Intro: The Simplest Bicycle Framebuilding Jig I Could Come Up With...
Here's a jig for building bicycle frames. This design is something I've been slowly picking away at for several years. Being both a cheapskate and not having much in the terms of machine tools and/or skills to use them, I've slowly come up with this design.
It's based off the easily available 8020 extrusion, and repurposes a lot of the 8020 small parts. I've tried to make every part requires as little machining as possible. In many cases, I've designed individual parts so that they can be initially made with as little tooling as a drill press, and slowly replaced with nicer machined parts as time/money/tooling allows.
If you want to machine all the parts yourself, you will need a small lathe (a 6" mini lathe is big enough) with a four-jaw chuck and a milling machine (or a milling attachment for the lathe) to make some of the parts. All the machining is simple and would make good practice in a community college machining class, if you don't have the machinery. If you do have to take it to a machine shop, it shouldn't be too expensive.
This Instructable doesn't give exact dimensions or step-by-step instructions. I've left it slightly vague to encourage the builder to customize it. If you want to build BMX frame or XXL 29ers you might want to change the overall dimensions. I've also some included some fancier or alternate parts to consider, if you want to make a nicer jig, practice your machining, or just have an easier time spending money than I do.
In case you're wondering, I don't have plans for sale at the moment. However, if you like the design, you can show your appreciation at my Amazon Wish List. If you get stuck on a part let me know in the comments and I could post more detailed plans. I have some parts drawn in Autocad.
This design and accompanying directions are licensed under the Creative Commons Attribution/Share-alike license. That means you can do anything you want with this design, as long as you give me credit, and allow derivative works to share this license. If you want to machine parts for this design and sell them, go for it. Heck, if you want to build full jigs and sell them, that's fine with me. Just give credit, and maybe buy me a beer if we ever meet. If you do make and sell new and improved parts for this design, I'd love it if you could send me one of your parts too.
Step 1: The Frame
The frame is built out of 8020 #1530 extrusion. I used the standard 1530 extrusion - there is also a 1530L extrusion that is a little lighter that you could probably use, especially on the adjustable beams that hold the tubes. This would make the jig a little lighter and cheaper
You could probably even build it out of the #1020 extrusion and it would be just fine.
The overall dimensions I used, which should work for most mtbs and large road frames, are 30 inches tall and 51 inches wide. If you want to build 29ers with long-travel forks, you might want to scale everything a couple of inches taller.
At the lower right hand corner the two frame beams are held together with a #4334 Inside Gusset Corner Bracket, so the lower beam is shorter - 48 inches long.
The other three corners are held together using just 16mm M8 buttonhead bolts and 8020 Double T-nuts. The beams are drilled to provide allen wrench access to the bolt heads.
Assembly is easy. Use a T-square to get each join at a 90 degree angle. Tighten it down.
You can order 8020 extrusion and parts from a variety of resellers - see 8020's web site. If you're in the Western US, I highly recommend F & L Industrial Solutions.
We'll look at the rest of the parts in the order they would be adjusted to set the jig up to the right dimensions.
Step 2: Dummy Axle
The first thing to set on the jig is to adjust the dummy axle to give you the correct bottom bracket drop.
The dummy axle is machined from threaded rod. I used 5/8" rod but you can use whatever is sitting in the remnant bucket at the local recycler. It should be bigger than 1/2" but smaller than 7/8". The bigger you go, the stiffer it will be, but if it's too big it might not work well with the smaller shrouded dropouts like the Ritchey socket dropouts.
Two recesses are turned into the rod at the appropriate distance apart. You'll have to make one dummy axle for each axle size. The end of the rod needs to be turned to press fit into the base plate - which conveniently is either an 8020 Manifold Plate (#2350) or a Base Plate (#s 2140, 2141, 2145) . The location of the hub centerline of the dummy axle will vary depending on how you make the bottom bracket post.
The general order of operations is this:
1. Center in the 4-jaw chuck.
2. Bore a hole to a convenient size smaller than the root diameter of the threaded rod you're using.
1. Turn the press-fit section to size (about 1 thousandth over your bored hole, I believe) and part off so it's about .7" long.
2. Clamp the press-fit section in the chuck and rough out the closer recess.
3. Finish the recess to 10mm OD and face off the inside flat where the dropout will be clamped, so that it is located the correct distance from the frame centerline.
4. Flip the piece around and turn and finish the other recess. Since you flip the piece you can alway make your cuts close to the chuck so you don't have to turn it between centers. This is also handy if you have a mini lathe that wont fit the threaded rod through the headstock.
The clamping nuts are turned down so one side can fit into shrouded dropouts.
You could also modify this design to be adjustable over several hub widths by making the recesses wider and using multiple center sleeves. They would be held on center by drilling the sleeve and axle and fixing them together with a clevis pin.
It a (lathe-less) pinch, you could just start out with 3/8" threaded rod (or 10mm if you can find it) and use doubled-up nuts on the inside of the dropouts. The base plate would just be drilled and threaded for the rod. It won't be as sturdy as the above design but it wouldn't require any lathe work. You could always reuse the base plate later by boring it out to a larger size as shown in the original design. It actually wouldn't be a bad idea to have one dummy axle assembly built this way in case you end up building something with an asymmetrical rear end or an unusual rear hub spacing. See the second picture.
You can see in the last picture how the bottom bracket drop is set. A piece of a measuring tape from a Dollar Store tape measure has been glued to the end of the frame upright. A notch has been filed and inked at the center of the base plate as an indicator.
Step 3: The Bottom Bracket Post
The BB post is machined from another 8020 manifold or caster plate. At the center it is drilled and tapped for a large bolt, anything around 1/2", though as I mention below it wouldn't be a bad idea to use the same size bolt as the threaded rod you used for the dummy axle. The bolt head is slotted for a washer to be brazed or welded in place. A piece of flat stock with a hole in the center serves as large washer to clamp the BB in place.
On a lathe two grooves have been cut to fit the inside of standard and eccentric bottom brackets. Since the plate is 3/4", or 19mm, if you cut the groove 3mm deep, it puts the centerline of the frame at 50mm off of the adjustable beams (19-3 + 68/2 = 50!). Extra material can be removed outside of the outer groove, and you can even taper it back away from the front face a bit.
On the milling machine you should remove a little bit of the outside flange for clearance on certain bottom brackets. You can mill a bit more off the rear edge for even more chainstay clearance. See the second photo.
Since the groove is turned 3mm deep into the face of the plate, you will either have to drill the 4 mounting holes on the plate a little deeper, or you can grind or turn the M8 button head allen bolts heads to give them a lower profile.
If you need an latheless alternative, You could drill the plate to take 4 spring pins or socket head allens. This would work to hold the bottom bracket shell on center. This would however move your centerline out 3mm from the above design. See the third photo.
These two methods require the bottom bracket shell to be faced accurately. The third photo shows a nicer design that you could have turned that would align the shell to the threads. It attaches to blind-mounted allens, much like the main frame is fastened together.
The beam pivots around an M8 allen head bolt that sits inside of the central cavity of the 8020 beam, as shown in the fourth photo. This gives you room for the clamping bolt of the BB post to thread into the base plate. It looks funny, with so little contact, so if you have an end mill you could at least mill a small flat for the bolt head to seat on. But it seems to just work fine without it. The bolt threads into a 8020 T-nut. You can load the bolt from the end of the extrusion so you only need to drill one hole through the beam, big enough to allow the bolt to pass through and meet up with the T-nut. It will be big enough to access the bolt with an allen wrench to tighten and loosen the bolt through the center hole of the BB post.
There is a little slop between the bolt and the t-slot. In practice, I don't think it makes much difference, but you could always turn a little spacer out of brass rod to take up some of the slack.
To set the chainstays, take the chainstay length and subtract half of the dummy axle width and half of the fixing bolt width. If you're clever and used the same size for both, you just subtract that number. Then just measure from the outside surface of the dummy axle to the edge of the hole. Slide the seat tube beam until it's correct and then tighten down the allen bolt.
If you wanted to build tandems, it seems like you'd just need to make 2 of these adustable beam assemblies with BB posts and seat tube cones, and a longer main frame to fit the length of the tandem frame. Or if you don't mind building the frame in segments, just fit the front adjustable beam with another bottom bracket plate. It won't interfere with single bike production, but would let you build the front triangle in the same manner as a single, and then transfer it forward to the other beam and build the stoker's portion of the frame and the rear triangle.
Step 4: Seat 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.
Step 5: Angle Indicator / Beam Lock
The piece locks the beam at the correct angle, and works as an angle indicator. After you have set your chainstay length at the bottom bracket, you set the seat angle here. It should be self-explanatory, really.
It's a #4375 Inside Corner Bracket. I milled a slot connecting the outer two holes with a milling machine but you could drill a series of holes and then finish the slot with a round file. A jeweler's saw would work too.
The T-handle is a standard 3/8" model from my local hardware store. I used a short carriage bolt from the hardware store, cut to length and ground it to fit the slot. 8020 does sell T-slot Studs but they are 5/16" or 8mm, not 3/8".
I took the nerdy approach and mocked up the angle indicator in Autocad, printed it out, and used the paper pattern to file little notches in the end of the angle bracket. They line up with one of the 8020 Align-A-Grooves to indicate the angle. The groove is inked so you can see it better. It's probably faster to just set the notches by just using a protractor to set the angle between the adjustable beam and the frame.
The angle bracket attaches to the beam with yet another double T-nut and matching 16mm M8 button-head allen bolts.
Step 6: Front-Center Adjustment
The next step in setting up the jig for a frame would be to set the pseudo-front-center. It's not the real front-center (the distance from the bottom bracket to the front hub axle, but actually the horizontal distance from the bottom bracket to the steering axis.
I design my bikes in CAD so this is an easy number to find. If you don't use CAD you will have to draw a scale drawing to correctly set this distance.
You could certainly set the location of the head tube by trying to measure the top tube length from beam to beam but that seems a bit finicky and vague on this design. However, there are lots of grooves on the extrusion to give you something to measure off of.
This part is just another 3/8 T-handle and a modified carriage bolt. To give me a more accurate point to measure to, I chucked a 3/8 stud in my lathe, threaded on the T- handle, and drilled a small indicator hole on center. The T-bolt goes through a hole drilled all the way through the adjustable beam.
Like on the seat beam, even if you bore the hole through the beam to an exact fit, there will be some play between the T-bolt and the main frame. You could take up this slop with a small piece of brass rod turned to fit. However, for most garage builders, I don't think the slop will make much of a difference.
Step 7: Head Tube Cones and Angle Indicator
The head tube cones are just like the seat tube cone, only larger in diameter.
If you can't get cones made, you could probably rig up something like the simple version of the bottom bracket post, using four socket head allen bolts. You'll have to make one set for each headset size, and you might have to trim the lower corner bracket to make room for the down tube.
I have glued another piece from a cheapo tape measure to this beam so that I can locate the head tube at the right height. Just load in your head tube and slide the lower cone assembly up until you're at the right height.
The angle indicator is the same as the other one, too.
Set your head tube angle and you're ready to start loading tubes!
Step 8: The Mounting Bar.
At first, I was planning to just bolt the frame to a small engine stand. You can get them for under $50 from your local Harbor Freight or other discount hardware supplier. They make a sturdy stand that you can roll around and you can rotate the jig around for better access.
However, since you probably have a Park stand, you can just rig a bar across the back of the jig. I used some surplus 1.5" pipe and hacksawed half of the tubing off to leave a flat. It was then drilled and mounted with T-nuts. The advantage to this design is that not only can you spin the jig, you can also tilt it away from you if you want the frame horizontal. Since the bar is not at the center of gravity, you should to run an old inner tube or bungee cord back down to the rear of the Park stand to insure the jig doesn't slowly flop forwards while you're working on it. There is enough room to mount the bar on the front of the main frame uprights and it might balance better that way. I haven't tried that yet.
Step 9: That's It!
Go build a jig.
Like I said earlier, I'm licensing this design with the Creative Commons Attribution / Sharealike license. Do whatever you want with the design. If you want to machine and sell parts kits for it, go for it. Just give me and these instructions credit. And if we run into each other, you could buy me a beer or maybe hook me up with some 29" tires.