How do I ensure a straight edge is straight?

[Pete Costa]

I've got an opportunity to buy a used 72" Starrett Ruler C416R. I primarily want to use it to align my "new to me" 12 inch SCMI jointer (also a craiglsist find!). So here's the challenge - how do I ensure a the ruler is actually straight (within tolerances reasonable for this use)? I have a 12 inch Starrett square and a 24 inch straightedge from Woodcraft. I won't be able to bring it back to my shop without purchasing it and I don't want to joint a reference board on a jointer that I'm not sure is straight. Thanks in advance for your thoughts!

[Mel Fulks]

I didn't look up that item. But the rulers are not the same as straight edges . I think they sell some narrow rulers and
some wide ones. The wider rules are probably straighter than most others. To check a used one ,I would draw a line
with it ,then put the rule on the other side of the line to compare. Good straight edges can be made at home.

[Dave Sabo]

NIST Certification ?

[Andrew Hughes]

If the place your buying it from is a machine shop then ask to lay it on one of their surface plates.
Or up against another one that’s there.
If non of that’s possible and you really want to get it. I would do a visual inspection there shouldn’t be any dings or nicks in the edges or ends.
If it was stored hanging form a hole on one end that would be a good sign.
Good Luck

[Lewis Mills]

You're talking about a straightedge with a $970 list price (https://www.starrett.com/metrology/p...etail/C416R-72). Unless you get a 95% or better discount, pass on it if you can't take it somewhere that you trust to verify it. Personally, if it cost more a few bucks, I'd pass if I couldn't take it somewhere to verify its accuracy. Doesn't matter how much of a discount you get if it doesn't meet your accuracy requirements.

[Mel Fulks]

I don't think he's gonna get a 95 percent discount. Machinists buy ,sell, and trade good tools at much less discount. A
thin wire under tension is pretty reliable ...unless a tornado is in progress.

[Andrew Hughes]

I missed the part about it being a long ruler. It’s not the same as a precision straight edge that’s much better to have when setting long jointer tables .
I bought one from Suburban tools when I need to setup my jointer. It’s very good quality I just checked they have a 6’ for 450 plus tax.
Good Luck

[Kevin Jenness]

At .010" thick and 1.5" wide it is not designed to be a straightedge, though it might serve as one. My 72" Starrett straightedge is more like .25" x 3".

My advice is to make sure the knives in your new jointer are sharp and the outfeed table is set even with the cutting circle. Run a pair of flat test boards as long as the outfeed on edge with the fence set at the front and back of the tables and dial in the outfeed table height until your test pieces mate well. If you can't achieve that, then the fun begins and you need to look for a proper straightedge.

[Brian Holcombe]

Three surfaces must agree with one another to know if they are straight. So technically what you can do is joint two 6’ long boards, compare them to one another for flatness and when the straight edge agrees with both of them the group is flat. If they all cannot be made to agree than the straight edge is not flat.

Two curves can agree with one another but three cannot.

[Pete Costa]

I didn't look up that item. But the rulers are not the same as straight edges . I think they sell some narrow rulers and
some wide ones. The wider rules are probably straighter than most others. To check a used one ,I would draw a line
with it ,then put the rule on the other side of the line to compare. Good straight edges can be made at home.

thanks Mel and others. Even though I wrote "ruler", it didn't occur to me that a ruler was not necessarily a straightedge. I'm now wondering how practical it would be for setting up a jointer, given that it's only 1/10 of an inch wide, even if it was reasonably accurate

[Andrew Pitonyak]

I like these:

https://www.leevalley.com/en-us/shop...s?item=05N6203

Now, how straight does it really need to be? That depends on what you are doing. In 2011, I sent an email to Empire and asked them about the tolerance for their straight edges. For their standard straight edge, the straightness and flatness is 0.006" per foot. So, 36 inches is 0.018" in a worst case. Probably close enough for many wood working tasks. The Veritas version is 0.0015 and it is stress relieved.

https://www.empirelevel.com/straight-edges.php

I have used both in a pinch depending on what is handy and what I am doing.

[Malcolm McLeod]

Three surfaces must agree with one another to know if they are straight. So technically what you can do is joint two 6’ long boards, compare them to one another for flatness and when the straight edge agrees with both of them the group is flat. If they all cannot be made to agree than the straight edge is not flat.

Two curves can agree with one another but three cannot.

To be rigorous, it would be wise to spec that an edge should be compared to the other and then swap end-for-end and compared again. Certain shapes, symmetric about the center, could ‘pass’ this test otherwise (think about a gentle s-curve :: all 3 could mate).

Admittedly this would be virtually impossible to create on a jointer, but after the art-work-as-end-stop thread, I just had to toss this grenade over the fence.

[Justin Rapp]

You can test it on plate glass, like a sliding glass door's glass pane...

[Bill Dufour]

I would think a machine shop would check it against two of theirs for a box of doughnuts.
bill D

[Bob Johnson2]

I have a 36" Straightedge, if I need something longer I use a level. When I redid the kitchen cabinets the counter guy had an 8' straightedge, that must have been some $$.