# Is there a formula for finger joints and wood length



## vdotmatrix (Jul 28, 2014)

SO, lets say you want to make a box that is LxW.

How do you know how wide to make the finger joints so there are a perfect number of finger joints for the the box?


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## woodnthings (Jan 24, 2009)

*the length and width won't matter ...*

It's the height that will determine your spacing. The smaller the joints the more difficult it will be to hide a "split" joint. You can always make your box, allowing a double joint space for the top, if there is one, and saw it to width after it's completed. Larger fingers, IE wider ones, allow for more accuracy, but you will still need to plan out the width based on the spacing.

https://images.search.yahoo.com/sea...&fr=ytff1-gl-gen1&va=boxes+with+finger+joints

I don't know if there is any rule of thumb, it just depends on the width of the fingers appropriate for your design and it's dimensions. A blanket chest will have wider fingers than a cigar size box. Look at other designs to see what works best.
http://www.woodmagazine.com/woodworking-tips/techniques/joinery/making-box-joints/


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## epicfail48 (Mar 27, 2014)

Depends how wide the stock is, length has no real factor. It also depends how big the fingers are. 

For example, say you're making a box and want to do 1/4 inch fingers. This limits the width of the side to a multiplw of 1/4, 2 1/4 or 3 inches wide for example. With 1/4 fingers, you couldn't make the sides 4 1/8 inches wide without some spacing issues. 

If you're having issues determining the width of the fingers themselves, I've never really heard any hard and fast rules. I think its more dependent on what you have to cut width. If you have a 1/4 router bit, enjoy the 1/4 fingers. Beyond that, I think its more asthetics. My personal rule of thumb is at least 2 pairs of fingers, or 1 corner of the joint has 2 fingers that mesh into 2 fingers of the other board. Less than that and I look at other methods of joining.


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## UnisawGuy (Jul 20, 2014)

I cut my box side a little over width, then trim to fit after cutting the fingers.


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## vdotmatrix (Jul 28, 2014)

I needed a right angle brace for gluing up perpendicular pieces. the piece of stock was 3/4" thick and 6.5".
I just picked a 1/2" wide dado cut out of the blue and when I made the cuts on my IBOX, it was the perfect combination...So I was wondering if there was a system for arriving at the perfect combo for making these.


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## FrankC (Aug 24, 2012)

vdotmatrix said:


> I needed a right angle brace for gluing up perpendicular pieces. the piece of stock was 3/4" thick and 6.5".
> I just picked a 1/2" wide dado cut out of the blue and when I made the cuts on my IBOX, it was the perfect combination...So I was wondering if there was a system for arriving at the perfect combo for making these.


1/2" divides evenly into 6 1/2" so it worked out, otherwise it is a matter of sizing the fingers so they divide evenly or having different sized fingers at top, bottom or both.


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## PhilBa (Jun 30, 2014)

I'm no expert at finger joints but I think the finger size is only part of the equation. I want to have my fingers be symmetrical so that the top and bottom fingers are the same size. Also, I'd want the top and bottom to both be pins or tails (to use dovetail terms). The easy way is to make the width be an odd multiple of a standard finger size. Other widths take a little more effort. If you look at the incremental positioning machines, they go to great lengths to make fingers and dovetails symmetrical, even with partial fingers. Or, if using a table saw with a dado blade, you might pick a finger size that allows you to gain symmetry. Up to you how far you want to go.


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## vdotmatrix (Jul 28, 2014)

woodnthings said:


> It's the height that will determine your spacing. The smaller the joints the more difficult it will be to hide a "split" joint. You can always make your box, allowing a double joint space for the top, if there is one, and saw it to width after it's completed. Larger fingers, IE wider ones, allow for more accuracy, but you will still need to plan out the width based on the spacing.
> 
> https://images.search.yahoo.com/sea...&fr=ytff1-gl-gen1&va=boxes+with+finger+joints
> 
> ...


Here I am again!

The piece I am joining is 9.25" long.

I can make x.33" fingers or x.88" fingers.

No getting around some fractional leftover.

Question: do I want to end up with a .33 or a .88 fraction of a finger?

It doesn't have to be pretty but it does have to be technically sound.

Thanks in advance.


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## nmacdonald (Jan 13, 2012)

Great video right here from just the other day

http://youtu.be/NutwD7B6tmE


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## vdotmatrix (Jul 28, 2014)

nmacdonald said:


> Great video right here from just the other day
> 
> http://youtu.be/NutwD7B6tmE


Thanks for sharing that! This guys has to be a math wiz and I am sure he is.....4oooth of an inch...holy ****. the last 8 seconds he talks about why he uses 3/8th of an inch fingers that works out perfectly mathematically, and the little hole is covered in the end, but that doesn't apply for me.....and my last question...

HOWEVAH!!!!!!! I am cutting the same grooves he did for some 5/32" coroplast that is going into this project.

I have walked away with a great explaination of how blades cut narrower that stated widths..Nice Video-I subscribed....

I use the INCRA IBOX which allows me to dial in thousandths of an inch adjustments to the finger joints-I am having problems with the bigger math question......

Thanks again!

OH BTW: I love this guy's workshop. dust collection, fancy pants table saw everything neat as a pin....


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## vdotmatrix (Jul 28, 2014)

I will have to just break down and make a video to answer this question tomorrow.


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## BZawat (Sep 21, 2012)

vdotmatrix said:


> Here I am again!
> 
> 
> 
> ...



With a 9.25" board, you could use either 1/4" or 1/2" fingers - either would work out. 1/4 = 38 fingers, 1/2 = 19 fingers. I'd probably use 1/2", BC 19 is an odd number you will end with the same cut that you start with. IOW, if you start with a "finger" you will end with one - or if you start with a notch you will end with the same. Provides some symmetry.


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## vdotmatrix (Jul 28, 2014)

BZawat said:


> With a 9.25" board, you could use either 1/4" or 1/2" fingers - either would work out. 1/4 = 38 fingers, 1/2 = 19 fingers.


 I should preface my questions because I get very useful responses and people take the time to help us. This is a bee hive component. going outside. I wanted to keep the numbers of fingers low in order to shoot some brads in there as well. I was looking at , i fergit either 12.33 fingers or 13.88 fingers. I did the math I could cut 38 .25" fingers or even 19 fingers....i just wanted to know what happens if one cuts fingers that arent exact multiples and you are left with a fraction of a finger. i can't visualize this in my head ... thank you!!!!!


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## vdotmatrix (Jul 28, 2014)

Well I just went in and cut some joints with the following parameters.
9.25" wide
23/32" (.703)
to cut 13.2 fingers

To answer my own question the ".2" " appears as a fraction of a finger ( of course ). I know you can go in and put that extra in the middle finger some how or saw off the end if it had to be a certain dimension....

The VIDEO  was pretty helpful in that in it he says most stated blade cuts are around 0.10 " thinner than stated or something like that. This guy is precise and I like that.
to get rid of that ~.2", i think it was 45/64th" I will add a 0.012 shim and hopefully that will approach acceptable-ness; always a crap shoot sometimes. 

If I had gone with the .75 width dado i would have ended up with 13.88 fingers which could have been nicer looking but i don't care about looks for this application just trying to wrap my head around what is going on here. 

The point I think is, you want to get the biggest fraction of a finger to make it less noticeable-the fingers will still be cut within that space-just have to either get the PERFECT dimension or live with the finger.


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## vdotmatrix (Jul 28, 2014)

Well, the micrometer and TI-30 is my friend. Having divided the ~.2 left over by 13 i needed an extra 0.014 to each finger but added 0.012' shims and the results on this test piece make me happy enough. I went ahead and cut the work and have moved on to the next phase of this project....:thumbsup:


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## Mort (Jan 4, 2014)

My brain hurts.


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## vdotmatrix (Jul 28, 2014)

m i t a k e


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