So I was given an old workshop vac which came without a filter.  I was using an old bath towel in its place, but was sure it wasn’t filtering to a decent standard.  After emptying the drum and removing the old inlet, I attached a soil pipe which I had cut to fit upon the other edge of the drum.  The newer shop vacs seem to use this method to create a vortex effect, and possibly avoiding directly blasting the filter with sawdust.

To explain some of the processes I was unable to film; I made this part by cutting the profile of the drum in MDF, which I then screwed to the pipe and scrolled around with a band saw.  Alternatively I believe this could be done with a wide throated coping saw.  The fit was not perfect but a combination of contact adhesive and later hot glue created a strong and gap free seal.  To fit the hose adaptor into the soil pipe I padded the gap with a brush piled draught excluder.  Later the MDF ring and disc, which I used to build a custom HEPA filter were cut out using a CNC router.  These could be made using a hand router or even a jigsaw.  The original bag for the vac would have probably only reached 1 micron filtration, but this HEPA certified filter can remove 99.97% up to 0.3 micron particles.

“The original HEPA filter was designed in the 1940s and was used in the Manhattan Project to prevent the spread of airborne radioactive contaminants… HEPA filters are composed of a mat of randomly arranged fibres. The fibres are typically composed of fibreglass…”  The filters on in cheaper vacuums can sometimes be made from paper, which are more likely to tear.

http://en.wikipedia.org/wiki/HEPA

After carefully removing the rubber gasket, contact adhesive and later hot glue is are used to create a strong seal.  In addition, I bought a filtration bag for chip extractors which are normally made from non-woven polyester, to add an additional protective layer.

The additional inlet was taken from a vacuum that had broken, and which I was unwilling to repair.  To maintain the suction pressure either one of the inlets must be blocked when the other is in operation.  It also gives me the option to fit a waste bag, encase I want an easier method of disposing dust.

The final modification involves several holes, which can be used to monitor the capacity of the bin without lifting the lid.