Thread: Rule of thumb

Not trying to be funny, my rule of thumb is simply "don't."

Not for fine joinery, anyway. In housing construction it's a different matter though... and your two rules are appropriate there. Not that I'd use dovies in a house frame! :eek:

Hi Ivan,

I'll try to post a quick reply again(the forum software spat me out last time)

The 1:6 ratio is for structural dovetails adn the 1:8 ratio is for decorative ones. It doesn't matter whether it's hardwood or softwood. (This was my trade training - completely different to internet legend, I know.)

With regards to joinery, use the rule of thirds: "joinery is either cut 1/3 or 2/3 of the way through the thickness, unless there's the word half or halving in the name of the joint." eg: rebates - cut 2/3 out, tenon/bridle - leave 1/3 behind, lap dovetails - go 2/3 of the way into the thickness, halving joint -go halfway

As far as screws and nails, I'm really stretching the brain here, but you need to go further in if you're screwing into end grain than if you are screwing into face grain.

From memory, if you're screwing into end grain, from memory, you need to go twice as far into the end grain as the bit you're attaching is thick - eg: if you're attaching 12mm timber, go in a total of 36mm. For screwing into face grain, go into the second bit 1.5 times the timber thickness - in the above example, go in 30mm

For nailing, the ratios were 3.5 times and 3 times the thickness of the bit you're attaching, so a 12mm bit of timber is attached to end grain with 42mm (theoretical) nails, and into face grain with 36mm nails (1.5")

Mind you, these numbers are purely theoretical, and don't take any account of skew nailing, etc... but this is buried (deep) in memory from when I was trained.

Cheers,

eddie

Mixing hardwood and softwood (or any very different species) can be a cause for major warping, distortion, etc because of the dirrerential rates of expansion/contraction occassioned by moisture and temperature.

So, as Skew says, for furniture anyway, "don't" is the answer.

You might also want to consider the manner of cut for the boards, as this can also affect the way the timber moves.

Thanks Steve,

In my original reply, I'd said something similar - thanks for picking it up.

Cheers,

eddie
(who's wondering what else he missed, but is flat out like the proverbial lizard at the moment.)

I'm not entirely sure its always a problem crossing the two. Some cases definetly..... dependent where your using it I suppose.

I'm no expert....but I'll give an example of what I mean.
shrinkage and expandtion problems exists in joints anyway doesn't even when the timbers the same.......say a tennon and mortise.....shrinkage would increase the dimentions of the mortise, wouldn't it ? .....and shrinkage would reduce the dimentions of the tennon, yes ? both cause a loose joint........that tells me that introducing a different timber for either the tennon or the mortise that moves less would actually benifit the joint ........:confused: ...ie. two different timbers seem ok here.

most furniture is designed to take this into account anyway isn't it ?. uno, to cater for swelling and shrinking accross grain...cause thankfully along timbers length there is little movement.

I only mention this, because I'm a cheap bugger who keeps every offcut from the last 7 years , so I'm forever building stuff from all types of timber.
And sometimes it looks good crossing soft with hard.

As for using nails and screws, I think just common sense is good enough. If the thread of a screw is in the softwood, go deeper into it and pilot hole less of a diameter (ideally no pilot hole if your drills powerful enough to drive that screw)

I'm working on this old joint at the moment, and we use baton screws on the roof, to afix these hardwood batons to the hardwood rafters uno......well, no pilot drilling there....Wack in, hex drive drill only. Have to pre-drill the 3inch nails with 1/8" bit first. Well I have to..... not like my smartarse boss with his faultless hammer swing.

Originally Posted by apricotripper

....say a tennon and mortise.....shrinkage would increase the dimentions of the mortise, wouldn't it ? ....

Sorry Apricotripperbut shrinkage will actually DECREASE the size of a mortice. you have fallen into a common trap in assuming that the wood will shrink away from the hole, but it actually shrinks INTO the hole. look at it this way - if it was possible to remove the timber from the mortice in one piece by some mythical means and it was subject to shrinkage the piece of timber would become smaller. thus the size of the hole it was cut from in the piece of stock would likewise become smaller. as the timber shrinks the sides, ends and bottom, for the want of more correct terms, of the mortice all shrink as if part of the shrinkage of the overall piece of stock. i hope this is clear, i know its not an easy concept to grasp.

Benchdog

Are yes, I see know.....my mistake.

Of coarse, its just the area of timber above and below the mortise thats of importance. Your right.

So, whats the significance of it do you think in practise ? ....cause I've built quite a few things crossing soft and hard with no problems. yet

Maybe I've got lucky ? ......

I can see a great deal of movement problems in say a panel sitting in a frame, because of all that width.......but not so much in the frames joints, for example, cause the timbers used is so much thinner.

has anyone experienced major joint failure by crossing soft and hard ?....or is it one of things that one worries too much about ? uno.

Yup, Otherwise, if you made a mortise say 3/4 the width of the stock and the stock shrank by 25%, then you'd have a hole bigger than the wood it is in! I wonder if that's how the T.A.R.D.I.S. was built? :confused:

One of my early red-gum rocking chairs I used softwood for contrasting the stretchers and turned uprights in the arm-rests and back. Within 6 month, it was firewood. I was soooo proud of it, too.

TARDIS ?! ....so penswappers are doctor who fans ?

WELL THEN !!! you must have ####ed up the joints ! not like me......I made a table out of balsa and ironwood.....turned the balsa legs and....

I think you guys might be right

Originally Posted by apricotripper

...So, whats the significance of it do you think in practise ? ....cause I've built quite a few things crossing soft and hard with no problems. yet...

in practice, you still have to be careful even with the same timber seasoned for the same amount of time, let alone dissimilar timbers. the stock is most likely to shrink more across the grain (tennon) than it is to shrink along the grain (long axis of mortise). thats why furniture, particularly chairs (with their short rails and shrinking tennons sticking into long stiles with more stable mortices) tend to fall apart within a couple of years, unless the craftsman adheres to well established traditional methods.

this is why modern manufacturing methods tend to resort to "metal dowels" for a lot of joinery tasks.

i do use dissimilar timbers for various reasons in some projects, but i try to find contrasting timbers with similar coefficients of shrinkage. personnally, i cant think of any reason to use softwood and hardwood in the same project, except for as skew chidamn mentioned above, attaching hardwood to a pine house frame. if you have done it in the past and gotten away with it, more power to you, but not for me thanks.

benchdog

Well, I definetly must have been lucky then. I do pin my T&M joints as much as possible as well.

As for a reason to combine them........Well I reakon contrasting colours can look good ? yes ? which I guess was the motivation behind skews chair......Seen a few professional pieces made this way. I don't know if their still in one piece though .

So, sounds like I should stop jointing this way, and only get that contrast I'm after with unjointed loose stuff....stuff that floats, like table tops and panels.

I reckon you're safe if you can "match" the timbers so that they have about the same shrinkage rate. I know for a fact (now) that celerytop pine and redgum aren't even in the same ball-park.