2.77 – Week 5

This week, I experimented with different motion systems for my mill’s axes; started building my stiffness testing tool; and kicked around some thermocentric design ideas. (Sorry for being a little short on text this week – I’m teaching labs this week and next, and I need to get to lesson planning!)


PUPS 5/Lab 5

For PUPS 5, we were assigned to start experimenting with different concepts for our machine. Since my machine’s frame and bearings are already defined, my design work focused on the actuators, drivetrain & supporting structures for the system. I looked at a range of different actuation technologies, including ball screws, lead screws, belt drives and roller screws (which were NOT what I thought they were…)

Reviewer Comments

Post-Review Design Notes

Design Workbook

Error Apportionment: Cable Drive

Error Apportionment: Ball Screw

Thus far, my conclusion is that ball screws are the way to go. This isn’t really surprising – there’s a very good reason that all modern conventional milling machines use ballscrews – but it’s good to have gone through the process of verifying this (and experimenting with the cable drivetrain idea – I really like cable designs, and it could be appropriate for a lower-force machine). I’m also¬†very pleasantly surprised to find how inexpensive ball screws are, particularly aftermarket. I’m going to need to be selective and careful (especially given the limited space I have to mount ball nuts), but I’m cautiously optimistic that I’ll be able to make this design work under budget.


Final Project Development

I also spent some time this week developing my stiffness tester. I elected to make the tester out of aluminum: it’s easier to machine, and more than meets the deflection & yield strength requirements for my tester. The tester also has screw mounts to match the Benchmaster’s table, as well as standard Bridgeport-style machine tables. I waterjetted the frame this week, and am getting my load cell hooked up this weekend: next week, I’ll finish machining, and start testing!

Part Files (Onshape)



Finally, for Seek-And-Geek this week, I was inspired to experiment with thermocentric design by a discussion that came up the other weekend when I was visiting my parents. In my freshman year of college, I used my new welding skills to build a fire grate for my parents’ fireplace. The fire grate (shown below) uses steel cables strung between stainless bars. Unfortunately, the steel cables sag as they heat up – thanks, thermal expansion! I started thinking about how I could make a thermocentric retrofit for the grate.


Calculations & Notes

Unfortunately – it looks like the thermocentric design would be pretty clunky to execute with common materials. I’d really like to learn more about materials for thermocentric design, though. I’m aware of low-CTE materials like Zerodur and Invar/Super Invar, but what about¬†high CTE materials – particularly materials with a high CTE, but also a high melting/glass deformation temperature? Do those exist?

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