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bridge straight edge out of steel

Are there any real reasons why a bridge straight edge cannot be made out of steel?

I know cast iron is the material of choice, but in todays world cast iron bar is simply not something that is easy to get any more.. While in years gone by every town of a reasonable size had a foundry today they are more common in the third world countries then the western world, and when you do find one in the western world they are not interested in ones and two's...

I'm taking a chance of reviving this 13 year old discussion: Cast iron is hard to find, while steel is not.

I found this video on "An Engineer's Findings" youtube channel, pretty clever guy, he made a welded steel straight edge that stayed straight within a few microns afte two years:

 
Richard,

Have you bought any Durabar lately?
They have a $250 minimum you can order from them, maybe more, I haven't checked lately, this was last year. They stopped stocking most rectangular sizes so your pickings are limited. My Durabar warehouse in Ft Worth only stocks rounds and the smallest diameter round they sell is 3". Oh, and the price per pound has doubled from the pre-pandemic years.
Yeah, they are not the guys we once knew.
 
Richard,

Have you bought any Durabar lately?
They have a $250 minimum you can order from them, maybe more, I haven't checked lately, this was last year. They stopped stocking most rectangular sizes so your pickings are limited. My Durabar warehouse in Ft Worth only stocks rounds and the smallest diameter round they sell is 3". Oh, and the price per pound has doubled from the pre-pandemic years.
Yeah, they are not the guys we once knew.
I buy from Woodstock IL where it's cast. I buy sawed from a plate. I'll call them tomorrow and see what I normally buy quotes now. Last time I bought was 4 years ago.ill cut and paste both quotes.
 
I guess the implicit question raised in this thread is "would a steel weldment be dimensionally stable over time." Clearly most folks would prefer to scrape iron when given a choice. But, if you can scrape or grind a stress-relieved steel straight edge to good flatness will it remain flat?

The answer based on the creep testing of steel that I could find is similar to that for cast iron. At normal environmental temperatures, i.e. under several hundred degrees F, neither steel nor cast iron creep. So, if a steel bridge-style (camelback) straight edge is stress relieved and scraped or ground flat, it will not distort over time just sitting out in the shop.

I think it is good practice to back up claims with at least some form of reference. So here is one brief quote:

Steel Creep
"What Is Creep (Deformation) in Steel?
Creep deformation in steel is only considered a problem when its operating temperature reaches 40% of its melting temperature for long periods of time. Various creep mechanisms can present themselves depending on the load conditions and the type of material. "

My limited experience trying to buy Durabar has been similar to that of several posters here: Very high price, horrible shipping costs, and practically no interest in selling onsie twosies. I gave up trying after a few such failures. I would be interested if someone finds a decent source of supply at a reasonable price.

Denis
 
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How i would make a steel welded straight edge:

Use flat bar half as wide and thick as the flange, in a triangular pattern, temporary spacer rods clamped between the top and bottom flange during welding.

The welds are easy to reach, inside the triangle. Probably want to start in the middle and work outwards.
 

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I guess the implicit question raised in this thread is "would a steel weldment be dimensionally stable over time." Clearly most folks would prefer to scrape iron when given a choice. But, if you can scrape or grind a stress-relieved steel straight edge to good flatness will it remain flat?

The answer based on the creep testing of steel that I could find is similar to that for cast iron. At normal environmental temperatures, i.e. under several hundred degrees F, neither steel nor cast iron creep. So, if a steel bridge-style (camelback) straight edge is stress relieved and scraped or ground flat, it will not distort over time just sitting out in the shop.

I think it is good practice to back up claims with at least some form of reference. So here is one brief quote:

Steel Creep
"What Is Creep (Deformation) in Steel?
Creep deformation in steel is only considered a problem when its operating temperature reaches 40% of its melting temperature for long periods of time. Various creep mechanisms can present themselves depending on the load conditions and the type of material. "
Denis
Denis, I am not a metallurgist but here is a theory of mine. You can have a beam that has a very high compressive stress on one side, neutral in the center and high tensile stress on the other side, the part ends up dead flat after scraping. There is no easy way to know what kind of stresses are hidden in the part. Some materials can creep at high stress, room temperature and long time so you hang up the part and loose straightness over time. I suspect this is why some parts move a small amount over time when no significant external load has been applied. The solution is exactly as you say, properly stress relieve and rescrape the part.
 
I'm taking a chance of reviving this 13 year old discussion: Cast iron is hard to find, while steel is not.

I found this video on "An Engineer's Findings" youtube channel, pretty clever guy, he made a welded steel straight edge that stayed straight within a few microns afte two years:

Congrats on waking up zombies, but,but,but in any construction, you can design something where things counteract in a way that they stay in balance. The fact something is made of steel does not necessarily mean that it will bend into a pretzel. It is maddening to read these posts. Suppose you know what you are doing, you can design a fabricated structure that is 100% stable at any temperature. The reason straight edges are made of CI is that it is relatively easy to make a stable CI piece, whereas it is almost impossible to do the same in steel
 
Congrats on waking up zombies, but,but,but in any construction, you can design something where things counteract in a way that they stay in balance. The fact something is made of steel does not necessarily mean that it will bend into a pretzel. It is maddening to read these posts. Suppose you know what you are doing, you can design a fabricated structure that is 100% stable at any temperature. The reason straight edges are made of CI is that it is relatively easy to make a stable CI piece, whereas it is almost impossible to do the same in steel
So you are saying steel creeps at normal environmental temperatures? That is contrary to what engineering studies I have been able to uncover. Can you cite a study that supports that idea?

That is different than saying machining (whether with an end mill or scraper) will cause warpage of unstress- relieved steel structures whether weldments or rolled or cast.

Denis
 
I found this paper on low temperature creep of steel, abstract only. Kind of technical, I do not really understand the implications of it. Here is a short excerpt
"Small plastic and creep strains in eight austenitic steels and a low-alloy steel were measured in the temperature range from 4 to 573 K. Every steel including the low-alloy steel showed logarithmic creep strain at these temperatures when stress was high enough to produce plastic strain. Although the creep strain rate in specimens, JIS SUS316L, at 293 K was proportional to about the 7th power of stress when the stress was around 0.2%-plastic-offset stress, the order of the power decreased to 1 as stress decreased. The ratio of creep strain at 105 s to plastic strain was in the range 1–3 at 293 K and 0.5–2 at 77 K, though the precipitate-hardened steel JIS SUH660 had a lower ratio."

Again from a practical standpoint, if the steel straightedge is properly stress relieved after welding, I don't see how it ever gets stresses in the plastic range unless someone uses it as a crowbar. If there are not high temperatures or high stresses there should not be creep. At least that is my guess.
,

Here is a screen grab of the source

1713227896823.png
 
So you are saying steel creeps at normal environmental temperatures? That is contrary to what engineering studies I have been able to uncover. Can you cite a study that supports that idea?

That is different than saying machining (whether with an end mill or scraper) will cause warpage of unstress- relieved steel structures whether weldments or rolled or cast.

Denis
Nope, I'm not claiming anything about steel creeping in one way or another. As far as I know, steel is darn stable at a given temperature. All I am saying is that whatever uneven heat-related deformation is inherent to steel structures can be compensated by designing the structure to account for it. That is not impossible, but somewhat improbable by accident. Ci is less susceptible to the same type of deformation due to its mass and other heat-related properties.
 
I'm no materials engineer by any means, but part of the issue as I understand it between steel and iron is that steel simply has the capacity to distort and bend and keep that damaged shape while still being structurally stable. You could drop it and it would look fine but be warped a few tenths (for example), compared to iron that is much more difficult to permanently bend in the same way without compromising it's structure, such that if it was mishandled and didn't crack or break then it safe to assume it's still stable dimensionally.

Having saved a couple sick-puppy straight edges, I think most of the issues with problem castings are due to poor or no stress relief and as such can be corrected. Design of the casting has more to do with keeping it from flexing or temporarily staging while also keeping it light for it's size. A straight edge that is unnecessarily light without adequate structure behind the working surface might flex in use or be more fragile, while one with "too much meat" won't flex at all, but is a bear to lug around.

I think most of the issues with steel creeping or internal stress's can by remedied by thermal stress relief similar to iron, and the issues with it maintaining geometric shape staticky can be remedied with how you design the piece before hand, but I think by the time you weld on enough steel to eliminate the possibility of it deforming from typical handling or mishandling scenarios, it's weight and shape make it no longer totable as a straight edge.

If I was in the boat of not having access to new SE castings, I'd carve up an Asian lathes bed or similar casting before I'd weld a steel one together. It's much more labor than making a new iron straight edge by the time you get a bed cut and trimmed and stress relieved (several times), but at least it's a known and reliable material.
 
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Talking about chopping up a lathe bed or any other machine member into a straight edge. My dad made a straight edge about 18" long that was cut out of a machine member when I was in diapers. Later in life I got ahold of the straight edge and tried to scrape on it. Dad never did stress relieve it and every time I made a pass across it with the scraper, it would bend another direction. Someone else has it now.
Something not mentioned is the weight difference of the two materials. Cast iron has a density of .258 lbs. per cubic in., and steel is .283 lbs. per cubic in. So a cast iron straight edge will weigh less than a steel one would. :willy_nilly: for what its worth.
 
McMaster offers several types of cast iron bars. Gray iron bars are about $1.15/lb. https://www.mcmaster.com/8925K37
You sure about the per-pound price? McM lists1 foot bar of 2" CI would weigh 9.5 pounds and would cost 43 dollars plus shipping, for example. Oh, and that reminds me that Rich was going to check on Durabar pricing. I wonder what he found.

Denis
 
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If I was in the boat of not having access to new SE castings, I'd carve up an Asian lathes bed or similar casting before I'd weld a steel one together. It's much more labor than making a new iron straight edge by the time you get a bed cut and trimmed and stress relieved (several times), but at least it's a known and reliable material.
One nice thing about using a lathe bed or similar "massive" chunk of cast iron would be that it would be unlikely to to retain stress from casting. Castings that are thick in cross section cool slowly and therefore tend to relieve stress on their own. I suppose if the bed were cast and then shaken out of the sand still red hot and sprayed down with water, it could be full of stress. But that process makes no sense in terms of effort involved or cost savings. So it is very unlikely it applies. One way to get an idea of residual stress is if the saw cuts tend to open up or close as you are cutting then there is a fair bit of stress. But if you are using a 1/8 blade and the kerf remains at 1/8," the casting is likely pretty free of stress. Additionally, stress relieving a iron casting is very simple. Just putting it in a common pottery kiln and heat it over three or more hours to 1150F and holding there for a an hour per inch thickness and then just letting it cool off by turning off the heat to the kiln will do for any grey iron alloy even "high alloy" (uncommon) ones. Most common alloys are fine if heated to 1050.

, I'm not claiming anything about steel creeping in one way or another. As far as I know, steel is darn stable at a given temperature. All I am saying is that whatever uneven heat-related deformation is inherent to steel structures can be compensated by designing the structure to account for it. That is not impossible, but somewhat improbable by accident. Ci is less susceptible to the same type of deformation due to its mass and other heat-related properties.
Sorry, did not mean to grill you. However I am probably a bit hypersensitive to the garbage I continue to read about cast iron creeping and the need to store straight edges by hanging them. That is a myth unsupported by any science and our group would be well served if that myth could just be allowed to disappear.

Denis
 
You sure about the per-pound price? McM lists1 foot bar of 2" CI would weigh 9.5 pounds and would cost 43 dollars plus shipping, for example. Oh, and that reminds me that Rich was going to check on Durabar pricing. I wonder what he found.

Denis
I forgot one operation in the calc. Yeah, more like $4/lb. 12.25 x 2.75 x 12 x .283 = .114. $445 / 114 = $3.90.
Anyway it's available from the easiest supplier ever in numerous sizes.
 
That is a great video and he apparently did make a nice straight edge using steel and welding. I applaud him.

There was one thing that he said in the video that is NOT correct and that is a common misconception about the three plate method. He stated that three straight edges could be compared against each other and be made straight. This is not a proper application of the three plate method because it says that, three surfaces will only be able to be in intimate contact, in all three combinations (AB, BC, and CA), AND in ALL POSSIBLE ROTATIONS when all three are flat.

The ALL POSSIBLE ROTATIONS part is commonly not understood. The narrow width of a straight edge makes comparing/printing two of them all but impossible when they are at right angles to each other. So you are limited to only two orientations at 0 and 180 degrees to each other.

I discussed this at greater length in a comment under the YouTube video.

He did use a surface plate to judge the straightness of his straight edge so it should be OK.



I'm taking a chance of reviving this 13 year old discussion: Cast iron is hard to find, while steel is not.

I found this video on "An Engineer's Findings" youtube channel, pretty clever guy, he made a welded steel straight edge that stayed straight within a few microns afte two years:

 








 
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