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Step 4Seat tube cone
This part holds the end of the seattube at the appropriate distance from the seat tube beam. It's just an #4304 Inside Corner Bracket and a metal cone.
Unfortunately, unless you own a lathe, the cone is one of the few parts that I think you're going to have to let a machine shop make for you. I've heard that there are off-the-shelf cones available as part of an auto clutch alignment tool, but I've never had one in my hand to see if it could be made to work here. Sometimes there are cones for sale on eBay for building motorcycle jigs. I've even heard that some small showerheads can be used for cones.
If you're really desperate, you could make something out of two pieces of angle iron and a long bolt, as shown in the second photo. Hacksaw a taper on the angle iron and weld or braze them to the bolt. You'll then want to rig up some way to file or grind the taper so that it's on center.
Anyway, the cone is, well, a cone. If you plan to work with pointy lugs you might want to include an straight extension at around 26.4 mm to help center things on the cone. Stainless steel would be ideal but mine are just aluminum and they do the job.
The far end of the cone is tapped for an M6 bolt. I drilled the hole in the angle bracket slightly oversize so that I could tweak the exact location of the the cone to fine-tune the alignment.
At it's most basic, you just clamp the part to the beam using two Double T-nuts. There's a good amount of play in the unit, so you have to make sure it stays aligned on center with the beam, otherwise your seat angle will come out wrong.
One way to improve the accuracy of this piece would be to replace one of the fastening bolts with a smaller bolt that taps into a small nylon or brass 'gib' that would ride in the 8020 slot. This would keep the piece from rotating. See the third photo.
Another option is to space all the fixture parts off of the main frame using 3x3 spacer blocks of the same #1530 extrusion. Then you could use #4367 Joining Plates riding on the sides of the beam to hold the plate. This does limit how far up the beam you can slide the cone before you hit the angle indicator / beam fastener. You'll want to make your adjustable beams 3 inches longer if you go this route. See the fourth photo.
Unfortunately, unless you own a lathe, the cone is one of the few parts that I think you're going to have to let a machine shop make for you. I've heard that there are off-the-shelf cones available as part of an auto clutch alignment tool, but I've never had one in my hand to see if it could be made to work here. Sometimes there are cones for sale on eBay for building motorcycle jigs. I've even heard that some small showerheads can be used for cones.
If you're really desperate, you could make something out of two pieces of angle iron and a long bolt, as shown in the second photo. Hacksaw a taper on the angle iron and weld or braze them to the bolt. You'll then want to rig up some way to file or grind the taper so that it's on center.
Anyway, the cone is, well, a cone. If you plan to work with pointy lugs you might want to include an straight extension at around 26.4 mm to help center things on the cone. Stainless steel would be ideal but mine are just aluminum and they do the job.
The far end of the cone is tapped for an M6 bolt. I drilled the hole in the angle bracket slightly oversize so that I could tweak the exact location of the the cone to fine-tune the alignment.
At it's most basic, you just clamp the part to the beam using two Double T-nuts. There's a good amount of play in the unit, so you have to make sure it stays aligned on center with the beam, otherwise your seat angle will come out wrong.
One way to improve the accuracy of this piece would be to replace one of the fastening bolts with a smaller bolt that taps into a small nylon or brass 'gib' that would ride in the 8020 slot. This would keep the piece from rotating. See the third photo.
Another option is to space all the fixture parts off of the main frame using 3x3 spacer blocks of the same #1530 extrusion. Then you could use #4367 Joining Plates riding on the sides of the beam to hold the plate. This does limit how far up the beam you can slide the cone before you hit the angle indicator / beam fastener. You'll want to make your adjustable beams 3 inches longer if you go this route. See the fourth photo.
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Jun 30, 2008. 10:03 AMjimgskoop
says:
If making a cone on a lathe is difficult, could you use a BALL shape instead? Something like a small trailer-hitch ball? See this or this.
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Jun 30, 2008. 10:10 AM
drwelby (author)
says:
drwelby (author)
says:
yojimg- I don't see why it wouldn't work. I'm not sure how round and on center those ball hitches are, but I'd think they'd be close enough to get someone started on their first couple of frames. Good idea!
Reply
Jan 2, 2009. 3:31 PMkurtpaulrobinson
says:
might want to try out some harbor freight plumb bobs....6 bucks a pop, chop the fat end off then thread into it. not sure if they have a large enough diameter but they come in all sizes.....
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