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Brace Pocket Measurement?

Started by jander3, December 29, 2008, 10:31:15 AM

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jander3

Since the weather hasn't allowed me much time up north to work on the cabin, I've been practicing timber frame joints in the garage.  Yesterday's project was this table. 





After I cut the pegs and draw-pinned the joints, the table came out pretty decent.

Next up, I need to figure out how to cut a knee brace. I've read posts on this topic and cutting the brace looks pretty straight forward.   I can certainly layout the brace pocket taking the dimensions off the brace I cut, cutting the 45 degree angle in the pocket seems like it should work just fine.  However, I would like to understand how to determine or calculate the length of the brace pocket. Any ideas or reference to a post?



Jim_Rogers

Quote from: jander3 on December 29, 2008, 10:31:15 AM
However, I would like to understand how to determine or calculate the length of the brace pocket. Any ideas or reference to a post?


Did you do a search here and read all that I've posted about it?

Jim
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

jander3

Jim,

Yes I did.  Everything seems very clear, except I can't seem to find how to calculate the length of the brace pocket. 

For example: If the tenon is 2" deep, then the length of the angled portion of the brace pocket will be 2" for a 45 degree brace).  However, I can't find an easy method of figuring out the length of the remaining brace pocket.   I can figure it up using trig functions, just wondering if there was an easier way.

One book I have, states cut the first brace to sized and then accurately measure the tenon. 



Jim_Rogers

Ok, so that I understand what you want look at this picture:



Which line are you wanting to know the length of?

The line where the measuring point is to the other side of the brace?

Jim

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

jander3

On this drawing I am looking for the length of the shoulder.  I believe you can get this dimension by the following:

Cosine A = Adjacent/Hypotenuse

Cosine 45 degrees = timber dimension (flat on the right side of the drawing)/shoulder length

shoulder length = timber dimension/Cosine 45 degrees


Looking for verification and/or an easier method.

Jim_Rogers

The easiest method is to cut a brace and measure the shoulder line.
Or at least layout a brace and measure the shoulder line.

It is much easier than trying to figure it mathematically.

But if you want to do it then you need to understand the correct layout and measurements using the 3/8" layout line if you're doing this square rule with a 1/2" housing. Or with the timbers resized down at the joints to the next 1/2" smaller in size.

Here is a picture to show you what I mean:



In the above picture the brace is laid out and cut.
The layout line used for a 1/2" smaller timber is a 3/8" offset line from the arris of the brace.
This layout line makes the joint come together correctly.

So for our example the brace is a standard 4" x6" brace with the 3/8" layout line. So the legs of the triangle are 5 5/8" by 5 5/8" and the hypotenuse is the shoulder line.
My drawing program rounded it off and it shows this line as 8".

Most of the layouts we use when doing braces that are 4x6 is 8" for the shoulder line.

Next you have to understand that the 3/8" layout line is the hypotenuse of the right triangle that makes the brace work in the frame.
So as this 3/8" layout line is laid out at the correct distance from the arris of the brace there has to be some place in your layout to allow for the variables of the widths of your brace stock.

What I mean is if your brace stock is truly 6" then everything is ok.
But if your brace stock is 6 1/16", 6 1/8" or even 6 1/4" then the shoulder line has to be longer to allow for the variances in the widths of your stock.

So what we do is layout your points on your timbers for the three corners for your right triangle that make up your brace layout and then make your housing lines longer then the standard 8".

This is shown here in this photo:



We sometimes add as much as 1/4" to the shoulder layout line on the brace mortise housing to allow for variances in the widths of your braces.

Also you'll see in the above picture we draw the profile of the brace pocket on the surface of the timber so we understand what the brace mortise will look like inside the beam.
As these are just layout lines for visualization the lines are drawn as dotted lines. Solid lines are cut lines, dotted lines are just for layout or again for visualization.

When we have these lines drawn we then project the bottom of the mortise to the mortise opening so that we understand what part is at an angle and what part is straight.

As shown here:



I hope you understand what I mean....

If not keep asking questions.....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

ARKANSAWYER



  Shoulder should be the square root of 2 times the height of the brace.  So a square rule 4x6 brace is square root 2 (5 1/2)= 7.778 or just a hair over 7 3/4 inches.  Does this help any?



      I could not find a square root key   ???
ARKANSAWYER

Jim_Rogers

The legs of this small right triangle are 5 5/8" not 5 1/2" so the formula would be the square root of (5 5/8" x 5 5/8")+(5 5/8" x 5 5/8"). So that's 31.64+31.64=63.28
square root of 63.28 is 7.9548" or if we round the 48 up it would be 7.96"

Then we convert .96" to 16th's by multiplying .96 by 16 and we get 15.36 16th's to you could say that the should line is 7 15/16" to be exact.

But again this is only good if the timbers are truly 6" timbers.
There needs to be gap for the wider timbers usually found in square rule layout.
This is why we add a 1/4" to the housing line.... and mortise....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

moonhill

7-15/16", that's the 8" that my computer program uses too, in the case at hand, the length of the brace pocket.  If it was a rafter or the brace it's self I would go for the 15/16ths.

5.625÷.7071=7.9550 or 8".   Any time I need to find the HYP., brace length, rafter length, or square layout I just divide by .7071, it is a simple number to remember, the numbers are all the left side of the key pad and any cheap calculator will handle the problem. No square root button needed. 

Jim's drawings are the best and I like the follow up with the real thing.

Tim   
This is a test, please stand by...

jander3

Thanks Jim. 

I really appreciate the tips on marking the timber so that you cut the right stuff in the right direction.  It seems like it would be easy to get it mixed up.  Your marking scheme looks fool proof.

I did follow the math, I will not likely use it often, but now it makes sense.  I like the idea of layout of the brace and then measure the shoulder.  I will probably calculate the shoulder, then cut a brace and measure it so I can compare.   Looks like once you establish the type of bracing, then the shoulder measurements become standard for each type.


   

ARKANSAWYER



  Yep the square root of 2 times 5 5/8 = 7.9549 or just a hair over 7 15/16".  Most of the times I do not worry about the hairs.  The commander will take care of them.
ARKANSAWYER

routestep

I think your use of the trig function to find the line is a good procedure to keep. If you ever make a brace that is not a 45 - 45 - 90 degree type you will need the trig.

Also as Jim said, make the mortise a little over size. I had some wide variation in the width of the bracing stock and had to widen some mortises after the fact. I thought that was harder then just cutting them long to begin with.

Brad_bb

Jim, two questions.
1) On your example brace, there is a narrow section of the bearing face adjacent to the shoulder.  Is there some advantage to that small bearing portion?  I believe the braces I laid out and cut did not have that portion of bearing face.

2)The brace pocket, , which I believe was talked about, does not need to have a 45 degree, correct?  It would seem that making the pocket a rectangular mortise, would be easier to do(ie with a mortising machine) without significant drawback.  Would you agree with that?
Regards, Brad
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

moonhill

Truth be told one doesn't need math at all, it is just a different method.  The framing square will do the odd angles just as well with out a calculator.  I suppose the framing square would then be the calculator.    Just lay it out full scale and gain the number that way. 

I see no problem with Jim's brace example, that is what you end up with when the square rules.

I cut all my brace pockets square the only bevel is in the housed section. 

Tim
This is a test, please stand by...

Mad Professor

Here is a way to layout a brace by scribe.  If you want 6" brace the circle is set to 6" and everything else is laid out from that circle, except the width of the brace, which is laid out using another smaller circle (i.e. 4"). This smaller circle is also used to layout the width of the mortise.

The mortise length  is the length of the side of the big 1:1: root 2 triangle that is formed on the layout below, the mortise depth is 1/2 of that, as is the length of the beveled (45 degree angled) portion of the mortise.  These lengths can be transfered to the post using the divider

All this can be laid out with a divider, chalk line and plumb bob.  No need to calculate anything, it's all done off the scribed circle and can be done with irregular pieces of lumber/brace stock (see diagram).

Anyone else trained in scribe?  How do you layout braces?  I spent the better part of a day on a drafting table figuring the method I came up with.

swampfox

hi, i like tim's approach for using just one number when dealing with 45 degrees.  I use 1.414 (twice tim's number)

if anyone is interested here is how the number is found.

a^2 + b^2 = c^2

if you are dealing with 45 degrees you are looking at a square cut in half diagonally which means a = b

so the algebra would look like this

2 (a^2) =c^2

divide by 2 on each side

a^2 = c^2/2

get rid of exponents by applying square root to all terms

a = c / square root of 2

a = c/1.414

or if you are looking for the hyp

c = a(1.414)

Jim_Rogers

Quote from: Brad_bb on January 01, 2009, 11:53:43 PM
Jim, two questions.
1) On your example brace, there is a narrow section of the bearing face adjacent to the shoulder.  Is there some advantage to that small bearing portion?  I believe the braces I laid out and cut did not have that portion of bearing face.

2)The brace pocket, , which I believe was talked about, does not need to have a 45 degree, correct?  It would seem that making the pocket a rectangular mortise, would be easier to do(ie with a mortising machine) without significant drawback.  Would you agree with that?
Regards, Brad

1)That bearing end is to account for the housing. Although it does add to the surface bearing area..... The braces you're laying out don't have that if you're doing "mill rule" as there is no housing.

2)There is no reason that a mortise has to have a 45° angle cut, it can be done as a rectangular mortise, if one is using a chain mortiser.
If you're doing it by hand with chisels and boring machines it isn't necessary to remove all that extra wood, as it is extra work.
As Tim mentioned just the housing would be enough for it to look correct....

Jim Rogers

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Brad_bb

Thanks Jim.  I was thinking about it further and just before reading your answer, realized that it must be for a housed pocket.  You are correct in that I was doing mill rule on the braces I did.
Brad
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

jander3

Jim,

I appreciate the info.  I cut a couple of braces and pockets for a table I am working on over the weekend.  Calculations matched the actual pieces.  Once I cut a brace, it was clear that cutting the brace and measuring the shoulder is no problem.

The braces fit up fine. 

Alexis

Quote2)There is no reason that a mortise has to have a 45° angle cut, it can be done as a rectangular mortise, if one is using a chain mortiser.
If you're doing it by hand with chisels and boring machines it isn't necessary to remove all that extra wood, as it is extra work.
As Tim mentioned just the housing would be enough for it to look correct....

Is that the way most people do it? which type is the most used by timber framers? Which was historically the most used? Is there an advantage to the 45 degree mortise versus the straight one?

thanks

VT

Good reading ,
Gent's , what program are you using on the computer to do these calculations.
Meaning EG: Timber frame CAD-101 or something  .
Plan layouts or designing ??

VT

Jim_Rogers

Alexis:
It depends on what type of tools you're using.
If you're doing it with a chain mortiser then just hog it out square from one end to the other, and as Tim has mentioned just bevel the housing so that it looks right.

If you're doing it with hand tools, none power tools, then why do all that extra work?
Just do the 45° angle and be done with it......

It's really up to you, it will never show, unless the building is taken down later on.....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Jim_Rogers

VT:
I'm just using a regular hand or pocket calculator.....

Nothing fancy here.....

As we have laid out and cut many braces over the years and they usually are 6" wide stock, I always use 8" or 8 1/8" for my housing line.....

Just one of the rules you keep in your head and never really think about it......

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

swampfox

I wouldn't think there is any advantage to leaving the 45 rather than rectangular mortise.  That part of the mortise sees no effects from the brace.  The bearing surfaces of the mortise (furthest away on each end of the brace) are the important part along with the mortise width.  Its a lot of work chiseling that 45 when it will never be seen.  The rectangular mortise gives you a little more freedom when fitting up the brace as well.

Oops, forgot to look at page 2.  Jim already answered Alexis's question.

About the calculator.  I like the Construction master.

VT

Construction Master II here, But i was asking about computer programs, Windows stuff, I know that calculators are a sort of puter, but more along the lines of designing programs, to put brain fasts on a common area, Here my wife and I, have bits of paper with fridge pictures on them.

Thanks  VT

I do enjoy your post Jim_Rogers, Very good reading. I'm just learning how to get around on this forum, and trying to find the Stack and pack posting of timbers (storage after the mill till ready to work with, Can't remember what you called it )..

Anywhos , Great reading Jim.

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