Extending Table Legs. A CNC Cut End Grain Finger Joint

 I've got a short Parson's table with 2" square solid wood legs.  The table would be more useful if it was 6" taller, but that would require stretching or replacing the legs. Or would it?

The is a rendering of a CNC cut finger joint I came up with. As fancy as it looks the assembled appearance would look like interlocking 1/2" square fingers.   I need to make a trip to a nearby hardwood lumber source before I can actually cut this joint and extend my table legs.  

One Side

The Other Side

I was asked in a router forum if this could be done without using a CNC.  Perhaps with templates and a hand held router the top image could be duplicated.  To make the bottom image half the template would be more complex and need a separate center to allow cutting around the center tenon.  I came up with a version that would produce the same final look but could be made easier using a router table or the table saw and a jig to hold up the parts with a 45 degree twist. 

 

Slides together,

Both versions would need a small allowance between halves, perhaps .003" on one side. Room for glue and a slip fit rather than needing to be pressed together with no clearance between halves. 

Possibilities include using a contrasting wood for the feet.  The table is made from Cherry, and Walnut socks might be more interesting than matching the wood with cherry feet. 

When I get around to making these leg extensions I'll add a photo or two to this post. 

5/4/2024 Update:

I accidently cut one of the legs on my TV tray table too short.  The cutoff piece was about 1/2" longer than I needed it to be.  This joint was my salvation. Cut using a 1/8" diameter upcut spiral end mill. 

7/8" x 7/8" 

My TV Tray Table

Here is the table leg repaired using this joint:

Right leg in this photo.

 Questions and comments appreciated.

4D

Rabbet Joint. With a Lesson on Using Screws and Plugs to Cover Them.

A simple rabbet at a corner joint does at least provide one reference side when it comes time to glue up the corner.  Not much better than a simple butt joint, but it adds a little more end grain to face grain glue surface area.  Throw in two or three screws and this corner becomes quick to glue up. The screws serve to clamp it tight. The screw heads can be covered with plugs made from the same wood as the side or contrasting wood for some detail effect. This corner will be the hardest to break apart thanks to the screws. 

Marked out to ensure the right location

It helps to mark out where the cut will be made to make sure it isn't cut on the wrong board or on the wrong side of the correct board. The boards used here are 5/8" thick.  The rabbet width is thus 5/8".  Depth of the rabbet can vary, but here we decided to make it 1/8" deep to leave enough wood behind it to embed screw heads into.  A 3/4" diameter flat router bit used in a router table with 5/8" of it exposed outside the fence is a good way to cut the rabbet.  Backing up the cut with a scrap board is recommended.  .  Notice in the photo above that the length of the board, if the rabbet is only 1/8" deep, will be 1/8" shorter than the opposite board with the rabbet/dado joint. Be sure and account for the different board lengths due to the joint being used. 

Make sure the router is unplugged while you set it up. You can set the exposed width of the router bit easily using one project board and another scrap of wood in front of it.  Make sure the tip of the bit is as far out from the fence as possible.  You may need to rotate it to get there. Once the bit just touches the outer board make sure the fence is tightened down.  5/8" of the bit now projects from the face of the fence. . 

Setting bit exposure width

You can set the exposed height of the router bit using a simple ruler or any of several gadgets available  for setting blade or bit height.  One shop trick is to use two 1/8" drill bits with a board laid atop. It is an easy way to set the bit height to 1/8" above the table surface.  Raise the bit until it just touches the board above. 

Setting bit height

Counterbore vs countersink? We countersink flat head screws so the head is flush with the wood when it is tight.  To counterbore is to end up with the screw head tight but beneath the surface of the wood.  Below the surface of the wood you can use pan head screws rather than flat head screws.  Flat head screws act to wedge the wood open as they tighten down.   Near the end of a board the last thing you want is to have the wood split.  A flat bottomed counterbore bit such as this one: Amazon Link  is perfect for using pan head screws in the joint.  

The screws made for pocket hole joints are self-tapping pan head screws.  Care must be taken to avoid splitting the side board that the screws thread into.  For a good reliable screwed joint three different drill bits, or a single combination bit should be used. The three bits would include a 3/8" bit for the counterbore, a second bit for the through hole that the screw will slip through but not thread into, and a third bit into the end of the joining side board. This should be the size of the inner shaft of the screws so the threads will cut but not try and split the wood.  

One tip is to drill the holes for the screw before you cut the rabbet. Occasionally when drilling through a board the grain will blow out. In this case the rabbet cut done after drilling the holes will remove any damaged area. 

Lay out where you want the screws.  A line drawn 5/16" from the edge will be on the center of the joining board end. Locate where on that line you want the screws to be, and use a center punch to put a dent. The dent will help the drill bit start exactly where you want it. 

Layout and Centerpunch.

While you can use a hand drill to drill the holes, it is wiser to use a drill press if one is available.   A fence on the drill press bed can be set 5/16" from the drill bit center to ensure all the holes are the same distance from the board end. 

Fence set.  Laser Aligned

Some woods are brittle enough that they may tear out when hit with a spinning drill bit near the end. Proceed slowly. Hold the board firmly.  Start with a 3/8" spur bit, and set the depth stop on the drill press to drill no more than 3/16"deep. If you can clamp down your board after aligning the bit to drill, you can place a thin scrap of 1/8"plywood over your board to drill through. That should prevent tear out on your board.

3 Holes Counterbored

Once the holes are drilled go ahead and cut the rabbet.  Be sure and back up the cut with a scrap block.  Hearing protection strongly advised. 

The only thing left to do is drill pilot holes for the screws in the ends of the mating side piece.  That, and look for a plug cutter that will make 3/8" diameter plugs.  While looking you can contemplate what wood you want to make the plugs from. A small scrap is all that is needed. 

On the adjoining board end it is best to layout, mark, and centerpunch where the screws will enter.

Marked and Centerpunched

At the drill press, lower the table so the board can fit under the bit. Using a bed vise, or simply left assembled so it stands vertical, align the marks with the drill bit.  When drilling it is best to jog in and out as the bit progresses to let the chips clear. Otherwise the bit will become clogged and heat up. Set the depth stop to drill approximately one inch deep. Choose the bit size to match the shaft of the screws but NOT the thread diameter of the screws. Threads will cut but the screw shaft won't try to split the board. 

Pilot holes drilled

Screwing into end grain a screw with coarse threads is best.  The screws available for pocket screws are ideal. 

Pocket Hole Screws

These screws should drop through and not thread through the holes drilled in the rabbeted board.  That hole diameter should match or be just a bit larger than the thread diameter of the screws used.  When assembling the screws will pull the rabbeted board tight against the end of the mating board. 

Screws in place.

A plug cutter works best when used in a drill press.  Face grain plugs can be cut that match the wood they insert into, and might nearly disappear when in place. Here I've used a scrap of walnut to make plugs with.

Plug Cutter in Drill Press

There are better plug cutters available than the one I show being used.  Mine is a bit dull and struggles to cut into the walnut scrap, yet it still makes useable 3/8" diameter plugs.

Small Screwdriver

A small screwdriver can be used to break the plugs free from the scrap board. Be patient before gluing these plugs in over the screws. The box sides still need to have a slot cut for the bottom of the box. The screws can still be removed so all the parts can be laid flat on the router table.  

Once the box is finally glued/screwed together the plugs can be glued into the screw counter bored holes. Let them stick out. Wait  for the glue to dry/set.  The excess plug that sticks out can be rough trimmed off at the band saw, or with a chisel.  A flush trim bit in a router table can trim what is left flush to the board surface. 

Comments and Questions are encouraged!

4D

Splined Miter. A Simple Corner Joint that also Simplifies Glue-up of a Box or Cabinet.

Spined Miter Corner

This is the second of four different corner joints it is useful to know about.  Best cut using a  table saw with a miter gauge to add some safety to the cuts.  It helps if the table saw blade is close to the same thickness of 3mm thick plywood as plywood's alternating layers make for a strong spline.  You can use wood as the spline if you make sure the grain runs across the joint rather than parallel to it.  On long mitered sides it is best to use the same material for the spline as the sides. Wood expansion/contraction  as humidity changes can create a shearing force that will lead to failure of the joint if the spline won't expand with the sides.

Material used was 5/8" thick.  The spline slot cut was 1/8" wide.

You can use a router table to cut the spline slot on small parts, but it requires a jig to hold the sides up at 45 degrees.  That or a router table that can tilt up 45 degrees. 

The initial miter cut can be cut on the table saw, or on a compound angle miter saw standing up or laying flat.  The height of the sides limits the use of the miter saw.   I have close to 6 inches of cut height on my Bosch glide saw, and 14 inches or so of cut length.  I get better result standing the sides up to cut so my limit is 6" of side height to cut them on the miter saw.  

Table saw blades might leave a V bottom in their cuts.  If you can find a blade that leaves a flat bottom it will make a flat bottom spline slot. The blades in a dado set are usually flat tipped so it might be worth changing to a single 1/8" or 3mm wide dado set blade.  A simple test slot cut using the table saw will reveal how flat of a bottom it leaves the slot.

To leave the joint strong it is best to have the spline slot closer to the inside of the corner, and not cut any deeper than half way through the sides.  When setting up the table saw to cut this slot I marked where I wanted it and transferred the position to the top/outside of the board. This helped me align it with the slot already cut in the miter gauge face.

Slots Cut

Using a miter gauge on the table saw to push the board through the saw blade is best done and easiest to align if you add a fresh face board to the miter gauge for the cuts.  Some friction tape on the front of the face board will help keep boards from sliding left or right when being pushed through the saw blade.  

Slot in Gauge Face Board

A stop block clamped to the miter gauge board can ease alignment for the cuts.  It is important that the slot position matches on mating sides.  Many miter gauges have a built-in sliding stop block, and you may be tempted to use it.  On a simpler set up a board clamped to the face board will do.    Best place for a stop block is at the tip of the mitered corner. 

Stop Block Position

Is a splined miter any stronger than a simple miter with no spline?  For simple boxes, it doesn't matter as there normally won't be any forces trying to pull apart or warp the corner.  The advantage is in gluing up the box.  The spline keeps the sides aligned when clamping together.   The addition of glue surface area should offset any weakness introduced from the  slot cut.

Questions or Comments welcomed!

4D   

Making a Rabbet/Dado Corner Joint Using the Router Table.

Rabbet/Dado Corner Joint

Here lies some logic and tricks for making a simple corner joint for wood boxes. This joint is as useful for small boxes as it is for case good furniture corners.  When I taught I would demonstrate this process to my students. The work area was a small router table. Students gathered around had a limited view and inevitably needed personal help once they started cutting their own corner joint.  This post is a reference so anyone making this joint can understand the steps. 

Wood for this sample was planed down to exactly 5/8" thick.  The goal is to end up with a snug fit and the two boards flush on the outside.  The router bit used is a 3/16" diameter spiral upcut bit.  Box sides could be any thickness from 1/8" to 3/4", so scale up or down the bit used as needed for the box side thickness.  Some logic for why a 3/16" bit was used here:

The Dado Cut

The dado side is done first.  The router bit used will define the width of the dado.  The dado slot needs to be positioned where the inner edge is 5/8" from the board end.  This will allow the rabbet side to end up flush with the dado side.  Using one board standing up against the fence allows using a second board to create a reference edge to set the bit flush against.  Make sure you have the fence securely locked in place once set.

Setting the distance of the bit from the fence:

  

Then set the height of the bit. A small ruler can be used to help set the height of the bit for the dado cut.  There are also several height setting gadgets out there that could be used.  

You should feed the board, snug against the fence and firmly against the table, from right to left.  The direction that the bit spins will help keep the board against the fence as you feed it through. Feed from left to right (the wrong way!) and the bit spin will try to pull your board away from the fence. You should use a backup board to follow the cut when cutting across the grain. This will prevent the grain from tearing out as the bit exits the side.

To set up for the rabbet cut you'll need to reset the bit height and distance from the fence.

The rabbet side setup can be derived from the dado side it needs to mate to.  

It helps to mark what will be removed when cutting the rabbet end. A common mistake is to cut the rabbet on the wrong side.  You can see in the photo below the area that will need to be removed as it relates to the dado.

Dado End Marked

A scrap of 3/16" thick wood slipped into the dado will make setting the height of the rabbet cut easy.  Raise the bit up until it just touches the projecting scrap. Be sure to tighten the bit adjustment once you have the bit set at the right height. 

Setting bit height for rabbet cut

You can use the dado slot you just cut to set the bit distance from the fence for the rabbet cut. Loosen one end of the fence.  Stand the dado side up and slip the dado over the bit, then push the fence against the board.  

Be sure to lock the fence in place after you've set it!  If the fence moves during a cut it will ruin the project board.

This Rabbet/Dado corner joint is just one of four different joints created for this project box. As it slides together in only one direction it is a good choice for drawer boxes. The rabbeted sides are the front and back of a drawer.  The tongues that hook into the dado on the side pieces will pull or push the side,  which will pull or push the back piece.  An additional front board might be added to cover the rabbeted front box board, possibly extend out to cover added drawer slides, and dress up the drawer.  

The entire joinery box project.  

Any suggestions for making this information more clear/understandable would be appreciated.  I'll edit the post as I refine the videos and process flow.  Just leave a comment.

4D

Coasters. Everywhere I Look I see Coasters!

I'm having quite a bit of fun playing with the drafting features of my CNC software.  It has a polygon drawing tool. This lets you make polygons of any whole number of sides above 2.  Triangles to 45 sided polygons and above.  Of course once above 20 sides or so the shapes start to look more like circles.  

In a previous post I mention how nice this tool and a few others could be used to make complex geometric patterns.   One great use for such pattern work is to add detail to simple project surfaces.  To quash a bored few minutes I've been using the software to make patterns for coasters.  A simple shape.  Unlimited pattern possibilities.  Here are a few.

From simple to very complex. The tiny details on some could be left out with no loss of visual impact on a 4" diameter coaster. The patterns I've confined to the inner 3.5" diameter area.

While I like the shadow effects of the V-cut patterns, for practical use the cuts should be filled with epoxy or inlayed using the VCarve inlay technique and contrasting wood.  Clear epoxy might let you still appreciate the carved look.  The slight chamfered edge makes them easy to handle. I'd cut a circle of adhesive backed cork to stick to the bottom surface if I made them. 

A good project to do with a CNC using small scraps of wood and a short amount of time.   Even complex toolpaths don't take much time to cut on such small areas.  Four or maybe six could be cut from a strip of wood to make a set. Use the same pattern for all, or use unique patterns on each individual coaster.

4D