Magnalium Straight Edges / Parallels - any user experiences?
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    Default Magnalium Straight Edges / Parallels - any user experiences?

    Good day,

    I'm looking for a straight edge to try my hand at some machine rebuilding, unfortunately in this part of the world it's near on impossible to get your hands on cast iron straight edges. But to my surprise my local tool supplier ordered a Magnalium straight edge for another client and has a spare one in stock. Prices are not that high +/- USD350 for a 1 meter version. From a quick inspection they have scraped surfaces and the specs online do not seem too bad (5μm flatness and 8μm parallelism over the length). These types of straight edges seem to be made in China or Taiwan. My question is, has any one used one of these units, do they hold up (not wear too quickly) and will they work for scraping / machine alignment purposes (carry spotting compound, etc). I am in the process of making a cast iron straight edge from a old casting I have butchered, but my surface plate will only accommodate about 700mm of length and I need something a little longer. Thank you for any info.

    Regards,
    Alastair



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    I would be very wary of using an aluminum straight edge for scraping...

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    Quote Originally Posted by proventheory View Post
    will they work for scraping / machine alignment purposes (carry spotting compound, etc).
    I cannot imagine ANY alloy that could be WORSE! Expect it to be worn out of spec almost at once if you try to rub-transfer spotting media with it!

    Now.. for gently placing against a surface and checking with light or feeler gage stock?

    Better than a length of ignorant rope or a spaghetti noodle, so yah. Not QUITE "useless".

    Just close-to...


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    Great, thanks for the feedback, I suspected they might be too 'fragile' for the purpose. I will try get in contact with some machine rebuilders in South Africa and see if they have a lead on any cast iron straight edges.
    Cheers,
    A.

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    I'd be interested in hearing from anyone who has actual experience with on of these or one like it. I can understand the opinions to blow it off, but some company has gone to the trouble to scrape these in and they'd have abandoned it quickly as well if it just did not work out. Maybe.

    One might argue in their favor that when they need re-scraping it's less work to do than with cast iron. Is it possible that the speed of reconditioning makes the frequency multiplied by time used actually more economical. It sounds bizarre, but who knows?

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    It would move the second you touched it!

    There is a reason cast iron is still around. Heck sometimes cast iron is incredibly temperature sensitive

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    I think if it was a worthy idea it would have been done by the old line makers. Have you ever heard of an aluminum straight edge from one of these? Not me. This is a product of the "low cost import" age IMO. Aluminum moves twice as much as iron with temperature changes, and as noted above, even iron can be temperamental when it comes to the tiny amount of movement that can affect repeatability during scraping.

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    Quote Originally Posted by eKretz View Post
    I think if it was a worthy idea it would have been done by the old line makers. Have you ever heard of an aluminum straight edge from one of these? Not me. This is a product of the "low cost import" age IMO. Aluminum moves twice as much as iron with temperature changes, and as noted above, even iron can be temperamental when it comes to the tiny amount of movement that can affect repeatability during scraping.
    I once got a Busch aluminum straightedge in an auction lot that had a 1/4" steel face bonded on and scraped flat. Make that "flat," in quotes, because the surface plate said it wasn't. Not sure if it came that way from the factory or not, but it looked professional enough. Suffice to say, it got resold on craigslist immediately because I'm sure as hell not going to scrape anything to a bimetal thermostat just because it sorta looks like a straightedge. Life's hard enough even when you've got good tools...just imagine trying to originate a set of 3 bimetal straightedges if you want a picture of machinist hell.

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    I'm less skeptical of this straightedge than others. I think the argument that no old-time builders used it simply implies it may not be optimal for a a high-use application of straightedges. Even if it isn't better for their application doesn't mean it can't have a place. Also, aluminum was not a widely used material until the 1930s, so there can be a factor of conservatism in machine tool rebuilding circles. For example granite was not immediately accepted as a surface plate material, in spite of its accepted superiority for many applications today.

    Let's talk actual pros/cons of aluminum for a straightedge:
    Pros:
    - Low density (1/3 of CI), making it easier to make very stiff straightedges. An aluminum straightedge can be 3x or more stiffer than the equivalent cast iron straightedge of equal weight.
    - Low cost
    - High thermal conductivity (roughly 3x that of cast iron)
    - Okay specific heat (890 J/kg*K vs cast iron at 460 J/kg*K; volumetrically since alumin is 1/3 the density of cast iron, aluminum has roughly 2/3 of the heat storage capacity of cast iron per cubic inch. This means aluminum will track temperature changes in a room better than cast iron)
    - Does not corrode, or corrodes very slowly depending on alloy.
    - Will not dull scraping tools quickly.
    - Certain alloys (eg. MIC-6) can be procured with very low internal stress.

    Cons:
    - High coefficient of thermal expansion (roughly 2x that of CI)
    - Very soft.
    - Raises burrs and dents easily (see above)
    - Poor wear resistance.
    - Poor damping for dynamic components (this is why you won't find aluminum machine tool structures).
    - Softness allows abrasive grit to embed easily, turning the reference into a lap (this is mostly bad but could be used to advantage somewhere).

    To me these properties indicate that aluminum is actually a very good candidate for a reference surface that does not get used for spotting, such as a reference square or straightedge used for indicating. You can make it much stiffer than cast iron, and the high thermal conductivity and low specific heat mean it will come to thermal equilibrium much faster and suffer thermally induced distortion for less time.

    I believe aluminum can be blued and scraped easily enough, as I've heard of this done for engine blocks and turbine housings.

    Of course an aluminum straightedge used for spotting will require a vigilant eye for burrs and dents. It will also require regular checking for wear and rescraping. You will also likely want a special scraper geometry. And may need a different way to deburr it from regular stones. But all of this is quite manageable, and I would much rather handle a 6 foot aluminum straightedge than 6 foot iron one. And for a reference used strictly for indicating, aluminum is likely superior in all respects.

    Whether it's worth $350 is up to you. My comments here were free.

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    If I recall correctly, Busch Precision offered indeed at least a 6 ft parallel-type aluminum straightedge with cast iron strips bonded on both sides. Apparently, they were quite accurate and stable.

    Coming to the straightedges of the original post, even assuming that they are made of stress-free materials, is that the ratio of height and length is not enough and that I-beam would flex too much over the length, if not supported evenly enough. Essentially, they are parallels, not straightedges.

    Paolo

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    Quote Originally Posted by Straightedge View Post
    I once got a Busch aluminum straightedge in an auction lot that had a 1/4" steel face bonded on and scraped flat.
    I have a similar large square from Busch. Heavy aluminum cast frame with iron faces bonded on. I've been satisfied with it. Not too concerned about the thing being a pair of bimetal elements, given the thickness of the sections, and I can lift it (with both hands).

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    I don't know, dissimilar metals like that could cause even worse issues with temperature related warpage. At least when it's just one metal it can grow in a straight line. I'll stick with the iron. For scraping purposes anyway. For inspection where you're not handling the items much the aluminum and hybrid stuff might work fine.

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    Quote Originally Posted by eKretz View Post
    I don't know, dissimilar metals like that could cause even worse issues with temperature related warpage. At least when it's just one metal it can grow in a straight line. I'll stick with the iron. For scraping purposes anyway. For inspection where you're not handling the items much the aluminum and hybrid stuff might work fine.
    I feel the same way. I saw the Busch hybrids, and while I know they make nice stuff (I have a fair amount), those make me nervous unless you have good thermal control where they're being used, especially by the people handling them (insulators on the grab handles, etc.).

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    Quote Originally Posted by Milland View Post
    I feel the same way. I saw the Busch hybrids, and while I know they make nice stuff (I have a fair amount), those make me nervous unless you have good thermal control where they're being used, especially by the people handling them (insulators on the grab handles, etc.).
    Exactly. As elements in a controlled-temp environment and used at the same temp as when ground/scraped flat, the bonded tools should work fine. But in the real world where we don't always have the temp control we'd wish for, the differential expansion of dissimilar metals will unavoidably generate curvature.

    A guy needs tools that deliver the promised goods, so there's probably plenty of work where the bonded edges would work. When we use edges around here, it's usually with the aim of producing straightness to a tolerance of arcseconds and the effort of handling iron edges is insignificant compared with the scraping, lapping, and metrology.


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