On Monday, February 23, 2015 03:29:26 PM Marcus Bowman wrote: > I guess you will need a matt black > finish, so the tip about producing a matt finish is useful. Acetal > will hold a decent cut thread.
Please bear in mind that the matt finish isn't the deadest black in optical terms. In the tv business we usually used a black velvet for camera black level setting, and its usually "close enough for the girls" us tv engineers went with, but it isn't black. We could, if we really wanted a true black, buy a sheet metal contraption that had a frame to hold the std grey scale step card on its front mouth, but you cut a hole where the velvet was so the light could pass on thru, and into a seashell like spiral of sheet metal rolled up into ever tighter curls, all painted on both sides with the glossiest black paint around. Light went in, and because of the glossy, light was reflected at low angles of incidence until it had all been absorbed, making the hole in the grey card a true black hole but without the gravity. Light went in and wore itself out looking for an exit it never found so 99.99% of its energy was converted into heating the thing. But not too many tv stations actually owned one of them as the guy who made and certified them wanted about $2500, in 1985 dollars for it. In 1985, we weren't able to throw that sort of money out when we could get within 3 or 4% of the correct settings with the velvet from JoAnnes IF it was clean. Dust needed to be vacuumed away almost daily. The flat black used in optics where the real estate for the absorber is not available still reflects 8 to 10% of the light that hits it. The singular exception is the case on modern WA lenses where you will see what looks like a blacked thread cut into the wall of the entry, or maybe behind the first lens cell. Those groves are generally glossy, and designed to reflect the unwanted light back out the front by carefully controlling the angles of the cut so the lens, if it does see the walls, see's them as pretty black from its inside looking out view. > Marcus > > On 23 Feb 2015, at 18:59, Bruce Layne wrote: > > PS - I machine some rubbery plastic parts that have a durometer > > similar to soft urethanes. I freeze the parts to make them harder, > > so they cut instead of smearing and tearing. > > > > On 02/23/2015 11:46 AM, Kirk Wallace wrote: > >> I am using this site as a reference for making some telescope > >> eyepieces: http://home.fuse.net/astronomy/ > >> > >> Larry chose to use urethane to make the blanks for machining. My > >> experience with urethane is that recipes range from rubbery soft to > >> pretty darn hard, but not fully hard. I need something that can take > >> a .6 mm pitch thread and stand up to assembly cycles. PVC pipe is > >> the wrong color, threads okay, but the threads smear easily. Acetal > >> works very well, but can't be bonded or painted. Polyester resin > >> might work. I'm also considering injection molding blanks from PLA. > >> > >> I'm wondering, are there forms of urethane that have the same > >> characteristics as acetal? Are there other materials that would be > >> better? > > > > --------------------------------------------------------------------- > > --------- Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT > > Server from Actuate! Instantly Supercharge Your Business Reports and > > Dashboards with Interactivity, Sharing, Native Excel Exports, App > > Integration & more Get technology previously reserved for > > billion-dollar corporations, FREE > > http://pubads.g.doubleclick.net/gampad/clk?id=190641631&iu=/4140/ost > > g.clktrk _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users > > ----------------------------------------------------------------------- > ------- Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT > Server from Actuate! Instantly Supercharge Your Business Reports and > Dashboards with Interactivity, Sharing, Native Excel Exports, App > Integration & more Get technology previously reserved for > billion-dollar corporations, FREE > http://pubads.g.doubleclick.net/gampad/clk?id=190641631&iu=/4140/ostg. > clktrk _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=190641631&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
