A Parabolic Reflector

So, I want to play with a parabolic reflector to make a directional microphone, bit I don't want to spend a lot of money.  I can buy 1/32" polycarbonate sheets from TAP plastics, and maybe I could try forming that into a parabola.  I thought about just heating it and laying on top of a round trash can, letting gravity and maybe a small weight shape a curve.  Then I researched vacuum forming on the internet.  I think I could cobble something together...

There are instructions on the net for making parabolic microphones out of dollar store umbrellas, mixing bowls, and woks, but those aren't likely the right shape or size to really do the trick.  A parabola is a specific shape that reflects a set of incoming waves onto a point in the middle, so random bowl shapes won't necessarily give the concentration of sound energy I'm after.  The other issue is the size.  It takes a large reflector to deal with low frequency sound - an efficient reflector apparently needs to be twice as large as the sound's wavelength.  The speed of sound is about 1100 ft / sec, so a 2 ft diameter reflector is efficient to about 550 Hz, which is low enough for many wildlife sounds, and acceptable for human speech.  I also know that the $1200 Telinga parabolic mic that a lot of pros use is about 23", so mine will be one bigger.  Yeah.

For this purpose, a parabola is expressed mathematically by the equation:

y = x² / 4f

where f is the height of the focus.  I wanted the focal point to be within the bowl of the parabola, so that sound from the sides would be blocked from the microphone element, and I wanted the diameter to be 24".  After testing a few values for f, I settled on 4.5.  That creates a parabola 8" deep, with a focus 4.5" from the bottom.

I calculated a y value for every half-inch from -12 to 12 (to make a 24" diameter dish), and plotted out the shape on a piece of construction paper, using a straightedge for the x axis and sliding a carpenter's square along it to measure up for the y axis.

I connected the dots, cut out the paper, and transferred the shape carefully to a piece of melamine-coated hardboard.  Then I cut the hardboard on the band saw and gave it a light hand sanding to make a smooth curve.  I might have been slightly more accurate to draw it directly on the hardboard, but I think my error was less than 1/32".

Next I cut a 26" diameter circle of 1/2" OSB sheathing with a router jig, and mounted a 8" long scrap of 4x4 to the center of it.  I carefully located the exact center, drilled a small hole in the top of the 4x4, and stuck a small finish nail in.  I cut the head of the nail off, leaving a small pin as a pivot point.  I drilled a mating hole in the center point of my hardboard pattern, and cut a notch in the outer part to ride on the edge of the circle.  Now sweeping it around on the pivot will form a 3D parabola.

Next I cut some rings out of a chunk of 1-1/2" foam insulation scrap.  I calculated the outer diameter at 1-1/2", 3", 4-1/2" and 6" high, and cut circles at that size.  I stacked them up, glued them together with some spray adhesive, and made a base for a plaster layer.  I trimmed the corners off to start rounding the shape.

I coated the foam layers with strips of fiberglass drywall reinforcement mesh tape.  It's a little sticky on one side, but I stapled the longer pieces to the wood block at the top and / or the wood base.  I used the pattern to make sure there was around 1/2" gap everywhere.  I also put a big lazy susan bearing under the wood base, which makes it much easier to work on.

Then I applied a layer of setting-type drywall compound to the form.  This stuff is basically plaster of paris with some fillers and retarders to slow the setting rate.  The type I used solidifies in 40 minutes after you add water.  Again, I was sure to leave this rough surface 1/8" - 1/4" low of the final finished height.  I let the whole thing dry out for a couple of days at this point.

Finally, I wiped a couple coats of Zinnser shellac on the hardboard pattern to waterproof it, and then used it as a screed to apply another coat of the setting compound.  The resulting surface has a few holes and defects, but is overall a very accurate shape.

After some filling and sanding, it's pretty smooth and I hope ready to use as a vacuum forming mold.

Update:  to my chagrin, I never got to test this mold out.  It turns out I'd have to build a big insulated box to make an oven to heat the polycarbonate sheet up, if I want to form it.  My original quick and dirty forming oven didn't have enough capacity.  I might do another one of these, in a size that I can just use our household oven to heat the plastic.  Don't tell my wife.

DeWalt DWP611 Mini Router Review

I got myself a new tool today.  My big Bosch router refused to start, I really needed to get some little hinge mortises cut, and I've been eyeing these little pint-sized routers for a while. This class of 1/4" collet routers has evolved over the past few years from the laminate trimmer category.

This one is really a miniaturized fixed-base router complete with all the features you'd expect.  The motor has a 7A rating, which probably means it's got enough power to drive any bit you'd find in a 1/4" shank. It also has a clear sub-base, variable speed control, and a pair of LED lights around the collet.  I played around with a few models in the store before buying this one at Lowe's for $119.  The LED lights and much smoother adjustment system sold it vs. the Bosch that was my second choice. It's made in Mexico, which is OK with me.

It's 3" in diameter at the grip (it's meant to be held in one hand at a textured metal grip near the base) and about 9" tall, although that changes depending on the depth setting. Depth is adjusted via a ring (marked in 1/64" increments) above the grip that engages threads in the motor housing.  Very fine adjustments are easy to make.  The height is locked with a cam buckle latch, or two tabs can be squeezed to quickly remove the motor from the base for bit changes.

So far I've just cut a few 1/16" deep hinge mortises in a pair of cabinet doors, so I can't speak to the power of this little tool, but I don't think that's the main selling point anyway.  I bought it for control over detailed cuts, and it did a great job balanced on the edge of a 3/4" workpiece without being wobbly.  Tomorrow I'll cut the matching hinge mortises in the face frame of my new bathroom cabinet, and I expect that to be pretty easy, too.

One-handed grip is good

A desk light for the workbench

My woodworking bench has 3/4" dog holes for clamping work.  I made an adapter to fit those holes so I could install a swing-arm desk lamp.  It's really handy to get light right where I'm working, and I also use it to provide raking light when I'm planing or doing other surface work.

The adapter is simple.  I just took a 1/2" PVC  schedule 80 threaded nipple, filed the threads off until the end fit snugly into a dog hole, and then cut the end of the nipple off to leave a wide ring around the top.  The ring keeps the adapter bushing from falling entirely into the dog hole, and also holds the bottom of the lamp up off the bench so it doesn't drag and scratch.  Schedule 80 is usually dark gray, has a thicker wall, and the inside matched the diameter of the desk lamp post pretty closely.

My Thien Dust Collector

I wanted to upgrade my dust collection system.  I have a Jet DC-650 dust collector that I bought in 2003, with a good pleated filter on top, and a plastic bag on the bottom.  The filter is stock, and according to the particle counter I borrowed, it does a very good job of catching all the finest sub-micron dust.  However, I really hate emptying the bag, because it attaches with this sleek-looking but fiddly internal spring ring.  It takes a lot of bandsaw or table saw use to fill it up, but the planer especially will generate large amounts of big shavings.  I'd also like to have something between the blower and the intake because what happens if I accidentally suck up a larger chunk of wood or a nail?  Whang!  Into the impeller, and then it gets flung into the bag where it pokes a hole.

There are trash can lid separators, but they don't work well.  I made one of my own several years ago and ditched it pretty quick.  A cyclone separator would be good, but they're expensive and take up a lot of room.  Fortunately, a guy named J. Phil Thien came up with a pretty slick and simple dust separator that combines the functionality of a cyclone with the compact form of a trash can lid separator, and he put the idea up on the web for people to build for themselves.  What a guy!  I printed out a couple of pages from his website, and they've been sitting in my pile of shop papers for a few years.  Last week, I finally got around to building one.  I made it in a "top hat" style so that the separation happens in a separate unit above the trash can, and all of the volume in the can is available for debris.

I bought a quality 10 gallon metal trash can first, about 18" tall x 14" diameter at the top.  I had a couple of 16" x 15" pieces of 3/4" melamine particleboard left over from a shelving project, so those became the top and bottom plates, and I got a piece of 1/16" clear polycarbonate 8" x 48" custom cut from TAP plastics for $10.  Polycarbonate is flexible, and will stand up to impact a lot better than acrylic, so I chose it for the walls of the separator.  It's also nice to be able to watch the thing in action.  You could also use sheet metal, which would definitely be more abrasion resistant, but I don't think it will be a problem.  The inlet is a 4" plastic dust collection elbow, and the outlet is a 4" angled coupler.

I used a circle jig and router to do almost all of the shaping.  First I used a 1/2" bit to cut a dado in the bottom plate the same 14" diameter as the trash can, about 3/8" deep.  This fits over the rim of the can and holds the separator in place.  Then I used a 1/4" bit to cut slightly smaller diameter dadoes in both the top and bottom plates, about 1/4" deep.  This will hold the polycarbonate sheet I used for the sides.  Then I used the same 1/4" bit to plunge all the way through and cut the drop slot.  This slot is 1-1/4" wide, and goes 2/3 of the way around the circle of the lid.  I set the circumference so that the outer edge of the slot is about 1/16" inside the 1/4" groove for the sides so that the inside of the walls is flush with the outer edge of the slot.  I hope all that makes sense because I didn't stop to take photos.

Here is the groove to fit the trash can rim.  I've marked the ends of the drop slot.
I couldn't find my angle gauge, so I resorted to some late night trigonometry to figure out 120˚.
Too bad I didn't remember this trick.

Then I used a 4" hole saw to put a hole in the center of the top for the outlet, and another 4" hole in outer portion for the inlet.  I used three pieces of #10-24 threaded rod with nuts and washers to clamp the polycarbonate sheet in the grooves between the top and bottom plates, after putting a bead of caulk into the bottom groove.  I fastened the overlapping ends of the sheet by drilling holes and putting some #4x1/2" pan head screws through and into a strip of plywood.  I added a strip of self-adhesive rubber weatherstripping to the dado that mates the top to the trash can, because any little leak into the space below the drop slot plate is a big problem for this type of separator.

I hooked it up and the first thing I tried was cleaning out under my table saw.  I have a panel installed in the bottom of my contractor's type saw as part of the dust collection setup.  It accumulates a lot of dust and chips, and I stuck the hose in there and vacuumed it all up.  Just like a cyclone separator, it swirled against the clear sides in a downward spiral, and then dropped into the slot.  Pretty cool.  Only a layer of fine dust accumulated in the dust collector bag.  Here's a video.

Update:  I've been using it now for a couple of weeks, and it does just what I want it to do.  I've filled the trash can a couple of times and very little has made it to the dust collector bag.  I built this pretty fast, just to test the concept, and there are a couple of things I don't like.  The configuration requires a lot of elbows, which adds bulk, makes the hose fall off sometimes, and the inlet elbow inside causes a lot of turbulence.  I'm sure a side-inlet version would work a lot better, but I've got actual projects to deal with for now.

Dust Hood for my Miter / Chop Saw

I take dust collection pretty seriously.  The fine particles are a health hazard, and it makes a mess anyway. So I've been rigging up ways to improve the dust collection of my tools over the years.  The chop saw makes a lot of dust, and it's tough to collect because it won't work with a bunch of shrouds and things near the blade.  I tacked together a very quick hood to help my dust collector capture most of what it generates.  After using it for a year, I think it works pretty well.  It's just three pieces of 1/8" lauan plywood tacked to a top and bottom trapezoid of 5/16" osb sheathing.  ie, I made it from scraps.  Dust collection is pretty effective.  With about 600 cfm of air being sucked into a port directly behind the blade, the fine dust gets captured pretty well.  Some bigger bits bounce off the hinge mechanism and are thrown around the room, but I'm less worried about that.  I've thought about making some kind of angled deflector to put on the saw to reduce that, but haven't got around to it.

Miter Saw Stand

I built it as a small rolling cart, with a bunch of storage below.  There are three small drawers in the top section, plus a flexible storage scheme I devised where sheets of hardboard slide in slots.  This makes very compact little drawers, perfect for bits and other small items.  The lower area has three larger drawers where I keep my cordless drills and small clamps.  Yeah, I've only put a front on one of the drawers and I'm using screws as handles.  I'm classy.

Storage Below 

I attached a plastic dust collector fitting at the bottom of the dust hood, then a short length of 4" hose, with a quick-connect on the end.  I attach the hose from my dust collector to each machine as I use it for maximum airflow.  I tacked a piece of plastic gutter screen over the opening to keep from losing chunks of wood and other large debris down the pipe.

Dust Collection

Building a Gate That Won't Sag

Here's a plan for a simple gate.  I've built two of these for my own yard, and one larger one for a neighbor. Despite his kids occasionally climbing on it, it has held up very well.  No special hardware or fussy joinery.  Just a couple of miter cuts and a little analysis of the physics of gates.

Cedar Gate in my yard

If you just attached two pieces of wood at a right angle, one vertical (a stile) and one horizontal (a rail), and put hinges on the stile, the rail can swing back and forth as a very simple gate.  It probably won't last too long, because all the weight of the rail is trying to twist the joint apart.

By installing a support at an angle between the base of the stile and the far end of the rail, you form a triangle, which is a very stable shape.  The weight of the rail is now supported by the angled brace and the top of the stile, and there are no forces acting to twist the rail-stile joint and cause the gate to sag.

You could theoretically get the same behavior from an inverted version, where the rail is low and the diagonal brace runs from the top of the stile down to the end of the rail, but now the ends of the brace are under tension.  Without special joinery (or welded metal fabrication) the parts will tend to separate.

So take that triangle and hang whatever kind of decorative stuff you'd like to on it.  In my version, I start with two rails, about 1/2" narrower than the opening you're putting the gate into.  Put them far enough apart that the brace runs at a 45˚ angle or higher with respect to the ground.  A short, wide gate is under much more strain than a tall narrow gate.  If the opening is very wide, you might consider a double gate, with one on each post and the latch in the center, but those have their own problems.

You can see how the wooden components form the rail, stile, and support from the diagram above.  When I build mine, I just put screws through the decorative pickets into the rails and support.  I don't actually join the support to the rail, and there is no framing member to form the stile, since the pickets serve that function.

Cleaning Paintbrushes with a Fork

After using a brush with water-based paint, I've found that the quickest, best way to clean the brush is with a fork.  Run a stream of water over the bristles while combing them with the tines of a regular dinner fork.  The tines split apart the gummy paint that accumulates up higher in the brush, and lets the water quickly penetrate into the bristles so the paint rinses out quickly and thoroughly. 
Once the water runs clear, give the brush a good shake to get rid of most of the water (I like to do this outside over the lawn), smooth the bristles into their proper shape, and leave it to air dry.