by KCStudly » Tue Apr 21, 2015 11:01 am
... and while we're at it, check for any twist from side to side: select 3 equal thickness shims; place one shim on each of two opposing diagonal corners; pull a string taut over the shims; and use the third shim to gauge each of the xmbrs (use an additional or thicker shim, if required to adjust for differences in height from xmbrs to main rails). Repeat for the other diagonal. The gap should be the same everywhere (after compensation).
The purpose of the shims under the string is to reveal any positive height. In other words, if you just stretch the string over a surface and it touches everywhere, it does not tell you if the surface is flat or crowned, just that it is not shallow. By using the extra spacer blocks the string becomes a reference that you can gauge from. (BTW, this technique has been used with wire instead of string in large steel fabrication, such as for aligning critical machinery in ship building... like submarine steering and diving shaft hull penetrations and other shafting applications.)
KC
My Build:
The Poet Creek Express Hybrid Foamie
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"
TnTTT ORIGINAL 200A LANTERN CLUB = "The 200A Gang"Green Lantern Corpsmen