I am looking for a bit to bore numerous (over 100, so speed matters) 9/16 or 5/8 holes in 4x4 pressure treaed lumber, which will then serve as the uprights for horse jumps. The holes need to have a clean entrance, smooth(ish) inner surface and a relatively clean exit. I am trying to avoid too much tearing and feathering. Due to the volume of holes, I would like to avoid having to drill from both sides of the stock, so a relatively clean exit hole is helpful.

In the past when I have done this, I have used either a traditional twist drill or a spade bit, both of which have left relatively rough holes, particularly on exit, which makes placement of the jump cup pins difficult.

I am thinking that an Irwin Max Speed trident spade, a Blue-Mol Xtreme Power or a Bosch Daredevil Tube Auger may go the job for me, but since these things run about $15/each, I'd rather have some feedback.

Due to space and location constraints, these will NOT be done on a drill press, but rather with a hand drill and jig, so a bit that will hold straight and not wander is important.

Auger bit, but back the piece with scrap lumber for cleaner exit. The scrap supports the wood fibers on the other side instead of allowing them to splinter. Clamp it or some other way provide a force to hold it against the piece, don't hold it yourself.

Forstner bit is what you want. Good drill bits are not cheap, so expect to pay. Buy several. Remove the bit from the hole frequently as you drill to remove wood chips. Loose wood chips will start to burn from friction and ruin your drill bit. Use a tight backer and slow down to minimize back tear out.

I have backed the piece, and it does help significantly. Part of the problem is that pressure treat is soft and wet, and it tears more than a hardwood piece even when backed. I also have a set of forstner bits and they are the cleanest drilling I have used. Because of the need to back them out frequently however, they fall down on the "speed" part of this equation. Clean they are; fast they are not.

I am not sure "speed" and "clean holes" are compatible. I agree with cne32507 that a Forstner bit will make the cleanest hole.

quote:

Forstner bit

Nothing cleaner.

Forstner bit will work well, but they work best with a drill press. You didn't say how thick the lumber is you need to drill, but a hole saw might be another option. If you go that way, I'd suggest a carbide tipped hole saw given the number of holes you want to drill.

quote:

Forstner bit will work well

As others have suggested

Is the wood square? Meaning, can you stack several of them together and drill several at a time? That would solve the backing and speed piece of the equation.

If so, you can get a long bit.

Yes, a forstner would be a good choice, except that, because of the nature of treated wood, soft and wet as you mentioned, I think you would find that it will run hot and plug up frequently, so you might spend a lot of time cleaning and letting it cool down a bit. I know that you said you would like to not go in from both sides, but that's what I would do. Lay out as many as you have space for at one time, use a good Irwin either spade or speedbor with a stop so you don't have to guess, go down the line, roll them over and finish from the back side. A back piece helps, but limited as it's doubtful that you will have enough of them that are that flat to start with.

What I've done in similar situations, is drilled all of the holes through with a 3/16 bit, drilled an inch into one side, then flipped the piece over to finish.

A forstner bit will be very clean, but you'll still contend with chip clearing and blowout on the backside. Even with a spade bit you can wait for the tip to pierce the back and flip the piece to finish.

I wouldn't use an auger if it has to be 100% neat.

Blowout on the backside can be awkward.

A forstner bit would give you a clean hole, but like DexterII I think if you're using one on treated lumber you'll have heat and chip clearing issues.


Brad point bits give clean holes and would be a lot faster than using a forstner bit. If you're drilling all the way through, a sacrificial backer board will give you clean holes on the exit side.

 
I would use a modified auger bit in a drill press. Modify the auger by grinding or filing off the threads on the lead screw, so the bit doesn't pull itself into the hole. The auger bit will clear the chips nicely, especially a double twist auger, and especially since you'll be drilling through 3.5" of wood.
 
A backing piece is the best way to eliminate tear out on the exit side. Also best to clamp the workpiece to the backing piece on the drill press table.
 

I have to agree with the auger bit. It is cutting like the Forstner but long and open so to clear chips easily and increase speed. Have made a lot of holes with dad's old brace and bit.

Fastest would be a spade bit but you need to keep them sharp for the cleanest hole.

Sounds like you have the project triangle dilemma.
Good, Fast, Cheap, pick two.

quote:

Originally posted by SigJacket:
Auger bit, but back the piece with scrap lumber for cleaner exit. The scrap supports the wood fibers on the other side instead of allowing them to splinter. Clamp it or some other way provide a force to hold it against the piece, don't hold it yourself.

This. Scrap lumber backing is the key. A couple of times I've done a guide hole and drilled the exit hole before going all the way through.

Irwin speedbore. Cuts fast and clean. Use a backer to minimize tearout

Thanks very much for all the suggestions. Likely I will not have to buy another bit, and will use some combination of the techniques here to make the system work suitably well. I have every other kind of bit in my garage!

Unfortunately, soft, wet, PT lumber will never bore as cleanly as something harder or at least drier. I think you'll likely be in 'as good as possible' mode boring that stuff.

You wouldn't happen to be doing this work for Clarcona Horse Park? I help out with their events occasionally.