I had a project where I needed to drill holes in a piece that I had turned on a lathe. The holes needed to be perpendicular to the axis, and spaced evenly at 120º increment. Admittedly, there are plenty of ways to accomplish this, but I wanted a tool that was adjustable, adaptable, and accurate. I believe this drilling jig accomplishes just this.
Step 1: Tools and Materials
My lathe is a 1930s Delta Double Duty Lathe. It's on the small side, but it does have a 1"-8 spindle and a corresponding 1" tool rest. I have a fairly well-equipped workshop, but most of this project can be done with hand tools. I used the following:
-Welder; I used a TIG, but MIG, flux core or even stick would work
-Portable Bandsaw (hacksaw will work as well)
-Bench grinder with wire wheel
For this project, I used bits of scrap I had on hand already.
-5-6" of ø1" steel round bar. I used cold rolled
-4-6" of 3/4x3/4x1/8" angle iron
-1 steel hinge
-5/16-18 nut and bolt
Step 2: Cutting the Pieces
I needed to drill holes that were 1/4" in diameter, but I wanted to be able to drill other sizes too. I had a piece of 1" round bar with a 1/4" hole in it that I used as a drill guide. If you have a metal lathe, you can quickly make your own drill guide, or you can purchase whatever size you need from McMaster Carr. They're sold under the name of "drill bushings" and are made of hardened steel so they won't wear out with heavy use.
Cut the pieces of steel as shown in the photos. The lengths are not critical. Make sure that the 1" round bar (or whatever size you end up using) is long enough to fit in the banjo of your wood lathe and still reach the centerline of the lathe's spindle. I
Step 3: Welding the Post
With the toolpost notched, you may now weld the angle iron to the toolpost. Tack in several places and make sure the angle iron is square to the the round bar. This step is important for drilling holes that are truly perpendicular to the axis of the lathe. Adding several tacks ensures that the parts don't shift during the welding process. I used silicon-bronze brazing rod, as it requires less heat (less distortion), and is very effective when joining parts of different thicknesses (1/8" to 1").
Step 4: Assembling the Clamping Lid
Now you may weld or braze your hinge and the other piece of angle iron to the lower piece. Clamp the drill bushing between the two pieces of angle iron as shown in the photos.
The next step is to add the clamping screw. I used a piece of round bar that was already drilled and tapped, but you can also use a nut. A small piece of tubing was also welded to the upper half of the clamp. Use the clamp bolt to align everything.
Step 5: Making a Wingnut
I used a socket head cap screw on my drilling jig. Whenever possible, I like jig that don't require specific tools. Using a small piece of steel, I brazed on a tab to the bolt.
Step 6: Putting It in Operation
As stated earlier, the holes I were drilling were perpendicular to the axis of the lathe, and were on the centerline of the axis of the lathe. The adjustability of the drilling jig allows you to drill at angles, and and different heights.
If you're looking to drill perpendicular and on centerline, first set your tool height by placing the drill bit in the drill guide. Then, adjust the height until it matches up with the point of the live center, as shown in the photos.
Next, we'll move the banjo where we're going to drill into the part, and set it square to the part using a machinist's square.
Now we're ready to drill. My lathe has an indexing feature for the spindle, allowing me to drill in precise increments. There are other instructables that can help you with this upgrade if your lathe isn't equipped with this feature.