Ok, here is the problem. I am making 5 mahogany outdoor chaise lounge chairs. They incorporate a slat design for the horizontal surfaces. To aid with the project I purchased a Leigh FMT for TONS of joinery, about 1200 mortise and tenons. I have about 4 more days to work on the project and I started to assemble the chair backs and leg rests. I noticed there was a slight twist to the surfaces. All the individual members are straight and the mortise and tenons fit at 90 degree angles and such before assembly.
The picture included is the basic design. Slats enclosed in an upper and lower crossmember, and two frame members on the outside (not depicted). When the slats are attached to either of the crossmembers, they sit straight and aligned, like fingers on an open hand. When the opposing crossmember is then added, the twist develops. I have tried every combination of assembly and even tried other slats, crossmembers and orientations.
I have noticed that the fence on the FMT was off about .5 degrees, canting the joints. Although, when I oppose the members being joined, the angular difference cancels out between both crossmembers. Also, there was a slight (barely measurable) rotation in the mortises. I assume due to my dull bit pulling at the wood. So, with a new bit and different technique I fabricated a few new crossmembers. Since the upper and lower crossmembers are different I only replaced the lowers. One of the three new pieces assembled without any twist. The other two experienced an extremely severe twist, worse than the first occurrence. After looking at the successful piece, I cannot come up with any reason why it works.
I have been able to rule out the angular problem with the fence. The rotation in the mortises was taken out of the equation with the new crossmember. I cannot think of any good reason why there is still a twist developing. Does anyone have any experience with this type of problem? There is something that I am missing that hopefully one of you can pickup on. I am sure you all have some great procedures I can try to get this problem solved. Thank you for any help you can give.
-John
Replies
" have noticed that the fence on the FMT was off about .5 degrees, canting the joints. Although, when I oppose the members being joined, the angular difference cancels out between both crossmembers."
John, it's a little difficult to picture, but it may be that opposing the members may not cancel the difference at all. When you rotate one crossmember around 180 degrees to the other side of your frame, the slope of the cant becomes opposite to the slope of the first member. This effectively doubles the angular difference between the two.
If this is what's happening, you may be best off milling a new half set of members- this time with the fence .5 degrees "off" in the opposite direction, so that they will pair up with your first pieces without creating a twist.
Hope this helps!
Simply by the way the tenons were done, they are angled in the same direction. Exaggerating this, if you were to lay a slat down flat, the tenons would both be angled down/up in the same way. Therefore, since the crossmember mortises are also off by .5 degrees, all I do is insert the slats in a "desructive" manner. Essentially so the .5 degrees cancel out. As long as this is done properly, the slats sit in the mortises at a perfect 90 degrees. If I were not to do this, the .5 degrees is additive to 1 degree off. Simply taking a sqaure to the pieces shows they are pretty far off.
It is a hard thing to explain. I guess it is like cutting a straight board at some angle and being able to orient the cut so the board is straight. You can flip one of the pieces and the angle is additive so the board would no be straight anymore.
Thanks for the suggestion. I am not totally ruling it out, it is just not my top suspected reason. Maybe I am wrong, I dunno. Right now I am trying to determine if there is indeed any twist in the various members. So far no luck. Thanks again for the help.
-John
Found the problem. Since the mortises are symmetrical across the member, I was setting an end-stop along the fence line to make allignment go quicker. Butting the wood up to the stop set the postion for the mortise. So, since there is symmetry, I was rotating the member to make a mortise on the opposite end. This rotation caused half the mortises to be canted in one direction opposite the other half. Simply remilling the mortises to correspond fixes the problem.
However, since I now have a HUGE mortise and a small tenon the joint is pretty loose. I can drop the slat into the mortise without any resistance. I will have to try and figure out how to fix this problem now.
-John
Glue thin pieces of the same wood to the tenons and re-cut them.Dan T.
The one serious conviction that a man should have is that nothing is to be taken too seriously.
Nicholas Butler (1862 - 1947)
Or if they're uniform, which I suspect they are, just slice some real thin pieces to the proper thickness and glue them onto the tenon at the same time you glue up the M&T. It will work fine and eliminates the need for re-cutting.
If they're not too sloppy just use epoxy which is gap filling and has the added advantage of being the best adhesive for outdoor applications as well. I'm curious if the mis-alignment of the FMT was do to a normal adjustment or was it something that has to be tweeked to make it right? Seems to me for the price it should be dead on.John O'Connell - JKO Handcrafted Woodworking
Life is tough. It's tougher if you're stupid - John Wayne
Ya, you would think so. I just posted a new topic about that. Right now the reason for all of the twist in the piece is misalignment right out of the box. I don't think I explained it well in the new post, but the mortise and tenons are twisted off parallel. Supposedly an easy adjustment (Which I have not been able to accomplish yet) that is not necessary due to super precise factory settings. Whatever, a tool is always a tool. I have yet to see a tool trued up out of the box. Guess $800 doesn't change that.
-John
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