Trestle Table #12: Next Steps: Cut Tenons Using Twin Blade Joinery Method

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Blog entry by HappyHowie posted 09-17-2016 09:37 PM 771 reads 0 times favorited 3 comments Add to Favorites Watch
« Part 11: Next Steps: Cut Tenons Using Twin Blade Joinery Method Part 12 of Trestle Table series Part 13: Cutting Tenons with Spacer Blocks »

I began work in my shop this morning by squaring up the corners or ends of these router bit made mortises. As you can see I have an Irwin Marples 3/8” or 10 mm hand chisel as well as a Robert Sorby mortising chisel in the same size 3/8” or 10 mm.

For this blog entry I am writing what I plan to do next instead of writing briefly what I did in my shop. Maybe this way I will be more thorough and clear of what my steps will be.

I subscribe to Woodcraft Magazine. I love their articles.

Paul Anthony wrote a Woodcraft article on Twin Blade Joinery in the Feb/Mar 2016 magazine. In the article he instructs how he cuts his tenons on his table saw. He uses two rip table saw blades that leaves him flat tenon shoulders. He knows exactly what shims to place on his arbor to get the tenons widths that he needs for his mortise and tenon joinery. I decided to use this same method after analyzing many methods used by experienced woodworkers.

Since I recently purchased a Saw Stop table saw, I also needed to make a new tenon cutting jig. I wanted a tenon JIG that would ride and track on top of my Saw Stop’s T-Glide fence. I liked Bob Van Dyke's multi-use tablesaw Rip Fence JIG. His instructions were the magazine cover article for Fine Woodworking Magazine , issue #231. I am already enjoy using this rip fence JIG system.

To cut tenons using twin rip blades on my table saw I went as far as following Paul’s instructions to order a 1/4 inch aluminum plate. I even cut the shims out at my bandsaw like his article instructed. That was two much work. Messy. I was sweeping pieces up for days, or maybe even weeks afterwards. Plus my disks did not turn out as pretty as Paul’s, but they are functional. They will do the job.

It was after these disks were formed that I discovered in my local Woodcraft store that I could have bought a set of extra DADO shims that would have worked for cutting tenons. If I had done that I could have given my Diablo DADO set to my son when I gave him my Porter Cable contractor table saw. Maybe he will still end up with a free DADO set, but he better get using that saw. Right?

As can be seen in the photo below I use two Freud RIP table saw blades with shims I cut from the aluminum plate plus the shims I own in a Freud Diablo DADO set. I have labeled every shim with a unique letter from our alphabet. I also wrote the thickness I measured with my digital calipers. Those measure are close, or in the ballpark since every attempt measuring their thickness can vary a bit when getting precise to the 1,000ths or 10,000ths.

With a scrap block of construction Douglas Fir, I punched test mortises for the different size of hollow mortising chisels I own for use with my Powermatic bench top mortiser. I recorded in my shop journal on page 40 the combination of shims I used inside the twin Freud rip blades that gave me a good tenon fit in this test mortise block.

Today, with the 3/8” mortises I cut in this test board, I will check if my combination of shims will be a good fit to these plunge router bit made mortises. Either way I will record the combination of shims that give me the best friction fit. I suspect each mortise I cut will be unique, but the best shim combinations I record will give me a good starting point. I would rather have the tenon a bit thick instead of too shinny. I can always use a shoulder planer or a rasp file to thin a tenon in order to get my preferred fitting.

That is my plan for today or tomorrow. I will not only perform a test in this scrap Douglas Fir, but I will also test mortises in a cherry scrap piece of hardwood.

After all of these test I will begin cutting the mortises and tenons for my trestle table leg assemblies. I am hoping after all these test that I will have a reliable procedure to follow on my trestle table build.

-- --- Happy Howie

3 comments so far

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272 posts in 1367 days

#1 posted 09-18-2016 12:54 AM

Well, I performed these twin blade configurations with my Porter Cable table saw; not with a Saw Stop. It may be impossible to do with this saw’s safety brake functions. The magazine article was using 10 inch blades with spacers. I have found that after mounting the twin blades with spacers while the arbor mechanism is raised to its highest point that the brake signal system gives the “green” light okay signal. However, as I approach the lowest point of lowering the blades so I can mount the cover plate, something shifts forward causing the outer blade to touch the brake system. When the blade touches the blade would not be able to spin freely.

I will have to call Saw Stop’s technical support to determine if this twin blade technique is even possible with this table saw system.

Maybe with using 8 inch DADO blades and spacers it may be possible. I will also check on that alternative.

-- --- Happy Howie

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272 posts in 1367 days

#2 posted 09-18-2016 08:57 PM

Since today is Sunday I decided to submit my questions about the twin blade joinery method I attempted above in this blog to SawStop in writing. I submitted these questions to SawStop on their support web page on

SawStop Support I have three questions.

(1) Please provide a link to a list of tested and approved or preferred DADO sets so I can understand what manufacturer brands and models works best with the SawStop’s safety brake system. I need further help beyond your user manual’s description of DADO set types that work best or at all with your safety brake. What I am saying is that the user manual itself is not that helpful to me when I am researching DADO sets on the Internet to determine if they fit your descriptions. I thought I had seen such a list on your website, but maybe it was someone else’s list that I cannot find again.

(2) Does an override of the brake safety system only work after first placing a blade or DADO set and getting an all clear or okay check of the blade setup?

With my prior table saw, I could implement a twin blade joinery method as described in a Woodcraft Magazine Feb/Mar 2016 article. Is implementing a similar twin blade method for cutting tenons on my SawStop impossible to do with your brake system? Even if I want to override or bypass the safety brake system?

Or, in other words, is it possible to run my SawStop without a safety brake system installed? Can I simply run my SawStop like the Porter Cable contractor saw I gave away after purchasing my SawStop. With my much less expensive PC table saw, I could run this twin blade tenon cutting process. Now it appears that method is impossible on this safety minded, five times more expensive SawStop. It appears there is no override to run the saw without a safety brake installed. Not only that this SawStop appears that a 10” blade safety brake or a 8” DADO safety brake has to be installed and the computerized safety check has to be passed, even though I intend to override the safety brake system. Is my assumption or statement here correct?

Would there be any issue I might encounter if I attempted to implement this twin blade method using my Diablo 8” DADO blade set other than loosing a 2 inch shoulder height setting for my tenon cuts?

(3) I did try to setup this twin blade joinery implementation on my SawStop table saw using two 10” Freud rip blades with spacers between the two saw blades. The saw’s safety system processed its safety checks. It gave me a “green” light while the blades were lifted to the saw’s arbor highest position. I unplugged the saw. I could spin the blades while the arbor was high for the installation of these blades.. However, as I was lowering the blades and hand spinning the blades to make sure they spun clear of the brake system, I experienced that when the blades were about to go underneath the tabletop the safety brake and the outer saw blade touched so the blades and shims would no longer spin freely. I know that trying to get that kind of setup to work would not be a good idea. I did not plug my saw to power to run the safety check.

My question is: when lowering the saw blade under the table top, is it a design feature of the brake or the arbor holding the blades to move into a position that is closer together than when the blades are raised to their highest position above the table? Is what I observed a feature of the safety brake system; the brake and the blades pinch in together when the blades are lowered below the tabletop?

-- --- Happy Howie

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272 posts in 1367 days

#3 posted 09-19-2016 05:22 PM

I suspect that Saw Stop’s answer to my (2) and (3) questions above will be that the safety brake will have to be installed and its setup will have to pass the computerized self check prior to over-riding the brake system. This means the double blade or twin blade joinery method of cutting tenons on the table saw will be an impossible methodology on a Saw Stop.

I will begin testing an alternative method using spacer blocks for cutting tenons on my Saw Stop table saw. This method is presented in a couple of Fine Woodworking Magazine articles for online members. The earlier article written by Alan Turner was published in issue 226, “How to Speed Up Your Work with Spacer Blocks”. The second article was written in issue 229 by Timothy Rousseau in his “Float the Top” article.

The tenon spacer block’s thickness is made by using a caliper to determine the thickness of the mortise and then adding the thickness of the saw kerf. The spacer block will be thicker than the mortise by the amount of the saw blade’s saw kerf.

Professional woodworker Tom McLaughlin cuts his tenon’s by using this spacer block methodology.

-- --- Happy Howie

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