Straight Edge checking assumptions - Double check please

[Brian Holcombe]

I want to double check my assumptions;

The beam is consistent in thickness, so my check is to lay it on the bench check the center with a feeler gauge, then flip it over and check the gap with a feeler gauge again and compare. Same spot for the feeler gauge and the beam is placed along the same length in the bench.

If the amount it is out is the same on both sides then the curve is in the bench and not the gauge (bench is surprisingly flat) since the thickness is the same throughout the beam. Meaning if it were up on the ends on one side it would have to be up in the middle on the other and vice versa.

Is this correct?

[David Weaver]

That relies on it being accurately even thickness.

I'd plane two edges that match the straight edge exactly, the same length as the straight edge. Then put them together and check for a gap. that's a functional test that doesn't rely on anything other than the straight edge being straight.

[Brian Holcombe]

Thanks David!

[Pat Barry]

Brian's method is correct unless the straightedge is some kind of deformed thing. You could always lay it down and scribe a line with a marking knife, then flip the straightedge and rescribe the same edge and compare.

[Jim Matthews]

Is the straight edge a metal standard?

That should be easy to measure with a dial caliper, to view deviations.
If the beam is constant, over it's length then what you've indicated makes sense.

Are you trying to get a benchtop flat, in this exercise?

*****

You could also do this with a measuring cup and water, or a water level.
Keep the water level fixed, next to the bench.

Attach the readout end to a square and move it along the bench
to get a quick read of the highs and lows.

******

If it was me (and it isn't) I would set a rotary laser (http://www.amazon.com/Johnson-Level-Tool-40-0922-Rotary/dp/B00EV4WHP2%3FSubscriptionId%3DAKIAILSHYYTFIVPWUY6Q %26tag%3Dduckduckgo-d-20%26linkCode%3Dxm2%26camp%3D2025%26creative%3D165 953%26creativeASIN%3DB00EV4WHP2#productDetails) on something immobile,
and play that across the top. Move a fine square across the surface to make
your observations. Sort of like surveying, on a minute scale.

[Brian Holcombe]

Thanks Fellas.

This is an 8' precision ground extruded aluminum level. I managed to knock it off my bench, so I just want to check it's precision again.

I have smaller precision tool steel straight edges for smaller planing jobs, but I have three of them so I can use them to reference one another.

[Steve Voigt]

Hey Brian,
Do you have a micrometer that's big enough? If you can verify that the level is truly uniform in width, them your original method will work. If it's not uniform within a thou or so, then no. In that case, David's suggestion would be a good way to go.

Not that it's necessary, but here's a great reference - maybe you've seen it since you mentioned three straight edges:
https://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html

[David Weaver]

Thanks Fellas.

This is an 8' precision ground extruded aluminum level. I managed to knock it off my bench, so I just want to check it's precision again.

I have smaller precision tool steel straight edges for smaller planing jobs, but I have three of them so I can use them to reference one another.

I'd be pretty impressed if it was straight enough when new to put to two separate edges and not have light pass through when they're compared. 8 feet is seriously long. I can't vouch for aluminum, but I can vouch that if you drop a starrett edge a minor distance, the end might ding, but the rest of it stays straight.

[Pat Barry]

Thanks Fellas.

This is an 8' precision ground extruded aluminum level. I managed to knock it off my bench, so I just want to check it's precision again.

I have smaller precision tool steel straight edges for smaller planing jobs, but I have three of them so I can use them to reference one another.
Seems to me the question about it not being uniform width can be ignored. Your first method tells the story

[Pat Barry]

That relies on it being accurately even thickness.

I'd plane two edges that match the straight edge exactly, the same length as the straight edge. Then put them together and check for a gap. that's a functional test that doesn't rely on anything other than the straight edge being straight.'
The method you proposed is simply not an effective way to check a straightedge. First - how exactly are you going to match that straight edge EXACTLY? That in itself will be a feat to be admired. Perhaps you could demonstrate that for us or at least describe it clearly enough that we can all play along. Now that you have one of these, why in the world would you need two of them? Please elaborate on the virtues of two of these (3 including the suspect staightedge itself!). Having the one perfect copy, together with the original straightedge is all that is required.

[Prashun Patel]

i had to edit some posts below to keep things civil. Lmk if there is an issue.

[Steve Voigt]

Now that you have one of these, why in the world would you need two of them? Please elaborate on the virtues of two of these (3 including the suspect staightedge itself!). Having the one perfect copy, together with the original straightedge is all that is required.

Hi Pat
For an answer to this question, you might want to check out the link I posted in my reply to Brian.

[Prashun Patel]

David's suggestion is to use the straight edge to test joint two boards. Any gap will magnify the error. It's akin to the '5-cut' method to test tablesaw fences for square.

+1, Like.

[lowell holmes]

How about stretching a fishing line in a bow. The line would need to be high tension, but it should be pretty close to straight.

[Warren Mickley]

Peter Nicholson who was a cabinetmaker in London in the late 18th century and later became a mathematician offered this method for making a straightedge in 1812. I use this method. I think a level would be clumsy compared to a wooden straightedge.

Face off two boards and put them in the vise together [I use 3/8 thick]. Plane the edges together with the jack plane until they are flat by eye. Then take the trying plane or jointer and plane full length until the shavings are uniform. Take the boards out of the vise and test with each other. If they coincide the edges are straight. If they are hollow or rounded the irregularity will be doubled. Replane until the edges are straight.

Nicholson says that in making a joint the boards are planed individually (unless they are very thin), and are not previously flattened on the face.

[Brian Holcombe]

Thanks for all of the inputs, I've got this straightedge down to .003".

[Jim Matthews]

That's clever.

Simple, too.
It also explains why I dinnit thunk it, first.

kudos

[Malcolm Schweizer]

i had to edit some posts below to keep things civil. Lmk if there is an issue.

It is sad when a post about checking a straight edge can get uncivil!

Interesting stuff in this post, and I always wondered how they did this "back in the day." I jokingly have thought to myself, "Wht if the first tool wasn't perfectly flat, and so all other tools are now out of flatness?" Here is an interesting video on making the world's roundest object, which was used as the base unit of measure for a kilogram. What I find most interesting is that the object was polished by hand- not by laser or robot. Mad skills.
https://m.youtube.com/watch?v=ZMByI4s-D-Y

[Warren Mickley]

"Back in the day" I read an article about Starrett making their best squares. A guy sits at a table in a darkened room with a small light at the other end. He has a granite surface plate and a few test squares. He holds a new square up against one of the standards on the plate and looks for light coming through. Then he makes small adjustments with his tool, a stick with fine sandpaper. Not a million dollar machine, just a million dollar brain.