Making radius dish on pedestal drill
Just finished my 15ft radius dish! Quite amazed really, as I wasn't very hopeful when I began the project yesterday. Basically because the normal reaction of my ancient Fobco drill when presented with anything to do with sanding, is to drop its morse taper chuck.
However, it seems as if the load from this enormous sanding disc is more vertical than sideways, and the chuck stayed in place.
I opted for a 9" dia sanding disc for an 18" radius dish. This was partly dictated by the swing of my lathe (even more ancient than the Fobco), and also given previous sanding experiences I was wary about using the largest possible disc (10") and perhaps putting too much load on the drill.
In the event, I cocked up the fabrication of the sanding disc and ended up firstly with a plywood one 2" undersize
and then having to add an MDF extension. I couldn't then re-establish concentricity between the disc periphery and the 8mm bolt used for the arbor, and to get it all spinning sweetly again ended up partially machining the threads off the bolt .... having omitted to replace the locking nut. This had ramifications in the actual machining process as we shall see.
For abrasive, I used 80 grit SAIT decorators roll as that was the coarsest I had available. I stuck it to the disc with spray adhesive from the stationers. With the 80 grit, I thought MDF would be more suitable for the dish than ply, hopefully offering less resistance to being sanded.
The angled baseplate is an old speaker stand base which bolts to the drill table. I also use it (unsuccessfully) with a sanding drum, hence the recess in the middle.
I set it all up surrounded by two vacuums and an air filtration unit and tentatively started to lower the disc into contact.
My drill table has no mechanical raise/lower, so I used the spindle lock together with the depth stop to advance the disc a few thou at a time.
I quickly found my set up would only cope with a few thou cut - and any attempt to speed things up was met by heavy chatter and the sanding disc arbor unscrewing itself (remember the missing lock nut!).
This got worse as the sanding area increased - using the spindle lock and the depth stop to set the cut requires both hands ... which leaves nothing holding the radius dish. The sanding disc would grab the dish and chatter the arbor unscrewed in the blink of an eye. I experimented with clamps and a rudimentary brake, but releasing the clamps caused spring back from the dish and immediate chatter/loose arbor. So it all took quite a lot longer than it might have - about an hour and a half in total.
But hey - it worked!
Not only did it work, but the sanding media is hardly worn.
I didn't find the dish rotating of its own accord, probably because the disc periphery was bang over the centre of the pivot pin. I did find that I needed to do multiple dish rotations in both directions, starting off feeding with the disc rotation and then against it. Changing rotation would always result in more of a cut being taken, despite sanding apparently having ceased. For final finishing I was spinning the dish as fast as I could one way then the other.
I need to sort out a better arbor arrangement before I try with another one - any suggestions gratefully received, though the missing lock nut won't have helped.
Many thanks for the method!
Cheers
Paul