Author: pauledwardcarson

Update (08/02/20): PLA has really poor creep behavior, so these work great, but only for a week or so. After a month of use, I lost all of the throw on these flexures.  I still think this is a worthwhile project and if you’re not leaving a load applied on your 3d printed flexures, they’re still a great solution. I want to experiment with other materials or reinforced filaments, but that’s pretty far down my list at this time.

Original Post:

I am very slowly working on a self cleaning cat litter box. One of the problems with automating it is detecting when a cat has made a “deposit”. I initially thought that I could measure in increase in the weight of the whole litter box system. Now my thinking is to detect the increase in mass of the cat getting into the litter box and then leaving, since I don’t want to run the litter box with a cat inside. It also avoids needing precise calibration of the system weight.

My first thought on this was: “No Problem! I’ll just find some cheap load cells and go for it!” It turns out that “cheap load cells” are rare as hen’s teeth.

I needed a super inexpensive way of measuring force. Enter a flexure.  Flexures are awesome because they’re really easy to model with beam bending formulas that anyone can use. They also pair really nicely with a 3d printer and a parametric model. I had already built some flexure spreadsheets based on JPE Precision Point.

Flexures, spreadsheets, 3d printers, and parametric models are an amazing team. You can design your flexure for planned loads, displacements, and fatigue life in a spreadsheet pretty easily. My preference is to have a cell with the factor of safety calculated from the maximum moment and the thickness of the flexure blade with conditional formatting so I know my factor of safety stays above 2.  That lets me ignore fatigue life for most of the stuff I make.

Then you take those dimensions and put them in a parametric model like this one.

It turns a potentiometer via a live hinge in the model. I’m tempted to revise it to have the flexure push a gear rack that turns a gear on the pot, but this was also a fun case for a live hinge. It’s designed that way to make it easy to injection mold, or to make an extrusion in the event the market for custom made flexure scales takes off.

So all of that makes it possible to make a short throw scale that will turn a potentiometer to measure the load on a flexure. Neat!

Some time prior to 2010, I met a woman at a bar in Wilkes-Barre, PA. I chatted her up, and asked what she did. She replied “I’m a sculptor.” Curious, I asked where I could see her work, because I assumed that any sculptor would have outdoor public work. She replied that she didn’t have anything showing at the time.  Intent on making an ass of myself, I then started boasting about a “public sculpture” that I helped to design and build.  It was this pink camouflage tank in Philadelphia. Marissa, (pictured) is not the sculptor in question.

This thing was part of the Kensington Kinetic Sculpture Derby, in May 2008. It was driven by six bicycle cranks on the inside, with a driver and a turret operator. It was very slow. More photos here.

The lady from the start of the story and I went on to be good friends, despite my terrible manners. In the wake of our single abortive date, I decided that my claim to being a street art sculptor was too tenuous, and I designed and built this giant 12 sided die. I chose the 12 sided die because it was instantly recognizable to folks who had done role playing games, and because the twelve sided die is kinda useless in dungeons and dragons, as I recall. It’s in every drawstring velvet bag of dice, but you never really use it. It was also pretty easy to design, since all the panels are the same size and shape, and the angles between each planar side are equal.

Not knowing anything about art, big/red/shiny was very much in earnest.

The 6 foot diameter 12 sided die lived in the sculpture garden on Frankford ave for a couple years. I believe the city cut it and the pink tank up because they were a drop point for local drug deals.

This project satisfied my desire to make public art, or to pretend that I was part of some kind of circa-2008 street art conversation. At this point, I’m not willing to inflict mediocre sculptures on the world without permission. Making giant silly stuff is really fun, but it takes up a lot of space.

Once you’ve done all of the metalworking, it’s time to sand and polish your frame. There might be little lumps of filler metal from brazing, or stray file marks, or lumps in the castings of parts you’re using, or inconsistencies in the welding, etc.  All of these imperfections will show up through paint. If you care about those aesthetic considerations, any of those blemishes need to be blended out with sandpaper.

I like 1 inch wide rolls of sandpaper in varying grits.  They are nice because you can run them over the tubes in a shoe polishing motion, or wrap them around a file.  Be careful not to remove too much material by sanding. They smooth out the surface of the metal tubes by removing metal and it’s totally possible to sand right through the tubes.  Don’t create a thinner tube wall than you designed for by sanding too much.

This is particularly easy to do if you’re using sandpaper to smooth out fillet brazed joints.

There are a couple options for coating your frame:

Powder Coating.

Powder Coating uses electrically charged plastic particles to coat the metal frame, and then melts them all together in an oven.  Powder coat is more durable than wet paint, better for the environment, and almost as good looking. It’s also much less expensive and much less labor intensive.  It’s a common industrial process and most medium size towns will have a local powder coater. To get an idea of what colors are available, call your local powder coater.  You could also talk them into doing other colors.

Columbia Coatings and Eastwood sell DIY setups and materials for powder coating.  They are both good references for the colors available.

Automotive Wet Paint.

Automotive wet paint is also an option.  Talk to your local auto body shop and see what they can do.  Unless you want to seriously nerd out on painting, it’s very unlikely that you can do as nice a job as an automotive body shop at the same cost.

I have wet painted some motorcycle and scooter stuff, along with a few bike frames.  It requires control of the humidity in the atmosphere, scrupulous attention to detail, fastidious cleanliness, and a ton of sanding. Getting good at wet paint makes climbing the TIG welding learning curve look easy.

Spray Paint

This is the stuff you buy at the hardware store. Follow the instructions, use primer and a top clearcoat. It won’t be very durable, but you can do it in your garage.  Emptying a spray can releases lots of stuff you don’t want to breathe. Do it in a well ventilated area. I spray painted a lot of things in my parents’ basement when I was in high school.  It stunk up the whole house. I can’t believe they put up with it.

Spray.Bike

I haven’t used it, but it’s in rattle cans and marketed for coating bike frames.

Direct to Metal (DTM) Epoxy Paints

These are really durable coatings that you can apply with a brush. I rode two bikes painted with this stuff and while they weren’t pretty coatings, they worked great.

If you want to go for a post apocalyptic form follows function look, coal tar epoxy is a good option. It’s ugly, goes on with a brush, leaves brush marks, and doesn’t require primer.

Braze Ons are a catch all term for the extra little doodads that you attach to the frame.  They are typically attached by silver brazing. This is how cable stops, water bottle bosses, rack bosses, chain hangers, pump pegs, and so on are added to a steel frame.

I prefer to attach all of these little parts with clamps made from spring clamps. I bend chunks of welding wire and braze them onto spring clamps, then use the end of the wire to hold the braze on to the bike frame. This is a very rough and ready approach. There are a number of cool tooling options available.  Cobra Framebuilding and Sputnik Tool both sell clamps to attach braze ons.

Braze ons are a really easy way to ruin a nearly finished frame.  Done improperly or attached at the wrong location, they can create a weak spot in the bike tube that can crack and ruin the bike. Worse yet, this crack can appear after only a month or two of use.  I attached downtube shifter bosses to one of the first frames I built up. I rode it back and forth to my day job, 15 miles each way. One morning, I started to hear a creak from my bike. I still had 4 or 5 miles to go, so I kept riding.  The frame started to feel a little looser as I approached work. When I arrived at the office, I noticed a spiral crack working it’s way around the downtube from the area right next to the downtube shifter boss. I had to take the train home that day.  When I cut out that section to fix the frame, I traced the crack to two sources. The downtube shifter boss was attached in the thinnest section of the tube, and I had brazed it on with bronze filler metal instead of silver solder. I don’t know which of these mistakes caused the failure, but I didn’t repeat either of them again.

If you’re going to braze on your braze-ons, use silver brazing. Attach your braze ons in the thicker butts of the tubing if at all possible.

Purists will clutch their pearls, but adhesives are a better way to go.  Metal to metal adhesives ( JB Weld is my favorite ) are very effective with proper joint preparation, and they will not ruin the strength of your tubes. They’re also easier to use than silver soldering, and require zero practice.

Even the low heat of silver brazing will cause some distortion of the tube shape, making the tube less round. This is more of a problem with TIG welding and fillet brazing. To make your headset, seatpost, fork crown race and bottom bracket fit properly, you’ll need to do some post welding machining. Most of this machining can be done with hand tools.  The easiest is the seat tube. Seat tubes need to be reamed in order to have the seatpost fit properly. The seatpost reamer allows the builder to remove just enough material so that the seatpost has a nice sliding fit with the seat tube. If the fit is too loose, the frame isn’t as strong and the seatpost clamp isn’t as effective. If the seat tube inside diameter is too small, the seatpost won’t go into the seat tube, or will be a super tight fit that’s likely to get stuck.

To ream the seatpost, put the frame in a repair stand, remove any removable seatpost clamp, and put some cutting oil on the seatpost reamer. Turn the reamer clockwise while pushing it into the tube. It will cut away a small amount of the inside diameter of the seat tube, so the seatpost fits just right.

Remove the reamer while continuing to turn in clockwise. Turning the reamer backwards will dull the cutting edges.

Reaming and facing the head tube is a similar process. It requires the right size reamers for your head tube and headset combination. If you’re using a press fit bottom bracket, reaming and facing the bottom bracket is the same.  If you’re using a threaded bottom bracket ( which you should!) chase the existing threads in your bottom bracket shell with a bottom bracket tap set.

Park Tool is the biggest manufacturer in the US of these tools. I’ve also used Cyclus tools and IceTools. It’s also possible to buy reamers in just about any size from your favorite machine tool supplier.

Cold setting and Frame alignment

Keeping weldments straight is by far the hardest part of welding stuff. Because of the heat in the process, there frame experiences local thermal expansion and contraction anywhere you heat it up. This causes the phenomenon known as welds “pulling.”

Image from WeldingEngineer.com

Super tight joint fitup helps to mitigate this problem, but it’s still there.  Ideally, you’d check your frame on a surface plate at various points in the welding process to bend it back into alignment or go over your joints again with the welding or oxyfuel torch.  I have heard this referred to as “wizard wanding.” Manufacturers who need really flat weldments with minimal variation will go to great lengths to control those welding processes. They might have preset welder settings, super detailed instructions for how long any segment of weld can be, what order to weld each segment in, and so on. If you’re not already welding at a professional level, just go for it and accept that your frame is going to dance around a little bit.

There are tools to cold set dropout alignment and to check the frame alignment. Park Tool also sells a frame bending tool.  I’ve used it, it works. You may well need it for your first couple frames.

This is where the rubber meets the road. Your tubes are filed, fitted, and ready to weld. You have you favorite welding or brazing setup. How to turn this pile of metal pieces into a component that you can bolt parts to?

Commercial Fixtures:

There are fancy commercial fixtures out there. Anvil, and Sputnik are the two big names that I’m aware of, though I’m sure there are more. These are sweet. They’re made for pro framebuilders who need serious efficiency.  I’ve never used one to build a bike, but I have met many builders who swear by them.

Mid range Commercial Fixtures:

Joe Bringheli sells some reasonably priced fixtures. I have met many hobby builders who have used his fixtures, and they seem great. It’s unclear to me how easy they are to set or how repeatable they are.

DIY Fixtures

I love solving meta problems. I’ll happily make tools to make tools to make tools. This is sometimes referred to as yak shaving.  With that in mind, I’ve made a number of iterations of DIY fixtures. My first one was a straight ahead Andrew Hague copy ripped straight from an illustration in the Paterek Manual.

This worked well, but was limited to fixed tube diameters.   Some might poo poo the square tube used, but they show up very straight from your local steel distributor and are very inexpensive. If you’ve done any other welding fabrication work outside of bikes, you know how nice square tube like this can be to use.

A couple iterations on this landed the design at the fixture in this rambling video, where I set it up.

CAD Files for the fixture Assembly

Around 2014 I was trying to sell tools like this in an ill advised business. While the business never really took off, it made me refine these tooling designs into a coherent system using 80/20 extrusions. 80/20 extrusions are nice because they require much less machining and they’re nice and straight and flat. They’re priced for business to business sales, but they also have an ebay store where they sell off surplus at very reasonable prices.

(I still need to gather up these designs and post links to the assemblies.)

A flat surface.

The basic building block of all these fixtures is a flat surface with standoffs of known length. If you have both of those things and the ability to make precise measurements over the distance of your frame, you might not need a fixture. It’s entirely possible to make a nice straight frame using some blocks, a flat surface, and lots of care.  A full scale drawing helps with this too.

In the machining industry, flat surfaces like this are ubiquitous for measurement. They are referred to as “Surface Plates.” Vendors like McMaster, MSC, Shars, and Grizzly sell them in varying sizes and flatness ratings.

In the welding industry, great big flat tables are really useful for exactly the same task as welding bike frames. Lots of stuff needs to be aligned to a plane and then welded. Acorn welding platens are the canonical table for this task. Sadly, they are no longer in business. Weldsale sells a similar table. I have used them and they’re really nice. There are a couple of options geared toward casual users. Stronghand tools has a nice cart with a flat plate top. Certiflat is another more DIY option.

I’m excited about the potential for 3d printed parametric models and single use tooling. Done well, you could make plastic alignment tools that would allow just about any joint configuration to be held securely and welded with much lower costs than all of the tooling listed above. Validating the results would require a surface plate or other measuring devices, but those precise measurement tools could be reserved for measurement, keeping them nice and precise.  Nothing ruins a cast iron surface plate faster than weld spatter.

For your first bike, copy a bike you already like. Just about every manufacturer publishes the geometry for each model and size, so it’s pretty straightforward to copy.

Once you’ve decided to make something that you can’t buy, BikeCAD is the way to go. It’s a bargain at $500 (Canadian!). That’s under $400 US at the time this was written. BikeCAD has libraries of most commercially available parts, and it will create mitering templates to cut your tubes.

RattleCAD is an open source alternative to BikeCAD. I have used it a couple years ago and I liked how fully featured it was. I found it less user friendly than BikeCAD, but it may have improved in recent versions.

If you already have a favorite CAD software like Solidworks or Fusion360, just use that.

When I made my first frame, I built a spreadsheet based on the Paterek manual that would calculate all of the lengths and miter angles for me. I used this in combination with this tube coping calculator. Tim Paterek published the first edition pdf of his manual on his website, but it seems to be down as of 2020.

For the true retrogrouch, there’s always the option of making a full size drawing on a big piece of paper.  This has the benefit of working nicely with the build method of laying out all your parts on top of your 1:1 scale drawing. It’s a lot harder to revise than a CAD model though. If you really must do this, at least make a spreadsheet to do all of your fit calculations. I’ve made a couple full size drawings just for laughs and it’s super satisfying but not when you have to do it over again and again and again. There’s also the option of having a full scale drawing printed.  Even in 2019, there are still printing businesses for blueprints, because they’re still used in the construction industry. A sign printing business could potentially do this too. The blueprint businesses will be able to reliably print to scale. Signmakers are more of a gamble. If I were going this route, I’d make some reference features of known size on my drawing so I could quickly check the scale when I received the drawings from the printer.

A couple notes on safety:

Product liability follows you forever. Even if the end user abuses or misuses your product. If you’re going to sell (or even give?) bikes to other humans, you should carry insurance and do the stuff that the insurance company requires you to do. After making a bunch of bikes, I wish I had cut more of them in half once I had ridden them.  That’s not because I’m ashamed of them, it’s because my name was painted on the side and now they’re in the hands of who knows who. I’m responsible to those mystery third parties for bikes that I have no control over. Insurance is expensive, around $2000 per year around 2015.

Making your own vehicle is amazing. I encourage it, that’s why I wrote this. Be cautious about what happens with stuff you make, because it could get real expensive.

You’re building a steel bike with cheap tubes, because it’s a first try (if it’s not the first, you can call it an “early prototype”). Here’s a list of resources.

Tubes:

Make your first frame from straight gauge tubes. Fancy bike tubes are butted, which means they have thinner walls in the middle for weight savings and thicker walls on the ends for weldability. Those tubes are nice, but you want basic tubes, that have the same wall thickness all along their length. What you want is 4130 seamless tubing.

4130 tubes are sold by outside diameter and wall thickness. Bike materials are typically specified in millimeters, but aircraft materials are specified in inches. Some units back and forth may be required. 1 inch = 25.4mm.

Here’s a chart with common outside Diameters and wall thicknesses.

Here are some online vendor options:

McMaster. I could go on all week about how great McMaster is. They have everything you want for making stuff out of metal. They have great customer service and will sell small quantities. They have a reputation for being expensive, that isn’t always true at those small quantities.

Aircraft Spruce – They sell stuff to build airplanes. Some of that stuff is 4130 tubes.

Wicks – They also sell stuff to build airplanes. Some of that stuff is 4130 tubes.

Online Metals – The name says it all. They have 4130 tubes too.

Everything Else:

These vendors have brazons, dropouts, and other small parts.  If you must use butted tubes, they have those too.

Nova Cycles Supply – They have lots of good stuff and will sell framebuilding kits along with individual tubes. I’ve done a lot of business with them over the years and they’re good folks.

Henry James – They also have lots of good stuff and will sell kits along with individual tubes. Their braze ons, lugs, and other misc doodads are nicer than than the Nova ones.

Joe Bringheli – Bringheli is great, a super old school DIY framebuilding source.  I’ve ordered from him a number of times and it’s always a good experience.

Richard Sachs – Richard Sachs is awesome, and his stuff is top quality. Probably not a great choice for a first frame, but worth mentioning.

Paragon – Paragon Machine Works sells all the dropouts, brazeons, and other doodads you want. They have all the parts you want except tubes.
Torch and File – They sell Reynolds tubes and have some super nice tools.