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Rocking Chair: An Online Course #14: Twin-Blade Joinery

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Blog entry by HappyHowie posted 03-23-2016 10:36 PM 775 reads 0 times favorited 4 comments Add to Favorites Watch
« Part 13: Pattern JIG for Bandsaw Completed Part 14 of Rocking Chair: An Online Course series Part 15: Loose Tenons: Festool Domino or Mortising JiGs »

My instructor for this project suggested that I use spacer blocks to cut the second shoulder for the tenons to fit the mortises needed for this rocking chair. I was about to make those spacers when I received a new issue of Woodcraft Magazine. The article in the Feb/Mar 2016 issue was entitled Twin-Blade Joinery. With this method I would use twin RIP saw blades with disks or shims between the blades. With the right combination of shims I could cut tenons for the following mortise sizes cut with my Powermatic mortiser and hollow mortise chisels: 1/4, 5/16, 3/8, and 1/2 inches.

IMAGES:
These are the aluminum disks I made and will use with the existing 8” Diablo DADO set I own. I had already cut sample mortises in a block of wood: 1/4”, 5/16”, 3/8”, and 1/2”. I will later determine the shims needed to fit a tenon to a 5/8 inch mortise.

I labeled all the shims including those from the Diablo DADO set. From the combinations needed for these mortise sizes I have written a log to record the shim combinations for each of the mortise sizes.

I believe this will speed up my tenon cutting and mortise fitting process…

-- --- Happy Howie



4 comments so far

View CaptainSkully's profile

CaptainSkully

1437 posts in 3024 days


#1 posted 03-24-2016 01:53 PM

That’s an interesting technique and would be very useful for doing lots of non-centered tenons. If your tenons are centered, why not just adjust the fence the right distance from a dado stack and flip the piece? Am I missing something?

-- You can't control the wind, but you can trim your sails

View HappyHowie's profile

HappyHowie

330 posts in 1411 days


#2 posted 03-24-2016 04:21 PM

With this twin blades and shims technique or with a spacer block technique, I am expecting the result to be repeatable thickness of the tenons to within thousandths of an inch, every time.

When I cut centered tenons with my table saw miter slot jig I cannot hit the starting mark exactly correctll every time. I do not know how to hit the mark within thousandths of an inch every time. However, with this technique or a spacer block, i believe I can be quaranteed a exact thickness so the tenon will fit into the mortise each time. Well, maybe a small amount of filing or sanding would be needed if it is too tight.

I have a couple shoulder planes but i expect these tenons to be so close that they won’t be needed to remove that much material.

This will be a new technique for me . I am hopeful that it will give me great results besides speeding up the process.

I was going to make spacer blocks for use on my table saw, but when this Woodcraft Magazine article surfaced it made a lot of sense to me since I would be using specific metal shims for setup each time. It just seemed more difficult for me to create spacer blocks. For example, Tom McLaughlin, my rocking chair project instructor was gluing playing cards to his spacer blocks so to shim his out to the widths required. However, I still may make spacer blocks for similar work at my bandsaw. Furniture maker and instructor Timothy Rousseau uses spacer blocks for tenon cuts, etc made at his bandsaw.

Click on the link above to see Tim demonstrate his spacer block technique…

https://shar.es/1Y7fG9

-- --- Happy Howie

View CaptainSkully's profile

CaptainSkully

1437 posts in 3024 days


#3 posted 03-26-2016 02:28 PM

Right, but don’t you still have to put the fence in the exact right place to center the tenon in the stock? With the flip technique, the tenon is automatically centered and you still have to place the fence in the exact right spot. I love learning stuff even after all these years…

-- You can't control the wind, but you can trim your sails

View HappyHowie's profile

HappyHowie

330 posts in 1411 days


#4 posted 03-26-2016 08:05 PM

You are right about the tenon being centered, but what I am attempting to accomplish is to cut the tenon to an almost exact dimension so it fits into the mortise I made with my hollow chisel mortiser.

So let me turn your question back by asking how do you determine when cutting a center tenon that you have it sized to fit into the mortise? For me it usually meant that (as some describe the process) it is a multi-dimensioning process of using a file or in my case a shoulder plane to remove some of the material and then test it for fit. When it still is too thick, then I take more off, then test. Eventually, I will get it dimensioned so it fits well. This is probably the way it has been done for centuries: fitting tenons into mortises.

The twin blade method I am attempting to follow is a technique so that I can cut the tenon with one stroke over the blades and it should be very close to fitting, it is doesn’t already. Positioning the apron (if it is for a table) over the blade as you would have had to do for a centered tenon is probably a matter of marking the starting point with a pencil and lining it up with the table saw blade for a centered tenon. If the mark is right and the width is precise (as I have tested the blades and the shims) it should be center, or very close.

So how do you guarantee that you do not cut the tenon too thin by positioning the piece too far into the blade? You will have the tenon centered, true. But, it may be cut too thin so it is too loose in its mortise… Of course, you can fudge the mark so the tenon is always too thick and then shave some of each side until it fits. It is just that part that i am trying to avoid in order to speed up the process by using two blades and the proper amount of shims so the tenon width is darn close to the mortise…

-- --- Happy Howie

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