Tapered dowel end into a tapered hole.

[4DThinker]

The first job I had after college was for a furniture company in St. Louis that specialized in metal framed residential furniture.  For many of their table designs they used thin walled steel pipe, usually chrome or brass plated.  3/8" od or 1/4" od pipe, swedged ends.  As the ends were swedged they would trap threaded hex nuts in the end.  The small pipes were stretchers for table frames into larger diameter pipe/tube as the legs.  They could die-punch precision holes in the leg sides a hair smaller in diameter than the stretcher tubes.  On the opposite side of the legs a smaller hole for the shaft of a machine screw was punched, and pressed in to create a countersink for the flat head screws. As you screwed through the leg into the stretcher end the taper would draw tight and the stretcher square to the leg.  A very tight joint, no welding required.  Could be assembled by customers with an included screw driver. 

 

This is, in effect, a wood version using dowels.  3/8" dowel stretcher into a 1" dowel leg.  Test done using some dowel scraps.  

 

A very snug intersection of the dowels. I used a pocket hole screw.  Pre-drilled the end of the small dowel for the screw threads. A wood plug could cover the screw head. 

The small dowel wedges in tight, and the screw draws it even tighter, roughly another 1/32" or so.  Being nearly 1/2" into the 1" dowel the small dowel could never split from the screw.

3 degree tapered end mill.   Strategy could easily scale up for larger dowels.  Not a challenge to figure out the toolpath vectors needed. The 3 degree end mill has a 1/8"d tip and a little more than 1" of cut length. 

 

The company I worked for was James David Inc.   Google may have images of the furniture they made.  I don't think they are still in business. 

4D

Edited March 26 by 4DThinker

[4DThinker]

I like the fit of this connection well enough that I just ordered $120 worth of assorted cherry dowels. 1.5"d, 5/8"d, and 3/8"d.   I have some cherry boards on hand for a table top and lower shelf, so I plan to make a small end table with a dowel frame using the joint above.  Mission-ish in style. Possibly a drawer that resides under and slides out from the top. It'll be  a week+ before the dowels arrive.

4D

Edited March 26 by 4DThinker

[4DThinker]

Toying with Aspire to come up with a tapered dowel end joint that could only be inserted in one orientation. 

Ended up with an egg shaped tip, sloping back to the 5/8" diameter dowel. 

Had to make this with the fluting toolpath and a circular array of short vectors.  I can make the tapered hole for it the same way.  Tried doing it as a 3D model, but the 3D toolpaths either try to cut down the whole length of the dowel, or not complete the cut of the tapered egg end.  Render from Aspire.  I still haven't received the cherry dowels I ordered. 

4D 

[4DThinker]

A simpler keyed tenon that can be cut much quicker with the 3 degree tapered end mill I have.

This is for the ends of 5/8"dowels I'll insert into 1.5"d dowel sides. 

4D

[4DThinker]

Sample cut and tested for fit:

The small dowel will only fit in one orientation.  That will help my make sure cuts on the side for 3/8"d dowels will be perfectly aligned and parallel to the vertical leg dowel. 

When tapped together the small dowel can't rotate, fits tight.  Positive cut on the tenon matches the negative cut in the mortise. 

4D

[honesttjohn]

Like that!!!

[4DThinker]

Strategy.  1.5" square blocks.  Two for the keyed tenon ends.  Two with a 5/8" hole in them to slip over the dowels. 

This way I can verify that the ends have the tenon cuts aligned, and the intersecting dowel holes perfectly 90 degrees and parallel with the leg dowels the stretcher dowels join to.  Half the battle with such projects is figuring out how to be sure all parts go together true and square to each other, and how I'll cut the details that make that happen. 

4D

 

[4DThinker]

Did a test for the 3/8"d into 5/8"d dowel connection this morning.

A good, snug, clean fit.  The 3/8" dowels will run vertically between 5/8" dowel stretchers.  As they will be trapped in place with no chance of coming out I won't use screws or glue.  The 3 degree taper is so shallow that the wedge leverage once tapped in place makes it very difficult to pull the connection apart.  Saves dealing with glue squeeze out, although I could put a couple drops of super glue on each before assembling.  

4D

[4DThinker]

My mind has been dwelling on why these CNC cut tapered joints work so well.

Working theory:   Wood dowels are imperfect. Drill bits are imperfect depending on who made them but generally make round holes. A straight dowel end (likely a bit wider than thicker) into a straight hole drilled into wood is either loose in one axis, loose all around, or so tight you have to pound/press it into the hole. Pressing it into the hole will wear off the fat side or deform the hole to accommodate the oblong dowel end.  Use a CNC with precision down to 0.001" to recut the end to a "perfect" tapered shape and a "perfect" matching tapered hole. There is no abrasion between dowel and hole when inserting until near the bottom when the tapered sides start to press against each other. Tapping the dowel in the last 1/32" or so presses the sides together.  Uniform contact around the perimeter. Friction all around. 

4D

[4DThinker]

Render of one side assembly.  Both sides and the back will look the same.  Front won't have the center small dowels or the stretcher 2" down from the top. 

Top stretcher won't be a dowel. It will project in to be the rail a drawer will slide on. 

Cherry dowels will be here tomorrow or Thursday. 

4D

[Grandpadave52]

Looking forward seeing this one come to fruition. 

[4DThinker]

The cherry dowels arrived yesterday.  All look good and straight.  I only made one math mistake when ordering but I think I can deal with it.  Legs are 20" tall.  Bought two 36" long 1.5:" diameter dowels for the legs.  I suspect an earlier vision of what I wanted to make was going to be 18" tall with the last bit of height being the 3/4" thick top. 

This looks like a good use for my radial finger joint to extend the legs to 20".

 

I have some cherry I can glue up to cut 1.5" thick strips from and turn them into 1.5" dowel sections.  Alternatively I also have some zebra wood and some walnut I could make the extensions from. Cherry extensions may go unnoticed. Any other option would stand out in contrast as socks on the bottom of the legs. Or all 4 legs could have a different sock if I throw in some other wood for the extensions.  Any opinions?

4D

[Grandpadave52]

2 hours ago, 4DThinker said:

Any opinions?

It would be a lot more work and calculating but rather than make "sock" extensions, what about incorporating the contrasting wood leg extension in the middle (or somewhere along the length) of the leg as an accent? They could be longer sections eg 4", 6", or ??. The walnut with the cherry would make a nice contrast IMO. You could even add some detail grooves or whatever to the piece. I couldn't do it, but with your talents, equipment and especially the joint design would have little problem accomplishing.

Edited April 3 by Grandpadave52

[4DThinker]

7 minutes ago, Grandpadave52 said:

... rather than make "sock" extensions, what about incorporating the contrasting wood leg extension in the middle (or somewhere along the length) of the leg as an accent?

Dang you.  Now you've got my mind all stirred up trying to come up some logic/reasoning to put a contrasting section somewhere other than at the bottom or top. The table next to my desk that holds up a color laser printer is a mix of Zebra and hickory.  It's dimensions are what inspire the dimensions of this dowel based table. Zebra upper and lower stretchers and a drawer front.  Hickory legs and slats on the sides and back.  Bottom shelf mostly Zebra with 2 stripes of hickory down the middle and the top mostly hickory with 3 stripes of zebra down the middle.   If I middle the contrasting section in the legs I'm gonna want to add more contrasting wood in other middles of the parts. 

4D

[Grandpadave52]

3 minutes ago, 4DThinker said:

Dang you.  Now you've got my mind all stirred up trying to come up some logic/reasoning to put a contrasting section somewhere other than at the bottom or top. The table next to my desk that holds up a color laser printer is a mix of Zebra and hickory.  It's dimensions are what inspire the dimensions of this dowel based table. Zebra upper and lower stretchers and a drawer front.  Hickory legs and slats on the sides and back.  Bottom shelf mostly Zebra with 2 stripes of hickory down the middle and the top mostly hickory with 3 stripes of zebra down the middle.   If I middle the contrasting section in the legs I'm gonna want to add more contrasting wood in other middles of the parts. 

4D

I do what I can. Sleep well tonight.

[4DThinker]

Didn't sleep well at all.   Pretty normal with a distracted mind.  Of the different choices I have for extending the cherry legs, the Zebra scraps I have are closest to being ready to turn into 1.5"d dowel sections. I've also had the zebra scraps for likely 40 years waiting to find a use for them. Now is their time. 

3 of the 4 show here.  4th is waiting on glue to dry.  I have enough length to cap the top and bottom ends of the cherry dowel legs.  This way the zebra corners will compose with the cherry top around them.   Plywood 1.5" disks were cut on my CNC.  Glued to the end of zebra blocks with paper in between.  The paper will split easily to remove the plywood.  Inserted the plywood end into the 3-jaw chuck about 1/2 way, then set Z=0 on the plywood.  Raised the bit 0.1" and reset zero there.  Then made zig-zag profile on-the-line toolpath to cut 0.1" deep across the surface. .047 stepover each trip up and down the length. Took about 7 minutes each to cut square blocks that were just a little larger than 1.5" x 1.5" down to cylinders very close to a perfect 1.5" in diameter. 

4D

[4DThinker]

Side frame render with contrasting caps on top and bottom.

Haven't figured out yet how to model my radial finger joint connection.  Just assume I'll use it to connect the end caps of zebra wood. 

4D

[Grandpadave52]

4 hours ago, 4DThinker said:

Didn't sleep well at all.   Pretty normal with a distracted mind.

Sorry. I actually slept pretty good for a change.

[Grandpadave52]

1 hour ago, 4DThinker said:

Side frame render with contrasting caps on top and bottom.

Haven't figured out yet how to model my radial finger joint connection.  Just assume I'll use it to connect the end caps of zebra wood. 

4D

I like it a lot. 

[4DThinker]

First test of the radial joint.  Checking fit when one side is still on the CNC.

Recut with a larger -.004 allowance:

Still some work to do.  I think my 3/16" end mill may be undersized a bit.  Parts should have fit better on first try.  

4D