Sunday, April 28, 2013

Connection Prototype - Sidewalk Drawing Robot.

I have been enjoying building drawbots of late. I find that they are a great way to learn about motion control systems, robotics, machining, and programing. Plus you know, robots that will do my bidding, how can you go wrong.

My next project is going to be a scaled up version of my pen plotter, this time capable of drawing on a sidewalk. Now because I don't want a 6' linear slide sticking out the front of my contraption, I am going to instead use a series of levers to get an approximate straight line that stays parallel to the ground. Here is an animation:

This linkage was designed in Linkage Mechanism Designer and Simulator an AWESOME free bit of software by David Rector. It is a wonderful program for working out linkages, and a massive improvement on my traditional cardboard and pins method.

To give you an idea of scale, the plan is to build this so that it will reach out about 6' when extended, giving me a drawing area of about 7'x4'

The construction is going to be fun, I am using 1"x1" alumium tube with 1/8" walls (Yup, despite the fact that I am in Canada a lot of stock is still sold in imperial measurements, its all arbitrary anyway really). Rather than just drilling a hole through the aluminum and bolting it I felt the need to go fancy and use sintered bronze and steel bushings press fit into the tubes. This reduces friction and makes it a lot more durable.

It took me a bit to work out how to get an accurate hole for the press fit, it has to be done to about 1/1000th of an inch tolerance (oh yeah, my milling machine is imperial as well). So if someone else is in the same boat here is how its done:

First drill a hole as close the final diameter as you can, unless you are like me and need an almost 3/4 inch hole and only have a 1/2 inch drillbit.
Next zero a set of digital calipers on the part you want to fit into the hole and then measure the hole, this will tell you how much material you have to remove.
The tool I am using to enlarge the hole is called a boring head, its basically a bit holder for boring bars that has a dovetail slide and a lead screw. By turning the lead screw you can offset the position of the boring bar within the hole. It has a graduated dial but its worthwhile to note whether the dial is set up to measure change in diameter or radius. In mine, its the later so all movements get doubled. ie. 10 thou on the dial removes 20 thou off the diameter.

Only use this tool on a milling machine, drill presses don't have strong enough bearings to hold up to the offset and out of balance cutting forces this tool creates. Unless that is, you like excitement. 
Take a bit of metal off at a time until you get to your desired diameter, I was actually shooting for about 3 thou of interference on the fit, but forgot to account for the spring-back of the boring bar. It flexes ever so slightly under cutting loads so you have to do a few passes to make sure its the final size, I didn't do this on my penultimate pass so it caused me to overcut the final pass. Fortunately this is a test piece, and learning is fun.
Now if it was actually cut properly I would have had to use a press to put the bearing in, lets pretend anyway. This is a 2 ton manual rack arbor press.
Final step is to cross drill a steel pin. V-blocks greatly simplify this process.
Here is the joint assembled with cotter pins.
And a view down the end.You can see the bronze bushing on the right and the steel bushings on the left. The steel bushings are flanged so that the two bars don't contact over their entire length.

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