Help with a Thien top hat design?

[Kevin Schlossberg]

I'm trying to build a thien top hat for my shop vac. I posted on Phil's forum but wanted to xpost here for additional suggestions / help

My post:

I'm a novice when it comes to woodworking so please excuse any mistakes in terminology or technique.

I've been eyeballing the various builds for a while, but never came across any plans for making a top hat separator, so I'm winging it based off the drawings for the baffle insert linked in the stickied thread in this forum.


From what I've gathered, the 32 gallon Brute garbage can seems to be strong enough that a shopvac won't collapse it, so I'm using that for my build, along with 3/4" mdf and 1/8" polycarbonate for the sides.





In this picture, you can see the MDF sitting on the garbage can. I've routed a .25" deep groove into the bottom of the piece, .75" wide, and it fits snugly on the lip of the can

Visible in the picture is the 1/8" groove that the polycarbonate will be fitted to.



Here's a picture with a piece of scrap polycarbonate, test fitting in the groove. I pushed it back .25" away from the drop slot.


My question is, did I put too much of a gap there between the walls and the drop slot? Or will the separator still work?


Here's where I'm at right now:

Dry test fitting of the two pieces together before I create the intake funnel

Phil responded that the sidewalls need to be flush with the drop slot for it to work. I'm at a loss how to do that exactly. I can route out a rabbet into the top of the drop slot and continue that around the rest of the arc, but I'm worried about retaining the polycarbonate sidewalls when the suction is applied. The polycarbonate will of course be trapped in the 90 degree or so section of the arc that the supports the baffle itself, but for the remaining 240 degrees it'll just be resting in the rabbet on one side. Am I overthinking this?

[Mark Burnette]

Phil is pretty adamant that any sort of a step between the wall and the drop slot will interfere with the flow of dust down into the drum. There's no mechanism directing the dust downward--it's the low pressure "calm" air below the slot that attracts the particles to enter the slot.

I considered building mine using the same approach as you but I couldn't come up with a way to keep the transition clean when the tophat is sitting on a plate. I ended up extending my chamber wall down into the can and supporting the baffle from the top plate with threaded rod--much like the original Thien Baffle idea. Mine is of the "all-in-one" variety with a blower on top but internally it's identical to most of the others that apply suction through a hose. I haven't gone beyond the testing phase & cleaned up the look yet & I haven't bought a bag...but I'm nearly there. It does work well to separate the fines so they end up in the drum instead of blowing out the exhaust but at first it performed very poorly in that regard and was actually sucking dust up out of the can and spewing it out the blower exhaust. I added a ring around the chamber that could be sealed to the can with weatherstrip. I wouldn't have thought the puny leakage I had there from the snug fit of the chamber walls to the can would have mattered but after sealing it up I now don't get a dust storm out the exhaust when I feed it in the intake.

Bottom line--sealing everything up tight with weatherstripping or caulk MATTERS as does smooth transitions in the airstream.

[Kevin Schlossberg]

I'm wondering if a slope or taper between the wall and the drop slot would be a good work around

[Rick Thom]

Is the outer edge of your drop-slot perpendicular with the edge of your garbage can? It should be.
You can route a step into the MDF to hold the acrylic so that it is flush with the outer edge of the drop slot. Apply a good bead of adhesive to ensure the acrylic is well bonded to the MDF and will form an air-tight seal. I tried latex calking first and it didn't like acrylic so I used 'Goop" automotive weather stripping adhesive and that worked, but others have used different products... doesn't matter provided it works. You want the dust to circulate around, hit the wall and drop straight down into the pail, so ledges.
My top hat is used with a full size DC and has a 1 1/4" drop slot for 240 degrees. It works very well for a variety of saw dust and chips.. capture rate is in the high 90% range which is impressive. I don't know what width drop slot is recommended for shop vacs.

[Curt Harms]

I'm wondering if a slope or taper between the wall and the drop slot would be a good work around

I don't know but all the chips & dust ride along the vertical wall of the 'top hat'. If there's anything horizontal between the slot and the vertical side I could see that impairing the function of the baffle.