5. MCM Build and Test
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MCM Build and Test
2016
In the latest adventures of John SteinBot, I began to build John's Most Critical Module (MCM) - the X axis with the writing utensil. To do this, I finished my X Axis design of the rails and actuator, bought my materials, and began machining. However, I was not able to finish machining the actuator because of several learning opportunities that popped up. In the end, I tested my carriage and guides to compare my measured and predicted X axis stiffness.
First, I finalized my rails, structures, and bearings by going over the error budget one more time. I arrived at conclusion that my machine would be accurate to 250 microns over short distances and accurate to 1 mm over the entire range. This is accurate enough for something such as writing. I sured up what I was going to buy for the X Axis (leadscrew, nuts, bearings, rails, and structure) and ordered from McMaster for about $60. I predict the rest of the machine to cost $40-60.
When my parts arrived, I began machining. I machined my motor mount first and it was a good warm-up and review since the coupling would make up for any machining error-caused misalignment.I realized I had made a slight mistake in the part and had to enlarge a through hole. The drill press belt was slipping which provided me an opportunity to learn how to CNC circle pockets on the MakerWorks ProtoTrak mill. This allowed me to achieve hole diameters as accurate as 0.0005"! The same day, I also machined my bridge structure which was pretty easy (but sadly, it needs a little paint...).
I also finished cutting my linear rails. I didn't realize it was so hard to cut hardened steel and I had to use an angle grinder to do so (which discoloured and warped my rails). In the future, I might just buy my rails to size.
Then, I machined my bearing blocks. I learned even more CNC and was able to CNC the outer contours of my bearing blocks using a sacrifical plate. I also machined the bearing bores, but had to enlarge them once I realized they were undersized because the end mill diameter was under specification. In the future, I should measure the end mill or step-up the CNC'd holes.
Next, I began cutting my carriage. First I drilled and reamed the bearing holes . I had originally planned to mill the underside by hand, but it seemed tedious and I wanted to learn more CNC. It took me a while to figure it out, but in the end I got a very beautiful piece.
I spent about 16 hours in the machine shop this week, but unfortunately was not able to finish all the parts. Still, I assembled by bearing blocks, rails, sleeve bearings, and carriage and began to test them.
Immediately, I noticed I had a problem. My carriage felt like it had 600 N of friction on the rails! I realized this was due to the fact I had machined my rail mount holes with a drill bit instead of CNC. This meant my rail holes were larger than expected, which meant the rails were closer than expected and caused a lot of friction with the sleeve bearings in the carriage. Darn. But this is what an MCM is for. Afterwards, I tested the stiffness of my carriage/rails and compared predicted and measured values.
This week was a good week. Although I didn't get everything I wanted done, I was still able to learn a ton of CNC which should speed up manufacturing in the future. I also learned a ton of useful stuff about parallelism that will help me next week in the design of the rest of John SteinBot.