Saturday, August 4, 2012

TOBL2: Build Week 1

Enough waiting around. At the beginning of this week I received some of the last bits and pieces needed to finish TOBL2. Here's what I got:

The parts from the water jet turned out really nicely! Seeing these parts in isolation makes me realize what a bizarre hobby/design sense I have. But, I suppose the endless hours of SolidWorks dimensioning has paid off. The only slight hitch is that Big Blue Saw messed with the dwg file and forgot to include all of the outer plates - those should come in Monday.

Whilst I wait I've had the privilege of full access to an impressively well-equipped machine shop at work. They used to do production in-house so now there's just an overkill shop full of mills, lathes, a wire EDM, a water jet, and so forth. A MechE candy store, if you will. And the guys there are great. Direct quote: "Don't buy any more hardware or material, okay? We have anything you need."  This past week I've been scarfing down lunch and spending the rest of the break in the shop machining.

The first order of business was to clean up those inner plates.

With all those countersinks I tested the motor mounting assembly.

No interference! While playing with this something occurred to me: why the four letter words was I going to use tension springs to tension the belt? The thrust bearings are a perfect place to wrap torsion springs; which is a much cleaner solution. I guess you can't do all your design work in front of a screen.

But before getting too excited a little bit of math is in order. The Hitec HS-45HB feather servos are rated to provide maximum 1N of force for a 1cm arm (i.e. stall torque 1kg-cm). There is an arm on the motor mount too...about 1inch or 2.5cm where the servo will catch it. The motor is about 1.25cm from the point of rotation. On McMaster, the weakest spring that fits around a 5-40 screw is 0.64 in-lbf or 0.072 N-m. Then we do a simple moment balance:

ΣM = 0 = kθ + mgl - FL

θ = (FL-mgl)/k

If we make the safety factor 2, and assume the servo can only output 0.5 N, the spring will overcome the servo at about 1.2°. For safety factor 1 we can get up to 5.2° travel. Somewhere in that range should be enough lift to disengage the belt. If not, coils can be removed to decrease k. Here's a clearer picture of the plan (I realize this is already like plan C):

The next things that need doing are:

1. Clean up the outer plates when they come in
2. Press all the bearings
3. Machine the interfaces between the PCB shelf and inner plates
4. Servo linkage
5. Torsion springs
6. Assemble!

This is the plan for next week. In the background I'll be working on the new controller. I've been learning some python specially for the occasion and am hoping to be competent enough to write a controller that will use keyboard inputs (for now). Another new goal is to have TOBL2 operable enough to get video to submit for World Maker Faire New York. The submission deadline is in two weeks, on August 17th. Presumably I could always enter TOBL1 like last year and then bring TOBL2, but let's see if we can't do this honestly.

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