Re: a different way to do cross rods!!?
Posted: Wed Nov 13, 2013 12:47 pm
Thanks for that tip, Will. (I don't have a lathe.)
The drilled end of the 3/8" square bar seems plenty robust to me. I like that it avoids having an aluminum pin. I have a prejudice against aluminum bearing surfaces. The one in the photo has a brass pin in one end - to facilitate tapping screw holes into the cross rod for the knee lever cradle - and a steel dowel pin in the other. Both 1/4". They fit tight enough to require light hammering, but I epoxied them also. The rod mounts in nylon bearings.
I centered the hole just by measuring carefully. It seems to me that the concentricity is not actually all that critical. I did take a lot of care to clamp the rod perpendicular to the drill press table. My hunch is that the error is in the same ball park as the error I'd get with a lathing or annular milling process (if done in an urban tenement apartment like the one I work in). If I were doing a bunch of these, maybe I'd devise a jig that would better insure that the second hole drilled was in line with the hole at the other end (which is what's critical).
Re: hollow mills: I have a number of counterbores that act as annular mills if you leave the centering pin out. I don't have one large enough for a 1/4" pin. But maybe people shopping for hollow mills/annular cutters might want to check out counterbores too?
Jon
The drilled end of the 3/8" square bar seems plenty robust to me. I like that it avoids having an aluminum pin. I have a prejudice against aluminum bearing surfaces. The one in the photo has a brass pin in one end - to facilitate tapping screw holes into the cross rod for the knee lever cradle - and a steel dowel pin in the other. Both 1/4". They fit tight enough to require light hammering, but I epoxied them also. The rod mounts in nylon bearings.
I centered the hole just by measuring carefully. It seems to me that the concentricity is not actually all that critical. I did take a lot of care to clamp the rod perpendicular to the drill press table. My hunch is that the error is in the same ball park as the error I'd get with a lathing or annular milling process (if done in an urban tenement apartment like the one I work in). If I were doing a bunch of these, maybe I'd devise a jig that would better insure that the second hole drilled was in line with the hole at the other end (which is what's critical).
Re: hollow mills: I have a number of counterbores that act as annular mills if you leave the centering pin out. I don't have one large enough for a 1/4" pin. But maybe people shopping for hollow mills/annular cutters might want to check out counterbores too?
Jon