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Best way to execute an edge miter joint for plywood carcase?

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Forum topic by ADHDan posted 07-28-2016 07:25 PM 2051 views 0 times favorited 39 replies Add to Favorites Watch
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ADHDan

800 posts in 1571 days


07-28-2016 07:25 PM

Topic tags/keywords: question miter carcase carcass cabinet joining joinery chamfer

I’m going to be building two bookcases for my kids’ room – each 5’H x 3’W x 1’D – to match their Charles Webb bed set. Charles Webb furniture typically uses mitered edge joints for carcases, like this:

My challenge is, I’m working with oak-veneered plywood instead of solid oak. Meaning, I have even less room for error than in the already-challenging mitered edge joint. On the plus side, since the book cases are only 1 foot deep I just need to to get a straight and true edge miter across 12” spans. (Since I’m working with plywood, I’ll be putting a face frame across the bookcase.)

I currently have a cabinet table saw with a cast-iron router table wing, with give me a decent amount of support if I bring the work to the tool (i.e., use router table or table saw instead of handheld router or circular saw). I’ve brainstormed three ways of attacking this, each with advantages and disadvantages:

(1) Buying a lock miter router bit and using a lock miter joint.

Advantage: if done right, ensures alignment and good glue strength.

Disadvantage: requires buying a $50+ router bit, and isn’t easy to execute (I think it would require routing a 1’x3’ board vertically, on its edge).

(2) Buying a 45 degree chamfer bit big enough to cut 3/4” vertical (plywood thickness), using it to route a 45 degree chamfer on each board, and using biscuits to aid in alignment and strength for glue-up.

Advantage: easier to route a simple chamfer vs. a locking miter.

Disadvantages: requires buying a $40+ router bit, plus I’m not sure how easy it is to properly align biscuits for a mitered edge-to-edge joint.

(3) Cutting 45 degree chamfers on the table saw and using biscuits to aid in alignment and strength for glue up.

Advantage: no need to buy anything.

Disadvantage: harder to cut perfect 45 degree edges using table saw vs. router with chamfer bit (due to use of router fence for easy and accurate repeatability); same issue with biscuiting a miter joint.

Anyone have thoughts on these approaches, or alternatives?

-- Dan in Minneapolis, woodworking since 11/11.


39 replies so far

View gargey's profile

gargey

463 posts in 238 days


#1 posted 07-28-2016 07:38 PM

Don’t just use glue for plywood miter joints. Your last choice is best of those listed because the biscuits.

Fine Woodworking or some other wood magazine last month had an article about joining plywood miters with splines and/or with aluminum angles attached to wood that fit into mortises on each piece of plywood.

Much more work to build it, but it wont suck the way a plywood miter would with just glue.

View Aj2's profile

Aj2

689 posts in 1261 days


#2 posted 07-28-2016 07:42 PM

That’s some nice looking pieces your thinking about making.
All the methods you mentioned sound iffy to me.Im not that great with a router maybe your better.
I am very good with a skill saw and a straight edged when it comes to sheet goods.
So if I had to have a miter joint that’s what I’d do.
How about a butt joint and you cover the end grain with oak?
Miter joints are kinda ugly if they aren’t tight.

Aj

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ChefHDAN

808 posts in 2312 days


#3 posted 07-28-2016 07:45 PM

Definite thumbs down for the locking miter bit, I bought 1 and there is also quite a bit of jigging required for the vertical cut, I used it for box columns which worked but it hasn’t been used since.

If I HAD TO have the mitered corner I’d go with a good blade and a good sled and cut them at the TS (remember to score your line with knife first for the tearout. .

But, since you’ll be working with ply anyway, you’re going to need some sort of edge treatment, I’d also consider putting solid stock around my edges and then joining the corners without the mitre, which won’t be a big design loss because the face frame would hide that front edge… 5” tall … kids room…. even if your mitre wasn’t perfect it wouldn’t be as noticeable at that height and

Nope I say go for the mitre, check your tune up, and maybe run some test cuts first, and sneak up on the final cut,

-- I've decided 1 mistake is really 2 opportunities to learn.. learn how to fix it... and learn how to not repeat it

View Mk3supraholic's profile

Mk3supraholic

7 posts in 130 days


#4 posted 07-28-2016 07:50 PM

If the cost for the router bits are truly a disadvantage, why wouldn’t you just buy a cheaper bit?

If you only have a few of these cuts to do i would just do it on the table saw.

View ADHDan's profile

ADHDan

800 posts in 1571 days


#5 posted 07-28-2016 08:15 PM


Don t just use glue for plywood miter joints. Your last choice is best of those listed because the biscuits.

Fine Woodworking or some other wood magazine last month had an article about joining plywood miters with splines and/or with aluminum angles attached to wood that fit into mortises on each piece of plywood.

Much more work to build it, but it wont suck the way a plywood miter would with just glue.

- gargey

Wasn’t planning on just using glue unless I did a lock miter, which I’m leaning against. Assuming I go with a simple chamfer and some kind of reinforcement, I’m wondering whether it would be better to use a parallel spline, perpendicular spline, or biscuits.

I.e.:

(a)

vs.

(b)

Follow up questions:

(1) Easiest way to cut a 45 degree bevel on the one-foot edge a 5’x1’ plywood board: table saw, router table with chamfer bit, or circular saw?

(2) For strengthening the miter joint, would it be easier to use biscuits, a parallel spline (a), or a series of perpendicular splines (b)?

(3) Easiest way to cut a parallel spline slot (image a) in the edges of large plywood boards – table saw blade tilted 45 degrees, running the board over it flipped end-for-end (to cut opposite the edge miter)? Series of biscuit joiner cuts, with the joiner fence set to 45 degrees? Something else?

I’ll be using an 80-tooth full-kerf Freud plywood cutting blade, and I’ll score/tape the cut line – so I’m not TOO concerned about tearout (plus any tearout should hit only the inside of the case, which would be easier to hide).

-- Dan in Minneapolis, woodworking since 11/11.

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JBrow

818 posts in 383 days


#6 posted 07-28-2016 08:37 PM

ADHDan,

If you can keep the plywood running true through the entire table saw cut, this would be my approach. In addition to the router based disadvantages you mentioned, I would be concerned about chip out.

If my geometry is correct and there is space in the shop, the bevels could be cut as Complementary Angles by cutting the carcase top on one side of the blade with the good face up and the carcase side on the opposite side of the blade with the good face down. In this way if one bevel is 44.9 degrees, the other bevel would be 45.1 degrees allowing a 90 degree corner to be formed. A sharp plywood cutting blade and firmly adhered masking tape on the both faces and both sides of the cut line could help control table saw tear out.

If the Complementary Angles are as close to 45 degrees as possible, shallow mating grooves could be cut into the joining bevels at the table saw and a parallel spline used for alignment and to reinforce the joint. Perpendicular splines add plenty of strength but do little to help with alignment in the glue-up. Great care would be required to flush up perpendicular splines since the sides and top are plywood.

Even with biscuits or the splines, a glue-up that results in perfectly aligned tight fitting joints all along the joint line would be the biggest challenge for me. But with planning, some dry runs, and glue with a long open time, a good joint could result. I would also consider using band clamps, but I could end up using bar clamps.

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ADHDan

800 posts in 1571 days


#7 posted 07-28-2016 08:58 PM



ADHDan,

If my geometry is correct and there is space in the shop, the bevels could be cut as Complementary Angles by cutting the carcase top on one side of the blade with the good face up and the carcase side on the opposite side of the blade with the good face down. In this way if one bevel is 44.9 degrees, the other bevel would be 45.1 degrees allowing a 90 degree corner to be formed. A sharp plywood cutting blade and firmly adhered masking tape on the both faces and both sides of the cut line could help control table saw tear out.

If the Complementary Angles are as close to 45 degrees as possible, shallow mating grooves could be cut into the joining bevels at the table saw and a parallel spline used for alignment and to reinforce the joint. Perpendicular splines add plenty of strength but do little to help with alignment in the glue-up. Great care would be required to flush up perpendicular splines since the sides and top are plywood.

Even with biscuits or the splines, a glue-up that results in perfectly aligned tight fitting joints all along the joint line would be the biggest challenge for me. But with planning, some dry runs, and glue with a long open time, a good joint could result. I would also consider using band clamps, but I could end up using bar clamps.

- JBrow

This is all 100% in line with my thinking. Since I have a decent sized cabinet saw with extension wing, I think I have good-enough support to use your opposite-cut strategy for the sides versus top/bottom pieces. I’ll just have to wheel the saw into the center of my 11’x17’ shop :-).

My biggest concern is keeping the board flat, straight, and square while crosscutting an edge miter on a 5’ by 1’ board. I have a dead-on accurate Incra miter gauge with an extendable fence, but I’m thinking it might be better to build a custom sled for this project, for three reasons: extended support under and behind the workpiece, a zero-clearance backer to minimize tearout, and the sled gives me something to clamp the board to so it doesn’t slide during the cut.

That last point seems key to me – if these boards shift at all during the edge miter cut, the joints are probably toast.

-- Dan in Minneapolis, woodworking since 11/11.

View jbay's profile

jbay

813 posts in 362 days


#8 posted 07-28-2016 09:16 PM

I’m building a dresser right now that’s exactly the same idea. (Rift Cut Oak Ply)
Top is 82” long x 20 deep. Sides are 30” long x 20” deep.

Edit: Changed my mind.
I’m going to use my left tilt table saw, cut the two sides face up with the fence set at 30 3/4”
Then I’m going to cut the long top, face down with a fresh blade, on the other side of the fence using a sacrifice fence and bottom board for it to ride on. I’ll post pics when I do it.
Tape the joints, glue, fold. Should be plenty strong enough.

-- My “MO” involves Judging others, playing God, acting as LJs law enforcement, and never admitting any of my ideas could possibly be wrong or anyone else's idea could possibly be correct -- (A1Jim)

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ADHDan

800 posts in 1571 days


#9 posted 07-28-2016 09:34 PM


I m going to use my left tile table saw, cut the two sides face up with the fence set at 30 3/4”
Then I m going to cut the long top, face down with a fresh blade, on the other side of the fence using a sacrifice fence and bottom board for it to ride on. I ll post pics when I do it.
Tape the joints, glue, fold. Should be plenty strong enough.

- jbay

I kind of liked your initial idea for a sacrificial fence. I have a left-tilt saw with more tabletop on the right side of the blade than the left side. Would it be a terrible idea for me to use a sacrificial fence on the left side of the blade and use that as a constant perfect reference for my crosscuts? Obviously, you typically never use both the fence and miter gauge at the same time, but for my setup the only “trapped offcut” danger is a 12” triangular strip sandwiched between the blade and the sacrificial fence.

That does seem like a very good way to ensure I’m making the exact same cuts with the exact same measurements for each piece, and I can do it with my miter gauge extrusion rather than building a new sled.

Is there a hidden danger I’m missing, besides the possibility of a thin strip kickback (which wouldn’t be anywhere near my body)?

-- Dan in Minneapolis, woodworking since 11/11.

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jbay

813 posts in 362 days


#10 posted 07-28-2016 09:52 PM

This is how I’m going to do the long ends. Face will be down, I will leave about 1/64” from the point so that it won’t splinter/chip the face. The off cut will be above the blade, it might flutter a little bit but it won’t get trapped and shoot out like a missile. (also blade won’t be as high as pictured)

-- My “MO” involves Judging others, playing God, acting as LJs law enforcement, and never admitting any of my ideas could possibly be wrong or anyone else's idea could possibly be correct -- (A1Jim)

View Kelly's profile

Kelly

1113 posts in 2407 days


#11 posted 07-29-2016 12:37 AM

I just did some counters that measured 36” tall by 60” long by 34” deep. I made them from the $30.00/sheet smooth pine plywood the big box sells. I mitered the corners using my cabinet saw and, when necessary, a simple jig I made for the purpose, since my saw is limited to four foot cuts.

Once I had the cuts, I used the biscuit joiner to tie the beasts together.

Here is a page with information on the jig: http://lumberjocks.com/topics/166210

View JBrow's profile

JBrow

818 posts in 383 days


#12 posted 07-29-2016 03:23 AM

ADHDan, jbay

You are probably ahead of me but I will comment again anyway. Sometimes plywood can have a cup across its width. I would think the plywood needs to be kept flat across its width throughout the mitre cut for a well-fitting joint. If the top and sides are rough cut extra wide, a flattening caul can be screwed to the outer edges of the plywood top and sides. This flattening caul (perhaps a 1” x 2” or 1” x 3” piece of wood) screwed in place toward the rough cut edges of the plywood could help keep the plywood flat. If needed, several cauls could be used. Since the flattening caul is screwed in place within the sacrificial zones at the outer edges of the plywood top and sides, the holes left by the screws would be cut away when the sides and top are ripped to final width. If the crown of the cup is down, then the extra wide plywood could be screwed to a sled or a pair of cauls (or runners) of the same thickness that would rest on the table saw table.

The second advantage to making the mitre cuts on wider rough cut plywood is that the mitre cut can be a stopped cut, leaving the off cut attached to the plywood work piece. A riving knife or slitter could help keep the still attached off cut from getting trapped and/or torn from the work piece. The uncut area would then be cut off when the top and sides are ripped to final width.

There is an option for cutting the mitre that would leave plenty of room for the off cut to just fall away without becoming trapped while using the table saw fence. This option is the use on an L fence (an overhead fence) and a rub strip attached to the work piece parallel to the mitre cut line. While I have yet to find an application for this technique, I have kept it in mind for when it may come in handy.

The L fence consists of two pieces of wood attached to each other at 90 degrees. One leg of the L is clamped to the fence. The second leg of the L extends over and covers the blade. This second leg of the L fence must have long edges that are parallel to each other and hence parallel to the table saw fence.

A rub strip screwed to the extra wide plywood top and sides in the sacrificial zone would ride against the leg of the L fence that extends over the blade. If the leg of the L fence is wide enough, sufficient room is left between blade and table saw fence preventing the off cut from becoming trapped. If for some reason the offcut decides to come rocketing out, the second leg of the L fence over the blade will keep the off cut from lifting off the table. The key to a good mitre cut is an L fence that is accurately made and the rub strip attached to the plywood top and sides exactly 90 degrees to the edges of the plywood and thus parallel to the mitre cut.

View ADHDan's profile

ADHDan

800 posts in 1571 days


#13 posted 07-29-2016 01:07 PM



This is how I m going to do the long ends. Face will be down, I will leave about 1/64” from the point so that it won t splinter/chip the face. The off cut will be above the blade, it might flutter a little bit but it won t get trapped and shoot out like a missile. (also blade won t be as high as pictured)

- jbay

Wish I could do that, but with my table saw and shop setup I think I’m going to have to end up with a trapped piece. I just don’t have adequate table support to the left of the blade for a long piece of wood. At least the offcut will be far away from my body, and my hands won’t be anywhere near the blade.

My panels are long, but only about 11” wide – so I should be able to support them adequately with my extruded miter gauge.

-- Dan in Minneapolis, woodworking since 11/11.

View ADHDan's profile

ADHDan

800 posts in 1571 days


#14 posted 07-29-2016 01:13 PM



ADHDan, jbay

You are probably ahead of me but I will comment again anyway. Sometimes plywood can have a cup across its width. I would think the plywood needs to be kept flat across its width throughout the mitre cut for a well-fitting joint. If the top and sides are rough cut extra wide, a flattening caul can be screwed to the outer edges of the plywood top and sides. This flattening caul (perhaps a 1” x 2” or 1” x 3” piece of wood) screwed in place toward the rough cut edges of the plywood could help keep the plywood flat. If needed, several cauls could be used. Since the flattening caul is screwed in place within the sacrificial zones at the outer edges of the plywood top and sides, the holes left by the screws would be cut away when the sides and top are ripped to final width. If the crown of the cup is down, then the extra wide plywood could be screwed to a sled or a pair of cauls (or runners) of the same thickness that would rest on the table saw table.

The second advantage to making the mitre cuts on wider rough cut plywood is that the mitre cut can be a stopped cut, leaving the off cut attached to the plywood work piece. A riving knife or slitter could help keep the still attached off cut from getting trapped and/or torn from the work piece. The uncut area would then be cut off when the top and sides are ripped to final width.

There is an option for cutting the mitre that would leave plenty of room for the off cut to just fall away without becoming trapped while using the table saw fence. This option is the use on an L fence (an overhead fence) and a rub strip attached to the work piece parallel to the mitre cut line. While I have yet to find an application for this technique, I have kept it in mind for when it may come in handy.

The L fence consists of two pieces of wood attached to each other at 90 degrees. One leg of the L is clamped to the fence. The second leg of the L extends over and covers the blade. This second leg of the L fence must have long edges that are parallel to each other and hence parallel to the table saw fence.

A rub strip screwed to the extra wide plywood top and sides in the sacrificial zone would ride against the leg of the L fence that extends over the blade. If the leg of the L fence is wide enough, sufficient room is left between blade and table saw fence preventing the off cut from becoming trapped. If for some reason the offcut decides to come rocketing out, the second leg of the L fence over the blade will keep the off cut from lifting off the table. The key to a good mitre cut is an L fence that is accurately made and the rub strip attached to the plywood top and sides exactly 90 degrees to the edges of the plywood and thus parallel to the mitre cut.

- JBrow

The first part of your post makes sense, but my pieces are already cut to width.

The second part makes sense too. Although for my purposes, I’m thinking it might be even easier just to clamp a 3/4” plywood fence on the front half of the fence – right up to the blade. That will give me a reference for my cuts right up to the blade, but after that there will be a gap between fence and blade – meaning, no trapped offcut.

-- Dan in Minneapolis, woodworking since 11/11.

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ADHDan

800 posts in 1571 days


#15 posted 07-29-2016 01:15 PM

By the way, this has been one of the most helpful threads I’ve ever posted on LJ. Lots of great brainstorming and ideas going on here, and I really appreciate it!

-- Dan in Minneapolis, woodworking since 11/11.

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