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Precision straight edge
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=47966		 Page 1 of 1
Author:  Ruby50 [ Fri Jul 01, 2016 11:40 am ]
Post subject:  Precision straight edge
I am changing the shop around and moving things I have not seen in a while. I came across a “ruler” that is 24+ inches long. It has numbers up both edges, with the number 1” starting at the same end on both edges, 1/8” increments on one edge and 1/16” on the other. There is 1/2” of blank metal beyond the 24” marks. The thing is a full 3/16” thick and would make a great pit bull trainer. The edges appear to have been milled. The maker is Theo. Altenader and Sons Phila USA, but there are no markings to identify what model it is or what it is for.

Altenader was in business from 1850 to at least the 1950’s. They made, or offered for sale from other makers, drafting and drawing equipment, including this cool device, 2-1/2 times as cool as the ones I have seen for marking ukulele nuts:

http://www.worthpoint.com/worthopedia/v ... -105619792

Does anyone have a method for using a strung up guitar string and a feeler gauge for checking the precision of a straight edge? Other methods that use stuff in a shop?

Thanks

Ed
Author:  truckjohn [ Fri Jul 01, 2016 12:00 pm ]
Post subject:  Re: Precision straight edge
No. The correct way to check a straight edge is with a certified straight edge or a surface plate.

Do not verify a straight edge against plate glass, guitar string or other associated whatnot.

What you can do is to take a good whetstone and carefully stone off burrs and dings. Do not use a dremel or grinder. Don't try to level it - just knock down the burrs.
Author:  Chris Pile [ Fri Jul 01, 2016 3:44 pm ]
Post subject:  Re: Precision straight edge
Ed, what you have is a scale. If it has measurement markings on it - it is NOT a straightedge.
Author:  John Arnold [ Fri Jul 01, 2016 5:11 pm ]
Post subject:  Re: Precision straight edge
I use stainless steel rulers to check neck straightness and relief. They are about 0.030" thick....not nearly as thick as yours. I have an 18" one and a 12" one. When I check relief with a feeler gauge and flip the ruler over, the measurements are within 0.001" of each other. That tells me that the rulers are straight enough for my use.
Author:  Ruby50 [ Fri Jul 01, 2016 6:56 pm ]
Post subject:  Re: Precision straight edge
Thanks all

I don't really have the need for a precision straightedge, but thought it would be interesting.

Truckjohn - I can see not using plate glass and hoe a reference surface would be ideal, but why not against a guitar string? Is there anything straighter and more precise on diameter?

Ed
Author:  truckjohn [ Fri Jul 01, 2016 8:24 pm ]
Post subject:  Re: Precision straight edge
You can't count on a string to be perfectly straight. First - a string is pretty floppy. It deflects under its own weight like a power line does. Then - it deflects when you touch it... It's not nearly anywhere near good enough when you are trying to get way better than 0.005" over 24".

I would talk with a machinist friend and ask him to check it against his surface plate.
Author:  WaddyThomson [ Fri Jul 01, 2016 10:04 pm ]
Post subject:  Re: Precision straight edge
Chris Pile wrote:
Ed, what you have is a scale. If it has measurement markings on it - it is NOT a straightedge.


Not necessarily true. I have a precision straight edge that is marked on one edge.
Author:  Chris Pile [ Fri Jul 01, 2016 10:49 pm ]
Post subject:  Re: Precision straight edge
Quote:
Not necessarily true. I have a precision straight edge that is marked on one edge.


Lotta people and companies CALL something that measures a straightedge... But that is NOT the proper definition. Old machinists and tool makers like me know that they are 2 different tools, that do 2 different things. I'm right.
Author:  Clay S. [ Sat Jul 02, 2016 7:44 am ]
Post subject:  Re: Precision straight edge
Yes, there is a difference between a straight edge and a straightedge, but a straight edge is all we really need for guitar building. bliss
Author:  Tom West [ Sat Jul 02, 2016 8:52 am ]
Post subject:  Re: Precision straight edge
John Arnold wrote:
I use stainless steel rulers to check neck straightness and relief. They are about 0.030" thick....not nearly as thick as yours. I have an 18" one and a 12" one. When I check relief with a feeler gauge and flip the ruler over, the measurements are within 0.001" of each other. That tells me that the rulers are straight enough for my use.


John: Think this is valid as long as the edges of the ruler are parallel.
Tom
Author:  murrmac [ Sat Jul 02, 2016 9:42 am ]
Post subject:  Re: Precision straight edge
Tom West wrote:
John Arnold wrote:
I use stainless steel rulers to check neck straightness and relief. They are about 0.030" thick....not nearly as thick as yours. I have an 18" one and a 12" one. When I check relief with a feeler gauge and flip the ruler over, the measurements are within 0.001" of each other. That tells me that the rulers are straight enough for my use.


John: Think this is valid as long as the edges of the ruler are parallel.
Tom


I would have to disagree there, Tom.

John's criterion for assessing the straightness of his ruler is not quite exact, admittedly (theoretically the edges could each have an identical concave curve and still yield identical measures of relief) but the parallelism or otherwise of the edges is not really a factor.
Author:  WaddyThomson [ Sat Jul 02, 2016 9:46 am ]
Post subject:  Re: Precision straight edge
Chris Pile wrote:
Quote:
Not necessarily true. I have a precision straight edge that is marked on one edge.


Lotta people and companies CALL something that measures a straightedge... But that is NOT the proper definition. Old machinists and tool makers like me know that they are 2 different tools, that do 2 different things. I'm right.



You can be right, I don't care about that. I bought one similar to this, and use it as it is sold, as a precision straight edge. http://www.woodcraft.com/product/154073 ... 4inch.aspx

Works for me!
Author:  Tom West [ Sat Jul 02, 2016 10:25 am ]
Post subject:  Re: Precision straight edge
Murr:
What John is basically doing is comparing three edges to themselves. We all know that for them to be straight that when compared to each other,none of the combinations (side one to the frets),(side two to the frets) and finally (side one to side two) must not show any gaps. Of course the comparison of side one to side two can't happen with out cutting the rule. For standards suitable for guitar making I think one can say the sides have to be parallel. Two factors at play ,straightness and parallelism. As you pointed out,there could be two concave edges and one gets the same reading from both sides. These type of things can get carried to quite a far degree. Such as using a certified straight edge. But if that edge has not be checked for calibration how do we know if that edge is straight. It could have been dropped or used as a crow-bar by the unknowing etc,etc. Guitar work does not require much precision but it does require knowing what one is doing.But just using anything that looks like a straightedge as the proper tool unless one is sure of oneself my not give the desired result. I'm quite sure that John Arnold knows exactly what he is doing. My concerns are for other folks who may be less informed and cause themselves a problem.
Tom
Author:  murrmac [ Sat Jul 02, 2016 11:25 am ]
Post subject:  Re: Precision straight edge
Tom, I am still failing to see where "parallelism" even enters into the equation.

Imagine, if you will, a length of 3/16" thick steel, 13" long, which is 1" wide at one end and 2" wide at the other. Each edge is accurately surface-ground to be a straight edge. Either edge can now be used to check relief, and each edge will give exactly the same result as if the edges were parallel.
Author:  dzsmith [ Sat Jul 02, 2016 12:45 pm ]
Post subject:  Re: Precision straight edge
I cringe when I see someone using a homeless depot level as a levelling beam for fret work.
A feller using a plastic one made me chuckle.
Author:  Tom West [ Sat Jul 02, 2016 12:59 pm ]
Post subject:  Re: Precision straight edge
Murr: Think of lapping three surfaces against each other to get them flat. You have assured that two of your surfaces are accurate before you start. With a ruler you don't have that pleasure. So to see if you are getting the same results of the two edges against the frets, one can assume the edges have to be parallel. Not sure I can expand any more.
Tom
Author:  Barry Daniels [ Sat Jul 02, 2016 1:57 pm ]
Post subject:  Re: Precision straight edge
I have a hardware store level is quite straight. Been using it successfully for fret leveling for years. Not cringe worthy at all.

What makes me cringe a bit is seeing someone trying to level frets with a large file. I know it can work but it is a lot more fussy.
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