I use the APTC Chuck and at the time I wanted a Button Jaw Set their was only one size available the 250mm (10") model which was adequate but not suitable for some of the larger Bowls / Plates I often Turn.

Some time later Axminster introduced a Larger Button Jaw Set but this apart from being expensive, the size at 400mm (16") would present some problems of adjustment as the maximum swing over the Bed is 200mm and therefore I would not be able to use the full adjustment range.

I set about making a simple set of Extension Jaws to fit the original Jaw Plates of the 250mm system these would have a nominal diameter of 360mm  (14") with a holding capacity up to 345mm (13½").

I decided to use 20mm (¾") MDF as the Material, but you could easily use Quality Plywood or even Aluminum if you have any available, I started with a 400mm Square onto which I marked the Center and then using a Compass drew a 360mm Circle, and then dissected this Circle into 4 equal Segments.

At the same time I marked the Segments 1 to 4 in the correct Sequence on the BACK of the Jaw Plates. You also need to Drill a Hole in the Center of the MDF with a typically a 12mm Drill. Now carefully cut the 4 Segments on the Bandsaw or with a Jig Saw

With the 4 Segments now Cut to shape you need to take the corresponding Button Jaw Plate and place it face down onto the back of the corresponding Segment i.e. #1 MDF Segment with #1 Aluminum Plate, so that the Straight Sides are flush and then carefully mark through the chosen fixing points.

It is recommended that you use 4 fixing points on each Segment, the fixings in my Plates are Threaded for M6 Screws and I therefore would require to drill out the Holes in each Segment to 7mm which would allow for a little bit of leeway when finally fixing the Segment to the Plate.

Once the Holes are Drilled you will then need to Countersink or Counter Bore the Hole to take the Head of the Screw so that it is Flush with the Surface of the Segment, if you are using Screws that require a Counter Bore then it may be advisable to start with a Thicker Sheet of MDF as you do not want the point of fixing to end up to thin.

You now Screw each Segment to its relevant Plate and if necessary fine trim any of the Straight edges that are not flush with the Aluminum Plates using a Belt or Disc Sander.

Now mount each Assembly into the Chuck and using 6mm (¼") Spacers, made from Wood off cuts, between each of the Jaw Segments, tighten the Jaws up and therefore trapping the spacers in position.

The next operation is to clean up the outer edge of the MDF Segments by running the Lathe at a suitable Speed (typically 800rpm or slower) and using a Gouge true the Jaws.

The distance between each set of Holes in the original Plates is 13mm (½"+) and I therefore duplicated this spacing on the Extension Jaws by firstly carefully marking out each point with a Pencil and carefully rotating the Chuck by Hand and thus marking a Circle onto the Jaws, I repeated this for each of the 5 Circle of Holes I could accommodate.

There are two ways you can now proceed to mark out and Drill the Holes for the Bolts to hold each Button, either carefully mark the centre line, from the two fixing Screws used to fix the Segment to the Plate, out to the Circumference of the Jaws, or by using an Indexing Head and mark of at every 45°.

You can then Drill the Holes on the Lathe with a suitable Drilling Jig or use a Bench Drill, whichever method you use you will need to be accurate with the Hole Position as you only have one chance and you have something like 40 Holes to Drill.

Again you can have a choice as to what fixing method you use, I chose to use M6 Roofing Bolts through a plain 7mm Hole with a Washer and Hexagon Nut at the rear, this does mean you have a protrusion to the Rear of the Jaws, although this is no worse than the way the Buttons protrude at the Front, but there is a safety issue that you need to be aware of.

Alternatively you could use Threaded Inserts or even T Nuts providing the fixture is sound.

I can use a number of Button types, the Original Buttons that came with the APTC Set or Rubber Door Stoppers which come in various sizes, these can be shaped with a sharp knife to give a shape for better grip or a wider grip on special projects.

fig 1

fig 2

To finish off the Jaws I gave them a couple of coats of Varnish and painted the Indents in the back of each Segment (numbering system) Red for easy identification.

Fig1 shows how I mount the Aluminum Jaw Plates first and then Screw each of the MDF Segments in place due to the size it is very awkward to mount the Jaws assembled, Fig 2 shows the Jaws  fully mounted and ready for use.

These Jaws are in constant use and I have held in excess of 200 Bowls / Platters over the last few of years and they show little sign of any wear and tear.

Capacity Chart for Extended Jaws

The above Extension Jaws give the following external holding capacity when used with the Original APTC Buttons:-