Friday, April 13, 2012

One More Part Checked Off the List

The CAD software I use, Pro-Engineer, has an assembly mode where you can create simplified representations of the assembly you are working on.  One simplified rep I have defined is the 'Build' state where I blank parts that are finished and today I went into it and checked off 'arm-upper-bearing.prt'.  The current build status is:

Some days the state of this assembly is depressing and some days it cheers me up.  Today I was in good cheer.  Lots to do, but lots done.  The reason for my cheer is a pair of nice looking multi-axis parts:
As you can guess, this is the result of the second operation on some of the parts in the last post.  In the previous operation I had machined all of the critical functional features:  bearing bores and locating tongue.  this second operation was merely to remove material along the outer perimeter of the part.  Although it needs to be done carefully to leave a balanced amount of material, the tolerances are much looser than the +/-.0002" the bearing bores needed.  For comparison that is about 1/10th the thickness of a human hair.

The fixture was simple, a machined boss that would locate in a bearing bore and a threaded hole to clamp the part down.  I also had to machine a step to keep the part from spinning due to machining torque.
Here's the part bolted in and being machined.
After completing the outer profile I indexed the trunnion and drilled the hole for the trail adjustment screw thread:
Here's the part assembled to the steering stem:
Its starting to look really nice and I guess that's reason to be happy about the build image.  I'll do the same for the chassis pivot part of the arm assembly tomorrow and keep the ball rolling.


Progress on the Upper Control Arms

Ahhhhhhhhh.  Machining and fabricating sweet parts is nice but doing it for your own bike on a nice sunny day just seems to be that much better.  Something about the sun, sky, and the self determination of being on your own that makes the air taste a bit sweeter.   Or maybe its just some new type of pollution.

The upper control arm is going to be a 2 step type of part.  Because it is the main adjustment point for fast turning/front weighted bike to stable/rearward weight bias I needed to be able to quickly make and test changes while keeping a rigid part that will not introduce any vagueness into the system.  The solution I came up with is a tongue-in-groove sliding joint with a screw adjustment to keep the setting under heavy load.

This 2 piece design will allow quick and accurate trail changes in the hot pits with no disassembly or swapping of parts.  For our first round of bikes and testing this is what we will use.  The plan is that testing will reveal several preferred settings which we will then machine in a fixed length one piece arm:
This one piece arm will be lighter and, more importantly, more rigid laterally.  The overall stiffness will be similar to the 2 piece design but the introduction of a bolted joint into an assembly inevitably results in the introduction of tiny amounts of hysteresis which can possibly have a negative effect on performance and feel.  As a tuner it is nice to have a chassis with adjustments at every pivot point but the cumulative affect of so many bolted joints can compromise the overall performance of the chassis.  This one joint in the assembly should not have significant adverse effects and will let us quickly dial in to optimal trail settings for a variety of conditions.

Now, on to machining........

This is a process we've all seen before:  billet, vice machining, flip, subplate machining, then repeat.  The suspense is gone but the cool pics are all new.

Here's the pile of billet that will soon become two complete control arms:
Here's the mill with the trunnion and vice mounted on it with stock clamped:
 Machining in progress:
Done with the first step of upright end machining:
Now on to the chassis pivot end:
Repeat both a couple of times and you get the resulting picture:
Now I need to machine a couple of fixture plates to hold these parts and then run the second and final operation.  Will post again real soon.