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The sides of the mortises I cut with a drill press mortising attachment are rough, and the bottom has the drill bit center hole and circle. I was wondering if these should be smoothed out before gluing. I’m going to use a polyurethane glue.
Thanks, Dave
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Dave,
The sides should be smoothed and squared up using either paring or mortise chisels. I make my mortises about 1/16th deeper than the tenon length for excess glue, so I don't worry about the bottoms. FWIW.
Dano
*Hi Dave,Whether you need to or want to “adjust” the mortise, smoothing the sides, depends upon your method of work. For example, I cut my mortises with a mortising machine, than cut the tenons with a tenoning jig on a tablesaw. Once I set up and test fit one tenon, I can cut all the rest of the tenons without resetting the jig provided I haven’t “adjusted” the size of the mortises. I find this method quick, easy, and less risky than other methods. It allows me to make all the tenons interchangeable with every mortise – an important point for a production furniture maker. So in my case, it would be a big mistake to change the dimensions of the mortises in any way after initial setup and milling.Now if you’re going to cut the tenons by hand you’re going to have to adjust them individually (unless you’re very good), so it wouldn’t matter if you “adjusted” the mortises. Even in this case I don’t think it would be necessary, but it wouldn’t hurt.If you want mortises that are perfectly smooth, than probably a mortising attachement or machine is not the method you should be using to cut your mortises. A router-based system would give you better results. I don’t personally think it’s necessary to have perfectly smooth mortises. Historically the mortises that were cut by hand with mortising chisels weren’t smooth and using a mortising machine cuts mortises at least as accurate as those.Hope this helps.Regards,Kim Carleton GravesCarleton Woodworking
*Interesting question. I agree with Kim that cleaning up the sides of a mortise (however produced) just compounds the problems of tenon-to-mortise fit. But historically, weren't the joints pegged to resist withdrawal, and therefore strong regardless of whether glued or not? Being a cross-grain joint in almost all cases, how strong, inherently, is the MT joint, anyway?
*dfore,there's something going unnoticed here. have you tuned up/sharpened the hollow chisels that you're using in the machine? it's common for people to forget that hollow mortising chisels (and bits) need to be every bit as sharp as your hand chisels to be used effectively. tuning those up may improve your mortice sides enough for you to be satisfied.donald,the MT joint is both over and under-rated, I think. Over rated because people often think it will never loosen or come apart, and as you've stated it is a cross grain joint - it will likely take years, but it will loosen, particularly if there is racking pressure exerted on it, such as in a chair. Underrated because it is the perfect mix of qualities - it may be cross grain, but that is still a face grain to face grain glue surface, which is remarkably strong (if the glue used is somewhat flexible, like hide glue). there is also great value in the mechanical properties of the joint (don't underestimate those shoulders!), and the glue surface and the mech. features actually work to "protect" one another and last longer. Lastly, it is a joint that can be very easily repaired every 50 years (if the builder was wise enough to use a reversible glue). Oh, yeah, I was going to mention that wood movement is a contributor in loosening the glue bond, but not as much as one would think. tenons are generally only a few inches wide, and buried inside a peice of furniture which is presumably sealed and in a stable environment. the movement here is not generally enough to destroy the joint on its own...my twelve cents (I owe y'all a dime)...scott
*Scott-Good points.I have this notion, unsupported by any actual research, that the classic joints (M&T, dovetail, and the like) rely on mechanical properties to resist the forces to which they're most likely to be subjected. Glue would be a bonus.You've pointed out the main mechanical properties of the mortise and tenon--the shoulders. Add pegs to resist withdrawal, and you can almost forget about glue.Mortise and tenon is one of the principal joinery methods in post and beam house framing, and there are many of those joints still holding things together after centuries of use without the benefit of glue.
*I agree with the don't clean them up camp. I've used several bench top mortisers (Delta & Pemco) and a drill press attachment. I've seen articles that recommend the hop scotch method punching the ends and as many 4 sided holes as possible in between and then cleaning out the rest with 2 sided plunges. I've tried it both ways (just going from end to the other) and have noticed very little difference in smoothness. On the other hand I did notice a lot more soreness after punching more 4 sided holes in white oak all day. Do those of you with floor standing HC mortisers that have the cross slide table get smooth walls? I've been lusting after the Powermatic and wondering if it's worth it. Especially with all the white oak I use.
*I found that the fence supplied with my mortise attachment was nothing short of a piece of junk, and contributed to sloppy cuts. I made a new fence and things improved, but now I use a fast acting drill press vise. The cuts are now very smooth and true. One other thing to look for, is to be sure that your chisel is square to the length of the mortise, or you’ll get rough sides from the projecting corners. I agree that routers will make smooth quick mortises. I switched to the router for most of my mortising work. It took a while for me to reach that conclusion, since I hate routers.
*Thanks for all the replies. Rob Millard identified the problem. The problem was the chisel would occasionally twist for some unknown reason. Cheap tools probably. I think I'll file a flat spot on the chisel for future use. Also, I was using the cheap plastic fence that came with the machine. I'll replace that the next time.The mortises were cut into hard maple legs for the under structure of a workbench. I did clean up the sides with a router, so they weren't "too bad". The joint will be glued and then pinned with two dowels so I don't think it will be coming apart. That together with the use of urethane glue (which according to the bottle has some gap filling qualities) gives me some confidence that the joint will be OK. Thanks again, Dave
*Dave-Just remember that urethane glue, while it fills the gaps, does so with a foam that has essentially no strength.
*besides being non-reversible, messy, and generally nasty stuff, but that's a whole different thread, one which I would avoid completely...sb
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