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Frame Fixture

I am building a frame fixture using 8020 and some simple machined parts. This is based on my learnings from the simpler one that I built, plus inspiration from photos of the Arctos Jig.

My fixture has these design goals:
* Flexible so that it can be used for oddball bikes (cargo bikes, tandems, etc)
* Breaks down into small linear sections for storage
* Cheaper than a used Nortac or similar jig. It looks like my total cost will be about $300 and ~20 hours of machine time.
* Not annoying to use, but also not designed to be very quick to dial in like most professional jigs. I have no plans to become a professional framebuilder and don't mind spending 30-45 minutes setting up a jig.
* My first jig was too adjustable, which made it easy to get things off of the centerline. On this one I fixed the centerline at 2.5" and am machining parts accurately so that I don't need to make them adjustable.

I'm highly influenced by the Arctos jig since that had a lot of design similarities (with much nicer execution) to my first jig. The rear triangle dummy axle is going to be a complete ripoff of the Arctos since I never came up with a design that I liked before. The Arctos is much slicker when it comes to setting angles.
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Current jig progress.  Front triangle is mostly done.<br />
<br />
The main components are:<br />
* A center spine made out of 2040 extrusion (2" by 4")<br />
* Two diagonal supports, one for the HT and one for the seat tube.  Those are 1010 extrusion (1" x 1")<br />
* Connecting blocks to connect the diagonal supports to the main spine<br />
* Tube holders on the diagonal supports.
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Current jig progress. Front triangle is mostly done.

The main components are:
* A center spine made out of 2040 extrusion (2" by 4")
* Two diagonal supports, one for the HT and one for the seat tube. Those are 1010 extrusion (1" x 1")
* Connecting blocks to connect the diagonal supports to the main spine
* Tube holders on the diagonal supports.

8020bicycleframe fixture

  • Current jig progress.  Front triangle is mostly done.<br />
<br />
The main components are:<br />
* A center spine made out of 2040 extrusion (2" by 4")<br />
* Two diagonal supports, one for the HT and one for the seat tube.  Those are 1010 extrusion (1" x 1")<br />
* Connecting blocks to connect the diagonal supports to the main spine<br />
* Tube holders on the diagonal supports.
  • Loosen the middle bolt and you can move the headtube assembly forwards or backwards (changing the front center).<br />
<br />
Loosen the top and bottom bolts and you can move it up and down.<br />
<br />
Loosen all three and you can change the angle.
  • This is a tube holding tower.  There will be 3 of them, one at each end of the head tube and one at the top of the seat tube.  The blocks are built on 8020 linear bearings, model 6716 or 6715.<br />
<br />
The center of the cone is exactly 2.5" from the top of the 8020 extrusion.
  • A tube holding tower in place.  Notice the brake handle to lock it into place.
  • This is a better view of the tube holding tower.  The linear bearing is a 8020 piece.  I made the cone and the block which connects the cone to the linear bearing.  No rocket science, but locating the cone precisely is important.
  • This is what the inside of the linear bearing looks like.  It slides on UHMW.  The brake is just a knob with a 1/4-20 stud and an 8020 nut.
  • The cone is secured in place with a center bolt.  This one is sized for a 25mm to 30mm seat tube.
  • The bottom bracket post.  It fits onto the end of 80-20 1010 extrusion and is designed for a 2.5" working height (just like the tube holding blocks).  There is an adjustable width split collar for small height adjustments.
  • The bottom bracket post diassembled.  The key things here are getting the top and bottom of the mounting block to be flat and perfectly parallel.  The adjustable shaft collar came from McMaster Carr and made the whole design possible and easy.
  • The assembled bottom bracket post.<br />
<br />
It is important to make sure that the post is exactly perpendicular to the supporting block.  This is done by locking the bottom shaft collar and the top of the adjustable shaft collar.  Expand the adjustable shaft collar to lock the bottom against the supporting block.  Tighten the bottom of the adjustable shaft collar.  Since the supporting block is machined flat the tube will be perfectly perpendicular.<br />
<br />
Once this is done the top of the adjustable shaft collar can be opened and used for height adjustment.<br />
<br />
The cones are from my previous fixture and have a shallower angle than I'd use now.  They work fine though.
  • This V-block was made out of steel and will be used for both fork building and alignment.
  • This is a dummy axle clamp which can mount onto 80-20 20 series extrusions.  I think it should be pretty easy to make a batch of these using a 1/2" ball end mill.
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  • This shows the measurements needed to set BB drop and the lower head tube point.  It is pretty easy to setup the jig with those 4 measurements (A, B, C, D shown above) plus head tube angle and seat tube angle.
  • The "ANT Lyon" fork jig plans.  Jeff Lyon suggested the idea to me of making a fork jig where the fork legs curve down towards the jig instead of away from it.  This makes it a bit more compact if you are often dealing with large fork offsets.<br />
<br />
The actual implementation is highly influenced by Mike Flanigan's (ANT) fork jig made out of 80/20.  You can see that one here:<br />
 <a href="http://www.flickr.com/photos/antbikemike/2292009052/">http://www.flickr.com/photos/antbikemike/2292009052/</a>
  • Guest

    on January 21, 2009

    Hi Alex - I've been following your projects for a while; we seem to be following a similar path. I recently put together a similar frame jig, but it has one major difference. I arranged the HT and ST supports perpendicular to the main beam. It simplifies setup and gets the rear dropouts up to the level of the beam, making it easier to build the support.

    I've got some pictures posted to Flickr here:
    http://www.flickr.com/photos/jerryw62/3065751541/ -jig.
    http://www.flickr.com/photos/jerryw62/3208296131/ -headtube area.
    http://www.flickr.com/photos/jerryw62/3208296429/ -with a partial frame I'm working on(fork mounted temporarily to take some measurements).
    There are a few other random pics of it in my photostream as well.

    I'm not sure this will be a help to you, but I thought I'd throw it out there anyway.

    Regards,
    Jerry Wick

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