Spade drills

[SteveM]

I see spade drills on ebay. Not the kind for boring wood, but metal ones with a shank and replaceable blades.

What are the uses / advantages / disadvantages of them?

Steve

[Harold_V]

Lower cost. Large drills used to cost about $100/inch of diameter. I have not priced them of late, but I suspect that still applies, taking in to account inflation.

With a common shank holder, spade drills can be far less expensive, and they offer the opportunity of sharpening without removal from the machine, although I'm not convinced that's one of the reasons they might be chosen.

Spade drills lack a spiral flute, so chip evacuation has the potential to be troublesome.

Spade drills---the poor man's large drill option.

Harold

[LIALLEGHENY]

Just my 2 cents/opinion based on my experiences. Spades drills perform better at higher RPM than conventional drills, this means more HP is required. I have found that they don't work well if you already have a pilot or smaller diameter hole drilled ....again this means more HP is required. If you chip the insert, throw it away....a conventional drill you can easily resharpen. They also prefer to have constant pressure /feed so they are not well suited for use in a tailstock. They can be used if mounted to a tool-post using the carriage feed, but they do put a lot of strain on the carriage and gear train especially when you get to larger diameters.
I recently had a job boring out a piece of inconel. Not the nicest stuff to work with. I had to put a 3.5 inch hole through the part. A friend told me to try a spade drill, 2" diameter which he had so I tried it. It worked beautifully until I stopped the cut to check the hole. There was so much stress built up that the carriage shot back about an inch or so when I released the power feed. I decided this wasn't a good idea for the machine ( 19" leblond Regal) I didn't want the carriage or quick change box to explode. Back to the tried and true conventional drills I went....

Just my 2 cents,

Nyle

[earlgo]

All of the replaceable blade spade drills that we used in the '80s on a CNC mill had coolant passageways and a high pressure coolant feed thru. The coolant ejected the chips with authority so spiral flutes were not required.
We did not use a pilot hole, because the machine was a 10 HP Cincinnatti T-10 Horizontal with pallet changer and 90 station tool chain, so it had plenty of oomph.
One could drill to remarkable depths with that combination.

--earlgo

[Russ Hanscom]

We need a bunch of 1 1/2" holes in 1 1/2" plate at the RR. After pricing bits I have acquired some HSS and carbide spades from Ebay. We have a 3 hp industrial drill with power feed. I will report back in a few months!

[Gary Armitstead]

We used spade drills in our die sinking trade often. We would have to remove a large amount of material from hard die steel to drill for pin hole locks. Usually holes starting at 2 inches in diameter up to 6 inches in diameter. Only used on large radial arm drill presses with HP in the 20 to 30 HP range and 24 inch columns. We flooded the holes with coolant to help remove chips. If the drill wasn't sharp, you could see the radial arm "jump" when building up pressure from the dull drill. Exciting stuff!

[SteveM]

OK, sounds like my 1/2hp South Bend drill press doesn't have the horsepower to drive one of these.

Steve

[earlgo]

I'd bet it has enough horsepressure to use a hole saw followed by a boring bar. I've used 2" hole saws on my anemic Atlas lathe with success. Just don't expect to rush through it, use plenty of cutting oil and clean out the gullets frequently. Depth is limited to the depth of the saw cup.

--earlgo

[refinery mike]

the trouble with hole saws in a small drill press is that those drill presses generally do not go slow enough. That produces terrible chatter problems. (I added a reducing shaft and pulley to slow mine down. Also if you are drilling deep holes with a hole saw you should drill some half inch holes along the cutting circle (inside the line or outside the line depending on what you want to save.) otherwise the chips pile up in the curf. Not to discourage the use of hole saws, I do it all the time. Just some tips to make it practical.
as far as spade drills, i am pretty sure they were pretty much the standard drill to drill holes through the web or rail road tracks. Large holes through tough steel.

[earlgo]

Agreed, a jackshaft was added to my Delta woodwork drill and cut the speeds in half.

--earlgo

[Carm]

I see spade drills on ebay. Not the kind for boring wood, but metal ones with a shank and replaceable blades.

What are the uses / advantages / disadvantages of them?

Steve

Couple things that weren't mentioned.
Spade drills hold size very well. IME, within .002
They don't wander when stack drilling.
Since they use inserts changeover is easy and consistent.

Harold, I expect you will say that a properly sharpened twist drill competes with the above.
I agree. However, in a manufacturing environment where instead of machinists one has clock punchers, the things win in spades. (sorry)
Even a skilled hand will have difficulty sharpening dead center offhand, definitely a cause for wandering.

[Harold_V]

Couple things that weren't mentioned.
Spade drills hold size very well. IME, within .002
They don't wander when stack drilling.
Since they use inserts changeover is easy and consistent.

Harold, I expect you will say that a properly sharpened twist drill competes with the above.

Actually, no, as I am not a fan of twist drills. I use them because one must drill holes, but it was proven to me VERY long ago that a twist drill is a miserable cutting tool, having but one true quality---that being the ability to move metal at an alarming rate.

To clarify, twist drills are a great roughing tool, but to expect them to provide a straight, round hole, on location (no drifting in the process) is a huge demand. When all those criteria are met, it's more the luck of the draw than by design.

Still have doubts?

Drill a deep hole, say 3/8" in diameter, then try putting a 3/8" drill blank in the hole. Will it go? Remember, drill blanks are typically ground .0002" under nominal.

You say it goes? All the way through? Rattle fit? Snug fit? Look through the hole. Do you see a pattern? I expect you do.

The point is, twist drills, at their best, provide a compromise, although I'm not implying that that is a problem. For most cases, it is not. In cases where it does, one then resorts to reaming (better than a drilled hole, but not necessarily a great hole), or boring. Honing gilds the lily.

Harold