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Oak hairline cracks - mortise and tenon

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Forum topic by Mugsy posted 08-01-2015 03:30 PM 907 views 0 times favorited 15 replies Add to Favorites Watch
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Mugsy

46 posts in 793 days


08-01-2015 03:30 PM

I’m building some stools (Gary Rogowski design) from red oak. The legs are 1-3/4” square. I’m not doing through tenons because this is only the second project I’ve done with mortise and tenon and I don’t want tear out.

I’ve noticed that when I finish the mortise, I have hairline cracks on the pieces, typically on the opposite face of the mortise. I don’t have a vise so I am clamping them to my workbench while I do the mortise. I have sharp chisels (wet/dry sandpaper from 80 through 1200 grit) and I’m not using a lot of force with the mallet, letting the chisel do the work slowly but surely. This isn’t happening on every piece so I don’t know if it’s something I’m doing wrong or if it’s just something that oak does.

I’m in San Antonio, so it’s been around 100 degrees and as humid as you can imagine south Texas being. Not sure if that will help determine what the problem is. Has anyone else had similar issues?

-- Matt, San Antonio


15 replies so far

View jdh122's profile

jdh122

879 posts in 2279 days


#1 posted 08-01-2015 04:05 PM

Can you clarify: are you chopping the mortises entirely by hand? or drilling them first before chiseling? or routing them and then just squaring them with a chisel?

-- Jeremy, in the Acadian forests

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Mugsy

46 posts in 793 days


#2 posted 08-01-2015 04:06 PM

I am doing them entirely by hand. They are 1/2” mortises. I scribed the lines to get a nice knife line to start in.

-- Matt, San Antonio

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WoodNSawdust

1417 posts in 638 days


#3 posted 08-01-2015 04:47 PM

Would you post a couple of pictures showing the problem?

-- "I love it when a plan comes together" John "Hannibal" Smith

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Mugsy

46 posts in 793 days


#4 posted 08-01-2015 04:53 PM

Absolutely! The cracks are on the opposite face of the mortise. I marked the ends of the cracks with pencil lines so you can (hopefully) see better.

I just finished another leg (2 mortises) and no cracks. I’m just baffled. I assume it has something to do with me rather than the wood, but that’s always my assumption. :)


-- Matt, San Antonio

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bondogaposis

4024 posts in 1813 days


#5 posted 08-01-2015 06:45 PM

Are you sure those cracks weren’t there before you started? How deep are the mortises? You may be putting lateral pressure on the side walls of the mortise, try paring those a bit and maybe not making them as deep.

-- Bondo Gaposis

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Mugsy

46 posts in 793 days


#6 posted 08-01-2015 06:49 PM

Bondo – that is possible. I checked them after I did the rough-size cuts on the TS but may not have looked closely enough. I assumed that it was due to the work I was doing because they are on the opposite side. I probably really noticed them because I was getting ready to sand and was looking more closely.

Edit – mortises are 1/2” deep and 1” long.

-- Matt, San Antonio

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jdh122

879 posts in 2279 days


#7 posted 08-01-2015 07:12 PM

I’d be inclined to think that they were there before. Oak splits easily, but chopping mortises doesn’t put much pressure in the splitting direction. I hand chop all the tenons I do and have never had that happen. But maybe it was just waiting to split and only needed the force of your mallet hits to do it.

-- Jeremy, in the Acadian forests

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Bill White

4450 posts in 3422 days


#8 posted 08-01-2015 07:12 PM

Is the oak dry?
Bill

-- bill@magraphics.us

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Mugsy

46 posts in 793 days


#9 posted 08-01-2015 07:21 PM

Jeremy – yay! I helped. :) they don’t go very deep so that could very well be, as you and Bondo both suggested. I know oak is strong, but is this something that you run into often? Having cracks that you find after cutting into a piece?

Bill – I don’t have any way of checking the moisture content of them. It started as a 12’ piece of 8/4 from a lumber yard here in SA. Not sure if that helps, but if most lumber yards have a “typical” time to dry wood…

-- Matt, San Antonio

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jinkyjock

487 posts in 1036 days


#10 posted 08-01-2015 08:19 PM

Matt,
what you seem to have is called “Surface-Checking”.
It is quite a common problem in Oak that has not been dried correctly.
It is caused by tensions within the wood as it dries which reveals itself as hairline cracks.
I don’t think you are doing anything wrong.
Cheers, Jinky (James).

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Mugsy

46 posts in 793 days


#11 posted 08-01-2015 08:24 PM

Jinky – that’s great to know that it may not be me. I’ve been filling them with glue and then sanding over them when they were still not quite dry so that it mixes the sawdust. Since these are for someone else, I want to make sure that the cracks don’t get worse.

-- Matt, San Antonio

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jinkyjock

487 posts in 1036 days


#12 posted 08-02-2015 07:34 AM

Matt,
unfortunately you may find the some of the cracks re-appearing when
you apply a finish (moisture).
With the finer lines we applied a little Super-Glue to them before sanding.
This usually dried as a fine dark line which is hard to spot in the Oak’s natural grain lines.
Good luck. Cheers, Jinky (James)

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Mugsy

46 posts in 793 days


#13 posted 08-02-2015 02:29 PM

Great advice! Thank you. With the moisture causing the cracks (or at least contributing) would an oil based stain be better to use than water based? Same question for poly.

-- Matt, San Antonio

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jinkyjock

487 posts in 1036 days


#14 posted 08-02-2015 05:40 PM

Matt, I notice some saw marks on your Oak.
If you have a hand plane give them the lightest skim just to clean up.
If no plane make up a flat sanding block.
This may sound contrary to all I’ve said but then you need to get a clean rag,
soak it in clean water then thoroughly wring it out till just damp, not wet.
Give the Oak a wipe with the rag and leave for a couple of hours.
When you go back you will find you have “Raised The Grain”.
Do the repairs with the glue and sand the Oak flat.
Then you can apply your stain which as you say would probably be better Oil-based.

Once you have applied the stain I’m afraid it is then in the Lap of the Gods’
‘cos you can’t re-sand as you would then have an uneven finish.
Then again as you say an Oil-based Poly varnish, thin coats.
Once again good luck.
Cheers, Jinky (James).

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Mugsy

46 posts in 793 days


#15 posted 08-02-2015 05:45 PM

Yes sir, the saw marks were sanded off. I was doing the mortises before I sanded/planed so I could take off any “oopses” from my chisels. So far I have done 80 and 150 and I’ll finish up with 220 after the damp rag. I was going to use mineral spirits, but if water will work just as well, I’d rather stay away from the smell!

Here is my progress on the first one. The pictures with the cracks are from the second piece.

-- Matt, San Antonio

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