Help In Buying A Good Straight Edge

[Jeffrey Schronce]

I kinda got a 6' level as my straight reference. It was on sale at Lowes for around $20 a while back...

Maybe I should check my 48" Johnson Level to see how straight it is compared to my jointer beds.

[cabinetman]

Okay, this is gonna sound nuts, but I use wire under tension for my longer straight edges. Concave surfaces are easy. Convex surfaces will deflect the wire but you can shim on both sides to determine the degree of hump. You can either use a wooden stick to hold the wire, or get help from a family member.

The wire can be used as a conductor in a circuit that lights a bulb or sounds a buzzer when there is contact with a surface you're checking. So you can shim the wire with thin thermal paper on both sides (approx. .0015" thick) and then use a feeler gauge along the length for pretty fine testing.

Cheap, never goes out of tolerance, made to any length you like, and kinda fun.

I guess that is a very good system. But what if you're blind and deaf? Actually the way this thread is going we might as well discuss absolute straight. Since the Earth is round, creating flat requires a tolerance minimized by its length. You first have to create a fulcrum to support a balance beam made from forged titanium that is three times as high as it is wide, and 30% as thick. At the ends of the beam set a 1/2 oz bubble of mercury (should be exactly 12ft apart). Then, at high noon between the months of April and June in even number years, set up a transit level at least 50 ft from one of the vials. Sight across the two mercury bubbles and when you see only one reflection from the sun, you have a straight line. Take your machine made straightedge and match it.

I must add that in over 30 years of ww'ing 5 - 6 days a week, sometimes 7, I've never needed a metal straightedge more that a 6' aluminum ruler from the box store. And to check for straight, I bought two and placed them edge to edge for a visual inspection. They were under $10.00 each I think. If I ever needed a wider metal straight edge, I use a level, either 2', 4' or 6'.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"

[LCHIEN]

Maybe I should check my 48" Johnson Level to see how straight it is compared to my jointer beds.

well, it's strong, it's light, the I-beam construction makes it very stiff and resists bending under small loads or definately under it's own weight.
I used it to check my jointer beds, I flipped it over and used both sides; if it had a warp or bend then I think it would appear concave on one side and convex on the other when used against someting like a jointer bed, whereas if the jointer bed were out and the level were true then it (the errors) would appear the same way on either side of the level.

A 8' sheet of MDF has a very straight, true edge, but its just not practical.

[Stick]

Holy smokes! The Woodpecker straight edges are expensive! I was looking at the stuff from Lee Valley and thought it was expensive (though I really like their aluminum straight edge)!

I'll have to keep and eye on ebay. Been a while since I got something for pennies on the dollar on Ebay. Guess you can't go wrong with Starrett.

Thanks!
Jeff

You don't even want to imagine what a real machinist's straightedge is worth! $270 for a 6 footer would be a steal! We have a 6 footer at work that was something like $2000!

[Tom Slick]

a "real" 6' machinist parallel straightedge garanteed .0001" per 12" weighs only 54 lbs, is made of cast iron, and costs only $1300.
Mcmaster-carr has "regular" straightedges on page 2103 of their catalog or website.

This is wood working not machinist work, cabinetman is giving out very good advice. if we are being "really accurate" we work in 32nds, why fret a couple of thousands?

[vaking]

Measuring "how straight your straight edge is" is actually a simple task. I learned in grade school. Take a long sheet of paper (a roll of craft paper works well). Lay it on a flat surface and set a straight edge on top of it. Draw a pencil line along the edge. Now turn the edge over and try to align the edge against the line you just made. If you can - you have a good edge. If your edge is bowed or curved - you will see easily.

[lcm1947]

I was at Home Depot today and ran across the Empire " True Blue " level in 24", 48" and 72". They state that they are guaranteed to be within .00050 over the length of the level. Is that good enough for a straight edge? Cause if it is I just may buy one since I need a good level anyway and that would take care of two tools I need. Is even a true straight edge any better then that?

[Stick]

This is wood working not machinist work, cabinetman is giving out very good advice. if we are being "really accurate" we work in 32nds, why fret a couple of thousands?

Exactly! 1/32 being 0.03125", why worry about 0.001? We can't see that anyway. I just use a strip of panel with one edge being the factory cut edge, that's plenty straight enough for me for woodworking.

[scorrpio]

It would worry me greatly if my 78" Stabila level was not straight....

[RodKirby]

Couple of thoughts from Downunder...

(Let's say) I have a $10,000 48" straight edge (+/- zero error)

What about:

• My pencil
• My table saw blade
• My table saw fence
• The wood I am cutting
• My cutting technique

?

[jackellis]

Couple of thoughts from Downunder...

(Let's say) I have a $10,000 48" straight edge (+/- zero error)

What about:

• My pencil
• My table saw blade
• My table saw fence
• The wood I am cutting
• My cutting technique

?

...and just how much will the wood move anyway?

I'm looking for some better measuring and marking tools (1/64th resolution) but truth be known, the machine operator is by far the least accurate part of the process.

[SARGE..g-47]

ICM1947...

The "True Blue" is sufficient.Icm1974. I bought one last week as I need an accurate level also as stated in an earlier post. Before I bought it I carried a friend's TB to Highland Woodworking and gauged it against a Veritas 50" straight edge at $73. The True Blue got the call to use on table-tops, checking for bow, twist, cup in long rough stock and it gives me an accurated "level" for around the house projects.

BTW... the .0005 is for the liquid bubble and not the edge itself. But in order to get that the edge has to be straight as confirmed by the comparison I made. As the guys stated, a slight tolerance is not a big deal in WW, but with at least one striaght edge and one true square, it gives you a reference point to other measuring devices you purchase at a cheaper price (engineer squares, framing squares, etc., etc.). I have seen some that are not exceptable when building useless you like to repair gaps after the stock has been prepared and meshed.

If you do machining.... you have to have more accurate devices as sloppy machining tolerances relates to poor quality and possible failure down the road.

Regards...

[LCHIEN]

Exactly! 1/32 being 0.03125", why worry about 0.001? We can't see that anyway. I just use a strip of panel with one edge being the factory cut edge, that's plenty straight enough for me for woodworking.

Well, in the measurement industry, we generally consider we need an instrument with ten times better resolution and accuracy than the item we are trying to measure.

If your accuracy of your cut is +/- 1/32 and your measurement tools are +/- 1/32, then you may actualy be within only 1/16th of your true desired absolute measure.


So if the stuff you are making is accurate to 1/32nd then your measruing stuff should be about .003"

[Jeffrey Schronce]

Well, in the measurement industry, we generally consider we need an instrument with ten times better resolution and accuracy than the item we are trying to measure.

If your accuracy of your cut is +/- 1/32 and your measurement tools are +/- 1/32, then you may actualy be within only 1/16th of your true desired absolute measure.


So if the stuff you are making is accurate to 1/32nd then your measruing stuff should be about .003"

Not only that, but wouldn't the error be multipled by other errors? For example if your edge is off by 1/32nd due to jointer being off, then your table saw is off by 1/32nd, wouldn't the error be more than just 1/16th? Or is that my medication kicking in?

[lcm1947]

So I take it that .00050 is pretty accurate then. Anybody know what that .00050 would be in fractions?