Trimming mitered hardwood edging? + accidental photo essay
My current project is a pair of tiny stereo sub-woofers (Pluto+ design by Siegfried Linkwitz; spouse friendly cabinets by me). Mostly it was an excuse to graduate from MDF + plywood to hardwood accents and learn router joinery that's more exciting than half and cross-laps.
I'm at the point where I trim, shim, and glue everything together and contemplating the trimming part especially thickness wise.
The top panel is a glue-up where I'm mating 3/4" solid birch (heartwood) edging to a nice piece of 18mm baltic birch plywood with .050" face veneers I can actually sand. The edging mates to itself with 45 degree mitered corners and a half inch wide shiplap to the plywood. It'll have invisible glue lines except at the corners where I sniped the plywood or dropped (DOH!) the hardwood edging.
Beneath it there are the four usual box sides (lock mitered) plus a pair of shelf-braces joined by a shouldered edge-lap in a "+" configuration. I'd like to retain nice glue-lines on the sides. The top needs to meet the braces for stiffness reasons.
The bottom is another glue-up with an 18mm ply baffle inset in 3" + 18mm high routed feet joined by lock miters, the relevant part being that the two side assemblies measure 14 1/64" square and 14" x 14 1/32" while the bottom panels are a hair off with one at 14" square. I'm going to have a little lip on the bottom so a matching one on top won't be out.
I left the edging over-sized and need to undo that.
1) Over-wide by 1/16". The easiest way to build boxes from sheet goods is to cut 4-sides and trim to fit. Originally I thought I'd do the same thing here. I made the tops to measure (within 1/64th) of the actual finished size (less 1" of edging on each side) of the joined (via a lock-miter) sides so I thought I'd do the same and still end up with nice corners.
For this project, I'm leaning towards trimming the edges all to the same width and forgetting about trying to trim to fit. The corners will have nice sharp points, and the tiny lip will be like the one on the bottom. I'm a bit curious about what I'd do if I did't settle on that - I'd be too scared of breaking something if I tried to trim a completed top; although maybe a climb cut a couple inches in from each corner would avoid that? I also thought about temporarily installing the edging with brads through the edge into the plywood where they overlap.
2) Over-thickness by .030" - .040". It's within a few mils of 3/4", but there's 10 mils of narrowing towards the factory edge of my Woodcraft Baltic Birch. The Home Despot birch I used for the sides was uniformly within a few mils of .690" but wasn't as nice looking and had .010" face plies which were in real danger of sanding through if any work was required so I upgraded and got a different set of problems.
This is the more interesting one. I think I want to cut surplus MDF to size so I can sandwich the plywood and edging and trap it in relation to the plywood, probably with some double sided tape.
After that I'm leaning towards using my vertical sled with a 2" flush trim bit on all four sides. I'll revisit the four un-trimmed ends a second time after their neighbors have been cut. I'm a bit worried about what will happen to the glue line.
I could build a vertical trim base. I think that would have the same problem and be more work.
I could cut a rabbet around the outside perimeter with a handful of passes and a 3/4" straight bit to match the minimum clearance to produce a perfect glue line. Then shim one end of the brace or use a more gap filling glue (polyurethane? Epoxy?) to join the pieces. The glue joint with the braces might not be as nice. I'd leave the inside ~1/4" full thickness and notch the end of the braces to fit.
Here are the relevant pictures which are about halfway to a photo essay. The edge lap, bottom glue up, and lock miters are the fun parts and with those it's more of a show-and-tell work-in-progress. I have learned a lot so far and made a surprising amount of firewood + sawdust for such small boxes.
02_two.jpg: Most of two enclosures. Note top glue-up and the brace + side structure it mates to. Top has 1/16" overhang on all four sides.
08_inside.jpg: .030-040" extra on the bottom side. Originally I thought I'd put a couple brads in through the wide lip to hold things together while I trimmed the edge in-place. The ship lap is danger of being air-tight even if the joints aren't, keeps things from wiggling, and was easier to cut than a tongue-and-groove.
14_sled.jpg: Sled with 90 degree leg. Anything run through against the vertical fence is going to stay up-right. Two pieces of 3/4" stock are attached and routed parallel to the front face to locate my lock-mitered feet and prevent tear-out. While some scrap to prevent tipping into the fence was a good idea, it would have been smarter just to cut the joints before trimming to height than trying to re-use the same backing pieces. With the parallelism error it would also have been better to align the pieces with the table and clamp them wherever they ended up.
03_oversize.jpg: Illustration of top construction. Note 1/2" wide rabbets for shiplap. The easiest thing to do here seemed to be starting with the moulding 1/16"long as shown, trimming two opposing sides within 1/32" and sneaking up on the other pair with a micro-adjustable stop (Incra miter fence + shop stop).
I did joint .010" off the other top instead of cutting longer side pieces using my rabbet bit. That will not be an invisible glue line. Originally I ran longer chunks of wood through the router table to put an edge on so I wouldn't have as much to trim width wise. The miter gauge can be set for the scale to read the inside length; but that only works if the thickness was uniform, which it wasn't until I re-ran the pieces through the table saw 14" and change at a time so the round overs are not uniform. A 6" wide piece of 1/2" MDF flat against the table made a perfect sub-fence for that operation with plenty of room to slip in with push blocks.
07_corner.jpg: The approach seems to have potential for invisible glue joints.
01_moulding_scrap.jpg: Once re-rounded that part will look nice too. These scraps came from where I checked the lap depth.
02_edge_lap.jpg: Shouldered edge-lap cut a few mils under .690 (two passes with the undersized half-inch bit, fence moved relative to scrap wood stops with shim stock) and .500 (undersized bit) before squaring off the shoulders. It resists racking better and may have some use in knock-down furniture. A few minutes with scrap wood, double sided tape, and 1 1/8" pattern bit with jigsaw + forstner bit to hog out the scrap yielded a pattern with straight lines that met in perfect radiuses.
07_glue-up.jpg: Bottom glue up. Lock-mitered corners. The same rabbeted inlay. I made this one fit by trimming to width in the table saw and re-cutting the rabbet until it slipped in. Home Depot birch is not void-free. I used a straight fluted bit for the outside diameter and rabbet to avoid tear-out on the veneer and spiral bit to go the rest of the way through in a few passes. I don't like the vibration from the 1/4" spiral and am tempted to try something shorter (I don't need over 3/4"), fatter (3/8" or 8mm), and correspondingly stiffer.
08_one_set.jpg: One set of feet and one in jig. The 2" wide strip is about .690" from the edge just like the plywood. I learned the hard way that diagonal cuts into the grain break when I got to my second board which was somehow different than the first. I also realized that I'd have had an easier time and avoided squareness errors if I made the pattern to match the inside only and left a couple inchest at the ends, trimming to width on my miter gauge. I do like the clamps better than sticky tape. I learned about matching grain and color too - it would have been better to get long pieces and use adjacent cuts.
09_endgrain_routing.jpg: I tried to make that routing direction work with a little clamping reinforcement. It didn't work, although the broken pieces stayed in the same spot instead of flying across the driveway.
05_corner2.jpg: Lock miters work in plywood too. There's no glue here - the plywood seems to be a bit springy so the joint starts with a slight interference fit. The veneer has separated on the piece cut vertically - I'm done using that stuff for visible pieces. The exit side is visible for the piece cut flat. With the fresh piece behind it and multiple passes you can't tell the flat piece was cut and then lock-mitered instead of the other order.
02_straight-edge.jpg: A straight edge helps avoid crushing the wafer-thin corner or any tipping although double sided tape is tricky with thin veneer on Borg Birch. Two sides together plus a scrap to avoid tearout on the second.
Last edited by Drew Eckhardt; 10-07-2008 at 07:10 AM.