Introduction: Large Boring Bar for Turning Wood Hollowforms
I have a captive-bar hollowing rig for my wood lathe that comes equipped with a 3/4" diameter boring bar. This is suitable for most tasks,but I wanted the ability to hollow deeper vessels. This calls for a bigger boring bar! So, I made it at the Techshop in San Jose, CA www.techshop.ws.
Step 1: Reduce Diameter.
After cutting a 22 inch long piece of 1 inch diameter steel bar stock on the horizontal bandsaw I mounted it in the Jet metal lathe using a 1 inch collet. Facing off the end was the first operation.
This was followed by reducing the bar's diameter to 5/8" diameter for 1 1/2 inches. This reduced diameter slips into a receiver block on the hollowing rig.
Step 2: Swap Ends and Face Off and Drill
I turned the bar end for end in the Jet lathe and faced off the new end. I then chucked a center drill in the Jacobs chuck in the lathe tailstock and spot drilled to locate the bar center point and to provide a starting point for the twist drill. Replacing the center drill with a 3/8" diameter twist drill and, using lubricant, I drilled a 1 1/2 inch deep hole. This hole will accept a cutting tool or tool holder.
Step 3: Taper End
With a cutting tool mounted in the tool post, I set the compound slide to approximately 4 degrees. This enabled cutting the taper on this end of the bar. The goal is to taper the bar down to 3/4" where it will mate with the tool holder. With the compound slide set, I used the X and Y handwheels to position the cutting tool just to the right of the end of the bar and set to take about a .02 cut. I started the lathe and used the compound slide handwheel to advance the cutter into the bar. This cut a taper. I returned the cutter to the start position, went in another .02 or so, and took another cut. I repeated as necessary until the minor diameter was 3/4".
Step 4: Drill and Tap Tapered End.
At the Jet Mill I clamped the bar to the mill table and located the center line of the bar and the right end. I mounted a Jacobs chuck in the mill spindle and fixed a center drill in place. I spot drilled at 1/4" and 1" and replaced the center drill with a #7 drill and thru drilled the two holes. I then tapped both holes 1/4-20.
Step 5: Mill Flat
Without unclamping the bar, I moved the mill table to position the reduced diameter end under the mill spindle. I mounted a mill in the spindle and proceeded to take light cuts to produce a 1/16 deep cut. This resulted in the locating and locking flat for the boring bar.