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Doing the coring depth math

Kim Metzger

Kim Metzger

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I was unable to use the Explore feature to find previous discussions of bowl coring. No hits. So I hope this question hasn't been discussed ad nauseum. Just acquired the Oneway Easy Core system and am learning how to use it. I've found several explanations of how to set the tool rest for the right distance so you don't go through the bottom of a bowl using some rather complicated math. Another turner uses a laser. At the Wortheffort website, Shawn Graham doesn't show any measuring. He just cuts a small blank and a larger blank without moving the tool rest. The same goes for Ernie Conover. Help appreciated.
 
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Kim,
As an example, if you are using your 9" knife the radius of that knife is 4.5". So that means if you set up the centerpoint of the knife post say 1" away from the face of the blank, you will cut 3.5" deep into the blank. If that example, if your blank was a total of 5" thick (from blank face to tenon) you would still have 1.5" worth of bowl blank remaining. Same method for the other sizes of knives.
 
When I got my easy core set I didn’t think it through enough and ended up with a few cores that the bottoms was too thin. I didn’t lose any bowls but did end up with some that had a really small foot. Try to think about the finished bowl shape you want and leave the bottoms thick enough to achieve that. That’s one of the few drawbacks I’ve found with the fixed shape of the easy core
 
I made plywood spacers that go between the headstock and the baseplate for the coring jig. All of my chucks are oneway stronghold so I can change from chuck jaw size to another with no problem, To make the spacers: install a chuck on the headstock spindle. Put a coring knife in the baseplate . Move the baseplate toward the chuck until you get the space you want the bottom of the bowl to be ( measure between the tip of the knife and the chuck). That will give you the dimension for the spacer for that knife. Repeat the sequence for each knife. Mark the spacers for the knife they go to. Next time you want to core put the spacer for the coring knife you are using on the bed rails, slide the coring baseplate left until the spacer is tight between the headstock and the baseplate. Slide the baseplate in or out to set the tip of the coring knife on the circumference you want to cut. lock down the baseplate and go to town coring. Remember that each time you select a different coring knife you will have to select the spacer for that Knife. I set my spacers to give me a 3/4" bottom on every blank. Also if you have different chucks the spacer will have to be assigned to that chuck and that knife. Sounds like a lot of work. It isn't. Saves a lot of time if you do a lot of coring. Hope this make sense. Good luck. Ray
 
I tried doing math but had trouble getting it to come out right. (That's why I became a biologist rather than a physicist!) Now I just eyeball it and it seems to work just fine. The biggest bowl is often a little thick on the bottom, depending on the shape of the log, I just clean it up with a bowl gouge. I worry more about rim thickness when setting the jig and trying to maximize depth while keeping the rim thick enough to accommodate warpage. I'm relatively new to coring, but have done about 20 logs so far and have been able to get four good nesting bowls out of the biggest ones, 2-3 out of the smaller ones.
 
A laser, used similarly to how many people use them to check wall thickness while hollowing, can be very helpful in setting depth of cut for Oneway cores. No math or spacers needed, at least not when coring from the outside in (making the biggest bowl first).

I reused the laser that came with my Gizmo hollowing rig. Got a friend to mill a steel rod so one end fit in the hole in the Oneway base that normally holds the coring knife. The other end of the rod fits the bracket that holds the laser arm. I put the 11.5" coring knife in the Oneway base and moved it so that knife just touched the spindle on the headstock. Then locked down the base. Then removed the knife and installed the laser arm. Then adjusted the laser arm until the laser beam just touched the spindle and marked that position on the horizontal arm that holds the laser. Basically, this calibrated the laser arm so it would show where the bottom of the cut is with the 11.5" knife. Repeated the process with the other knives so now my laser arm is calibrated for each of the knives.

To use it, mount your blank on the lathe. Decide what knife you want to start with. Set the laser arm for that knife. Shine the laser on your blank and move the base around until the laser shows the thickness/position of cut you want on the sides and bottom. Lock down the base. Install the knife and go to town.

I go one step further for bottom thickness, especially when trying to go thin. I measured the distance from the head stock to to the outside face of the jaws for each of my chucks. So, for example, it's 4-3/8" from the head stock to the face of the 4" dovetail jaws on my Nova SN2. Say I want 5/8" bottom thickness. I move the base so the laser beam is 5" from the head stock on the center line of the lathe bed and lock it down. This means the bottom of the cut (inside surface of the largest bowl) will be 5" from the head stock. 5" inside minus 4 3/8" to chuck jaw face (outside of bowl) = 5/8" bottom thickness. I've been able to get very reliable thicknesses, +/- 1/8", using this method. I use 6" combination square held against the head stock to do the measurements when setting the base position.
 
I used the plywood spacers as well. I made mine into rectangles. One dimension for one blade and the cross dimension for another blade. I do like the Woodcut laser set up since it mounts into a center hole on the pivot point. You could probably do some thing similar with the Oneway. You can also select the blade first, and position it on your lathe, then make sure you have 1/2 to 3/4 inch of space between it and the jaws of your chuck. I still prefer the McNaughton...

robo hippy
 
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