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I did support the prototype plate under each end, pushed down in the middle with maybe 40 lbs of pressure and measured .003 deflection.
In the past few days I have read so much and watched so many videos about preparing CI straight edge castings by plaining, milling, and grinding my eyes are crossing. I've learned that plaining may be the ideal preparation for scraping a part this size. Whatever I do, I still see stretching a length of belt sanding paper on a plate and "lapping" or "grinding" from let's say .002 to .001 a legitimate option. There's no clamping distortion when doing that.
I also decided to add stiffness/support (before final surfacing) to my prototype by attaching legs. The end result will be a channel shape with the legs appx 3/8 thick and 3" tall running the length on each side. The legs can be removed for easier machining of the many changes to come (the shape and dimensions of the counterbore will be changed). Since CI stock of this size for legs is not available I planned to use cast aluminum. My suspicion that this may also be a legitimate idea has been confirmed by this:
https://www.penntoolco.com/a6100/
A precision parallel/straightedge that uses a cast aluminum "leg" to provide support for a thin CI surface with straightness of .0003 over 36". The fact they are mating these two different materials and expect the complete assembly to remain stable to .0003 ($2500 worth of .0003) is informative. Oh, and they are "ground or scraped".
The CI plate is at a nearby shop to be machined to accept the legs. Aluminum material for the legs is on the way. I'm working on the walnut mock up legs now which will do until the aluminum ones are done. I ordered my Biax hand scraper starter kit today. And the CI straightedge castings (for practice scraping) are being rough milled at another shop. He claims he can get them to .001 by milling. And I dropped a steel plate yesterday, crushing the small bone and fracturing the middle bone of my big toe. Hence, the marathon of research I've done today on the couch.
In the past few days I have read so much and watched so many videos about preparing CI straight edge castings by plaining, milling, and grinding my eyes are crossing. I've learned that plaining may be the ideal preparation for scraping a part this size. Whatever I do, I still see stretching a length of belt sanding paper on a plate and "lapping" or "grinding" from let's say .002 to .001 a legitimate option. There's no clamping distortion when doing that.
I also decided to add stiffness/support (before final surfacing) to my prototype by attaching legs. The end result will be a channel shape with the legs appx 3/8 thick and 3" tall running the length on each side. The legs can be removed for easier machining of the many changes to come (the shape and dimensions of the counterbore will be changed). Since CI stock of this size for legs is not available I planned to use cast aluminum. My suspicion that this may also be a legitimate idea has been confirmed by this:
https://www.penntoolco.com/a6100/
A precision parallel/straightedge that uses a cast aluminum "leg" to provide support for a thin CI surface with straightness of .0003 over 36". The fact they are mating these two different materials and expect the complete assembly to remain stable to .0003 ($2500 worth of .0003) is informative. Oh, and they are "ground or scraped".
The CI plate is at a nearby shop to be machined to accept the legs. Aluminum material for the legs is on the way. I'm working on the walnut mock up legs now which will do until the aluminum ones are done. I ordered my Biax hand scraper starter kit today. And the CI straightedge castings (for practice scraping) are being rough milled at another shop. He claims he can get them to .001 by milling. And I dropped a steel plate yesterday, crushing the small bone and fracturing the middle bone of my big toe. Hence, the marathon of research I've done today on the couch.
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