How do I know my straight edge is...straight?

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Forum topic by deeznutz posted 04-06-2010 11:39 PM 20107 views 1 time favorited 18 replies Add to Favorites Watch
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33 posts in 3831 days

04-06-2010 11:39 PM

I just received a 24” aluminum straight edge in the mail from Garrett Wade for just $20 shipped (link below). The reason for getting it was I’m starting to get into hand planes and needed a good, accurate reference edge to see if the soles are flat, the granite I’m using to fettle them is flat, etc.

Question is…how do I know the straight edge is flat? If it were a Veritas, I’d trust it. But being a cheaper version, and the fact that all the plane soles were not particular close to flat according to this straight edge…how do I know for sure? I’d like to be more confident it’s flat before I spend a week fettling my newly discovered non-flat Stanley #8!

Does anyone have any trick/tips so I can settle my mind and have a real reference edge finally? The best I can think of is picking up a new piece of float glass and checking the straight edge with that.

Any ideas would be greatly appreciated!

18 replies so far

View NBeener's profile


4816 posts in 3985 days

#1 posted 04-06-2010 11:55 PM

Can you

- lay it on a sheet of plywood

- draw a straight line with it

- flip it 180 degrees in the other direction

- draw a line extremely close to the other line

They should be exactly parallel, over their entire run—never diverging, converging, or crossing.

That’ll give you a good idea—at least—of whether anything is grossly out of whack.

-- -- Neil

View MedicKen's profile


1615 posts in 4273 days

#2 posted 04-06-2010 11:56 PM

I have heard that Garrett Wade stuff is nice. I would trust it. Most of the good manufacturers will have tools, especially precision tools from reputable suppliers. The lower end suppliers, think Harbor Freight, lowes, Home Depot will go with the lowest cost and not necessarily the best quality. Companies like Garrett Wade, Lee Valley, Lie-Nielsen will have their tools ground and precision checked to very tight tolerances. Something in the range of .0003-.0005

-- My job is to give my kids things to discuss with their [email protected]

View ocwoodworker's profile


209 posts in 3815 days

#3 posted 04-07-2010 12:01 AM

I would just add to NBeener that in order to find that it is ‘in whack’ place it on a known flat surface and shine a light behind it.

-- I'd like to believe Murphy's Law haunts my woodshop, because if it's Karma it would mean I had something to do with it. - K.R.

View Todd Clare's profile

Todd Clare

67 posts in 3796 days

#4 posted 04-07-2010 12:03 AM

You could rig up two jigs: one in the miter track of a table saw (etc) and the other to the fence. On one, mount the straightedge, on the other mount a dial indicator. Move one past the other and watch the needle.

This assumes, of course, your slot and fence are parallel and that there’s no play in any of the jigs.

-- Todd (Denver, CO -- Highlands)

View grumpycarp's profile


257 posts in 4557 days

#5 posted 04-07-2010 12:18 AM

. . . and you can almost count on it being straighter than the sole of a vintage #8. But then there are places where it matters and where it’s really not that important. Way more important on the elements of missile guidance hardware than say, an end table or the sole of a #4 smoother. Personally, I’d trust the straight edge for now and worry about more important things, like understanding wood movement and what affect that has on your joinery selection. Or just enjoying your time in the shop making something.


P.S. You could always go get one of those 5 dollar laser gizmos and experiment with shining that through a (increasingly narrower) slit in a piece of cardboard. Light doesn’t bend, at least not in a scale relevant to flattening the sole of a plane. Try raking the light above the edge of the plane’s sole, with the plane on its’ side, should give you an idea of the sole of the plane is flat or not. You could also use this to grossly check your granite flat plate. However, the aluminum straight edge is too floppy for this to work.

View deeznutz's profile


33 posts in 3831 days

#6 posted 04-07-2010 12:43 AM

Thank you all for the advice. I’m thinking the straight is probably pretty good. I like the dial indicator idea but don’t have one at this time. The first suggestion of the two lines idea seems to be as parallel as my eye can determine. (I should have thought of this idea…I do the same thing to check to see if my squares are square).

In this case, I guess I can count on this straight edge being the flatest (straightest?) edge in my shop. For $20 I feel comfortable recommending it to anyone.

The downside of this…the #8, and the my #4 as well, seem pretty grossly out of flat. I’ll have to pick up some feeler gauges to see just how much. Any ideas on how much is too much in the sense that it might not be worth the time to fettle it by hand?

Thanks again!

View TopamaxSurvivor's profile


19352 posts in 4487 days

#7 posted 04-07-2010 12:55 AM

You could just look down the edge, your eye will detect the slightest deviation.

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

View Gofor's profile


470 posts in 4598 days

#8 posted 04-07-2010 05:39 AM

NBeener is right about how to check it. Only, use a knife blade to draw the line. If you get three different edges to perfectly match, they must all be straight (I won’t go into the math and physics, but outside of the quantam physics arena, it is true). The edge you bought is edge 1. The first line scribed is edge 2, and the second scribed line when you flip it is edge 3.

I have made long straight edges by ripping plywood and matching three different edges, tuning them with hand planes, until no light will shine between the edges for the full length. I use two,(they will get worn by the marking tool, awl, scribing knife over time) and keep the third for a reference to make more.

My primary use is to check straightness on glue joint edges for making panels


-- Go

View hazbro's profile


109 posts in 3801 days

#9 posted 04-07-2010 06:29 AM


that’s it. stone/dust age tech.

-- measure once, keep cuttin' til it fits

View docholladay's profile


1287 posts in 3870 days

#10 posted 04-07-2010 06:51 AM

It is really prett easy to test for straightness. Much the same as testing a framing square. First, lay it flat on a table. Draw a ine down one edge. Then roll it over to the other side of the line you just drew, but with the other flat surface on down. Check it against the line that you just drew. If it matches perfectly, then it has to be straight. Any amount of out-of-true-ness will be clearly visible.

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

View GWTech's profile


7 posts in 3782 days

#11 posted 04-07-2010 11:15 PM

I’m a technician at Garrett Wade. I had used these straightedges around the shop and studio long before I was asked to test them for QC purposes. I mostly used them for layouts and for cutting sheet material with a razor blade. For checking flatness of machinery or a workbench top, I always reached for the Starrett.
Not too long ago, in response to customers’ inquiries, we tested all of the straightedges that GW currently sells. A dozen or so of each item was randomly pulled from stock and compared to a high precision machinist straightedge (Snap On). The results surprised us. While finding some issues with the steel straightedges, we couldn’t get a 0.001” feeler gauge between the aluminum straightedge and the Snap On anywhere along the whole length.

I would be very happy to hear your thoughts after checking out these straightedges. I can be reached at [email protected]

-- [email protected]

View Gofor's profile


470 posts in 4598 days

#12 posted 04-08-2010 03:26 AM

I will have to check out your product (Garrett Wade). Due to access and income, my recent experience has been with the cheap stuff you get in the big box stores, and I have yet to find a straight “straight edge”, level, or “square” square. Admittedly, most are fine for construction, but leave a lot to be desired in furniture grade work. My old machinist tools bought back in the 60’s (Starrett and Luftkin) are all dead on.


-- Go

View scrappy's profile


3507 posts in 4241 days

#13 posted 04-08-2010 08:49 AM

I agree with Gofor, Dad used to be a machinist and I have some of his 50’s and 60’s tools. Still the straightest tools in the shop. Now, if only I could cut a straight line. haha


-- Scrap Wood's the best...the projects are smaller, and so is the mess!

View Blasthoff's profile


2 posts in 3427 days

#14 posted 10-22-2014 12:01 AM

One way to have a straight edge to check against would be to stretch a wire. An unwound guitar string tightened between two posts will be very straight and the wire very uniform. Used with a strong light you could easily spot any deviation well under .001

Just be sure to use light thin enough to see a close tolerance, if the light is too thick, use some light thinner LOL!

View hoosier0311's profile


706 posts in 2836 days

#15 posted 10-22-2014 12:30 AM

I just laid mine on the cast iron saw top and put a flashlight behind it. A long as I don’t see any light it’s good enough for me. I never measure down to the .001 anyway.

-- atta boy Clarence!

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