When I built this dust collector in 2012, I hadn't intended to use it with my thickness planer. It's not quite powerful enough to always keep up, but it's better than having the shavings on the floor and dust in the air. I like using the small dust collector because the hose fits right on my planer and the 5-gallon buckets are easy to empty. But one time recently, after planing some wide maple, I noticed there wasn't pulling much air anymore.
Lifting up the top inspection cover, I could see the problem. Even though the bucket was only partly full, the cyclone was completely filled with shavings...
.. and the filter area was also completely clogged up!
Those really wide shavings, especially from maple, have a tendency to get stuck at the end of the slot in the Thien-baffle cyclone. The slot then fills up all the way around, and then the cyclone itself starts to fill with shavings, and then everything just passes right through to the filter.
I had put a piece of felt in front of the furnace filter as a pre-filter. This worked fairly well. The felt, especially once it got completely caked in fine sawdust, was moderately effective at filtering out microparticles. However, it also made for quite a bit of air resistance, so I was getting about 4 cm of pressure loss across the filter.
What was impressive though was that the furnace filter behind the felt looked very clean except for a few spots where air had been able to get around the felt!
I figured if I could have more area for the felt, and let that get caked in sawdust, I might have a reasonable compromise between filtering and airflow.
So I made a sort of pleated support out of a metal wire mesh (hardware cloth) by folding it in a zigzag pattern.
I mounted that metal zigzag in a wooden frame. The mounting wasn't the most elegant.
Next I pushed the felt into the folds of the wire mesh.
Of the sorts of fabrics that are easy to obtain, I think felt is the most suitable for filtering. Woven fabrics are not ideal because they consist of dense threads of fabric with relatively large holes in between. All the air just goes through the holes, and the fabric acts more like a sieve than a filter. But felt has lots of individual fibers for dust to get entrapped into.
I needed more room in the filter box for my new pleated filter, so I added another 3 cm of thickness to the box.
I put the furnace filter back in (after blowing it out), with a piece of felt in front of it, just like before. The felt, if not caked with sawdust, has quite a low air resistance.
In front of the furnace filter and felt goes my pleated filter, made from the same type of felt.
The idea is that the first felt filter will eventually get caked with sawdust. In fact, it needs to get caked to become effective at filtering out microparticles. Seeing that the felt in front of the furnace filter kept the furnace filter at least looking clean, the first layer of pleated filter, once caked, should keep the other layers from getting plugged up. I'm not sure if it all makes that much sense, but the air resistance of all these filter layers, when not caked, is quite low, so no harm in having them in there.
I also de-tuned my cyclone a little bit to get more air flow. I made the inlet port a little bit larger so it would not shoot the air into the cyclone quite as fast. Slower speed in the cyclone decreases separation performance, but also decreases back-pressure a lot (see here).
I figure this way, it should still do at least as well as the average Thien-baffle. At least my cyclone has a bit of a nozzle and nothing to obstruct the circular motion of the air.
I also slightly widened the slot where chips fall down into the bucket, and added strips of felt as gaskets to cut down on leakage.
All put back together. I then proceeded to clean up my shop with the modified dust collector. It sucks more than ever, so that's good. But according to my Dylos air quality monitor, the fine particle count in my workshop went up quite a lot while I was using it. So initially, my filters were are not very good at catching microparticles.
After some use, the felt is now partly caked with fine sawdust, and the particle counts coming out of it are now lower, while airflow is still very good.
I'm disappointed that the furnace filter after the felt filter isn't doing much to catch the microparticles.
Another improvement I thought of was to add a coarse metal screen (hardware cloth) to the channel that takes air from the cyclone to the filter. That way, if the cyclone gets plugged up again and passes shavings, I get a blockage in the channel instead of getting the filter full of shavings. And that actually has happened to me since. I should have thought of that sooner!
Only one problem now...
I don't have any more of that green paint to paint the bits that I replaced.
Update - 7 months later
Seven months after switching to the pleated felt filter, the filter has become completely caked in fine dust. Airflow is down a little, but the dust collector has become quite good at filtering out particles down to 0.5 micron.
So the idea of using felt (which on its own wasn't that great a filter for fine particles) and letting it get completely filthy seems to work. But ideally, I would have an even larger filter area.
Oh, and I painted the new wooden parts when I painted my homemade table saw