Open Segmented Bowl – The Jigs Part I

Now that I had the design and a cut list I needed to build four different jigs to cut and assemble the pieces.  Rather than show a completed picture and say, “I built the jigs.” then let it go from there I had planned on spending a little time going through the jig building process.  The jigs are important since if they are not right the piece won’t be either.  However, as you can see this post is titled The Jigs Part I which means there will be a Part II.  As I developed the jigs the build process got a bit more complicated than I thought and the information grew.  It finally got to a point where I needed to break the material into two posts.  So this post covers the jigs that get mounted to the lathe.

The process starts by making some drawings to get the design down and figure out what I need in the way of materials.  Three of the jigs get mounted on the lathe. 

Drawing of Jigs for Lathe

The first and easiest is an indexing wheel.  It along with the second jig is used to accurately set the individual segments every 30 degrees and to offset the adjacent layer by 15 degrees.  I printed out a paper copy of an indexing wheel made for a 12 segment design, glued it to a piece of thin plywood cut it out on the bandsaw and sanded the edges smooth.  

Index Wheel Drawing and Paper Glued to Plywood & Cut Out

The jig is mounted on the rotating part of the lathe.  Its location is shown in the left photo by the red line on the spindle shoulder.  In my case all I needed to do was drill a hole the same diameter as the spindle shoulder and set it in place, middle photo.  When I thread the face place onto the spindle it will act as a clamp and hold the jig firmly in place, right photo.

The second jig is used to align the guide lines on the index wheel to a specific point and as an extension of that the bowl blank mounted to the lathe.  When clamped to the index wheel it holds the bowl blank in a fixed location so the segments can be located and glued in place.

Alignment Guide Jig Parts

There are just a few pieces in this jig:

1.  The alignment guide made out of clear plastic.

2.    A plywood backer to support plastic.

3.    The top crosspiece that goes across the lathe ways to which the alignment guide and its backer are screwed to.

4.    A spacer that goes between the ways to keep the jig square to the spindle axis and hold it a fixed distance away from the lathe headstock.

5.    A clamping bolt that when tightened holds the jig in place.

6.    The bottom crosspiece which goes below the ways that the clamping bolt is threaded into.

Only a couple of pieces need to be cut and located carefully.  The top of the alignment guide needs to be right at the centerline of the spindle.  You want the line on the index wheel to be parallel with the top of the alignment guide.  If the alignment guide top is above or below center then it will never be in correct registration.

Alignment Jig & Index Wheel in place

The other piece is the spacer that goes between the lathe ways.  To keep the jig square it needs to be moveable but not a sloppy fit.  I spaced mine the thickness of a playing card (about .01”) less than the ways spacing.  The piece in the photo is long enough for both this jig and the next one. 

Alignment Jig and Gluing Jig Spacer

Its length needs to be cut so when it butts against the headstock it holds the plastic alignment guide about 1/16” out from the index wheel.  Lastly, the spacer and top crosspiece are fastened together at 90 degrees.

To make the clamping bolt knob here is the process I go through.  I start with a template to layout the shape, bandsaw it close, finish shaping on the disk sander, cut a square mortise for the carriage bolt lugs, drill a hole for the threaded shaft of the bolt, round the edges, sand smooth and finish.

The clamping bolt goes from the top crosspiece through the spacer then on through the bottom crosspiece and typically into a nut.  To me that’s just one more piece to lose or fumble around trying to get my hand between the ways without dropping the nut so I can get it started on the clamping bolt.  For me an easier way is to use a metal threaded insert set in the bottom crosspiece.  Here is what it looks like installed.

The inserts often come with a slot that would lead one to believe they can be easily installed using a screw driver.  To that all I can say is, “Fat Chance!” I tried and it was a disaster.  I have had good luck with this method:

1.    Threading an insert on a threaded rod and holding it in place with a jamb nut

2.   Mounting that assembly in a drill press

3.    Use the spindle feed handles to start the insert and put some pressure on it.

4.    Turn the drill press chuck by hand to run the insert in to the desired depth. 

One thing, just don’t turn the drill press on to run the insert in.  If you do it will be thrilling but not the kind of thrill you really want.

The last jig that gets mounted to the lathe is the Gluing Jig.  It makes sure the segment is set parallel to the layer center line and that it is at the correct distance from the layer center point.  While the actual size of the jig is not critical I made it large enough to be rigid when clamped in place.  My base ended up being 12 ½” x 14”.  As with the last jig there are a few things that are important such as keeping the pieces square and plumb with the lathe ways. 

Gluing Jig

However, the location of the aluminum angle at the top is the one item to really pay attention to.  The top corner and its horizontal cut edge needs to be right on the centerline of the spindle because that is your reference point for setting the outside edge of the segment.  If it is off then so will your layer diameter which impacts the gap size and the size of that little glue joint which holds everything together.  The long part of the angle needs to line up with the center line across the face plate.  If it’s out of parallel with the ways, too high or too low then the segments will be twisted when you look down on the ring. 

Aluminum Angle Aligned to Centerlines

This completes the jigs necessary to accurately locate and glue the individual segments in place.

Completed Jigs Ready to Use

Next up – The Jigs Part II