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finding the center of board thickness

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Forum topic by treesner posted 07-11-2016 06:03 AM 830 views 0 times favorited 22 replies Add to Favorites Watch
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treesner

167 posts in 431 days


07-11-2016 06:03 AM

Curious how you guys find the center of a board thickness. Like a piece of plywood?

When you measure its generally not an easy number to half. I usually just set my adjustable square and go back and forth until the line i draw is even on both sides. kind of a slow process though


22 replies so far

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bandit571

14632 posts in 2150 days


#1 posted 07-11-2016 07:16 AM

Well, use two dowels, and a stick. Drill and install the two dowels into a length of wood scrap….say 1/2” thick 1” wide, maybe 6” long. First, find the exact center of the stick, drill a small hole to allow the point of a pencil through.

Next drill the two holes for the dowels….equal distant from the center hole. Dowels can go all the way through…or just stay on one side.

In use: Lay this jig on the edge of a board, rotate the jig until both dowels touch the board. pencil marks the center of the board’s thickness. You can leave the pencil in, and slide the jig along, as long as both dowels still touch the faces of the board.

One could even make one using metal parts.

-- A Planer? I'M the planer, this is what I use

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bearkatwood

1214 posts in 479 days


#2 posted 07-11-2016 11:29 AM

When I am making saw handles I need to find the center of a very short flat and the great jig mentioned above won’t fit so I use calipers to measure the overall thickness and try to cut that in half as best I can and then scribe a thin line flip the board and scribe a line again and the difference is center. I use this quite a bit and the poor man’s version is to hold the pencil in your hand and use your index finger as a stop and guess at half the board
s width. Scribe a line, flip the board and do it again and you will be surprised how accurate you can get for center. One other way is to make two parallel marks running the width of the board and then connect the corners. Where they meet in the middle should be center. I don’t have the jig mentioned, they work if you can get the holes drilled dead right, but even if it is off it can be used with the flip trick too. Hope that helps.

-- Brian Noel

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JBrow

819 posts in 387 days


#3 posted 07-11-2016 11:57 AM

treesner,

I do not have a center finding gauge like described by bandit571, which, if accurately made, works well.

My method is similar to yours. I set the combination square to what I think is the approximate center. I mark the edge measuring from one face and then the other. Then I re-set the combination square to the midpoint of these two marks, make a mark, and double check by re-measuring from both faces.

Another method I rarely use, but is fast and as accurate as your ability to mark accurately, is to lay a ruler diagonally across the edge, aligning nice even numbers with each surface. The center measurement is half this diagonal distance. For example, a ruler is laid across the edge with the ruler’s 2” mark aligned with one surface and the ruler’s 4” mark aligned with the other surface. Wherever the ruler’s 3” tic mark lands is the center. Measuring from a surface to this point yields the distance from the surface to the center of the board.

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Tennessee

2410 posts in 1981 days


#4 posted 07-11-2016 12:10 PM

I measure the thickness with a MM rule. Mine is a Starrett.
I divide in half. Mark with a fine sharpened #3 pencil.
For me, if I need tighter tolerances than .5MM, then I go to calipers like Brian but that is not the norm.

-- Paul, Tennessee, http://www.tsunamiguitars.com

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DrDirt

4169 posts in 3209 days


#5 posted 07-11-2016 12:31 PM

you also needent struggle with finding ‘half’. A trick that the lap siding folks (many others I’m sure) use is to tip the measureing device to a slight diagonal..

so as an example if you are finding the center of some 8/4 stock that is at 1 7/8 planed.

just lay your ruler at a slight diagonal where the tick mark on your ruler for the 1” is on on side and the 3” mark is at the other side.
The center is the “2 inch” mark. Fundamentally the same as making a stick with dowels as a center finder.

House Siding folks use their story stick this way. If they want 6 inch reveals, they have a stick with 6 inch spaced marks on it.
However:
But say they want to “Squeeze” the spacing so that they don’t have some pencil thin strip left at the top of the window…

They lay their stick “off vertical” so that the overlap is even at the top and bottom, then snap chalk lines for where each e.g. 5-13/16 piece spaced section lap siding goes, with no re-measuring.

The ‘center’ of the diagonal – - is the same as the center of a line straight across

-- 'Political correctness is fascism pretending to be manners' ~George Carlin

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ScottM

347 posts in 1614 days


#6 posted 07-11-2016 12:35 PM

I’ve asked people that same question and the answer is always to make a gauge. BUT, you can’t accurately make a gauge without being able to find the exact center; width AND length. So I would say, to be truly accurate, you’ll have to buy a gauge. Rockler has them for about $10.

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bondogaposis

4037 posts in 1818 days


#7 posted 07-11-2016 12:46 PM

You can use the diagonal ruler trick. Say you have some 1/2’’ plywood that everyone knows is not 1/2”. Take your rule and hold it so that the 1 inch mark lines up with one face then rotate the rule so that the 3 inch mark lines up with the other face. The 2 inch mark will be at the center of the plywood.

-- Bondo Gaposis

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a1Jim

115206 posts in 3044 days


#8 posted 07-11-2016 01:43 PM

Another vote for using a combination square simple and quick.

-- http://artisticwoodstudio.com Custom furniture

View MadMark's profile

MadMark

978 posts in 920 days


#9 posted 07-11-2016 08:01 PM

Why are you measuring plywood thickness? The thickness is specified! Divide by two and mark that measure.

Now the question comes as to what are you doing that requires the center to be that precise in the edge of a sheet of ply?

M

-- Madmark - Madmark2150@yahoo.com Wiretreefarm.com

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Nezzerscape

25 posts in 330 days


#10 posted 07-11-2016 08:20 PM

I use a compass and a square. Draw a line perpendicular to the edge. Then draw an arch with the pin right at the edge where the line is and another from the opposite side. Where they cross is the center.

Similar method for creating the jig. Take a random stock and draw a line down it. Pick the center and place a mark there. Set the compass point on that mark and draw across the center line at both ends of the stock. There you have three points in a line, with the two end points equal distance from center.

View dhazelton's profile

dhazelton

2326 posts in 1764 days


#11 posted 07-11-2016 11:23 PM

Double the denominator – that’s half the thickness.

But when you don’t know what it is you do the diagonal rule trick – it’s carpentry, not rocket science.

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Plain

157 posts in 166 days


#12 posted 07-11-2016 11:40 PM

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Plain

157 posts in 166 days


#13 posted 07-11-2016 11:42 PM

Say thank you to stubborn heads who rejected the metric system to just prove the point.

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Combo Prof

2385 posts in 744 days


#14 posted 07-11-2016 11:45 PM



I ve asked people that same question and the answer is always to make a gauge. BUT, you can t accurately make a gauge without being able to find the exact center; width AND length. So I would say, to be truly accurate, you ll have to buy a gauge. Rockler has them for about $10.

- ScottM

Yes you can. You do it this way:


I use a compass and a square. Draw a line perpendicular to the edge. Then draw an arch with the pin right at the edge where the line is and another from the opposite side. Where they cross is the center.

Similar method for creating the jig. Take a random stock and draw a line down it. Pick the center and place a mark there. Set the compass point on that mark and draw across the center line at both ends of the stock. There you have three points in a line, with the two end points equal distance from center.

- Nezzerscape

Basic geometry wins.

-- Don K, (Houghton, Michigan)

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AlaskaGuy

2406 posts in 1776 days


#15 posted 07-12-2016 12:09 AM



Double the denominator – that s half the thickness.

But when you don t know what it is you do the diagonal rule trick – it s carpentry, not rocket science.

- dhazelton

What the denominator of 19.7mm

-- Alaskan's for Global warming!

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