Coring with Bill
Bill Grumbine shows how he cores bowls on the lathe.
It was a beautiful September Saturday in eastern Pennsylvania as I pointed my truck toward rural Helfrichsville, about 25 miles to the northwest of headquarters. I was supposed to be away for the weekend, but a change in plans allowed me to attend Bill Grumbine's day-long seminar on coring bowls using the Kelton Hollowing Tool system. Since this had been a subject of intense interest on the messageboards in recent days, I figured it would be good to stop by and see if I could figure out what the issues are. It turned out to be an enjoyable and informative affair.
When I arrived around 10:30 am, Bill was just truing up the bowl blank he was preparing for the first coring demo of the day. It was a nice hemisphere of cherry, about 12" in diameter. There were about 10 or 12 turners and spouses there, mostly from New Jersey and the Maryland/DC area, but also including Bill and Nettie Turpin, who made the pilgrimage all the way from the mountains of North Carolina for the event. They thoughtfully brought up a big basket of apples to share with everyone. Mmmm, great apples.
Bill's first step after preparing the blank was to use a parting tool to turn a 2" tenon in the center of the trued-up face of the blank. The groove was about 5/16" wide, a convenient size for either a compression or expansion grip on the tenon as needed later. Bill explained that there were basically two approaches to coring. In the first, simplest way, you start with the smallest core first and progress to the larger ones sequentially, without removing the original blank from the chuck. If you plan to use this approach, you don't need to turn the tenon in the face.
The other method -- the one Bill prefers -- is to core the largest bowl first, since that one is likely to be the most desired and valuable piece. If you start with the smallest bowl first, you run some risk of ruining this outside bowl through accumulated errors in the earlier coring steps. The downside to this approach is that you need to rechuck the most recent core on the lathe for each subsequent core. The tenon allows you to reverse-chuck the core each time to turn a new spigot on the bottom.
Once the blank is prepared, the next step is to set up for the first coring cut. The Kelton Hollowing System Bill was demonstrating has a steel cage mounted in the banjo of your lathe that must be positioned in such a way that the knife will follow the desired cutting arc relative to the blank. But, before you can position the cage, you need to choose a coring tool with the appropriate curvature for the cut you want to make.
This is where things get tricky. Let's say you're following the conventional wisdom of having a wall thickness of 1" per 10"-12" of diameter: On a blank like the one Bill was using, you'd want a knife whose radius of curvature is about an inch smaller than the outside radius of the blank you're cutting. In the best of worlds, you'd happen to have that exact curvature handy. In the real world, though, you have to approximate. Bill holds the most likely knife up against his blank to eyeball the situation.
To illustrate some more of the complexity of planning your cuts, Bill drew the same curve four times on a piece of paper, each line with a different entry angle. Regardless of the outside curvature of the blank you're coring, you can orient the inside curve however you want. Any differences in radii between your coring knife and the outside curve will result in a bowl blank that is either thicker in the center or thicker at the edges. It will only be uniform in thickness if the knife curvature is parallel to the outside blank curvature. So, in other cases, you need to plan in advance for thicker and thinner areas of your blank. Of course, it is my understanding that the knife sets are graduated so that the curves "nest" nicely, with some uniformly fixed difference in radius from one knife to the next. I haven't done the research on this, so correct me if I'm wrong.
Once the appropriate knife is chosen, the next step is to set up the gate so that it guides the knife as accurately as possible along the cutting path you want. As Bill explained, the tip of the tool should be aligned perfectly with the horizontal centerline of the blank. He also stressed the importance of holding up the back end of the tool so that the top of the knife is up against the top of the gate opening. This helps assure that you won't have an unwanted surprise in the middle of a cut, where the handle jumps up at you at several times the feet-per-second speed of the blank. Not friendly.
To align the gate properly, Bill said you want to move it as close as possible to the face of the blank, and position it so that the cutting tip of the knife is aligned with the inside of the line you've drawn to mark the border between the bowl and the core. If I understood correctly, the direction the knife will cut into the wood is governed by a combination of the orientation of the gate posts and the way you shade the cut as it progresses. The cutter has a natural path it wants to follow, and if you set it moving in some other direction, it will bind in the cut sooner or later. I think this is where a lot of people have trouble with this system. Bill did give me a chance to try a cut, and I must say that it went pretty well. I handed the tool back to Bill to finish the core.
When the knife was cutting smoothly, particularly at the beginning of the cut, Bill needed very little pressure or exertion to control the tool. Once he got further into the cut, though, the knife bound up on several occasions, even slowing the lathe momentarily, because of excess shavings building up in the kerf, so clearly, it is advantageous to blow out the shavings before they get a chance to accumulate. I don't know if this is possible or advisable while coring. Bill stopped the lathe and cleared them away, and then he resumed cutting.
At one point, he noticed that the side of the knife was starting to rub against the outside of the kerf, so he had to widen the entry point of the kerf a bit to allow for greater clearance. He also realigned the gate slightly to orient the cutter a little differently. This was the mysterious part of the operation; knowing how and when to compensate for cuts that don't go quite perfectly, and apparently this happens to the best of 'em, Bill included. The point is that it is fairly routine to make these sorts of judgments once you have acquired some familiarity with the way these cutters work.
Several of the attendees had brought along their coring tools for comparison. I took this shot of the two tip profiles -- the spear point (top) of the most recent versions of the Kelton jig and the raker-style point of the previous version. The consensus seemed to be that the new profile is an improvement over the older one.
Once the core had been removed, Bill took the opportunity to trot out his newfangled depth gauge so he could check how thick the bottom of the remaining bowl blank was. This deluxe gauge is an improvement over the rough-and-ready "two-dowels-and-a-stick" gizmo he used to use. It uses a laser pen to show the precise depth on the outside of the bowl. Made by Sierra Mold, this gauge sells for $95 direct from Bill, laser not included.
All in all, I wished I could have stayed for the rest of the day. I heard that everyone had a chance to try out the hollowing rigs and learn more about the challenges and tricks involved. It was great seeing everyone. I'll look forward to Alan Zenreich's images and movies. I'm sure they will add a lot to this humble report of mine.
. . . Ellis Walentine
And, here is a short video that Alan Zenreich posted on YouTube of the first core of the day. Thanks, Alan.