LMS 3990 Servo CNC Conversion — Part 1: Unboxing the Servo Kit
Specifically, unboxing the 3-Axis TWO NEMA23 350oz in, One NEMA34 1125 oz-in, 60VDC/20A PSU, G320X Gecko Driver kit from Automation Technology Inc.
I bought a Little Machine Shop 3990 (LMS 3990) mini mill in June 2018 as a birthday gift to myself. I previously had bought a Harbor Freight 93212 7" X 10" mini lathe and quickly found the bed length too short to meet my needs. I had wanted to make a drum weapon for a planned Firestorm III, but the short bed length forced me to redesign and revisit the whole idea.
It also convinced me to not go for the absolute cheapest Sieg X2 mini mill I could find when I wanted to buy one (which would have probably been the Harbor Freight mini mill (HB 44991) but instead to go for the more expensive LMS 3990.
In retrospect, I am glad I did. Despite being based of the same Sieg X2 design, the LMS 3990 has some aspects which are better than the Harbor Freight mini mill, such as:
- A longer bed ( 18.1" x 4.7" for the LMS 3990 vs 15.9" x 3.7" for the HB 44991)
- A solid column which adds more stiffness
- A gearless spindle drive, which allows for continuous speed changes throughout the whole 50–2,500 RPM speed range
I didn’t fully appreciate all these things at the time I bought it, but I like a lot of them now. The stiffness and gearless spindle drive also are convenient for CNC conversion.
Now, strictly speaking, there is no reason for me to convert my machine to CNC. I do not do machining in any professional sense, so efficiency doesn’t really come into play. But the idea captured my imagination after seeing a couple of YouTube videos on how to do it, and the definition of a hobby is something you are doing just for fun, so why not?
I initially thought I would buy the cheapest lathe stepper conversion kit I could, and got the 2 axis stepper kit, thinking I’d just dip my toe in.
My initial plan was to buy a cheap old computer off of Craigslist and use that to drive the steppers using LinuxCNC. I found an old Thinkpad WS510 for sale and got that, thinking the touchscreen might be useful.
After some experimentation with LinuxCNC though I found that the laptop I’d bought had too much jitter to be used for CNC. When I installed LinuxCNC and let the latency test run, I was getting latency numbers well above 100,000 nanoseconds, which according to LinuxCNC mean you can’t use the PC to drive the steppers.
I also managed to break both the stepper drivers I had bought when I tried to wire them up to the break out board that came with the kit. I did something wrong, shorted both of them out and fried them.
I was contemplating whether to buy more stepper drivers and try again when I came across a post on CNCCookbook.com that talked about some of the advantages of servos vs. steppers. Even though the article concludes that beginners should start with steppers and not servos, I was persuaded by the article that servos are better than steppers for accuracy.
I also came to the conclusion that if I am going to do all this work I want to make sure that the results are the best they can be, and that I wanted to start with the mini mill and not the lathe.
The reasons the mill and not the lathe are that I don’t have a digital read out (DRO) for the mini mill, which is a device that tells you exactly how much you’ve moved each axis of the machine relative to a specific point you determine. I don’t have one for the lathe either, but I found that not having one on the mill is a pain, probably because I use the mill more often. Right off the bat, having CNC of the mill will be like having DRO on all axis and having power feed, even if I don’t use any of the automation aspects of CNC.
And so, I talked myself into buying the aforementioned servo kit from Automation Technology.
Below are the videos of my unboxing the various hardware pieces when they arrived.
I am particularly taken with toroidal power supply. This things looks like the kind of thing a mad scientist would use; I love it.
