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How tight should tenons be?

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Forum topic by 1tacoshort posted 05-25-2016 03:50 AM 851 views 2 times favorited 18 replies Add to Favorites Watch
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1tacoshort

7 posts in 340 days


05-25-2016 03:50 AM

Hi,

I just finished my first furniture project (an arts and crafts bench for the end of my bed) and am working my second (a mudroom bench—also arts and crafts). My tenons were pretty tight—I had to wiggle the tenons back and forth and pound with a hammer to get them to seat properly. I resolved to make the tenons looser on the current project but I don’t know what will make for a good, strong joint. Does the recommended tightness change when it’s a through tenon vs. a blind tenon?

Thoughts (and, thanks!)?

-- Wade


18 replies so far

View Loren's profile

Loren

8301 posts in 3110 days


#1 posted 05-25-2016 03:55 AM

Ideally they should go together with firm hand pressure,
come apart with pulling and wiggling. If you need a
mallet, it’s too tight, if you can push it in with your
hat, too loose.

They never all come out the way you want of course.
Modern glues do a decent job filling gaps.

Try to make the shoulders accurately because gaps
at the shoulder can allow wiggling.

View BurlyBob's profile

BurlyBob

3663 posts in 1727 days


#2 posted 05-25-2016 03:57 AM

Exactly what Loren said !!!!

View 1tacoshort's profile

1tacoshort

7 posts in 340 days


#3 posted 05-25-2016 04:11 AM

Thanks!

-- Wade

View conifur's profile

conifur

955 posts in 613 days


#4 posted 05-25-2016 04:11 AM

”’Modern glues do a decent job filling gaps. ”
Yes it does fill it, yes it fills it but no strength there, do to the molecular structure of wood glue, it would need to be Epoxy to do that.
Wiggling a dry M and T joint to pull it apart weakens the tenon, and miss shape the Mortise, a straight pull is what is recommended.
Blind or through makes no difference, just a cosmetic thing, of course the longer it is, more gluing surface.

-- Knowledge and experience equals Wisdom, Michael Frankowski

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conifur

955 posts in 613 days


#5 posted 05-25-2016 04:30 AM

Think of it this way, regular wood Glue, Tite Bond ect, I think it is called a PVC type glue, if it could fill gaps and have strength, you would not have to clamp it and put pressure on the faces when doing a flat panel glue up. You cant do that with a M&T joint on the flat grain that is what holds it, not the shoulder end grain of the Tenon against the flat grain of the mating mortising piece.
Nothing worse then junk info, it is like buying a $300 phone and use it as a camera. Buy a $75 phone and a $225 camera that can do a better job of taking pics.

-- Knowledge and experience equals Wisdom, Michael Frankowski

View jmartel's profile

jmartel

6565 posts in 1612 days


#6 posted 05-25-2016 01:18 PM

Generally a good test is that they should go together fairly easily with hand pressure, and if you pick up one end and hold it upsidown, it shouldn’t come apart.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

View bonesbr549's profile

bonesbr549

1176 posts in 2529 days


#7 posted 05-25-2016 03:21 PM

My rule is it should not have to be pounded in and not sloppy and fall out. Never had one fail.

-- Sooner or later Liberals run out of other people's money.

View JCantin's profile

JCantin

165 posts in 2874 days


#8 posted 05-25-2016 05:51 PM

One of the best tips – and not mentioned here so far – is to undercut shoulders to help with fit.

The Secret to Perfect Mortise and Tenons
Trim hidden areas to quickly produce a flawless fit
fww.com

View 1tacoshort's profile

1tacoshort

7 posts in 340 days


#9 posted 05-27-2016 03:57 AM

Thanks, everybody! I’ve got 40 M&T joints in my current project and this advise will come in handy.

-- Wade

View splatman's profile

splatman

558 posts in 861 days


#10 posted 05-27-2016 04:15 AM

Cut the tenons slightly oversize, then shave to fit.

View AandCstyle's profile

AandCstyle

2566 posts in 1719 days


#11 posted 05-27-2016 11:53 PM

Wade, I make my floating tenons to fit too tightly, then I fine tune them by holding them against a belt sander for a couple seconds and test the fit, repeat as necessary until they slide in with some effort, but do not require the use of a mallet. If they fall out after being turned upside down, glue a piece of veneer to the side of the tenon and try again. HTH

-- Art

View Matt Rogers's profile

Matt Rogers

69 posts in 1432 days


#12 posted 05-28-2016 01:53 AM

Actually, I recently read a scientific paper on the effects of tight and loose mortises using resorcinol, plastic resin, and pva glues.

The tight glue fit was in the range of 0.002” gap. Medium fit was about 0.006” and loose fit was 0.010”.

The resorcinol glue did require very tight mortise fit for the greatest strength due to the fact that the glue is proved to require very high clamping pressures and thin glue lines. It lost strength of about 25% with the loose fit.

Plastic Resin glues did well with both tight and loose fits.

PVA glue actually had higher strength with the loose fitting mortise than with the tight fit. This could be the result of a glue starved joint in the tight fit (a known problem with PVA) and the fact that the PVA is a flexible glue as opposed to the rigid glue lines of plastic resin. With the joint able to move a slight amount in the mortise, it is better able to distribute forces along the entire joint as well and to other adjoining pieces, making for a slightly less stiff, but more resilient structure.

The overall strength of a properly prepared resorcinol joint was great and there is no dissing the different glues, they all work, but PVA was shown to prefer a slightly loose joint (still only 0.009”). Enough for a good amount of glue but tight enough for good alignment.

Wish I could remember the link for the study. I found it when I was looking up specs and advise for sizing domino tenon stock. The Domino XL does not even come with a single sample wood domino so I did not have a sample to measure, so I needed to know how tight the fit normally is and how tight or loose to make my stock.

Hope that helps, if you believe that I recounted the study correctly.

-- Matt Rogers, http://www.cleanairwoodworks.com and http://www.cleanairyurts.com

View jdh122's profile

jdh122

879 posts in 2279 days


#13 posted 05-28-2016 10:57 AM

C ould it be this article (http://www.woodresearch.sk/articles/9-36-103939_WR_4_2007_07-Tankut-WR_4_2007.pdf)? It says that with PVA glues there is no significant (“negligible”) difference between tight and loose-fitting mortises. Mathias Wandel’s testing suggests the same thing: “The mortise and tenon joints failed at 170, 175, 140, 180, 185, and 190 pounds. There wasn’t any detectable difference between the loose, snug, and overly tight joints that I made. My conclusion is that the fit, to the last few thousandth of an inch, is not that important to the actual strength of the joint, so long as there is glue in the joint everywhere.” https://woodgears.ca/joint_strength/

-- Jeremy, in the Acadian forests

View Redoak49's profile

Redoak49

1944 posts in 1450 days


#14 posted 05-28-2016 11:32 AM

A great thread with some real data on how tight to make a joint. I re-read the Mathias article and he did a great job with his shop testing. Unfortunately, the link to the other article did not work for me.

View jdh122's profile

jdh122

879 posts in 2279 days


#15 posted 05-28-2016 02:54 PM

Somehow the parenthesis got included in the link. This one should work:
http://www.woodresearch.sk/articles/9-36-103939_WR_4_2007_07-Tankut-WR_4_2007.pdf

-- Jeremy, in the Acadian forests

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