Mortising in general

Re: Mortising in general

Tim of Livermore

>"Thinking of the old guys using mortising chisels, however, means that the mortise sides were not smooth either and their furniture, OFTEN PEGGED, is still together. "

I capped the part I think is most important. Glue will eventually fail when stressed. Hence....joints like aprons to legs or rails to stiles will survive longest/best when mechanically reinforced. Pegging is one method...drawboring is more of a 'special' case of pegging. Wedging - fox wedge or tusk tenon - is another typical mechanical lock used with mortise and tenon.

The mechanical reinforcement/interlock is why dovetails function so well....and fox wedged mortise and tenon is essentially just turning the tenon into a dovetail.

I'd guess the good folks at Woodworking Magazine would agree with the above....since they've kept posting on how dado joints require reinforcement for longevity. My supposition from what they've put on the BLOG is that they'll suggest nailing the dado....and being sure to toe the nails - metal cut nails or wooden dowel nails.

Tim

Re: Mortising in general

Adam Cherubini

>I'm not sure if i understood your question but...

A smooth faying surface is said to make better structural bond than a rough one. BUT:

Hide glue extrudes out of tight joints, leaving so little glue that little or no strength results (at least in my experience). Hide glue prefers a discernable bond line. The rough surfaces resulting from a hand sawn tenon and hand cut mortise provide the tight mechanical fit as well as the pockets or recesses necessary for hide glue. Its a fact that the same approach is taken with structural using epoxy. A thin scrim or open weave cloth is often added between the mating parts to retain the glue in the joint.

On the flip side, PVA wants smooth surfaces and 250psi of clamp pressure. Since achieving that sort of pressure in typical furniture-sized M&T joints is next to impossible, one wonders about the sense of:

1) machining these joints

2) using PVA in them

So if we're just thinking about strength. I have trouble seeing the benefit of table saw cut tenons, perfected with shoulder planes, in machined pockets (that's really what they call them nowadays isn't it?).

Adam

Re: Mortising in general

William Duffield, on the Cohansey

>No, your HCM should not be leaving arcs on the side walls of the mortises. There could be several reasons why it is doing it, including sloppy manufacturing tolerances, improper adjustment of the drill depth, dull chisels, improper sharpening of chisels, quill runout, chisel not aligned parallel to fence, etc. You'll should be able to get lots of in-depth advise on troubleshooting this on the Main Messages forum.

Do not assume that the old guys could not make precision joints with hand tools, because many of them were very skilled. I have seen one set of 19th Century hand made door joints with pegs but without glue, whose mortise walls were smooth and square and fit precisely, with no detectable play. The doors still worked perfectly. The only reason I got a chance to examine this joinery was because the owner of the corner cupboard needed it shorter, to get it in a room with 8' ceilings.

Your Machine is Sprung

Bob Smalser, Seabeck, WA

>Having used half a dozen large mortising machines in commercial shops, none of them had bits that projected beyond the chisel.

What's causing that in your machine is very likely runout from worn bearing surfaces. That's common in commercial mortising machines when users apply more and more pressure to the overarm instead of changing the dull bit. Not only does this wear out the bearing surfaces prematurely, it only takes one hamhanded soul to spring the arm out of alignment, producing a perpetually fussy machine that can't be fixed.

If you need a mortising machine, buy new or check it out before you buy...and you need two sets of bits...one set being sharpened while you use the other. Only takes one dull chisel and too much pressure one time to wreck a big machine, let alone these itty-bitty homeowner models.

Re: Mortising in general

Frank D. in Montreal

>On the flip side, PVA wants smooth surfaces and 250psi of clamp pressure. Since achieving that sort of pressure in typical furniture-sized M&T joints is next to impossible, one wonders about the sense of:

1) machining these joints

2) using PVA in them

Hi Adam,

Your post really got me wondering. Could 1 & 2 make sense taken together? As with using PVA and cleaning up tenons with planes? I wonder because what you say makes sense but I've also seen m&t furniture that was assembled with PVA glue but still holds together fine after over 30 years. I've heard the same comment about clamping pressure but if a joint fits properly (tightly) could the moisture in the glue, by making the wood swell, not provide ample pressure for the glue to bond? Just think about the limited time you have to assemble a mortise-and-tenon joint (with PVA) before the wood starts to swell and make things really hard to fit without strong clamps to close the joints fully (biscuits bond the same way, although because they are compressed they can tolerate more slop inside their pockets...hehe).

Just a few questions from someone who uses PVA glue...

Frank

Re: Mortising in general

Adam Cherubini

>Hi Frank,

I use PVA too by the way and like it.

I don't think the wood is swelling- I think the glue starts to tack and you get stiction.

Even if the wood did swell, I doubt you'd get 250psi.

But let's say this;

1) if you want to use PVA in a M&T (which is practically unclampable)- you are probably better off with smooth surfaces and tight fits.

2) Tho the manufacturer told me unclamped PVA had no properties, I suspect it has some. (clearly it has some) Glue manufacturers are interested in ultimate strength in "ultimate" conditions. They are also interested in working and not answering nosy engineers' questions. PVA is good for like 3000-3500psi. If you got 500psi of properties on one side of a 1" tenon, you'd need 500 pounds to pull it apart.

3) The shrink and swell cycle probably does more to defeat any glued M&T than the strength of the glue.

Hide glue does fill gaps better. I offer this as an explanation of why the "sloppy" joinery of the preindustrial woodworker, though not necessarily intentional, worked out okay because of the glue he used.

make sense?

Adam

Re: Your Machine is Sprung

Gerry Mayberry

>To Bob, William, Adam,etc. Thanks for very informative information about mortising chisels. I build reproduction furniture for my family after having been an industrial chemist for many years. I will have to admit to sometimes not using a really sharp chisel. However, my mortiser has not really seen enough work to likely affect the bearings although I could be wrong about this. The comment that no commercial mortiser produces anything other than smooth walls is telling. Any comments on the Powermatic mortiser and about mortising chisels that would leave smooth walls? Where to find such chisels?. I would like to do all by hand mortising and save money but I am 71 and trying to develop perfect chisel technique at this stage of the game would be daunting. I can cut pretty decent dovetails! Considering the comments from you folks, the mortise walls I have been achieving are anything but smooth. I will make a change! Thanks again for the helpful information.

Thanks Adam

Frank D. in Montreal

>for taking the time. Yes it does make sense. I'm a bit disappointed my swelling theory doesn't hold water, I thought my joints had the full stregth of the glue. I guess that doesn't really make them any weaker, though.

Thanks again.

It's Not the Chisels

Bob Smalser, Seabeck, WA

>Your drills are going a different direction than your chisels, or they are wobbling around inside them, hence the marks outside the chisel cut line. If this just happens on one side, you may be able to adjust it out.

If the machine is that new, call the manufacturer.

M&T data from Frankin Glue company

bill tindall

>About once a year these issues come up. Will someone get this data into the Articles section so I don't need to type it again?

Source: Glueing and furniture Design-Franklin Chemical Indsutries summarized from original paper by Hill and Eckelman, Furniture Desigh and Manufacturing, December 1973.

Reletaive strength of adhesives in M&T application

phenol-resorcinol 1.00

hide 1.05

Urea formaldehyde 1.24

PVA 1.32

Effect of tenon fit on relative strength

gap relative strength

0.000 to 0.002" 1.00 (need to tap together)

).003-0.005 0.94 (snug fit)

0.006-0.008 0.89 (this is a very sloppy joint)

Effect of tenon lenght on strength

length relative strength

.5" .48

.75 .80

1.0 1.0

1.25 1.12

1.5 1.28

1.75 1.46

2.00 1.66

it would be nice to see a graph of these data. I looks like it is nearly a linear function beyond .75"

Strength of a M&T: hard maple, PVA adhesive 2" wide M&T 1" deep- 4100 inch pounds

Maybe an engineer can think about this further but it would seem that the shear strength of the adhesive is what is important in a M&T joint while tensile strength is what is important in edge glueing. Over time it will likely be the flexibility of the adhesive that will determine failure of the M&T as a result of wood movement in the joint.

Re: M&T data from Frankin Glue company

Adam Cherubini

>How does fit effect the numbers? Is this saying all glues' degrade in properties with an increase in gap? That doesn't make sense to me and doesn't reflect the sense I got when I asked this question last year! :)

Adam