No Measure Turned Joint
Ron Browning
The following is a method that I use to produce either a glued joint or a slip fit joint for woodturnings such as Christmas ornaments and end grain boxes. I came to use this method when I realized that when using even the best and most accurate ruler available there was only one mark on it that I needed for the joint that I was about to make. Once that I realized that there was only one mark for each project then I was free to make that mark where it was needed. Here is how;
First decide on your project, well that is obvious! Once
that is done you will know whether it is a glue joint or a slip joint. I
usually make slip joints at least ¼” long while glue joints can be as small as
1/8 “. For this explanation let’s describe a slip joint, later you can confuse
yourself by gluing it shut if you like.
Take your blank and make it round and cut a tendon on each
end for your chuck. Speaking of chucks, look at the inside of the jaws, are
they straight, dove tail, toothed? When you make the tendon you should make the
shape of the tendon to fit the inside of your chuck jaws. In my opinion the
worst case would be if you used a Talon type jaw and didn’t cut groves for the
teeth. Why you ask, because the teeth will bite into the wood fibers and
actually break them, then what you have is very short sections of broken wood
fibers under compression trying to hold on to the wood that they are broken
from. In addition to this the tendon should be shorter than the depth of the
jaws so that the front faces of the chuck jaws touch the wood that you are
turning. So, look at that face and make sure it is smooth and not all torn up
like what you get from a dull parting tool! You may have to use the point of a
skew to clean up this face. I do all of these operations between centers, this
will leave a very nice center mark for your use later.
Next determine where the part between the top and the bottom will be and mark it with a pencil.
Put the blank into the chuck. I usually use the center mark from the live center or drive center to help register the blank into the jaws of the chuck by using the live center to put pressure on the blank prior to tightening the chuck jaws. This will ensure that the front faces of the chuck jaws are tightly registered against the wood shoulder that you took so much time to clean up earlier. If you had any burs on this front face you would embed them into the wood and therefore have a good fitting blank.
Ok, now roughly shape your project on the end that is in the chuck. If you remove the line marked earlier, you may just have to make that mark again or remember where it was. Now use a sharp parting tool, here I am talking about a diamond cross section parting tool that is 1/8” wide and sharpened straight across so that the cutting edge is 90º to the side of the tool, to part a grove about 1/8” deep leaving that part of your blank that is in the chuck the proper size. This depth is controlled by the design of your project. If your project is to have 1” wall thickness and you want the slip joint in the middle of the wall thickness you will need to cut this grove ½” deep.
Next make another parting tool cut adjacent to the first cut and to the same depth, thus making the grove ¼” wide. Then do this again making the grove 3/8” wide. Then make another cut to make the grove just a little over 3/8” wide (25/64”)
Next you should pay attention to the quality of the cut which forms the sides of the grove. You can make an almost invisible joint if these two sides are cut clean. Caution you may need to sharpen the skew to achieve the quality of cut that you want! Now look at the bottom of the grove, is it exactly flat and exactly the same diameter at both sides? It has to be so that the rest of the operation will work. This is not hard to do, just make sure that the parting tool is 90º to the axis of rotation and move it back and forth lightly on the bottom of the grove.
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1/8” |
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¼” |
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This will become the ridge or line that you will use to hollow the bottom so the lid fits the first time! |
This will be a sliding fit joint so my design configuration makes the tendon on
the top half. I do this so that when it is used any damage will occur on the
bottom of the tendon which is not seen when the lid is on the box thus
preserving the quality of the joining line of the two halves of the turning.
Next, you will part off the top. Hold on there, not so fast, back up, don’t touch that parting tool, this is the important part so pay attention! You will want to start the parting cut about 1/64” above the headstock side of the grove that you made and trued up earlier. When you get to the depth with your parting tool that you want to make some room, be sure to do this on the tailstock side of your parting cut. The idea is to leave a ridge 1/64” high on the half that will become the bottom. This ridge is the only mark on your ruler that counts and you don’t even need a pencil to mark it, what a deal!
Ok hoss, here is where your turning skill will be put to the test, you will need to hollow the bottom of your project without making a funnel and stopping about 1/8” before you get to the ridge that you made during the last step. For end grain hollowing I prefer to use the only tool that works, a hook tool. Do what you have to do and make the hollow as good as you can, sand it out if you have to, but just don’t touch that ridge.
For this step I use a sharp right hand skew scraper that is
ground with an auxiliary cutting edge on the left which is ground at 90º. Make
a pencil mark to mark the depth of the tendon on the lid, or just guess and make
it a little deeper. Start removing wood from the 
inside,
using the auxiliary cutting edge, toward the ridge taking a little at a time and stopping when you get close to the ridge. You are going to keep removing wood
in a deliberate manner until the ridge just goes away. Wait a minute now, no
heavy hands here. Chances are that the blank moved a little since you turned
the grove. This will result in a point where some of the ridge is gone while
some is left behind. You will be at the exactly correct diameter when half of
the ridge is gone. 
Don’t
believe me, try it, then try fit the lid. Tight huh!
Ok,
now put the other half into the chuck. First mark the #1 jaw position, I know
the bottom isn’t finished on the outside. Once mounted in the chuck, hollow the
top to fit your design, remember that that little tendon hanging out is finished
and you do not want to turn it away or you won’t anything to slide into the
joint. For this project I plan a ¼” wall thickness so I would leave the tendon
1/8” thick. Take the top out of the chuck.
Put the bottom back into the chuck aligning the #1 jaw and the mark you made. Put the top into the bottom and hold in place with the live center. Shape the outside of your box, you can stop the lathe and take the halves apart to check the wall thickness as required. You should do this before you make the outside smaller than the inside.
If you are going to make a finial top you can rough shape it then use masking tape to hold the top and the bottom together to finish the finial. Take the top off of the bottom and take the bottom out of the chuck.
Put a piece of soft wood into the chuck and turn a jamb
chuck for the inside of the top. I make the jamb chuck about 1/8” and usually
make it slightly tapered, smaller toward the tailstock. Sneaking up on the size
I try fit the bottom to the jamb chuck then when it is about right you will be
able to see a shiny line on the taper. That is the correct diameter and it is
at 90º. Jamb the top onto the jamb chuck, no heavy hands, you can split your
turning very easily. Uses the live center in the mark, to make everything spin
right, then finish the bottom. When you get close to the center you will need
to back off the live center and lightly cut that little nubbin off of the
turning. Don’t break it off, you will 
make
a hole in the bottom of your vessel.
I
am using the clear wrapping tape here to protect the finished area because I
will be spraying lacquer on the freshly sanded portion and I don’t want the
intersection to be visible when finished.
Here is some after thoughts. If your wood has a wavy grain or otherwise distinctive grain which makes a noticeable difference between the top and the bottom you can hide this by making a slight round over at the joint. This will result in a rounded “V” at the joint. You could make a couple of these to hide the joint. The curving at the joint will pull your eye away from or hide the discrepancy in the grain.
The sliding joint does not leave room for a built up finish. This project is best made out of some very nice hard wood which you buff up on a Beall system. If you are going to use a build up finish like varnish you will want to make room for it prior to removing the bottom from the chuck for finishing turning the bottom. You know what you are going to use, so make the sliding fit area just a little bigger, then turn it over and finish the bottom.
If you are going to use this as a glue joint, I suggest using the minimum amount of yellow glue and putting it only on the inside bottom of the bottom half of the sliding joint, that way any squeeze out will be on the inside and not seen. The glue will stain the joint line, CA glue is the worst!
The above pictures are of making a hook tool. The following is a link to Allen Lacers excellent handout for making this tool:
http://azwoodturners.org/summer_2005_basichooktool.pdf
You can reach me by e-mail at woodturner01@yahoo.com or by phone 407.222.1542