I need to drill 3/8 holes as straight as possible end to end from the bottom in twenty finials like the one pictured. They are just a hair over four inches tall for size reference. I have tried a couple different jig ideas but neither have worked very well. The first really didn’t work at all.
The first involved having a slightly undersized hole in a piece of wood that was cut in half and acted as a vise around the waist at the bottom. The piece of wood rested on the top of a U with the bottom of the U being drilled for the top ball of the finial to set in to keep it centered.
The second jig was similar but used vee blocks.
Because of the end use, keeping the hole centered for the entire length of the bore is going to be very important.
The first jig sounds like it should do the job but I would do a little bit more. Drill the bottom hole exactly the diameter of the base. Then cut a small, 1/8", strip away so the when squeezed with clamp it hill hold the base securely. Next, you should make a second jig to mark the top where the drill will start. Center punch it and drill with a Forstner bit. The bit will probably not be long enough so the drilling can be completed with a conventional long drill bit.
Let me see if I can change your thinking. Drill a 1/2" hole for the 3/8" dowel, then mix some epoxy as thick as peanut butter to act as a filler for any inaccuracies. When you glue the parts together, just slowly wiggle the part together to suite your eye.
then make a box about 1 inch larger all around it. Place it in the center in a plastic bag. Spray some insulation foam in the box and let it harden around it. Cut the foam in half so you can place one in at a time.
This should keep it centered and supported all around. I do like the idea of drilling an over sized hole then filling it with a dowel with the correct size hole in the center of it.
Keith L these are going to be used for the ends of poles on an 18th century style marquee. I will be using them differently than the ones shown in the photo in that a loop of rope will be in the groove between the base and the large ball. I don’t want to remove any more of the wood (looks to be maple) than I absolutely have to for strength’s sake.
Since posting this I had another idea but I’ve had to wait for a few glue ups to set before I can try it.
A machine lathe might be the tool of choice here, but here's a notion of how I might do this job without it. First, two things: You need to hold the piece solid as a rock, and use a suitable guide bushing for your drill bit.
The bushing must be referenced absolutely on the work-holding fixture and be immovable during the drilling operation.
For the work-holding fixture, I envision turning a tapered recess in a very stable block, maybe laminated plywood or something that isn't going to move on you. Without knowing if your pieces are uniform, but assuming they are all circular in section, a tapered hole would have to work, but it might take some trial and error.
Then I'd rig up a housing for your drill bushing (e.g., https://www.mscdirect.com/product/details/07124050) that would be registered absolutely concentrically with your holding fixture , probably with two or three tapered pins, so the alignment is repeatable. Use a 3/8" x 6" twist drill in a drill press, with your work-holding fixture securely clamped directly in line with the bit. Test it first, making sure that that the bit is perfectly centered with the bottom of the tapered hole in your fixture.
If all this fails, try Keith's notion. It's in keeping with the latest and most forgiving hardware theory, where you drill oversize holes and let epoxy fill the voids.
Chuck the big end up center on the ball on top of the finial and drill say 3 in. down then make you a piece that would be the same size as the small ball cut it in 4 pieces put on ball and clamp in a chuck. find the center on the bottom hold in place while tightening with live center and drill from the bottom to meet the hole previously drilled. If you have the centers correct then the hole will be perfectly centered.
You could do one on all of them at one time and then the same for the other end.
Is the bottom surface reliably perpendicular to the centerline of the finial? If so, make an open bottom box, drill a centered hole in the top slightly larger than the needed diameter in the finial. Choose a method to clamp the bottom of the finial to the underside of the box. This could be a split donut, four strips of wood extending in through holes in the sides near the top, . . . Mark the center of the bottom of the finial, clamp it to the underside of the top of the box so the center of the finial bottom is centered in the hole in the box top, and drill. Hard to explain, hope this can be followed.
Are you turning the external finial shape too? If so, drill the hole first, and then turn the finial on the same center. Pen turners do this all the time. Even if the finials are already turned, you are much better off drilling the hole on the lathe. You mount the finial in the headstock, so it turns, and use a brad point bit held steady in a Jacob's chuck in the tail stock, and advance the bit into the turning finial. This is the best way to get the bit to follow the center of the turning.
