EDITED 17/09/2017 TO INSERT:-
Please do not make one of these until you have read the instructable,"Universal Dust Extractor" which I found after I had posted mine. It is faster, cheaper and better. I have uploaded extra photographs of fitting it to the sander. I have also thrown away my hard plastic adaptor. Mine worked, but Balloon007's way of doing things was _so_ much better.
BACK TO THE ORIGINAL (NOW SUPERSEDED) INSTRUCTABLE.
The dust extraction port for my belt sander is about four inches across. The hose into my shop vac is about two inches across. I go fed up wasting a yard of duct tape kludging it up every time I wanted to make the connection, so I decided to make a re-usable adaptor.
This used an old plastic milk bottle, a few staples from my desk stapler, a wire coat hanger and about as much duct tape as a single bodge-up.
Step 1: Large Diameter Section
The first section slips over the large diameter post on the sander. To get a sheet of flat plastic, I cut the lower part of a plastic milk bottle and unrolled it.
Cut a strip from the lower section of the bottle which will give a couple of inches of flat material.
Wrap this around the dust port and use a staple to hold the right diameter. It shouldn't be as tight as it can be, but just a little looser.
Once the cylinder has been formed, cut off the scrap plastic and use another couple of staples to give some stability to the join.
Step 2: Tapering Section
Take the upper part of the milk bottle.
Cut off most of the handle, leaving a couple of inches of stub.
Cut a V-shaped chunk out of each corner, leaving the four flat sides. The cut-outs here were too big, which meant that gaps had to be sealed later. That's not a problem, but a cut about an inch deep and half an inch wide at the base would have been better.
Seal the stub of the handle with a little duct tape.
Step 3: Joining the Sections
Using very short strips of duct tape, fix opposite flaps from the tapering section onto the tube.
Then tape the other two flaps down, and use tape to cover up the join and any holes. No need to skimp on the tape at this stage as the finished adaptor will be used and re-used.
Step 4: Fitting the Vacuum Adaptor
My shop vac came with a variety of nozzles which never get used. One of them was sacrificed to be the permanent connector in this adaptor.
Offer up the nozzle to the bottle neck, secure with tape, and then cover off any gaps with more tape.
Step 5: Attachment Brace
As shown in the first photograph, the adaptor tended to sag off the dust port under its own weight and the weight of the vacuum hose.
Use a short section of thick wire to hook onto a _SAFE_ location on the sander. The other end fits through a hole put through the tape on the end of the handle stub. This was unplanned, but worked really well as the handle gave a good angle to transmit the stress.
As a final tidy-up, use the flame from a candle or a match to soften off any sharp edges from the cut end of the adaptor. This plastic bottle melted much more quickly than expected, so do be careful. I used the cut-off scraps as training material before attacking the adaptor.
The end result works really well at interfacing the vacuum to the sander. It is cheap, light, easy to fit and effective. I intend to make a couple of others to fit different pieces of equipment.