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Forum topic by MarkTheFiddler posted 11-30-2014 01:08 AM 1091 views 0 times favorited 12 replies Add to Favorites Watch
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MarkTheFiddler

2053 posts in 1653 days


11-30-2014 01:08 AM

Topic tags/keywords: tip trick

Howdy,

You might be able to take advantage of this math equation. Its a little basic algebra that can really help.

How do you turn this:

Into this?:

I’m not talking about the rabbit cuts. I’m talking about how to calculate exactly how much to rip so when you glue up the two parts you get the piece that is as thick as it is wide.

Take a few measurements first.
Width of board.
Thickness of board.
Thickness of blade or kerf.

What we are going to do is cut the width narrower, then add the offcut to the thickness so width = thickness. We are just talking about 2 sides here. ;)

Here we go.

Width minus kerf minus offcut equals thickness plus offcut.
Or
width – kerf – offcut = thickness + offcut.
Or
W – K – Oc = T + Oc

So we have.
1\2(W – K – T) = Oc

let’s try it with something that is very conveniently simple.
Width 3 and 1\8.
Thickness = 1
Kerf = 1/8

We now have
1\2(3 and 1\8 – 1\8 – 1) =Oc
We can work it through a little.
1\2(2) = Oc
1=Oc

Let’s check it out.

We had 3 and 1\8. We cut off 1 inch and lost 1\8 kerf so its now a width of 2.
We glued that 1 inch offcut to the thickness and got a thickness of 2.

That one was easy because we can see it in our heads. When you are using what you have available. The math comes in handy. No need to guess and your calculator can make quick work of the fractions.

Hope it helps.

-- Thanks for all the lessons!


12 replies so far

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MarkTheFiddler

2053 posts in 1653 days


#1 posted 11-30-2014 01:39 AM

I found a picture that may help explain a little better.

The offcut is sitting right next to the width. I simply glued it to the thickness. I really thought I took more pictures. Ah well.

-- Thanks for all the lessons!

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Rick M

7922 posts in 1844 days


#2 posted 11-30-2014 06:35 AM

When I tested this equation with a board that is 6” wide by 1.5” thick, I get

1\2(W – K – T) = Oc
Width = 6
Thickness = 1.5
Kerf = .125

.5(6-.125-1.5) = Oc
3-.0625-.75 = Oc
2.1875 = Oc (or 2 3/16”)

A square cross section would be 3×3.

-- http://thewoodknack.blogspot.com/

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MarkTheFiddler

2053 posts in 1653 days


#3 posted 11-30-2014 03:41 PM

Howdy Rick,

Thanks for checking it out! Your Math is correct. If you took your offcut and glued it to the thickness, you will have a corner that is as thick as it is wide.

You revealed a weakness in my explanation and I’m happy to correct it.
My original formula is:
W – K – Oc = T + Oc

Let’s solve each side individually with your measurements and calculation.
6 – .125 – 2.1875 = 3.6875
1.5 + 2.1875 = 3.6875

Your board and kerf will produce a corner that is 3.6875 by 3.6875.

The 3×3 answer you give reveals thinking I call pop math. I like pop math.
It allows me to go to the grocery store and approximate how much I will spend.
I can approximate travel times with unknown variables like road conditions, traffic delays and health breaks.
It allows me to buy lumber without knowing exactly how much I will need.

It’s good stuff and it got you close to the correct finished dimensions.

Again, thank you very much for trying it out.

-- Thanks for all the lessons!

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MarkTheFiddler

2053 posts in 1653 days


#4 posted 11-30-2014 03:54 PM

By the way, this formula only works for butt joints.
An easy miter will give 3×3.
A miter cut through without flipping the offcut is a little smaller.
A miter cut using the flipped offcut is wider. Another formula to come. ;)

-- Thanks for all the lessons!

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Rick M

7922 posts in 1844 days


#5 posted 11-30-2014 10:35 PM

Ah, you are gluing boards at right angles to make a square profile, that was not clear to me. Well you have condescension down pat but don’t quit your day job to become a technical writer. This might come in handy if you are trying to build from what you have and making it up as you go along.

-- http://thewoodknack.blogspot.com/

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MarkTheFiddler

2053 posts in 1653 days


#6 posted 12-01-2014 01:11 AM

Rick,

Sometimes I hate the written word. It doesn’t convey sincerity. On top of that, I sound rather condescending at times and I don’t mean to at all. I think its when I go off on an explanation. I have a tendency to go off on elementary explanations that every one already understands.

I’m sorry I conveyed condescension. I did not mean to make any part of my reply negative at all. Please forgive me. I’ll keep working on my communication skills.

As far as making it up as I go along, you got it on the first try. I had some really thick hunks of oak counter top. At first, I was just going to use the entire width you saw It would have eaten up too much real estate on the front of the desk I was making. Then I thought I might use the wide legs on the sides. That wasn’t working for me. I went with the offcut solution in the end. As you said, I used what I had.

Last thing and this is funny. I have written so many technical documents it will make your head spin. I hate doing it. Fortunately for wood-workers, they are all software and computer industry based. I hand my documents to another person who puts them in English and will come to me when I leave important information out.

-- Thanks for all the lessons!

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ElChe

630 posts in 801 days


#7 posted 12-01-2014 01:45 AM

Darn. I thought I was going to see a math fight. ;
What did one math book say to the other? Leave my alone, I’ve got my own problems.
Math puns are a sine of mental instability.

-- Tom - Measure twice cut once. Then measure again. Curse. Fudge.

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MarkTheFiddler

2053 posts in 1653 days


#8 posted 12-01-2014 01:50 AM

Oh Tom. You just had to go there. Now I can’t resist.

There are 3 kinds of people in this world.
Those who are good at math and those who are not.

-- Thanks for all the lessons!

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Rick M

7922 posts in 1844 days


#9 posted 12-01-2014 06:06 AM

I was snappish earlier. I tend to be grumpy until my belly is full. Last night it didn’t occur to me that you were gluing the boards at right angles. The 3×3 is not “pop math” but the square cross section from a leg glued from 6×1.5 stock (less the kerf) but your example would not have a square cross section, it would have an L shaped cross section. I misunderstood your intention.

-- http://thewoodknack.blogspot.com/

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MarkTheFiddler

2053 posts in 1653 days


#10 posted 12-01-2014 06:35 AM

Rick, it’s all good sir. I think I should have displayed the L in one of the pictures. Instead I just displayed what the corner looked like from the outside.

About being grumpy when your hungry, my wife tells me I’m really bad about that. ;)

-- Thanks for all the lessons!

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GrandpaLen

1643 posts in 1737 days


#11 posted 12-01-2014 03:31 PM

At first I thought I understood…
...but now I think I’m confused.

Is it better to eat before you do the math…
...or before you do the glue ups?

...or before you write the formula?

...or before you read the post? ...just askin’.

Work Safely and have Fun. – Len ;-)

-- Mother Nature should be proud of what you've done with her tree. - Len ...just north of a stone's throw from the oHIo, river that is, in So. Indiana.

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MarkTheFiddler

2053 posts in 1653 days


#12 posted 12-01-2014 07:12 PM

Len,

I believe I can answer that question if I replace the ‘or’ with ‘and’.
Absolutely;)

-- Thanks for all the lessons!

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