Garrett Lambert
02-15-2003, 10:43 PM
I wasted a few hours yesterday calling into several machine shops to get a face-plate bored out and re-threaded for my new lathe. Basically, none of them wanted to do it. I persisted because it's the one I use for vacuum chucking, converted from a 12" d. heavy aluminum sander platen I found in a tool store for $15. The first time I had it re-bored was easy because that lathe had a 1" x 12 tpi spindle, a standard tap size. My new General 26020 has the standard "big" wood lathe spindle size of 1 1/4 x 8. (Unfortunately, a standard 1 1/4 tap is 7 tpi, hence the reluctance.)
The obvious solution was a new platen out of MDF and plastic laminate screwed to a face plate. However, with nothing to lose, why not do the same thing with the aluminum one?
Since I didn't think of posting about this until after I’d finished, this picture shows me holding the cut-off hub in position to give you an idea of what I started with.
<IMG SRC="http://www3.telus.net/GLI/Platen/1a.jpg">
<BR>
With a Super Talon and medium jaws I chucked the hub externally, and enlarged the centre hole with a scraper and parting tool using light cuts at about 150 RPM.
With aluminum, the speed has to be kept down and only very light cuts taken. Any chisel, including a gouge – I didn’t try a skew! – works but scrapers are easiest. However, even at slow speeds aluminum melts and builds up on the cutting edge so kept the grinder running for light touch-ups often.
With the centre hole enlarged, I reversed the platen on the chuck and used the mini-jaws internally. A dial gauge helped get it running true. With the chuck as tight as I could get it, I clamped a pair of Vise-grips on either end of a hacksaw blade - I couldn't get a hacksaw frame in close enough - and cut the hub off with the lathe running at about 200 RPM. That wasn’t particularly difficult, and the cut was fairly clean.
Once that was done I tried to fair the surface with a scraper, but although it cut well enough I realized I'd never get it true by hand. As the next photo shows, I dug out my el-cheapo X-Y vise, made a block to raise it to the right height, and clamped a boring bar with a metal cutting bit into it. (I originally bought this kit to hollow out deep vessels.) At about 300 RPM it took almost no time at all to create a true flat.
<IMG SRC="http://www3.telus.net/GLI/Platen/2a.jpg">
<BR>
The next problem was to figure out how to perfectly centre the 3" face plate on the platen. That turned out to be simple. I have a OneWay rotating tailstock with a screw-on cone which I reversed, i.e. small end forward. I slipped the 3" faceplate onto it and brought it up to the platen with the end of the cone entering the hole in the platen. The faceplate mated up tight to the platen in exactly the right position. Now I was stumped as to how to mark the holes accurately - no room to get a drill bit or even a pencil into the holes nor any way to clamp it in place. I finally decided to just scribe the circumference of the plate on the platen.
Removing the platen from the chuck was point of no return, because I'd not be able to re-mount it true again. With the platen face down on my bench and the faceplate inside the scribed circle, I clamped the two pieces together and to the bench. With a Vix Bit (self-centering hinge bit) in a portable drill, I started pilot holes using the counter-sunk faceplate holes as the guide. (This proved to be a minor error. For better alignment I should have drilled only one, tapped it, set the plate back in place using that screw to hold it, and only then drilled and tapped the other two holes.) I then used my drill press to drill out the three holes without going through, and tapped them for 6 mm machine screws, the same as used for the Talon chuck jaws.
Here's what it looked like reversed on the Talon:
<IMG SRC="http://www3.telus.net/GLI/Platen/3a.jpg">
<BR>
With the platen back on the lathe via the faceplate, I was dismayed by a very obvious wobble. My first thought was to shim it out, but when looked closer under a stronger light I saw an almost imperceptible gap where one of the 3 holes I'd drilled was not perfectly lined up. The best solution would be to rotate the faceplate 60° and drill and tap 3 new holes. With that option in reserve, I decided to enlarge the offending hole ever so slightly with the drill press, and to see if the steel machine screw would still bite and pull it in. It did, and when I remounted the plate and put it back on the lathe, it ran remarkably true, i.e. perpendicular to the spindle. However, although my placement within the scribed circle was out only infinitesimally, at the rim of the 12" diameter it was magnified it to about 1/32. A scraper with light cuts quickly trued that up.
Here's the finished platen. All I have to do now is glue on the thin rubber sheet.
<IMG SRC="http://www3.telus.net/GLI/Platen/4a.jpg">
Cheers, Garrett
The obvious solution was a new platen out of MDF and plastic laminate screwed to a face plate. However, with nothing to lose, why not do the same thing with the aluminum one?
Since I didn't think of posting about this until after I’d finished, this picture shows me holding the cut-off hub in position to give you an idea of what I started with.
<IMG SRC="http://www3.telus.net/GLI/Platen/1a.jpg">
<BR>
With a Super Talon and medium jaws I chucked the hub externally, and enlarged the centre hole with a scraper and parting tool using light cuts at about 150 RPM.
With aluminum, the speed has to be kept down and only very light cuts taken. Any chisel, including a gouge – I didn’t try a skew! – works but scrapers are easiest. However, even at slow speeds aluminum melts and builds up on the cutting edge so kept the grinder running for light touch-ups often.
With the centre hole enlarged, I reversed the platen on the chuck and used the mini-jaws internally. A dial gauge helped get it running true. With the chuck as tight as I could get it, I clamped a pair of Vise-grips on either end of a hacksaw blade - I couldn't get a hacksaw frame in close enough - and cut the hub off with the lathe running at about 200 RPM. That wasn’t particularly difficult, and the cut was fairly clean.
Once that was done I tried to fair the surface with a scraper, but although it cut well enough I realized I'd never get it true by hand. As the next photo shows, I dug out my el-cheapo X-Y vise, made a block to raise it to the right height, and clamped a boring bar with a metal cutting bit into it. (I originally bought this kit to hollow out deep vessels.) At about 300 RPM it took almost no time at all to create a true flat.
<IMG SRC="http://www3.telus.net/GLI/Platen/2a.jpg">
<BR>
The next problem was to figure out how to perfectly centre the 3" face plate on the platen. That turned out to be simple. I have a OneWay rotating tailstock with a screw-on cone which I reversed, i.e. small end forward. I slipped the 3" faceplate onto it and brought it up to the platen with the end of the cone entering the hole in the platen. The faceplate mated up tight to the platen in exactly the right position. Now I was stumped as to how to mark the holes accurately - no room to get a drill bit or even a pencil into the holes nor any way to clamp it in place. I finally decided to just scribe the circumference of the plate on the platen.
Removing the platen from the chuck was point of no return, because I'd not be able to re-mount it true again. With the platen face down on my bench and the faceplate inside the scribed circle, I clamped the two pieces together and to the bench. With a Vix Bit (self-centering hinge bit) in a portable drill, I started pilot holes using the counter-sunk faceplate holes as the guide. (This proved to be a minor error. For better alignment I should have drilled only one, tapped it, set the plate back in place using that screw to hold it, and only then drilled and tapped the other two holes.) I then used my drill press to drill out the three holes without going through, and tapped them for 6 mm machine screws, the same as used for the Talon chuck jaws.
Here's what it looked like reversed on the Talon:
<IMG SRC="http://www3.telus.net/GLI/Platen/3a.jpg">
<BR>
With the platen back on the lathe via the faceplate, I was dismayed by a very obvious wobble. My first thought was to shim it out, but when looked closer under a stronger light I saw an almost imperceptible gap where one of the 3 holes I'd drilled was not perfectly lined up. The best solution would be to rotate the faceplate 60° and drill and tap 3 new holes. With that option in reserve, I decided to enlarge the offending hole ever so slightly with the drill press, and to see if the steel machine screw would still bite and pull it in. It did, and when I remounted the plate and put it back on the lathe, it ran remarkably true, i.e. perpendicular to the spindle. However, although my placement within the scribed circle was out only infinitesimally, at the rim of the 12" diameter it was magnified it to about 1/32. A scraper with light cuts quickly trued that up.
Here's the finished platen. All I have to do now is glue on the thin rubber sheet.
<IMG SRC="http://www3.telus.net/GLI/Platen/4a.jpg">
Cheers, Garrett