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A homemade straightedge

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Project by wch posted 05-16-2010 10:50 AM 2952 views 10 times favorited 20 comments Add to Favorites Watch

This is a straightedge I made from a 1×2” piece of clear pine. I based it on a description of a straightedge from Toshio Odate’s book, Japanese Woodworking Tools.

I’ve made a few other tools. This one is the simplest and yet also in some ways impresses me the most, because it taught me that it’s possible to make something that’s straighter than any object you have to begin with. You do need a hand plane, but it doesn’t have to be a great one; the resulting straightedge can be straighter than the plane’s sole.

The straightedge actually consists of two pieces of wood. They’re placed side-by-side and planed together, and then “unfolded” and held up to a light to check for gaps. If there are gaps, you shave away the high spots in the wood until the gaps are gone.

One property that might actually be an advantage over a store-bought straightedge is that this straightedge serves as a reference for itself. To illustrate, I have a cheap combination square, and I’ve never been sure if the blade was straight, but I had no good reference object to compare it to. If I compared it to another straight object and the two were slightly off, I couldn’t be sure which one wasn’t straight, or if both of them weren’t straight. But with this two-piece straightedge, I don’t need a separate, better reference object; I can just compare the two pieces to each other, and if they disagree at all, then I just plane them flat.

For more information on how it’s made, click here.

Update: From reading some postings on another forum, I realized that if there are any visible gaps, you can use a sheet of paper as a feeler gauge to test the amount of deviation. A sheet of regular 20 lb. paper is 0.004” thick, so if it won’t fit between the two pieces of wood, then the straightedge is good to within 0.002”. Playing with my micrometer, the smallest gap I can see is just under 0.001”, so I think that if there are no visible gaps, the straightedge is probably good to within .0005”. Bear in mind that you have to hold them together lightly—if you press the two pieces together with any appreciable force, you can easily bend them this much and close a gap.





20 comments so far

View naomi weiss's profile

naomi weiss

199 posts in 2148 days


#1 posted 05-16-2010 11:32 AM

cool! i have been too scared to try it but you’ve inspired me!

-- 'Humility is a duty in great ones, as well as in idiots'--Jeremy Taylor

View docholladay's profile

docholladay

1287 posts in 1813 days


#2 posted 05-16-2010 01:40 PM

If you applied the same concept to make both edges straight, you would end up with an instant pair of winding sticks. Interesting concept on referencing one piece to the other. So simple and clever.

-- Hey, woodworking ain't brain surgery. Just do something and keep trying till you get it. Doc

View BlacksheepWW's profile

BlacksheepWW

9 posts in 1718 days


#3 posted 05-16-2010 02:43 PM

That is a great idea! it makes perfect sense.
Good shop project.

View Cory's profile

Cory

724 posts in 2173 days


#4 posted 05-16-2010 02:43 PM

What about wood movement? How often do you need to check them?

-- The secret to getting ahead is getting started.

View daltxguy's profile

daltxguy

1373 posts in 2668 days


#5 posted 05-16-2010 02:43 PM

Cool! The tool genesis problem has always been fascinating to me. ( How do you create better and more accurate tools from lesser accurate ones) This technique is clearly an important link. Thanks for posting.

-- If you can't joint it, bead it!

View a1Jim's profile

a1Jim

112939 posts in 2331 days


#6 posted 05-16-2010 05:41 PM

I’m concerned about wood movement also

-- http://artisticwoodstudio.com Custom furniture

View stefang's profile

stefang

13633 posts in 2088 days


#7 posted 05-16-2010 06:48 PM

Yes, with any wood, but especially with pine.

-- Mike, an American living in Norway.

View Dusty56's profile

Dusty56

11688 posts in 2442 days


#8 posted 05-16-2010 07:07 PM

Where do you live ?
If I used Pine around here , I would be chasing the proverbial straightedge until the Pine was gone …LOL
Sounds like a nice idea though : ) Thanks for sharing it with us !

-- I'm absolutely positive that I couldn't be more uncertain!

View wch's profile

wch

45 posts in 1712 days


#9 posted 05-16-2010 07:42 PM

@docholladay: It occurred to me too that you could do something like this for making winding sticks. I think you’d have to plane them together, then flip one of them so that the other side is up and the opposite ends are lined up, and plane them together again. If you did something like this, I think both sticks should be a consistent thickness. However, if you have a tendency to plane with lateral unevenness (like I do), taking off more on the left or right side, you’d probably have to swap the positions of the two pieces to make sure you don’t end up with a trapezoidal cross-section. Although now that I think of it, for winding sticks it might not matter, as long as you always sight with the acute edges facing you.

When it comes to wood movement, I’m not sure exactly how much of a issue it’ll be. In the sketches in the book, there are a couple of details that seem relevant, although Odate doesn’t talk about them. The straight edge of the wood is tangential to the growth rings. That means that the wood movement that could distort the straightness is in the radial direction (although I suppose twisting could also be an issue). Since wood movement in the radial direction is typically about half that of the tangential direction, this would help minimize effect of wood movement. Also, when viewed end-on, the rings on the two pieces of wood are mirror images. My guess is that this is done so that any wood movement will happen the same way in both pieces, making it easy to detect.

My sense from reading this book is that part of the philosophy behind traditional Japanese tools is that they’re ephemeral and not permanent; everything eventually gets used up and requires some maintenance (and skill to perform this maintenance) along the way. So this straightedge will probably need testing and truing periodically, which is why the two pieces are stored together. It wouldn’t be a drop-in replacement for a zero-maintenance metal straightedge, but it sure beats what I had before, which was a cheap combination square that I never really trusted, and rightfully so, I now know. :) If I eventually find that truing up the straightedge gets annoying, I may break down and buy a metal one, but I’ll still be glad I learned how to make this one.

Regarding the use of pine, it actually has one of the lowest expansion factors of all commonly used woods. Hardwoods like poplar, oak, and maple move more than pine does.

View wch's profile

wch

45 posts in 1712 days


#10 posted 05-16-2010 08:02 PM

@Dusty56: I’m in Illinois – I guess I’ll find out how much of a problem it’ll be!

I had another thought, which is that I could have made this out of some high-quality baltic birch plywood. The hobby and art stores around here sell it in relatively small pieces. That stuff has a lot of layers, and while it might move a tiny bit, I’m sure it’s much better than a solid piece of wood.

View PurpLev's profile

PurpLev

8476 posts in 2402 days


#11 posted 05-16-2010 08:11 PM

very cool, simplicity at it’s best

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View Karson's profile

Karson

34916 posts in 3154 days


#12 posted 05-16-2010 08:44 PM

A nice tool. Great job.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Southern Delaware karson_morrison@bigfoot.com †

View stefang's profile

stefang

13633 posts in 2088 days


#13 posted 05-16-2010 09:26 PM

Hi WCH. I admire that you are trying this out. It was interesting to learn about the expansion of pine vs hardwoods, but I was thinking more about stresses created in a drying situation. No matter, what you’ve done sounds logical and well thought through. You can easily test your metal squares by setting it against a straight edge (factory edge on Baltic Birch for exampel) by drawing a line and then flipping it over on the same edge and comparing it to the first line, then sandpaper or file off the amount need to make it true.

-- Mike, an American living in Norway.

View wch's profile

wch

45 posts in 1712 days


#14 posted 05-17-2010 03:17 AM

@stefang: the experiences that I’ve had with pine warping has been with construction grade lumber, which usually comes from the store with a much higher moisture content than the “quality” woods that they sell (hardwoods, clear pine, etc.). I made this straightedge from a piece of clear pine from Home Depot that has been sitting around for a few years, so the worst should be long past. But I agree that it’s a good thing to be aware of: if you were to make a straightedge today from a 2×4 that you just bought, it probably won’t be straight tomorrow!

I’ve had limited success with the “draw a line and flip the ruler” technique to get a straight edge. If the goal is to have a straight edge for layout and marking, that will do the trick. But in my experience, when it comes to checking a plane sole or sharpening stone for flatness, you need something much more precise than what a 0.5mm (or even 0.3mm) pencil line will let you see.

View TopamaxSurvivor's profile

TopamaxSurvivor

15090 posts in 2430 days


#15 posted 05-17-2010 04:43 AM

Looking for light pasing is teh truest test :-) Interesting project and explaination

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

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