I’m building some end tables with exposed through mortises out of mahogany. I’ve been having problems with the mortises not being tight all the way around the tenon. So far I have tried cutting them with a dedicated mortising machine and well sharpened bits, also have tried a router in a mortising jig with an upcutting solid carbide bid with limited success.
I am looking forward to any suggestions that may be coming my way as I am always learning.
Replies
"mortises not being tight all the way around the tenon"
You mean loose fit of tenon and mortise, inconsistent fit, or uneven fit at the shoulder?
Thanks for the reply Bob, best I can describe it is when pushing the tenon through and it begins poking through the exposed side, now looking at the tenon head on the mortise doesn't fit tightly all the way around. Its as if when using the mortising machine the workpiece is moving causing a "rocking" motion thus uneven fit. However, to prevent this possibility initially I clamped the wood in place. Now, I know that that the possibility of "error" is added each time I touch the workpiece to move it.
Blueline
Not being exact is a potential problem when using through mortices. I always cut the motice a tad small and the tenon a bit large. That way you can size the mortice sligthly larger with a chisel and downsize the tenon with a shoulder plane. You can take away easily, but you can't add to after the cut has been made.
Regards...
sarge..jt Proud member of the : "I Rocked With ToolDoc Club" .... :>)
I agree with Sarge....no woodworking machine is a precision instrument like in working metals....that square blade is reacting to the grain of your workpiece and might be flexing just enuf to bring your mortises to something less than plumb.
I've used several 3PH commercial machines that I was never entirely happy with, given the relatively difficult bit sharpening chores required when working difficult wood. Maybe you're trying to cut them too fast, driving the bit off line as it catches grain. Maybe your bits aren't as sharp as they need to be.
Frankly, I don't use mortising machines much any more...for a single piece I chop them by hand with mortising chisels....from marks on both sides for thru mortises. For production work I use routered slip tenons...or more often, sliding dovetails.
Edited 11/15/2003 9:51:03 PM ET by Bob
Sarge's advice is probably the best. On an occasional mistake you can pin the mortice with wedges, but only in the direction of end grain of the mortice (other wise you might split the board). Obviously, you would have to pin all the mortices to make the piece look correct. Some times you just have to make a new tenon piece and try again. I guess that is why it is good to mill up extra material.
Rod
Check that your mortising chisel isn't bent. I inadvertently bent a small (1/4") chisel while adjusting the table on my powermatic mortiser.The vertical part of the table pushed against the chisel, bending it. Didn't realize it until mortises starting turning out skewed.
And a good way to get a sloppy one to fit w/o scrapping the piece is shown in the pic:
http://pic3.picturetrail.com/VOL12/1104763/2594266/26720357.jpg
Bob
Pretty clever. I'm sure craftsman have been using that one for about 5000 years, but I've never seen one wedged that way. I'm kind of sheltered. ha..ha..
I"m making a mental note of that one. If you used a different species of wood in another contrasting but complimenting color, might just look as it was a design plan. Could be an effective "eye-catcher".
Thanks...
sarge..jtProud member of the : "I Rocked With ToolDoc Club" .... :>)
That one is just a crude saw vise made from scraps....wedged them that way because hard cherry tenons under camming stress might crush the soft pine mortise and break the glue bond...so I did the crushing ahead of time with the wedges and it's held up fine for almost 20 years of use with casein resin glue.
I don't know the specifics of your operation, but here is a quirk of table top mortisers that I found the hard way (two twisted batches of cabinet doors.)
In a mortiser, the base is parallel to the head and the chisel descends at 90 deg to both the base and the head. But, the head is cantelivered out over the base; that is, the connection between the head and the base is at the back of the machine, not in the middle. When you press into wood, the downwards force slightly lifts the front of the head relative to the back. Thus, the angle the chisel makes is still 90 deg to the head, but is no longer 90 deg to the base and the workpiece - the angle to the workpiece has opened up (relative to the back)an amount dependant on the tolerances your machine was built to. Thus, the mortise is not 90 deg from the surface. Let's call this angular error the "individual error" for each mortise.
If you cut one side of a single mortise, then turn the piece around to do the other side, no matter which sides we're talking about, the angular error on the two sides of the mortise (of the mortise on each end of a surface) are 2 times the error out of square with respect to each other. The tenon will not fit well. Make the first cut on a wide mortise, then move the fence and make the second cut accurately without flipping the workpiece.
If you're making a door, it's even worse! The errors on each style will be equal, but opposite. The final angular mismatch between styles will be FOUR times the individual error per mortise.
In practical terms, picture a style for a door (the vertical part with the mortises (innies), versus the rails (horizontal) with their tenons (outies)). If the mortises are symmetrical, that is, shaped the same and the same distance from the end, you can use one set up with a stop block on each end. You could cut the first mortise, then, flip the board and cut the second. Easy. But, you've multiplied the individual error in the angle of the mortise by FOUR and your door will be twisted.
Instead, move the board along the bed of the mortiser only one way, do not ever flip it. Do this even if you need to make two separate setups. The angular error will be only one times the individual error and, better yet, the two door styles will have the same error and be parallel. That is, no twist!!
I may or may not have addressed your problem, but there's a bit of my hard-earned wisdom about hollow chisel mortisers that I can share. I hope I've not been too obtuse.
Odd that I never thought about it that way...but I believe Mike is spot on.
Mortice chisels can have a mind of their own. If the bevel is sharpened slightly off center they can run. Of course you have to check your machine set up to ensure that the chisel is 90 degrees to the table and fence. I try to cut the two farthest ends first and then take some out of the middle. I try to make sure that the chisel is supported on both sides when making all but the clean up cuts. If you start at one end and make sucessive cuts toward the other end the chisel will wander and maybe even bend a little. How are you cutting the tenons? If you are using dado heads and cut in from each face then consistent board thickness is a major issue. Any slight variation will be multiplied by two. In any case, wood movement will loosen the joint. That's why they either get pinned or wedged
I'm not sure how thick the wood is, but you can get long router bits, and if you use a plunge router taking small "bits" each time, you'll get a perfect mortise.
First, your mortise has to be square. Either use a drill press or a mortiser to accomplish this and check it frequently.
The Tenon is simply made a tad oversize and trimmed to fit.
Regards,
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
The problem is not with the mortises. Make them first, then make a tenon that's slightly too big and bring them to a piston fit with a shoulder plane or a chisel.
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