# Making a 36" straight edge from scratch - ideas?



## Lignin (Mar 12, 2013)

Hi there,

I would like to make a 36" straight edge from some 50mmx5mm cold rolled steel I have.

I know the theory suggests making two together, which slide along each other with no 'gapping' once they are true. However, what is the best method to get from a 'gapping' pair to a 'non-gapping' pair.

Also, surely a matching pair of convex and concave edges would slide along each other with no 'gapping'.

Any ideas appreciated.


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## Richard King (Mar 12, 2013)

I am not sure what you mean by Gapping?  You mean if you lay it on a surface the gap from a bow or a bent bar?

Steel straight edges are used in the auto engine rebuild business and can be surface ground to .001" .  

http://www.starrett.com/metrology/m...urrentPage=1&displayMode=grid&sortBy=none/asc

I see them on Ebay all the time.  Probably be better to buy one then make one if you don't have a surface grinder.


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## Halteclere (Mar 12, 2013)

The best method I've seen described to create a strait edge is to make three strait edges. Here's a link that describes the process: 

http://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html.

With two strait edges both pieces can have the same irregularity but fit tight together. With three strait edges all three pieces can only fit tight together when all three are strait.


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## PurpLev (Mar 12, 2013)

Halteclere said:


> The best method I've seen described to create a strait edge is to make three strait edges. Here's a link that describes the process:
> 
> http://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html.
> 
> With two strait edges both pieces can have the same irregularity but fit tight together. With three strait edges all three pieces can only fit tight together when all three are strait.



That was an interesting read. thanks for the link!


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## Richard King (Mar 12, 2013)

Halteclere said:


> The best method I've seen described to create a strait edge is to make three strait edges. Here's a link that describes the process:
> 
> http://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html.
> 
> With two strait edges both pieces can have the same irregularity but fit tight together. With three strait edges all three pieces can only fit tight together when all three are strait.



In my opinion, It's not the best now-a-days, but for years before you could buy super accurate granite surface plates it was the best and only way to generate a flat surface.  The Moore book has a section devoted to 3 plate scraping.   I've scraped and lapped 3 plates that way and it takes a long time compared to scraping one to a granite plate.    Rich


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## PurpLev (Mar 12, 2013)

Richard King said:


> In my opinion, It's not the best now-a-days, but for years before you could buy super accurate granite surface plates it was the best and only way to generate a flat surface.  The Moore book has a section devoted to 3 plate scraping.   I've scraped and lapped 3 plates that way and it takes a long time compared to scraping one to a granite plate.    Rich



It makes sense that scraping using a granite plate as reference would be faster, but for those of us without a 36"+ granite plate this might be a viable option. unless I'm missing something and one could use a <18" granite surface to produce a 36" straight edge (eager to learn)?


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## Lignin (Mar 12, 2013)

Halteclere said:


> The best method I've seen described to create a strait edge is to make three strait edges. Here's a link that describes the process:
> 
> http://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html.
> 
> With two strait edges both pieces can have the same irregularity but fit tight together. With three strait edges all three pieces can only fit tight together when all three are strait.



That makes perfect sense.....probably what I had heard in the past, but just remembered wrongly!

Thanks for the link too.


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## bleonard (Mar 13, 2013)

Lignin said:


> Hi there,
> 
> I would like to make a 36" straight edge from some 50mmx5mm cold rolled steel I have
> 
> ...


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## Richard King (Mar 14, 2013)

I was just thinking if you need a straight edge rule to use a feeler gage or light gap, why not go down to a local sheet metal shop or welding shop and have them shear you a  1/8", 3/16 or  1/4" x 3" x 36".
Today's sheet metal shears can cut precision parts.  I bet within .002"  You originally said .0015.  Close enough.  Heck they might give it to you.
Just file the sharp edge and there you go.


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## Olin (Jun 9, 2013)

Greetings All

Well I haven't done this myself so... 

I purchased just about all the various metalworking books Lindsay Publications put out, a sad loss to me when they closed-up. 

The  re-prints of older texts talk of generating surface plates, precision straight edges, and squares from scratch. They all use a three piece method. As I understand it, you get two to match fairly close and then take the third and match it to the others. You keep exchanging and comparing the three to each other, as you remove finer and finer discrepancies you will finally end up with a flat surface plate, matching straight edges and matching squares. The greater care you take, the finer the degree of agreement between the various items. In case it's not clear you do surface plate to surface plate, square to square, and straight edge to straight edge. I don't recall any of the of the old books stating this, but my thought is; if you did this with sufficient care then the straight edges and the limbs of the squares would also match each other and the surface plate also.

Matching shorter pieces of thread in a similar manner and then using that to generate a longer thread is the method that was used to make the first precision threads. Precision meaning that the threaded piece matched another threaded piece very closely. 

all the best

Olin


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## Lignin (Jun 10, 2013)

Olin said:


> Greetings All
> 
> Well I haven't done this myself so...
> 
> ...



Thanks. That seems to be the received wisdom. Those old timers were sure patient - a quality which is fast disappearing.


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## oldgoaly (Jun 12, 2013)

I was borrowing a 3' to make some tests (cylinder heads), but I wanted one of my own. I happened across a 24" long 1/4"by 2" pc of cold rolled that was fairly close.  How I was told was 3 panes of glass with fine sand paper (garnet paper?)  the variations evened out the differences. I would check it against the 3' Starrett and it was good. It took time but then I had more of that than money. Years later I came upon a bargain for a Starrett pink granite 18 by 24, got the straight edge out and check it on that by golly it is as flat as the granite!  btw I did the narrow 1/4" edge. A couple of oldtimers told me how to do it, I believed them, always thought I better listen so I don't have to go buy some pc of junk from a 3rd world country. Every time I use it I think of Bill and Greg, there are many things they showed me how to do and sometime  I hope to show my son or any young folk who have more time and no money, it might just keep them off those silly video games for a few minutes!  Go ahead and try it!


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## Optic Eyes (Apr 28, 2021)

Olin said:


> Greetings All
> 
> Well I haven't done this myself so...
> 
> ...


This can be done, I have done this, it is not easy, but start with stress relieved steel


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