If you have a Taig CNC milling machine odds are that you have been playing with your machine for a long while before reading this post.
I highly recommend that anyone playing with CNC machines who had no prior experience like myself use very soft materials at first. I must confess having an engineering education makes me more of a coward than average… since I am a visual thinker I could imagine quite vividly the types of damage I could do to my own flesh or to two thousand dollars worth of equipment if I am even an instant unwary.
So for my first two years of ownership I played in my spare time on softer materials such as pure bismuth and tufa rock, for making molds for rubber casting and silver casting respectively. I just didn’t believe I was ready to cut aluminum or steel yet! I also felt I could handle balsa wood, but I couldn’t see a need yet.
And of course I lurked on CNC forums and asked lots and lots of questions.
Last year, in early 2008, I had put the machine in mothballs to move it to Seattle, where I tried for the final time to find software engineering work. I kept everything boxed during the entire spring and summer that year in order to recover from the ordeal.
I then tried, starting in Fall 2008, to attend classes part time for a year afterward for a possible doctorate computer science. But after my depression kicked in with a vengeance during midterms for each of the Spring and Fall semesters I felt I could not proceed any further. My grades were okay, but each semester costed me more in emotional stability than I had realized. Plus, I had a lot of other very practical reasons for quitting… the trip was physically exhausting at more than 300 miles each way once per week!!!
The main price I had asked of my wife in exchange for leaving the doctoral program and committing all of my SSDI money toward my family’s needs rather than toward quite distant odds of a doctorate, was the rental of some storage out of town so that I could use the entire shed at home as my crafting area rather than have to share with storage. Her only condition was that my daughter’s trampoline would need to winter over in the shed, replacing my card table. I could tolerate that condition because the card table was more of a staging area than a work area anyways, having a disassembled trampoline ring would be a minor nuisance in comparison to only having the north half of the shed available in the winter.
So I finally got my Taig and all related equipment out of boxes during the summer of 2009. I felt a whole lot safer being able to actually walk around and assemble my equipment without bumping into boxes and bicycles. Plus, I could even set up my drill press and bandsaw in separate stations.
After I finished setting up I stil had a couple of weeks left over before my family was due to leave for my wife 20th high school reunion.
I felt readier than ever to carve silver, and soon aluminum, as my next metals. But before I did, I wanted to deal with a true pain-in-the-butt I had encountered over the last couple of years:
Typically in a Tufa mold I would mill a pocket for holding the silver, and then I would carve the pattern into the pocket, like this:
A CNC carving pattern on Tufa
This requires two endmills: a larger one for roughing the pocket (0.25 inch) and then a much smaller one (0.05 inch) for carving the pattern at the bottom of the pocket. But I needed both mills to use the same origin point. It was difficult in the extreme to get the tip of endmill #2 at precisely the same XYZ coordinate as where the tip of endmill #1 left off.
I learned a lot about edge finding and Z-axis touchoff in making those molds, it was a lot of repetitive and tedious effort which frankly I had gotten quite sick of doing. So for about an hour or two each day before vacation I planned, built, and installed a set of home switches. For my ambitions I didn’t need the full monty of home and limit switches because I only dealt with small areas in which to carve.
All I really wanted was a consistent origin for my carving pattern, so all I needed were home switches for each of my three axes. The gives my computer the ability to determine the zero coordinate (“home” coordinate) for the measurement for each axis of movement and return to that same place each and every time (barring switch measurement hysteresis, gear backlash, computer real-time measurement limitations, etc, but typically accurate to .0001 even with equipment such as mine).
Notice my assembly for the X and Y axes home switches:
Homemade X and Y home switches for Taig CNC mill
If you zoom into the picture on your browser, you may notice that each axis has a special type of switch, called a microswitch (at least they were “micro” back in the 50′s when they were manufactured for much larger machinery, I got these switches at a hamfest when I was still in my twenties and held onto them ever since! Radio Shack offers smaller ones that can do the same thing).
The milling table moves leftward in the picture for decreasing X, and downward in the picture for decreasing Y. I mounted a wooden post on my milling cabinet so that the switches actuate as they contact the top right corner of the post as X and Y reach zero. The surfaces of the wooden post themselves define X = Y = 0.
The milling table moves rightward in the picture for increasing X, and upward in the picture for increasing Y. Therefore, as my carving coordinates vary from X = Y = 0 the switches move away from the post.
There are clearly more far elegant ways to install home switches on a tabletop CNC milling machines. What you see a crude, fast, easy, and effective lashup whose installation is nondestructive to my Taig, preserving the Taig’s possible eventual resale value. I sacrifice some range for X and Y but since most of what I am doing occurs within the space of a 3X5 card I don’t consider the reduced range to be a serious matter, for now.
The switches themselves are normally open, connected through 10 kilo-ohm pullup resistors via some salvaged keyboard cable to dedicated a pair of terminals on my Xylotex 4 axis control board, these terminals in turn connect to spare lines on the parallel port connecting my Xylotex board to my computer running EMC2/Axis under a real-time (RTAI) patched Ubuntu 8.04.
(For precise connection details, consult the Yahoo Xylotex user group, and the EMC2 manual).
Caution:
You really want to arrange the placement of your switches, wooden post, and Taig table so that perhaps a half inch remains between what the computer considers zero, and where the hard stops on the Taig table actually are. In other words, you need to make a logical zero for your computer that is some minimal distance away from “hard zero” on your travel axis. You really don’t want your Taig to reach hard stops while it is trying to seek home via the switches, because when that happens that could cause physical damage to your ballscrews!
[ But then, that's what limit switches are for, which is too advanced a topic for me to deal with at this stage in my development as a "precious metals and materials processor" (I don't dare call myself a jeweler right now.) ]
Once you feel more confident (or if unlike me you started out as a seasoned professional like many of the masters on Practical Machinist) you could reduce the safety distance down to perhaps a tenth of an inch (go ahead and ask them and see if they think I am still being overcautious (har har)).
[ Commenters, please let me know if I need to post a diagram here explaining my concern! ]
So well and good. The board along with the software allows the computer to find a consistent origin point for X and Y axis travel.
Meaning:
1) Once I mount my carving stock to the vise I can tell the computer to jog my endmill to the starting corner of the stock.
2) I can record the computer’s coordinates for my endmill’s location on paper.
3) If I should turn the computer off because my wife says it’s bedtime, I can command the computer to Home Axes, return to the machine the next day, then rejog to the same coordinates that I had written down earlier, accurate to a thousandth of an inch or better.
4) Likewise, I can simply home axes, change my endmill, and then rejog. This opens possibilities for more intricate carving patterns involving more than a single size of endmill.
But wait, there’s more!!! I also want a home switch for Z axis. Take a look:
Breadboard and mounting bracket for Z axis home switch.
If you zoom closely, you should be able to see that the switch is designed to stop the Z axis vertical column from hitting hard stop (and if the software or connection doesn’t work, the vertical column will crush the switch. Hopefully the process of crushing the switch will allow me enough time to turn off master power to the entire shed (kzinti-style grin))!
I had to be a little more creative about mounting the breadboard onto the Z axis vertical column. Fortunately, there were a few places along the right hand side of the column where there were several long screws, and I placed a metal mounting bracket on the column by way of washer on one of those screws.
But beside the obvious benefit of the Z axis home switch preventing us from running into the hard stop or dying in the attempt during a pattern carving, there is now the the added benefit of a repeatable touchoff!
Once I have the stock mounted, and once the Z axis is homed, I can command the computer to jog the Z axis (I will later contract this to “command the Z axis”) to set the end mill’s tip to the surface of the carving stock, minus the thickness of a sheet of paper.
The masters in the Practical Machinist forums can tell you how to do a paper touchoff in detail if you ask, but essentially I place a small piece of paper on the stock, then command the Z axis to touch the paper rather than to stock, I want it close enough so the paper barely moves. That point, minus the thickness of the sheet of paper (measurable by micrometer), can define a Z axis touchoff.
I then record the touchoff point on paper, command the Z axis upward so I can remove the sheet of paper, command the Z axis downward to my recorded point, and then I can start carving. Later, if I need to change the endmill, or continue the carving into the next day, I can use my recorded reading to move the endmill tip back to the carving stock.
I’m so glad I made these switches this summer. For a little investment of my time and few parts from my junk box, I know I will save myself a lot of effort in what I hope will become some fairly sophisticated milling operations.
Lordy, it feels so good to have a place to call home at long last!!!
