Saturday, April 02, 2011

Stem Jig

I've been wanting to do stems with frames and along with that make a batch of sizing stems for fitting purposes to keep around the shop and loan out as needed.  I came up with this jig from mostly scrap around the shop with some 4130 tubing and a trip to copper state nut and bolt to buy way more hardware than I ended up using in the final product.  The steerer clamp is 1" pipe brazed to the angle iron and I've just been using a 1 1/8" sleeve for the bigger steerers.  I made two bar clamp pieces for 1" and 1 1/4" bars.   The jig is straight as best I can tell with machinist squares off the angle iron.  I've had mixed results with the stems as far as straightness...this is HARD....or rather building a stem is easy, building a straight stem is hard.  Especially with hand files.  I've made four stems so far.  I've been trying to dial in the most efficient process and have that part down I think, but now I'm going to go back and do some blanks of just tubing without binders to figure out the best tacking sequence and hopefully get a repeatably straight end result.  To check straightness (handlebar twist in one hand rests higher/lower than the other) I used an extruded piece of aluminum rectangle, a straight 7/8" "handlebar" about 30" long and some shims from PMW.  I clamped the center of the bar in the stem and clamped the faced steerer clamp to the aluminum and measured the gap between the two at both ends of the bar with calipers.  Two (first and fourth) were 2mm off centerline, one (second) was 7mm off, and one (third) was dead center.  Whoa!  I've been trying to figure ways to check other planes of alignment, but they just don't seem as important.  If you have any ideas (BOBBY) I'd love to hear them.

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