My new frame fixture

I started on this fixture a couple of months ago, then back burnered it.  A couple of weeks ago I started to get active again and this weekend I was able to put the finishing touches on it.  I’m really excited about how it’s come out. 

It is highly insipired by the Arctos Frame Jig that was designed by Gary Helfrich.  The details and implementation are different, but it is setup in the same way and I copied the basic overall shape and virtual BB pivot of the Arctos.  I always liked the idea of the Arctos Frame Jig because it was made of T-Slot extrusion (in my case I’m using 15-series 8020).  That makes the fixture a bit more modular (so that I can reconfigure it for different needs) and kept the largest pieces that I needed to machine relatively small.  It doesn’t make it easier or cheaper to make the fixture compared to other designs, there are still over a dozen custom cut parts on here which required high precision and 8020 extrusion is not cheap.

A key feature of this fixture is that every adjustment is locked independently.  My old fixture had many parameters locked with the same few bolts.  With the old one if I screwed up a miter and made a tube 1mm too short I’d spend 20 minutes rejiggering everything and getting it all aligned again.  On the new fixture I’d just need to unlock a couple of handles, slide one part to make the small adjustment, and lock them again. 

It was really important to me to have a fixture that makes it easy to install and remove the frame. This fixture makes it easy, you just raise the upper head tube and seat tube cones, release the dummy axle using the quick release, and remove the shaft collar that holds the bottom bracket in place. Many other designs have solid rods which run through the head tube and use many more clamps around frame tubing, but I didn’t want mess with all of that.

This new fixture is much easier to configure than my old one.  The basic measurements required are:

  • X and Y from the center of the bottom bracket to the bottom of the head tube (438mm Y, 451.7mm X in the drawing below)
  • Head tube angle
  • Seat tube angle
  • Bottom bracket drop
  • Chainstay length
  • Head tube length
  • Seat tube length

Here is a drawing for the next frame that I’m building which shows these dimensions (in red).  It also lists dimensions for mitering (in cyan).  That is basically everything that I need to make the frame.  This bike will be my new commuter, replacing my Novara Fusion.

The fixture will have scales which allow for direct reading of the first 4 items listed.  Currently there isn’t a permanent scale for the X offset, but I have a simple solution of clamping a ruler in place.  I did make direct reading scales for the head tube angle and seat tube angle, shown here:


The bottom bracket drop also is set from an easy to read scale.  The alignment of the bottom of the dummy axle with the scale shows you the bottom bracket drop (74mm in this example):

It isn’t obvious from photos, but a tricky part of the Arctos Jig (and this one) is that the BB position stays static as you adjust the seat tube angle.  The same is true for the bottom of the head tube.  The seat tube angle adjustment has a virtual pivot point around the bottom bracket despite not having anything connected to that pivot point.  That is done with two slots that are concentric around the bottom bracket.  I cut these on my baby sized CNC mill, this would be a much harder operation to do on a manual machine.  The HTA block has a real pivot which is aligned to be directly under the bottom of the head tube.


The rear triangle setup is pretty easy.  There is a rear triangle tower that has a quick release dummy axle holder that is centered with respect to the tube cones.  It can be slid back and forth by opening two handles, and then you can read the rear chainstay length using a ruler:

Arctos Jigs use solid aluminum for the standoffs to this rear tower.  I hate drilling really deep holes in solid aluminum, so I used 80/20 which already has a hollow cavity down the middle.  The end plates that I made keep the threaded shaft for the locking T-Nuts in place:

I used the bottom bracket post from my old jig, but I’ve always been proud of this design.  The key is using an adjustable locking shaft collar (about $30 from McMaster Carr).  That shaft collar gives me a fine adjustment for the offset so that I can handle 68mm to 73mm bottom brackets easily.  Removing the outer locking shaft collar frees up the frame.  The rod running through the bottom bracket is solid 5/8″ steel and is plenty beefy. 

I’ve already been asked if I’m going to make this fixture as a kit.  The simple answer is no for two good reasons:

  1. It isn’t my design.  I wouldn’t feel comfortable selling anything that was so heavily copied from an existing source.
  2. There are a lot of parts in this fixture that were done on my manual mill and which require a good precision.  I think that a realistic estimate would be 15-20 hours of labor per fixture.  I just don’t have that amount of free time and would rather use what I have to build frames, spend time with my family and friends, ride my bike, etc.

However I am helping Alistair Spence make one of these fixtures for his shop.  I’m excited about the possiblities of us both having compatible fixtures, it opens up options to make unusual hybrids for building tandems or cargo bikes somewhere down the line.

I’m writing this post while on limited sleep, so hopefully it makes at least a little sense.  If you have questions leave comments here, on my smugmug (permanent), or my flickr (free account, so photos will fall off over time).  There are more photos published in both locations than in this blog entry, and you can find high resolution versions of all photos there.


  1. Pat S says:

    I don”t have my head around 100% of the machining and setup intricacies you describe, but a fair amount. What I can totally appreciate based on your comment that you”re running on limited sleep is that you had a chance to get deeply buried in the shop this weekend. It looks like a very friendly and versatile fixture – great to hear that you”re so happy with how it came out.

  2. Alex Wetmore says:

    Thanks Pat.

    The limited sleep wasn”t from late night machining sessions, but from going to parties Friday and Saturday nights and staying up way too late on Saturday. I did get a reasonable amount of shop time this weekend though.

  3. Phil Brown says:

    I don”t think Gary would mind. At any rate, he’’s easy to contact. Email me if you don”t know how.

  4. Zip says:

    I”ve been watching your posts for the past few years from the background. Very nice work. I really like what you”ve done here. If I may, a few questions:
    –When you marked the angles of things on the jig, did you do that after the face with a scribe and an good angle gauge? (or did you plan ahead, and program those into your CNC pieces?)

    –I”m a little confused about the measurements on the head tube. Its not clear to me where the pivot is. Is the allen bolt at at the bottom of the mount? I”m confused on how you are finding out EXACTLY where the bottom of the head tube is (fore / aft, as well as up / down, relative to your datum zero. ) It looks like you get it close, then tweak up down, fore/aft to get it to match your original bike frame CAD layout. It looks like with a cone shaped HTA bottom mount, you”ve got to re-scribe reference lines for every option of Head Tube diameter.

    If you had used a flat bottom head tube base (perhaps with bolt in top spacers to handle a variety of head tube ID’’s) AND used a two pin / curved slot virtual pivot point on the head tube set (same as the BB set up) wouldn”t you have been further ahead?

    Or am I missing something?

    Don”t mean to be critical with my comments.. You”ve got my head spinning with the work (particularly the two pin slot phantom pivot BB thing…) Very nice work. Many thanks for all your work!

    Metro Detroit…

  5. Alex Wetmore says:

    Good questions. The scribed lines for the angles were done with the CNC machine. Some of the reference lines (like the one mentioned below) were done by hand.

    The bottom of the head tube is located exactly at the pivot. To locate this I have a scribed line that is centered on the pivot on the side of the head tube tower. The lower cone is moved until the bottom of the head tube (or head tube lug) is square with that line. You can see a photo of this here:

  6. Pete says:

    Alex, this is wonderful. I have been looking at replicating the Arctos design because it uses commonly available 8020 extrusions, and it also looks like it would be simple to modify/extend. Would you be willing to at least post a materials list (if you could include 8020 and McMaster/Carr part numbers, that would be great), and perhaps dimensional drawings of the machining that you ended up doing for the jig? I understand not having the capacity or interest to create this as a kit, but I do think there’’s interest in, and a market for, a decent and reasonably priced DIY framebuilding jig.

  7. Grant says:

    Hello Alex,

    I am interested in learning to do some fillet brazing to create stems for my various bikes. Wondered if you know of any Seattle area framebuilders or metal works instructors who might offer such a class.

    Thanks, Grant

  8. Theo says:

    Alex, like very much following all the stuff you are doing. Thank you for posting!
    Right now, I am designing “my” jig. Before I order the 80/20 stuff (actually, european 40mm profiles) I would like to know if you used just the 80/20 standard sections ( ) or the light or Ultra-light sections. IMHO the load on the fixture is not very high. So I would take the light version which I can get here easily. Did you experience any deformation? Deformation after applying the heat for brazing? – Theo

  9. Alex Wetmore says:

    I used the standard weight stuff, not the light weight. My jig is built with the SAE dimensioned 80/20 extrusions because it is hard to find other metric sized materials in the US.

    The light sections would probably be fine if that is your only option, but if you can get the heavier stuff then there isn”t too large of a downside to them.

  10. Hello Alex,

    I have been searching and scouring the internet for a DIY frame jig, Yours looks to be the simplest minus the custom parts.

    Do you have detailed photos of the custom parts and possible the dimensions?

    Are you selling jigs? Would you? :)

    I would love more detail on the jig, I see you have a few versions of it.


  11. Alex Wetmore says:

    Sorry, I”ve published all that I”m going to on this design. Since I copied it from a commercial design (the Arctos) I don”t really feel right giving away detailed dimensions and drawings. My photos show some more details than the ones that Arctos provide, so they do make it a little bit easier than what has already been out there.

    I enjoy making bicycle tooling, but have had to step back from selling it to keep up with my day job and other hobbies.