# Accuracy of tenons



## CajunRon (Dec 24, 2015)

I just retired and decided to try my hand at woodworking. I recently made a kids table and chairs for my grandson for Christmas and decided to try tenon and mortise joints instead of the easy but weak socket screw joints I had used in the past. I used a router for the mortises and used the table saw technique for making the tenons but could never get a proper fitting tenon. Either the joints were too loose (would fall apart when trying to assemble) or too tight (had to use a lot of force but never had to resort to a mallet). I read an article that indicated an accuracy of +- 0.005 " was necessary. Trying to eyeball the edge of the saw blade to the edge of the pencil line with 66 year old eyes was impossible for me to achieve anything close to this accuracy. Is this degree of accuracy really necessary? I see so many YouTube video demonstrations where they get a perfectly fitting tenon and mortise...how do they do it?


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## Steve Neul (Sep 2, 2011)

You have to leave some room for glue however there should be a little friction when dry fitting the joint. How sharp are your blades. A blade that is a little dull is difficult to make accurate cuts. The blade can bend back when making the cut with even a blade that is only slightly dull.


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## GeorgeC (Jul 30, 2008)

Your tight fit joint was probably OK as you did not have to use a hammer. I like my mortise and tennon joints to be a "snug" fit. That is some resistance when pushing the joint together.

00.005 accuracy is strictly not needed. 

George


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## kramar74 (Sep 21, 2015)

I typically cut my tenons first and then use those to lay out the mortises. Much easier to get a good fit that way. If the tenons are to tight, you can use a wood rasp or sharp chisel to sneak up on the fit.


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## Getting better (Dec 3, 2009)

I have never had success making finished sized tenons straight off of the table saw using a tenon jig. What I do instead is cut them slightly oversized the fine tune with hand tools.


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## woodnthings (Jan 24, 2009)

*I do the opposite*



kramar74 said:


> I typically cut my tenons first and then use those to lay out the mortises. Much easier to get a good fit that way. If the tenons are to tight, you can use a wood rasp or sharp chisel to sneak up on the fit.













Since making a mortise is more difficult, I make them first .... using either a plunge router or a hollow chisel mortising machine, which has a given width. The tenons can be made close using a tenon jig on the table saw, or I use a bandsaw. I set the fence to make the tenon the thickness I need and set a back stop on it for the length. I make one pass, hit the stop and flip it over and make another. This makes a kerf an equal distance in from the fence setting. It's real quick way to make them. The shoulder is another fence setting with a stop. 

You will need to make several attempts at the fence setting using practice pieces, BUT once you dial it in you can make them all day long and have them a perfect fit.

*A self centering guide for a router* This will make beautiful mortises all day long. You can either round over the tenons or as I did square up the corners:


Notice the invisible repair on a mortise made on the wrong side of the leg in the bottom photo. 
Attached Thumbnails


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## Toolman50 (Mar 22, 2015)

When making mortise and tenon joints, I will cut all my mortises first using a router. I rough out all the tenons including one or two sample ends on the table saw or radial arm saw. The final cuts are made on the table saw on the sample ends first. When I get the tight fit I'm looking for on the sample end, I cut all the tenons at that setting. Depending on the project, I sometimes round over the edges of the tenon so I don't have to square up the edges on the mortises.


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## BernieL (Oct 28, 2011)

I'm with the camp that cuts the mortise 1st and then a slightly proud tenon because taking wood off a proud tenon is easier the adding wood to fat mortise. 

Sounds like you've got a pretty good mt joint CajunRon. When I get my tenons that close, I use a metal file if I think they're a bit too tight... or sand paper. I prefer the file because it's easier to keep it level (To achieve a flat surface).

As suggested - practice the mt joint (or any other joints with your scraps. Oh yes - welcome to the world of retirement. Keep busy and enjoy...


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## epicfail48 (Mar 27, 2014)

Ye olden woodworkers didn't have micrometers to make sure their joints were accurate to .001 inch clearances, and their furniture is still hanging around. I wouldn't stress about making sure your clearances are withing some arbitrary number, certainly not 5 thou. You'll get more than that in wood movement from day to day, so it doesn't make a lot of sense to try to make cuts that accurate. The rule of thumb seems to be that a joint should be loose enough to go together by hand, but tight enough to not fall out on its own.

For what its worth though, sometimes I'll cheat and settle for a slightly looser fit. Not enough to have a significant amount of play, but slightly loose. Wood glue will fill small gaps with no trouble


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## Jig_saw (May 17, 2015)

BernieL said:


> I'm with the camp that cuts the mortise 1st and then a slightly proud tenon because taking wood off a proud tenon is easier the adding wood to fat mortise.
> 
> Sounds like you've got a pretty good mt joint CajunRon. When I get my tenons that close, I use a metal file if I think they're a bit too tight... or sand paper. I prefer the file because it's easier to keep it level (To achieve a flat surface).


I cut the tenon first with a back saw and then outline the corresponding mortise with a pencil. I drill on the inside of the marking and clear the mortise with a chisel progressively outward, making sure the tenon fits tightly. This way, there can be no mistake of a loose-fitting joint.:no:


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## Jig_saw (May 17, 2015)

epicfail48 said:


> For what its worth though, sometimes I'll cheat and settle for a slightly looser fit. Not enough to have a significant amount of play, but slightly loose. Wood glue will fill small gaps with no trouble


 Yes, glue often saves a loose fitting joint.:yes:

Do you glue all your mortise-tenon joints, or do you use dowels?


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## epicfail48 (Mar 27, 2014)

Jig_saw said:


> Yes, glue often saves a loose fitting joint.:yes:
> 
> Do you glue all your mortise-tenon joints, or do you use dowels?


If its a joint and its not meant to be a knock-down joint, I glue it. Even something like a drawbored m&t, which pretty much secures itself, I still glue


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## d_slat (Apr 10, 2012)

woodnthings said:


> *A self centering guide for a router*  This will make beautiful mortises all day long. You can either round over the tenons or as I did square up the corners:


You have my attention. The only self centering guides I have seen use 2 pegs and you have to apply torque to the handles on the router to keep it centered. The jig in your picture appears to have some adjustable guides with lock handles. Do you have a link to plans, or can you post some more pictures of it so we can see how it works?


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## woodnthings (Jan 24, 2009)

*as usual, I winged it ....*

I came up with this jig possibly after seeing some on You Tube, I really can't remember. It's a parallelogram kinda thing which has adjustments holes for the gross dimension. You then use the parallelogram feature to fine tune it for the width and fore center. Notice the holes visible on the right side toward the front ... there are 3 on each side. It is currently in the center hole.




I also used a standard Porter Cable edge guide and it is screwed to the rear bar while the front bar can be adjusted. Once everything is centered, by using the threaded knob on the PC, you just tighten down the black T knobs and off you go.

Notice there is another block of wood at the end of the PC guide which is just a support to ride on of the correct height/thickness as the workpiece .... a scrap.

The Irwin clamp is used for a stop block. I'm sure a better arrangement can be made, but this worked OK for me.... :yes:


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## Zircon (Aug 1, 2009)

I get perfectly centered mortices with the plunge router just using the edge guide. I set the guide so the bit is as close to the center as possible and make the mortice with the guide on the left side of the stock. Then I plunge again with the guide on the right side of the stock. If I wasn't perfectly centered on the first pass I end up with a slightly wider mortice after the second pass but it will be perfectly centered. Then I cut the tenon on the bandsaw to match the mortice. I set the tenon stock over the mortice and drop a six inch steel ruler in the mortice and use that as a straight edge to mark the side of the tenon.


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## BernieL (Oct 28, 2011)

After using a doweling jig for centering my mortises - this post and what woodnthings wrote convinced me to build a simple mortise jig for my router and WOW. What a difference and so easy to use. I know I've seen previous posts over the years about these router jigs and never tried any :huh:


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## MJJ (Jul 14, 2012)

I have gone ahead and assembled quite a few M&T joints over the years that could only have been described as sloppy, and once you get them glued up you never know. If you're worried, you can use an epoxy paste that is a good gap filler. Likewise with paying too much attention to being "perfectly centered."


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## BernieL (Oct 28, 2011)

MJJ - I've cut many sloppy joints in my time and yes, the nature of the joint does tend to cover up the slop, and yes, my sloppy joints are still holding. One factor working in our favor is that the moister of the glue will swell the wood a little bit.

But cutting the joints using a router loaded with a spiral upcut bit mounted onto a simple centering jig not only assures you a perfect mortise - it makes the work a lot easier and faster. After cutting 12 mortises, I've already saved the time it took me to make the jig and yes they are now accurate.

Yes, I do agree with your statement MJJ - but I don't intend on wasting my time to make the joint sloppy. I'm happy to have learned something very useful to me from this post. :thumbsup:


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