As previously explained, the spade bit derived from the old brace's bit called the center bit.
The spade bit on the right is a modern Irwin Speedbor 2000.
Not all spade bits bear these improvements.
The original spade bit has only a flat bottom filed at an opposite angle on each side of the center pin.
That one is a Irwin Speedbor 88, notice the groove cut into the pin,
it is to facilitate its cutting action with less pressure. Older ones have a straight pin, no groove. The hole often found on the spade is to help fish wires for electrician.
It is designed for high speed operation in a drill and has more of a scraping action than a cutting action. As a result it is often looked upon as a rough cutting tool, and indeed it is often used by electricians for cutting holes thru framing timbers for passing wires (electrician or installer bits). It leaves a roughly defined hole with splinters, when used as such, but it is quick and cheap and if you hit nails, no big losses
But it does has its uses in our workshop. They are easily customized for odd sizes jobs, they can be made to cut small rotary profiles etc. (think on top of a knob) They are very versatile and the new versions like the Irwin Speedbor 2000 introduced a few refinements to make them cut better and faster with less effort. The older plain ones are easier to modify BTW.
Speedbor 88 Plus
The improvements made are:
The groove in the center pin makes it cut faster, longer (88)
The side spurs on each sides help score the outline before the scraper action, resulting in a neater hole (2000)
The way they made the spurs, also changed the cutting action of the scrapers edges. More of a curve cut than a flat cut on the smaller sizes.
They are a lot easier to sharpen than the center bit owning to its simplified cutting geometry. Especially the older ones with no side spurs.
If you touch up the center pin, careful not to offset the pin off center by removing more material on one side. You are better off to simply touch up the flat part of the pin to restore the sharp arris of the pin than messing up the angled parts.
Often all you need to do to restore the scraping edges is to touch up the flat part of the body. Unless you nicked the edge...
Note that the newer Speedbor 2000 requires a small rat tail file to sharpen instead of a flat file if you need to touch up the cutting edge in the smaller sizes.
HSS bit brazed to a carbon steel shaft. MF Buck Rogers bits.
The scrapers are not flat but slightly angled inward,
effecting a side spur cutting action.
Top Stanley Power bore (now reproduced by LV).
Middle Forgot its name, no longer available, but its short pin, beefy round back wings allow you to change directions easily while cutting. Great bit.
Bottom Ridgid woodboring bit. Some sort of hybrid bit.
It claimed to be up to 6X faster than a spade bit.
I bought this bit to cut the dog holes on my maple top bench.
It grabbed too fast and quickly drained my cordless drill :-(
Finished that job with a Forstner bit with a corded drill...
Incidentally, the next logical progression from a simple center pin was to make it threaded. It solved two problems.
1- It negate the user having to sharpened it and buggered up its center
2- It facilitate the bit to be pulled down without using undue pressure
But it also introduced a small drawback. Depending on the cutting geometry of the bit and the type of wood used, the thread used may pull the bit too fast. Such of what happened when trying that new fancy bit from Ridgid on my 1-1/2 in thick maple bench top in a cordless drill...Fails!
That's why, Jennings and Irwin came with three different thread pitch:
1- Coarse, for softwood
2- Medium, happy compromise for both soft and hardwoods
3- Fine, for hardwood
There was brace center bits made with a thread center pin (rarely seen) but I never used one. Bosch came out recently with a modified spade bit with spurs and threaded pin.
Bosch Daredevil bit
Reputed to be 4 times faster than traditional spade bits...
The body seems a tad wider than the spurs.
Probably would thread too fast and easily jam... 4 times faster :-)
I'll stick to my Irwin's Speedbor thank you.
And finally this style of bit is easy to modify to make it ''Expandable'' : the expansive bit (no it is not a typo, it is not EXPENSIVE :-)
Irwin No 21 expansive bit
By changing the wing cutter you change the range of adjustment.
Bob, the ace of spades