Originally Posted by
repechage
When I looked at the fixture I wondered too about height adjustment- consider though:
not all tubes are true
over time I am sure there has been a transition from metric to imperial OD dimensions
a bottom bracket shell would vary in width from as delivered to dressed finished
Somehow those differences might be accounted for- perhaps using a height gauge off the surface plate?
even inefficient systems can become time efficient with repeated use.
From a quick look, I would work from the headtube "center" to the bottom bracket tube socket centers then the seat lug center of sockets. All else just has to match those.
( assuming tube bows are accounted for and not effecting lateral displacement )
I'm going to assume that he has a "surface gauge" which I presume is the same as what you described as a "height gauge" to check alignment in various stages of a build.
It looks like his fixture can do tandems so that would mean it can accommodate different tubing diameters. I'm willing to bet the Vs on his fixture are height adjustable, it just isn't obvious to me how they adjust. The V bases has a knurled disc that I'm suspicious can be turned to adjust height somehow (either by raising the V or raising part of the base to move the V higher or lower).
Part of his fixture represents the center of a front axle. I'm sure that is for measuring purposes. Its position on the fixture, Oliver said, is established after making the fork. It can move up and down to place BB drop and can be measured from to know where the bottom of the head tube is located. The down tube position would then be established with a lug positioned a determined distance from the representative of the front axle. That system always make more sense to me than using trig because it is not based on assumptions but the actual materials themselves. The value of the DT/HT center varies between lugs.
I'm wondering if more fixture parts stored below the table are what holds the head tube or if he uses the "hockey stick" method of putting a frame together.