Angled tenon and length

[ByronBlack]

Hi

I need to make some angled tennons, and have no idea on how to work out the angles, or how to work out the length of the timber.

This is for a greenhouse (lean-to) project, I have an upright which is higher at the back than the front to create a slope (to allow for water run off). What I can't work out is how to work out the length the timber needs to be from the back stick to the front. The width between the two will be 500mm but because it needs to rise from the front to the back i'm assuming it needs to be longer - is there a calculation to work this out? And finally, once I work out this length, how do I work out what angle the tenon needs to be to enter the two uprights at 90 degrees?

I hope that makes sense

 

[Freetochat]

How about laying out a rod for the roof section and working off that. Tommy Walsh done something similar when building his roof for the 'ultimate workshop'.

 

[SketchUp Guru]

You could draw it out in SketchUp and interogate SU for the angles. Or you could send the numbers to someone you know who could draw it in SU and figure it out for you.

 

[Anonymous]

I'd concurr with freetochat 100%

If you draw it out full size, with the spann and rise etc + all the length's for your raftes, everything, you will be able to transfer them off the rod onto each component, even for angles etc, where to start and stop mortic's etc. The dimensions will create themselfs you dont even necesaarily need to know it was so many inche's or mini meters. It might seem like its crude and low tech but it is foolproof as potential construction problems can usually be observed and resolved at this drawing stage

 

[Nick W]

...If you draw it out full size, ...

... you'll need one big piece of paper!

 

[Freetochat]

...If you draw it out full size, ...

... you'll need one big piece of paper!

A couple of sheets of cheap shuttering ply or hardbord would do the job.

 

[ByronBlack]

Thanks for the tips guy's. I did think of using a 'rod' or storystick, but wasn't sure if there wasn't a sure-fire calculation. I have an 8'x4' sheet of ply in the shop, so i'll probably do as suggested, and draw it out fullsize and make some rods/storysticks etc..

 

[Anonymous]

...If you draw it out full size, ...

... you'll need one big piece of paper!

Paper is the last thing I'd draw on :wink: . I dont think Byron's project is that big though. I once was talking to a joiner and he said a bloke rang him up can you come and fix my greenhouse please? It turns out the bloke ran a market garden and the "greenhouse" was over 80 foot long :lol: :lol:
As freetochat said, plywood is fine, osb, mdf etc, or do like the French roofer's and draw it out directly onto the floor, (even a car park or a wall at a pinch) Or lay out a few scaffold board's where the lines will be and use that, make a giant 3/4/5 set square etc for 90 degree corners. Marc Guilhemjouan's site might give you a few idea's also. The trouble with relying on abstract calcs is that any mistakes usually show up after you started cutting, with a rod mistakes tend to show up at the drawing stage what you see is what you cut

 

[Sgian Dubh]

I can't work out is how to work out the length the timber needs to be from the back stick to the front. The width between the two will be 500mm but because it needs to rise from the front to the back i'm assuming it needs to be longer - is there a calculation to work this out?

I hope that makes sense

Possibly. It sounds like you're dealing with a right triangle, i.e., one where the hypotenuse (sloping side) is opposite the right angle.

The maths is simple. The length of the hypotenuse is found by adding the the length of the short side squared to the length of the long side squared and square rooting it.

For example, take a 3,4,5 triangle where a 3 ft leg and 4 ft leg form a right angle.

3² = 9
4² = 16
16 + 9 = 25
Find the square root of 25, thus
√25 = 5 ft, the length of the hypotenuse.

The sums can usually be done quite easily even on a fairly basic calculator, especially if they have a ² key and a √ key, but even without these a bit of additional calculating will get you there. Slainte.

 

[Freetochat]

It is possible to use a program like autocad to calculate dimensions, or just mathmatics, but IMO there are too many variables that would not allow precision.

 

[engineer one]

amazing what you can do with old fashioned trignometry :lol: :lol:

do they still teach it.???????
:twisted:

paul :wink:

 

[j]

Good old Pythagoras' theorem, who'd 'ave thunk it

J

 

[Modernist]

You need to be a bit careful with the trig as the actual length of timber may not be the actual hypotenuse as you have to allow for the width of the verticals and the fact that both ends will be oblique.

I would draw it on Autosketch or use a hardboard template

 

[engineer one]

yes but the hypotonuse is within the uprights, and then as normal you allow the excess for the tenons, just maybe 15% not 10 as normal.

paul :wink:

 

[ByronBlack]

All this maths has done me in - just can't get my head round it all. So i've decided to scrap the idea of an angled tenon and have gone with a straight connection and will have a tapered section that reduced the height from the back stile to the lower front stile, makes life a lot easier, won't look as nice but atleast the build will be a lot easier.

 

[Benchwayze]

Either a mock-up Byron, or a drawing on a ply sheet as has been suggested.

Using a story-stick is Frank Klaus's favourite way in the shop. Once you get the hang of it, it is simple. Even so, trig is useful in any woodwork shop.

Now, how did I do it?
In the late-sixties I knew virtually nothing about building sheds etc.
So, I fixed the wall-plate first, using car-filler paste for the studs. (Maybe I should have patented it, for Tommy Walsh!) Then I located one upright over the quoin and temporarily fixed that, with a horizontal 3 x 2.
Next, I offered up the 'rafter' and moved it about until it looked right. I made pencil marks on the rafter and the upright. and cut the rafter as needed. The remaining rafters I made from that first one.
After that, using a spirit level, everything else fitted into place. It was suck it and see I suppose.

I moved on from that house, but the workshop is still there after 39 years. The last I heard it was being used as a 'summer-house' where people actually sleep in the warmer weather. Who knows? The new owners might even have rented the place out.



Happy Head Scratching

John

 

[Adam]

All this maths has done me in - just can't get my head round it all.

Come on, don't buckle yet, we can get you through. OK, so maybe you went to sleep in that maths lesson, but this is standard stuff.

I think what you are saying is how long do you make the sloping piece?

Like this? First you need to work out how long you want to make the horizontal piece. I assumed 2meters (2000mm). You need to work in one single measurement type for this to work.

Lets call them in letters.

You might need to dig out your windows calculator to run this, if you don't have a handheld that has the square root function. Its under start>programs>accessories>Calculator - or something like that.

Or just draw it in sketchup!

 

[Benchwayze]

Yep. Like I said:

Suck it and see.
Only do it on paper firrst!

Well explained Adam.
My extra space is just a metre wide. is it worth the bother I ask myself!

John

 

[ByronBlack]

Hi Chaps. Thanks for the explanations. Adam your way of doing it makes it all a lot easier to understand, cheers!

However, i've already decided to avoid making an angled tenon by making the part horizontal and then attaching a wedge shaped piece to the top to create the slope - lazy? Yes - but practical - definitly