joe greiner
27th November 2007, 01:47 AM
These gauges facilitate checking the size of a spigot/tenon or socket, or selecting an appropriate chuck, for later mounting. Thus, regular calipers are freed for adjustable measurements. On an earlier thread ( http://www.woodworkforums.ubeaut.com.au/showthread.php?t=60883 ), member littlebuddha posted a pic of a spanner used for checking tenons. I proposed a rigid caliper set to contain representations of all four jaw conditions: smallest and largest expansion, and smallest and largest compression. This concept had each jaw condition on the edge of a rectangle, with a rectangle for each jaw set. From the outset, it appeared somewhat unwieldy, at least for the larger jaw set. We'll call that Version 1.
Intuitively (a bit unreliable, as we'll see), my first improvement was to place two conditions on each side of a stick, with another stick for the other two conditions. The sticks were joined by a slotted joint along their centre lines. That was Version 2. Joining the sticks was a mistake, because the tailstock end of the blank is too crowded. Luckily, I hadn't glued the joints. I separated the sets and drilled a hole near the end of each stick for Version 3. I married all of the sticks with a piece of electrical wire; a bead chain could replace the wire some day. Versions 2 and 3 are cut from thin plywood on a scroll saw.
The lump shown for the test isn't the final tenon for the blank. It's just what was left after turning the blank to a cylinder and squaring the ends.
For Version 4, I used the entire jaw profiles themselves, with the expansion and compression ranges combined, and reduced the spine separation to about 1/2 inch. I duplicated the expansion bevel at the far end of each stick; seemed like a good idea at the time. Version 4 is cut from polycarbonate, about 4.5mm thick, on a bandsaw (1/8in blade, 16tpi). Polycarbonate is slightly more robust than acrylic, but neither of them cut very well on a scroll saw, because of melting behind the cut so as to trap the blade. On the bandsaw, some melted polycarbonate piles up below the workpiece. In non-critical areas, I just let the workpiece ride up on the melt; elsewhere, I backed out and broke off the melt. I burned through the lettering on the pattern with a low-wattage soldering iron; a wood-burning pen was too aggressive. A PITA nonetheless.
Transparency may be good for Florida's "government in the sunshine," but it's not so great for calipers. Version 5 has the same patterns as Version 4, but executed in 1/8-inch-thick opaque HDPE instead. HDPE cuts very well on a scroll saw, with almost no need for finishing. It's soft enough to break the corner edges with a thumbnail. It's also less brittle than polycarbonate. At this stage it finally dawned on me that the lettering wasn't needed; I figured that if I couldn't tell which gauge corresponded to which jaw, I probably should take up a less intellectually demanding sport than woodturning. I suppose it'd be different if I had more chucks and jaws, though. Versions 4 and 5 are connected with binding posts for now.
I have no burning desire to commercialise this tool, so the design is free for all. This is the sort of accessory that could probably be provided by the chuck manufacturers. At worst, they'd have accurate measurements of the jaw profiles. I found a chart of some jaw dimensions here:
http://www.cnew.org/tips_techniques/oneway_chuck_chart.png
Spigots/tenons or sockets shouldn't be formed at the travel limits of the jaws. Some travel should be reserved for tightening the jaws, and there's usually an optimum setting for best purchase on the tenon or socket, subject to variations for different species of timber, spalting, etc.
Joe
Intuitively (a bit unreliable, as we'll see), my first improvement was to place two conditions on each side of a stick, with another stick for the other two conditions. The sticks were joined by a slotted joint along their centre lines. That was Version 2. Joining the sticks was a mistake, because the tailstock end of the blank is too crowded. Luckily, I hadn't glued the joints. I separated the sets and drilled a hole near the end of each stick for Version 3. I married all of the sticks with a piece of electrical wire; a bead chain could replace the wire some day. Versions 2 and 3 are cut from thin plywood on a scroll saw.
The lump shown for the test isn't the final tenon for the blank. It's just what was left after turning the blank to a cylinder and squaring the ends.
For Version 4, I used the entire jaw profiles themselves, with the expansion and compression ranges combined, and reduced the spine separation to about 1/2 inch. I duplicated the expansion bevel at the far end of each stick; seemed like a good idea at the time. Version 4 is cut from polycarbonate, about 4.5mm thick, on a bandsaw (1/8in blade, 16tpi). Polycarbonate is slightly more robust than acrylic, but neither of them cut very well on a scroll saw, because of melting behind the cut so as to trap the blade. On the bandsaw, some melted polycarbonate piles up below the workpiece. In non-critical areas, I just let the workpiece ride up on the melt; elsewhere, I backed out and broke off the melt. I burned through the lettering on the pattern with a low-wattage soldering iron; a wood-burning pen was too aggressive. A PITA nonetheless.
Transparency may be good for Florida's "government in the sunshine," but it's not so great for calipers. Version 5 has the same patterns as Version 4, but executed in 1/8-inch-thick opaque HDPE instead. HDPE cuts very well on a scroll saw, with almost no need for finishing. It's soft enough to break the corner edges with a thumbnail. It's also less brittle than polycarbonate. At this stage it finally dawned on me that the lettering wasn't needed; I figured that if I couldn't tell which gauge corresponded to which jaw, I probably should take up a less intellectually demanding sport than woodturning. I suppose it'd be different if I had more chucks and jaws, though. Versions 4 and 5 are connected with binding posts for now.
I have no burning desire to commercialise this tool, so the design is free for all. This is the sort of accessory that could probably be provided by the chuck manufacturers. At worst, they'd have accurate measurements of the jaw profiles. I found a chart of some jaw dimensions here:
http://www.cnew.org/tips_techniques/oneway_chuck_chart.png
Spigots/tenons or sockets shouldn't be formed at the travel limits of the jaws. Some travel should be reserved for tightening the jaws, and there's usually an optimum setting for best purchase on the tenon or socket, subject to variations for different species of timber, spalting, etc.
Joe