all. Just trying to contribute with something I do know a thing or two about, since you guys will know more about oil than I ever will, and I've taken much more from this site than I've given.

Cheers,
cheers3.gif
 
yea i had to bore holes 3/4 to 1 1/4, in steel mill fixiures. that had to be = or - .0002/.0001. these are called sine key holes, for pins to aline the the mill fixture, for mounting on cnc mills. i ran a manual machine. some of the plates were 3 ft by 8 ft. not only the diameter had to be that close the distance between holes had to be that close, some times in aluminum 200 inchs apart, in a non air conditioned room. had to compensate for temp.
 
to: tmoto yea your right about machine operators. i call them "cycle start guys" the machine i ran had 240 " of travel in x. and that wasnt the largest machine boeing had. my machine had a readout that had a steel tape for digital measuring, so if i had a aluminum part i had to compensate the expansion DIFFERENCE between steel and aluminum























'
 
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