Pilot hole for hollow form depth guidance

[Robert Marshall]

I've been using a fairly long 1/2" spade bit (guessing about 15" total length) to drill pilot holes in hollow form blanks, as deep as 8 to 12 inches in some cases. I've mounted the bit in a drill chuck with a Morse taper #2 base, and used the headstock and quill to advance it. I've also tried it with the chuck in hand. In either case, I get some wandering as the drill bit advances, with the result being that the far end of the pilot hole is off center. This makes the final stages of hollowing somewhat tricky, as the off-center hole makes the cutter jump when it is encountered.

Most of these hollow form blanks are mounted spindle-style (so, drilling into end-grain).

I'm looking for recommendations as to how to minimize this problem. Thanks in advance!

Robert

[David Walser]

Robert -- Spade bits tend to wander. So do regular twist drill bits. Forstner bits drill straighter, but they tend to have short shafts. However, after you drill the the maximum depth of a Forstner bit, you can extend the depth by using a drill bit extender. These have some flex in them, but I've never had the Forstner wander off-center when using them.

Rather than using a Jacobs chuck mounted in the tailstock, I use a 3/8" MT#2 collet. The collet works with all my Forstner bits (none have a hex shank) and drill bit extenders. The collet is more accurate and takes up less room than a Jacobs chuck.

[Richard Coers]

I really like a gun drill for doing that boring. I found one at an industrial surplus dealer. I've also used a high helix twist drill. But even with those specialty bits, it's very hard to bore 8-12" and stay on center.

[Chris A Lawrence]

Forstner bits are going to get you the straightest hole because once the hole is started the side walls of the bit guide it straight. So if you start with a straight hole you should finish with one. Like David said start with just the bit to max depth then add the extension. I have 2 size bits i use the most that i ground the points off. I start with a bit with a point then use the one without the point to finish. Less material i have to remove from the bottom to get the little hole the point makes that way.

[Robert Marshall]

Thanks for the replies!

Question - what diameter forstner bit do you use most often?

[Chris A Lawrence]

Thanks for the replies!

Question - what diameter forstner bit do you use most often?

The 2 i use most often are a 1" and a 2 1/8" depends on the size hollow form i am making.

[David Walser]

Allow me to add: When using Forstner bits, go slow and pull the bit out frequently. Forstner bits have a lot of surface area and heat up quickly. Allowing the chips to pack around the bit will cause the bit to heat up even more quickly. So, pull the bit back to clear the chips frequently -- more frequently than you otherwise might think required. This will clear the chips and allow air to cool the bit, allowing your Forstner bits to stay sharp and last longer. In the long run, this will save time. You should also learn how to sharpen Forstner bits. It's easy to do once you've learned how. All that is required is a small diamond sharpening card.

[Peter Blair]

I used to use spade bits but now use forsteners of varying sizes and extensions to get close to the final depth then I use a spade bit with the point removed and grout to a round shape to do the final little bit. I have ground several common sizes and if it fits nicely in the hole from the Forster bit it tends to run true and leave a nice round bottom.

[Dave Fritz]

Anyone use an auger bit? They're pretty stout.

[Kyle Iwamoto]

Anyone use an auger bit? They're pretty stout.

I do, but they are not very big, diameter wise. I guess you could use a Forstner after.
IMO, if the pilot hole is smaller (diameter) it's way easy to accept the wobble that it has, since it's small. Once the hole is "straight" the hollower can cut pretty fast. When the hole wobble is big, I think it's harder to get the hollower to engage/follow and cut well. I don't turn fast RPM wise, so if speed helps in that, you could dial up the speed.
I do use a captured rig. Hand held would be even scarier to try and follow an off center hole.

[John K Jordan]

Allow me to add: When using Forstner bits, go slow and pull the bit out frequently. Forstner bits have a lot of surface area and heat up quickly. Allowing the chips to pack around the bit will cause the bit to heat up even more quickly. So, pull the bit back to clear the chips frequently -- more frequently than you otherwise might think required. This will clear the chips and allow air to cool the bit, allowing your Forstner bits to stay sharp and last longer. In the long run, this will save time. You should also learn how to sharpen Forstner bits. It's easy to do once you've learned how. All that is required is a small diamond sharpening card.

For the heat and chips: when drilling with Forstner bits I direct a constant stream of compressed air into the hole for the entire drilling depth. This not only clears most of the chips but keeps the bit cooler. Sometimes it clears so well I don't have to stop and back the bit out.

For large diameter depth and material hogging cuts that don't require smooth sides I prefer inexpensive carbide Forstner bits which drill about forever without sharpening.

I haven't used spade bits for marking the depth of a hole. I use a much smaller twist drill, say 1/4" or 3/8". The hole will stay straighter if done in stages.

For twist drill bits it's best to start with the shortest bit first since it will wander more. I drill for depth with normal twist drills. I first start with a machinist's center drill or starter bit in a drill chuck, then use the shortest bit I have (stubby or screw machine bits are short and stout), then switch to a jobbers bit then to a long bit if needed. I like a fair amount of speed and slow bit advance.

Center drills:
center drills.jpg

For marking relatively short depths, say for lidded boxes, I simply use a twist drill bit in a wooden handle. I put a depth collar on the bit with the desired depth so I simply drill until it hits the collar. If the bit wanders it doesn't affect the hollowing of a small piece.

My favorite bits for on the lathe are taper shank bits. These fits directly into the Morse taper in the tailstock and eliminate the Jacob's drill chuck for a shorter overall length with less chance of flexing. I keep taper shank bits in a variety of sizes up to 1". MOST of my drilling on the lathe now is with these bits.

Taper shank bits:
taper_1_IMG_20160919_094408.jpg

The length saved by eliminating the drill chuck:
taper_2_IMG_20160919_094945.jpg

JKJ

[Alex Zeller]

I'll point out one more thing about Forstner bits. They are very similar and often confused with self feeding (or auger) bits. The self feeding is for construction use and has a screw in the center while the one you really want just has a small point in the center. The screw style one will pull it's way into the wood and can cause cracking as well as pulling itself too far into the wood.

[John K Jordan]

...The self feeding is for construction use and has a screw in the center...

The self feeding bits are the best thing ever when running romex through joists and studs!

Very aggressive, especially with an impact driver. I'd hate to go back to using spade bits.

JKJ