Paul Carson Designs

First off, this article series  (Book?) is about hobby building. It’s intended to share what I learned over a number of years of hobby frame building and some commercial framebuilding (that was really just hobby building with much higher costs.)

No one believes that their first frame will be trash. I didn’t when I built my first frame. Embrace the fact that your first frame will be trash. Give that trash a hug! Buy extra tubes ( or better yet, but straight gauge tubes). Copy an existing design.

Why copy an existing design?

If you’re enough of a bike nerd to want to build your own frame, you probably have an older steel bike that you like and are familiar with. Copy this frame. Copy this frame because you already know you like it. Frame design and geometry is a guessing game until you’ve made a number of frames in your size and then ridden them. You might also have a complete set of components for that old steel frame. If you copy it, you can move all of the components over to your new handbuilt frame and ride it. Chances are that you’ll love your new frame just because you made it, even though it’s a piece of crap. Embrace this bias and ride the wheels off of your new shitbox. Then break it down, build the original older steel frame back up, and realize that your first frame was inferior to the old production donor frame in every way.

Why a steel frame?

Steel is the easiest bike frame material. It’s light, strong, easily weldable, easy to braze, has a long fatigue life, and it’s inexpensive. If steel was invented tomorrow, the inventor would get a Nobel prize the next day.

But what about [some other material]?

New high end bike frames are made of carbon fiber. If you’re insistent that only carbon fiber will do, you should go to Dave Bohm’s school in Tucson. (Hat tip to Dave Bohm for getting framebuildingschool.com )

Aluminum frames require welding, and post weld heat treatment. Also, welding aluminum is less forgiving than steel because of aluminum’s reactivity with oxygen and other contaminants, not to mention that you have to go a lot faster. It’s doable, but is a level 2 ( maybe 5?) skill. Heat treatment for 6000 series aluminum requires high temperatures that you can’t do at home and 7000 series can be hard to source for the little bits unless you also have a machine shop to make aluminum in any shape you want.

Titanium frames require professional quality welding and fastidious cleanliness. Titanium is also a much more expensive material. I’ve never built a titanium frame because I never had the money to buy the tubes and a TIG welder at the same time.

What kind of frame?

Your first frame should be as simple as possible. A track frame is ideal, even if track bikes are a little silly. A road frame also makes sense. A rigid mountain bike frame is also good. Single speed bikes are a great choice. Also, Single speed bikes are rad. You should make one so you own one and ride one.

I made this video as a guide for finishing Form 2 prints in the lab at work. Check it out.

2.810 uses a plastic gear that can be tough to source, and not all of them have a set screw, which makes them harder to use.  I wanted a mold making challenge and the ability to make gears at will, so I machined the molds.

My design goals were:

1. Mold a gear that meshes with existing RC car drivetrain components

2. Insert mold a set screw into the part.

Link to the CAD file.

The first goal was pretty easy. Fusion 360 has an add in gear generator. I knew the pitch of my gear and the number of teeth.  Find this generator under Design – Tools – Add-Ins – Scripts and Add Ins – Spur Gear.

When building the molds for the gear I only scaled the generated gear by 1%, as that’s a low estimate for polypropylene shrinkage. The mold could always be remachined to be bigger, so starting at the low side for part shrinkage made sense.

Creating the side pull is mostly about adding an extra parting line. I chose the centerline of this gear because it’s symmetrical, but any line that makes sense for your part is fine.  The side pull mounting holes and the hole that holds the set screw were machined first, then the side pull and rest of the core were machined as a set to avoid steps, mismatch and flash.

Die springs operate the side pull. These were found in a drawer of misc springs in the shop. They’re really strong – I can’t close it without a press. The side pull is closed in the injection molder with an angled plate. There’s a bronze plate behind the side pull to prevent aluminum to aluminum galling as the two parts slide past each other. The side pull has a section of allen wrench pressed into it, with a magnet behind it to retain the set screw.  It’s a little fiddly to put the set screw on the mold, but it stays on there well and releases well in the machine.

TL:DR – 1.9%.

When injection molding parts, the mold sizes need to be slightly bigger than the final parts to allow for thermal contraction of the plastic. How much bigger? I wasn’t sure.

I wanted to create a lot of data fast, so I designed molds to create four parts with the same thickness, and I (arbitrarily) chose 0.100” as the thickness.

After molding around 60 parts, I measured them all in this scheme:

All of my measurement data is in this Google Sheets Link.

I used a scatterplot and linear regression because I wanted to see the effect of part size if it was present.  The data show a nice linear relationship between the mold size and the finished part size, with a shrinkage factor of 0.981.  A typical workflow for drawing molds includes scaling up the original part and subtracting it from the mold blanks. The amount to scale the part is 1/0.0981, or 1.019.

TL:DR – if you need machined features, make the part a rectangular prism.

All about Axle Blocks:

In 2.810, many teams choose to make lots of parts out of waterjet aluminum plate, and then post machine those plates.  This is a pretty intuitive strategy, but isn’t the fastest. I created a couple sample parts and then filmed myself doing a time study for how long each one took.

Onshape Models and Drawings

The waterjet is still an amazing tool with great capabilities, but if you’re going to do any post machining (except maybe tapping waterjet holes), you’re better off machining the whole part.