I developed and built the Fume Coffin - Laser Cutter Exhaust Vent Filter to assist with filtering the exhaust vented from my 50 Watt CO2 laser cutter.

This new Instructable is an optional add-on for the Fume Coffin project.

The Fume Coffin design has been successful at filtering the exhaust from my laser cutter, and the cost of major consumables is quite reasonable.

But it's not easy to service the Pre-Filter.

In the original Fume Coffin design the Pre-Filter lines the interior of the cylindrical HEPA filter. And accessing it requires opening the Fume Coffin and disassembling the HEPA filter assembly.

This project replaces (or supplements) the original Fume Coffin's Pre-Filter with an inline Pre-Filter that's designed to be easily monitored (it's see-through) and easily serviced.

Step 1: New Pre-Filter Design

The Pre-Filter is the first filter element encountered by the laser exhaust. The Pre-Filter's role in the system is to capture the large particulates and protect the HEPA filter from becoming fouled and gummed up.

But the Pre-Filter gets dirty quickly. The Pre-Filter uses low-cost fiber filter cloth. And it is intended to be serviced often, like every week or two.

The new Pre-Filter is entirely modular. It just plugs into the exhaust chain before the Fume Coffin.

The Pre-Filter consists of two parts -- a lower plywood filter frame and a clear acrylic outer lid. It is designed to hold a piece of 15 inch-wide filter cloth over a frame with air holes. The acrylic lid then fits over the cloth and frame and holds it tightly in place. It is sized for filter cloth between 15 and 15.5 inches wide. The material must be at least 21 inches long. So a standard 15 by 24 inch piece of filter cloth can be installed. 3M Filtrete Air Conditioner Cloth (Amazon) or Duck Brand Air Conditioner Cloth ( Amazon) are two examples and cost about $8 to $10. Longer filter cloth can be cut to size.

The base is constructed from 5mm plywood, and the outer lid is made from 5mm clear acrylic. It uses two "DryerDock" 4-inch duct ports (Amazon) (Home Depot) to facilitate quick disassembly and re-assembly.

It is also designed so all the major parts are cut with the laser cutter rather than traditional woodworking tools.

The design was created in Fusion 360.

The model is available here. It can be downloaded in Fusion 360 and other CAD formats.

Step 2: Build the Plywood Base

I've included DXF files for the plywood parts. (I've also included AI versions of the internal panels because my laser cutter control software had trouble with the complex Voronoi patterns in the DXF files.)

Cut the following parts from both-sides smooth 5mm plywood.

1 each Bottom panel - bottom_ply.dxf,
2 each Internal vertical panel - internal_vertical_ply.ai*
1 each Internal horizontal panel - internal_horozontal_ply.ai*
2 each Lower lip panel - lowerLip_ply.dxf
1 each Left panel - left_ply.dxf
1 each Right panel (with port) - right_ply.dxf

* = dxf versions did not work in laser cut program.

Assemble the plywood parts starting with the bottom panel.

I used both hot glue gun or wood glue to assemble the panels. Check the fit and assemble the panels before gluing. Make sure you mark the orientation and outside surfaces before gluing.

Insert and glue the internal vertical panels and the internal horizontal panel.

Insert and glue the 2 lower lip panels.

While the joints are still somewhat pliable assemble the left panel onto the end.

Make sure all the seams with exterior exposure are well sealed. Hot glue makes a good air seal. But you can also use silicone-latex sealant caulk (e.g. DAP Kwik Seal Plus). Take care not to leave large gobs of glue on the inside of the lips where the acrylic top will attach. (It is not hard to remove hardened hot glue or caulk with a putty knife.)

Finally, assemble the right panel on the other end.

Step 3: Build the Acrylic Top

Cut the acrylic top panel and two side panels with the laser cutter.

1 each Acrylic Top - top_acrylic.dxf
2 each Acrylic Sides - side_acrylic.dxf

You will glue the acrylic panels and pieces using Acrylic Cement (TAP).

Cement the top and sides square. Make sure you place the top panel on top of the side panels and make the edges flush. Allow the glue joints to harden.

The acrylic end-lips and hold-downs are a little tricky to cut and fit to the completed plywood base. The acrylic lid must be easily removable. (That's the whole point of this Instructable.)

My original design called for each end-lip to be a single piece of acrylic (endLip_acrylic.dxf). However, I found it easier to fit the end-lip by attaching three acrylic pieces to the acrylic top and sides (see photos). The pieces were cut from 2 each of endLip02_acrylic.dxf and 1 each of endLipTop03_acrylic.dxf.

Cement the lip assembly for one side in place using a piece of 5 mm plywood scrap as a jig for the spacing of the overhang of the top and sides acrylic piece.

The lip assembly for the other side will need to be fit to match up with the actual size of the plywood assembly. Position the lip pieces with enough play so that the completed acrylic lid assembly can be easily removed.

I used a Dremel tool with a cutting wheel to cut and grind the acrylic pieces to fit as needed.

I used small trigger clamps to hold the acrylic pieces in place while the Acrylic Cement is applied using capillary action.

The hold-down pieces run the length of the lid and fit into the side lip area. They serve to hold the filter cloth firmly against the lower plywood assembly. The hold-down pieces may require trimming to fit. (Again, I used the Dremel tool.)

Cement the hold-down pieces in place.

Step 4: Attach the Duct Ports

The DryerDock duct ports facilitate disconnect and reconnect of the exhaust ducts. The DryerDock consists of two parts -- one part with 3 screw holes in the flange; the second part has no screw holes.

The DryerDock part attached to the acrylic lid assembly must be trimmed down to fit, as is it will hit the plywood filter and obstruct the exhaust air flow. There needs to be a large air gap and filter cloth between the DryerDock and the plywood frame.

Insert the part of the DryerDock with the screw holes into the acrylic lid assembly from the outside. Put painter's tape around the portion of the port that extends into the acrylic box and mark a line approximately 3/8 inch from the flange where you will trim the port so that it will enter into the box just past the lid by about 1/8 inch.

Trim the DryerDock piece at the line with a Dremel tool and the cutting wheel or a hacksaw.

Insert the trimmed DryerDock piece into the acrylic assembly. Add a bead of silicone-acrylic caulk at the flange if it doesn't fit tight.

I fastened the DryerDock port in place using 3/16" stainless steel pop rivets, but small nuts and bolts will work just as well. Make sure the rivets or bolts do not extend above the countersunk holes in the DryerDock flange.

Position the other DryerDock to the plywood assembly.

Important note: The DryerDock must be positioned with the screw holes at 2 o'clock, 6 o'clock and 10 o'clock. If you position the DryerDock the other way (hole at 12 o'clock) the screw holes will run into the plywood filter frame.

Mark and drill holes for the 3 rivets or bolts.

Attach the DryerDock with the rivets or bolts.

Step 5: Assembling the Pre-Filter Element

To assemble the completed Pre-Filter Element you will need 6 each 3/4 inch 1/4-20 thread button-head bolts and 6 each 1/4-20 thread wing nuts.

Place the bolts into the holes in the plywood lip from the inside out (with the bolt heads on the inside). I added a drop of hot glue on the underside of each bolt head to hold each loosely in place.

In order to help creates a grippy rubbery surface for the filter cloth, I put a bead of hot glue along the edges of the internal plywood filter frame and allowed it to harden. I also ran a bead of hot glue along the sides of the air holes in the frame.

Stretch the filter cloth, such as 3M Filtrete Air Conditioner Cloth, over the filter frame. Center the cloth so that there is an equal amount on each side.

Place the acrylic lid over the plywood frame leaving a couple inches before the bolts. Thread the filter cloth up into the gap between the acrylic lid and the holddowns. The filter cloth should be on the inside of the acrylic lid.

Evenly press down on the lid so that the acrylic lid fits over the lip and the bolts fit into the slots in the acrylic lid. The lid should fit all the way down and there should not be gaps larger than 1/16 inch all around.

Screw on the wing nuts.