Replies

1. 
   b1gcountry | Sep 26, 2002 12:34am | #1

   Say we have a 45 degree miter on a 2" wide piece of stock. One edge will be, say, 20" long and the other edge will be 18" long. Now since wood doesn't expand lengthwise, these lengths (20" and 18") will not change. The width, however, will change in width. This will effectively change the length of one leg of the triangle, but not the other, so it will change the angle of your miter from 45 degrees to less when the wood expands (summer), and more when the wood dries out (winter). This will open up the miter on the outside in the summer, and on the inside in the winter. The amount the miter opens up is usually counteracted by the glue in small miters, but, in large miters, the glue can't hold up to the stress, and the gap will become appearant. The width depends upon the seasonal change in humidity you expect, the type of finish applied, and the width of the boards mitered.

   A 7" miter would probably gap. It will gap less with QS wood. You might be okay with QS Mah. finished with shellac (good protectant for moisture exchange). I haven't pushed this enough to know where exactly that magical point really lies...

   Tom

2. 
   SgianDubh | Sep 26, 2002 12:58am | #2

   Yes. If the timber takes on moisture in service it will expand and it's likely that the outside of the mitred corner will open. Conversely, losing moisture will cause shrinkage and the inside corner of the mitre will probably open. This may take a few months or years if the parts are well polished. One way, apart from sealing well, of reducing-- but not guaranteed to eliminate-- the likelihood of this opening happening might be to make up your 7" width out of two pieces about 3-1/2"+ wide, tongued and grooved together along the length with a decorative disguising moulding at the tongue and groove. Assemble the two parts dry with plenty of masking tape across the joint to hold them in alignment, mitre the corners as required. At glue up time make sure the tongue and grooved part remains unglued all along the length, and in the corners. Now you have two 'frames', each with mitred corners, where one is 'independent' of the other, thus reducing the length of the corner mitre. Any opening up will be less obvious in each frame than a wider mitre. Of course, the long grain tongue and groove detail may be visually unacceptable. Slainte.

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1. 
   JeffKurtz | Sep 26, 2002 01:09am | #3

   Some friends of mine have a table that intrigues me.  It's made by Henredon (good stuff?) and has a 2.5" mitered and molded strip that runs around the entire perimeter of the table.  How do they keep it from splitting/opening?

   Jeff

1. 
   SgianDubh | Sep 26, 2002 01:24am | #5

   Jeff, over the long term it probably will open, maybe only a little, but it is hard to be categorical. My guess for why it hasn't happened yet, assuming it hasn't, is that the mitre is well glued and/or reinforced with a biscuit/tongue, etc., and the body of the table top is veneered board of some sort which is relatively-- but not completly-- immune from expansion and contraction. And good polishing helps too. Slainte.Website

1. 
   JCalvetti | Sep 26, 2002 12:48pm | #6

   Thanks for the responses everyone and to Sgian for the solution.

    

1. 
   sorethum1 | Oct 30, 2002 01:33am | #7

   I have tongue and groove around my 5' x 3' desk top, with 1-1/2 solid mahog edge, mdf tongue.  Where should I put glue?  The fit is snug, requiring slight taps with mallet.  I assume glue throughout, esp since I want a tight joint showing on the top (which I may rout a groove on later anyway).  Todd

2. 
   SgianDubh | Oct 31, 2002 08:17am | #11

   Sorethum, presumably a reasonably generously thick mahogany foreedge around a veneered man made board allowing for trimming down the mahogany after glue set up? If so, then yes, squeeze glue into both grooves, insert the tongue material into the main top and 'squidge' the tongue about to spread the glue onto the tongue. (Your loose tongue shouldn't be too 'snug' a fit, for when the timber from both mating parts expands a bo'hair with the moisture from the glue-- assuming water based-- it might become too 'snug', i.e., tight.) What squeezes out from this assembled part you then run round with a wee brush and slather onto the exposed part of the tongue and onto the edge of the board. Add abit more glue to the edge of the board if needed.  Slop a wee bit of glue onto the mitred ends, assemble, and cramp up. (I'm assuming you're mitreing the corners explaining why your question appears in this thread.)

   Cramping up all four sides in one go can be difficult requiring lots of cramps top and bottom side and co-ordination, and liquid hide glue gives you a fairly generous 20 minutes open time to fiddle about at about 70°F, and longer if your workshop is cooler. Yellow PVA only gives you about 5 minutes if you're lucky. White PVA gives you about 7-10 minutes.  Liquid hide glue has a use by date stamped on the bottom, but it's handy stuff for indoor furniture-- Franklin make a good version, and follow their instructions to the letter printed on the bottle.

   You might consider cramping up just the long edges first-- very carefully so that the mitres are perfectly lined up with the corner of your veneered board, and fitting the short edges the next day.  In this situation, this is a common strategy for me, and I do fugg up from time to time requiring a bit of creative bodging and fixing, ha, ha. It's not an easy task, even for old dogs like me.

   If you are applying this foreedge around a solid wood panel rather than a man made board panel I'd reccomend you reconsider because the expansion and contraction of the panel will cause problems with the 'breadboard' or 'clamped' end and the mitres you form, or the panel will split.

   This advice might be too late as I see your question was posted nearly two days past. I kind of missed it as I was busy doing other things, but maybe this helps, even if only retrospectively. Sorry if I'm too late. Slainte.   Some stuff I've made.

3. 
   toddhunter | Oct 31, 2002 05:02pm | #12

   SD, don't apologize.  This is the kind of step by step I'm in need of at the moment.  To clarify, my top is mdf with a 1/4" ply top.  The top is tongued while the edge is grooved, solid mahogany, 3/4 thick, 1-1/2 top to bottom (as it sits on the edge).  I think my fit may be a little tight, though it is mdf tongue.  I don't think I can move the edge back and forth once fitted.  So I will consider removing a slice of the tongue.  I'm inclined to miter both ends of the front, then clamp without glue.  Then fit a side piece, no glue. Then the back, etc.  As I go, I expect to make adjustments, probably with the miter saw (hacksaw style) and maybe with the dozuki or chisel/block.  Most of the shooting boards I've seen are for face moulding, but I suppose I could create one with more depth.  Does all this make sense?  Or would you do it differently?  Todd

4. 
   SgianDubh | Nov 01, 2002 02:50am | #13

   Todd, A good strategy I find is to prepare all your foreedges or lippings with the corners mitred and dry clamp it all in place fully assembled. This does require cramps that will bridge over each other, using edge cramps, or some other strategy. Then release the clamps holding the long edges, apply the glue to the tongue/groove omitting to glue the mitre. Now clamp the parts back in place. Almost immediately, release the short edges dry clamped on, clean off any glue in the corners, and set these parts aside until the long edges glued up joints are cured. Later, attach the short foreedges permanently.

   This is a two stage process, so it takes more time, but it’s a lot less stressful than doing all four edges in one go. It sounds like you were thinking of going along this route anyway, and it might be an idea to take a wee shaving off your stuck MDF tongue. How you cut the mitres is up to you. You can cut them with anything from a handsaw and a knife mark, plus a bit of trimming with a chisel, to using a sliding table saw (and still perhaps a bit of trimming.)

    What caught my eye as interesting was what appeared to be an unbalanced table top. Did you say you had one face only of 1/4” ply attached to a ground of 3/4” MDF? That could lead to cupping problems if I read your description correctly. On the other hand you might have plenty of reinforcing members on the opposite side of the ply, so you might be basically okay. Slainte.Some stuff I've made.

5. 
   sorethum1 | Nov 01, 2002 07:49pm | #14

   Great suggestion.  Makes sense to me.  It does assume I can cut the miters correctly of course, but it's a plan for gluing.  I also figured that with the tongue, it might be more difficult to cut the miter after gluing.  Last night I spent some time with a sanding block thinning the tongue and things fit much easier.  This was laborious though, and may have been easier if I just routed a little more.  But I think part of the problem may be slight bowing of either the top or the edge, so that they don't align precisely.  I'm pretty sure thinning the tongue through the sticky parts will do it.  As for the sandwich, yes there is quarter inch ply on the top, but 1-1/4 inch of mdf (3/4 and 1/2) underneath.  The ply itself is mdf between two good sides of mahogany.  I'm hoping cupping doesn't become a problem with this proportion of mdf, the hardwood edging and the fastening to the base.  The entire top, which is very heavy, will be attached with screws and metal brackets as the original top was.  My biggest concern was whether the mdf would be able to hold the screws adequately or whether I should add some strips of wood for the screws to grab.  I'm not too concerned about that though.  Todd

6. 
   SgianDubh | Nov 02, 2002 08:35am | #15

   Todd, and Sorethum seem to be a bit of a split personality. Do you (guys) have more than one screen name? It sounds like you're (both, ha, ha) in generally good shape to proceed.

   Sanding that tongue thinner is a tough way to do the job well-- it tends to round off and 'smudge' the profile. If your budget in the next year or so can stretch to a shoulder plane from manufacturer's like Clifton, Stanley, or Lie Nielsen(sp?) you'll find those wee trimming jobs much, much easier, and much more precise too. Shoulder planes, and their cousins, rebate (rabbet)  planes make short work of final fitting things like tongues, tenons, rebates, etc..

   I'd be hesitant of the laminated structure you have for the top, the MDF, plus more MDF, and the mahogany ply mix due to the imbalanced nature of the structure, but it seems you have quite a few reinforcements underneath to hold it all flat. If you want to do the maths, MDF weighs about 50 lbs.±. a cubic foot, therefore, a piece 96" X 48" X 0.75"/ 1728 = 2 cubic feet, or ~100lbs-- 1728 is derived from the sum, 12" X 12" X 12".

   MDF holds screws pretty well if they are screwed into the face, but a coarse aggressively threaded screw works best. Standard wood screws don't really hold any better than a sick monkey's fcuk into the end grain of MDF and it will tend to delaminate allowing the screwed on part to fall off, unless it's also glued. Slainte. Some stuff I've made.

7. 
   toddhunter | Nov 02, 2002 04:11pm | #16

   Toddh and Sorethum are the same, home and office, and we (I) appreciate the help.  I hope to do a little mitering this weekend, but the shoulder plane sounds mighty tempting.  Todd

2. 
   Marcello | Oct 30, 2002 04:52pm | #8

   In a related topic, I have been thinking about long miters holding up in the past few days since I am in the process of putting together one of these monsters.  A table top, 14" long x 20" wide to be mitered to its leg, 30" long x 20" wide.  Attached is the combination 1/2 blind dove tail / 45 deg miter I came up with.  Do you have any thoughts on it?  The finish of the piece will be tung oil and shellac combo, if that helps any.

   
1. 
   SgianDubh | Oct 30, 2002 05:44pm | #9

   TM, what you've drawn seems to be no more than a standard secret mitred dovetail, in the past typically used in premium quality cabinets and chests where the top meets the sides. This is used whether or not the top is veneered. In veneered work the advantage of the secret mitred dovetail is that no end grain of the pins in a lap dovetail, or both the end grain of tails and the end grain of pins in a through dovetail can telegraph their position through the veneer due to shrinkage of the dovetail joint in service. In addition, the join between the veneer and the solid wood underneath is all long grain to long grain, not long grain to end grain (where the end of the tails and/or pins come through.) In solid timber, the advantage is a clean joint with no distracting joinery-- it depends on the look you are after-- some find the look of exposed joinery attractive, and I agree that it is in the right place, but clean lines are appropriate too for other work.

   Bear in mind that the secret mitred dovetail, and it's nearly as difficult associates the secret lap, and secret double lap dovetail require a big step up in skill level and execution from the simple through or or lap (half blind) versions, which might partially explain the fondness of many workers/designers for exposed joinery (dovetails.) Essentially, plain through dovetails, and common lap dovetails are  reasonably simple to execute with some practice, if a little tedious and time consuming.

   In your case the timber of your end slab (and presumably of the top)  is only 1" thick, so your mitre is not long, only about 36 mm (1-7/16") in the diagonal measure across the one inch board, but it is a wide mitre at 20", so no significant problem.

   If you haven't already got a good reference book, I'd recommend that veritable font of information, The Technique (US Dictionary) of Furniture Making by Ernest Joyce as a good one, available online too if you want to get a copy. He shows this joint, and a whole heap more, but watch out for the British'isms. If, like me, you own almost no woodworking books, [I have just 20" of shelfspace covered with woodworking books] then the few you own need to be pretty good, ha, ha. I'm not a pack rat and hate clutter. Slainte. 

   PS. In edit mode. I thought I'd mention that the hand cut secret mitred dovetail is one of only two versions of the dovetail where the pins must  be cut first-- the other is the secret lap dovetail. In every other version I can think of offhand the order of cutting tails or pins first is optional and down to the workers preference. I cut them both ways depending on my mood and other sundry factors.

   Some stuff I've made.

   Edited 11/1/2002 12:25:52 AM ET by Sgian Dubh

2. 
   Marcello | Oct 31, 2002 12:49am | #10

   Thanks for the wealth of info.  I'll look up the book and get it. .....and I thought I came up with that joint....silly me.

   Marcello

2. 
   SgianDubh | Sep 26, 2002 01:16am | #4

   Joseph, then I recalled that somewhere I had a scanned image that might help to clarify my description. At this 'click-on' the outer section of the frame parts is thicker than the inner section, and no fancy moulding shown on the inner edge of the thick part, but I think itadequately  illustrates the principle. Slainte.Website