Tuesday, September 21, 2010

DRO-350 Enclosure Progress & More Info on Taig Mill Modifications

I was sure I would've gotten more accomplished on the enclosure, the scale cables, and the scale mounting and protection brackets/covers for this project by now, but we didn't have class today so I couldn't make any progress on the mounting/protecting/cover brackets for the scales (the X axis bracket runs the entire length of my mill table like the one shown in the seventh pic from the top or the very last pic here: Taig Mill X Axis Aluminum Angle Digital Scale Mounting & Covering Bracket Installation) and I still haven't found all the info I'd like to have to help me decide which end of the cable's shielding should be terminated at a common point.  Two of the scales I have take LR44 1.5VDC batteries and the X axis scale takes a CR2032 3.0VDC battery, so I'll probably have to build a small voltage regulator board that I'm hoping I can fit within the enclosure to reduce the 5.0VDC regulated power supply to 1.5V for the Y and Z axis scales and 3.0V for the X axis scale so I can remove the scale's batteries and replace them with a 100uF electrolytic capacitor and a 0.1uF cap to reduce jitter from low and high frequencies respectively.  A resistor may also need soldered in place between the caps and one of the battery connectors to drain the energy stored in the caps should I encounter any problems with the scales "freezing" or locking up, but I'll probably hold off on installing the resistor (56K ohm seems ideal from what I've read, but I would think that more current would be consumed by the scales when they're in operation with the resistor in place; more heat would be generated in the general area due to heat radiation from the resistor, so I'll test the scales without it first and only add it if needed).

Anyway, my first attempt at milling plastic wasn't a failure.  I only took one practice cut inside a display pocket and was pleased with the results, but I really think I need a higher spindle speed and faster feed rates to mill the enclosure material more efficiently; 10,600RPM, the fastest stock Taig pulley setting is a bit slow for an 1/8" end mill in this material, and trying to count hand crank revolutions while attempting to maintain a feed rate of over 20IPM (which equates to over 6 2/3 hand wheel rotations per second) is extremely hard to do on a manual mill without a digital readout already installed.  I probably would've set this enclosure up in one of the CNC machining centers at school if I hadn't spent so much time aligning it to the table of my Taig...

Here's a couple of pics of the milling operations completed on the front of the enclosure:

This plastic is messy, and the burrs left by a less-than-adequate feed rate and spindle speed had been removed by hand with a razor knife so the wavy edges you see in the pics were the result of using the razor.  The overlay will cover and hide the less than perfect edges, so those won't bother me.

I'd like to finish this up tonight, but it's too late now.  The only operations left to perform on the front of the enclosure are drilling a total of 23 0.500" diameter holes, which I'll most likely use the 5/16"-7/8" Irwin step drill bit to complete since I can control the depth of the countersink and deburr each hole without changing tools in one operation.  This won't take but maybe an hour tops, IF I have to (or just want to) go back and triple-check my calculations for the coordinates so I can drill the holes in the most efficient sequence possible, so I'll probably start on that shortly after returning to the house tomorrow.  Once that's completed, I'd like to take a few measurements to verify the 4-pin XLR style receptacles will fit in the recommended locations for the DIN connectors that are supposed to be used with this kit.  They should, but it never hurts to verify...

I'm going to keep the auxillary input on this DRO for use with both an electronic edge finder and a tach sensor, but I bought an extra 4-pin XLR receptacle and connector for it too.  The SFPM display feature is a nice touch incorporated into this DRO and it requires a single PPR from an EO sensor, IR emitter/detector, or even an IR sensor and encoder wheel with a single slot mounted on the spindle or spindle pulley or within the range of the sensor away from the spindle/spindle pulley.  The biggest drawback of this feature is that you lose the display of the Z axis (with the mill firmware), so I believe I'll build a junction in the sensors cable.  The wiring that will connect directly to the DRO in the AUX port will not be optically coupled to the sensor; it will remain hard-wired, but I'll build an additional display consisting of at least five seven-segment LEDs in a custom enclosure for continuous RPM display of the spindle speed.  This will be a PIC microcontroller based unit of my own design and will remain fairly simple; with the correct 40DIP MCU (having a minimum of 35 outputs), I can dedicate an individual output pin to each segment of each display for continuous versus "flashed" or refreshed control of all segments in each display.  The displays on the DRO-350 are pulse width modulated at a frequency that cannot be detected by the human eye (similar to the way television and computer monitors work), which is fine for most individuals and applications, but there is a possibility that it could cause seizures in some people, and getting the right frequency, figuring out the wiring connections, and all sorts of other conditions relating to that method can be a pain in the butt when building a unit like this from scratch, so I'd rather use an MCU with enough outputs to dedicate each segment and only really have to worry about the refresh/update rate and the timing/counting pulse rate when writing the source code for this dedicated display. 

I've really taken advantage of free samples for students from a large number of semiconductor manufacturers, too, including Microchip's PIC samples, so I'm pretty well stocked up on any PIC MCU I'll need for the next few years anyway.  I also have more electronic components (nearly everything I'll need for any kind of project that I could dream up at this time) than I do machine tool accessories and tooling, so I need to put some of those to good use ASAP.

I'm going to start on a new base plate for the Taig pretty soon, too.  The main reason I started this blog was to document the radical modifications I'm going to undertake to it, but I've only been covering more common projects lately.  With a new 1/2" thick base plate, ground on at least the top side (I'm still going to use shims of various thicknesses in steel, alumnum, brass, and rubber and also cork and rubber gasket material between the base plate and bench top and between the mill's mounting feet and base plate to both reduce noise from vibration and level the mill and base), I can reposition the mill on the plate so that I'll have an equal amount of room on both sides of the Z column for adding the base mounted retainers of my "turnbuckle" style supports, more room on the plate behind the Z column for use with the adjusting/leveling feet designed to help support the massive GE DC motor (if I do use that for my spindle motor), and a more solid base to start the series of coming modifications.  None of the structural and supporting components will be made from aluminum; my current plans involve not only function but form as well, so the actual turnbuckle (adjusting mechanism for the Z column supports) will probably be built from 1.375-1.500" stainless hex bar, bored through and threaded both left hand and right hand 20TPI threads at equal depths to accept left- and right-hand threaded supports with a minimum of 0.625" diameter (although I'd really prefer to use 0.750" or 0.875" diameter 20TPI left- and right-hand threaded rods for the adjustable sections of the supports).  You'll soon see what I'm referring to, but I need to finish the DRO and scale mounts first before starting on those.

I'd like to try multiple spindle motor options, too, but my plans remain to keep the spindle motor securely mounted to the rear of the Z column and stationary while the Z carriage is free to move along its length of travel when the spindle motor is powering the spindle.  This is not a very simple configuration, but I feel it should greatly assist with the issues I've had squaring and aligning both the Z column and headstock/spindle because of the weight of the spindle motor hanging off to the left-hand side of the headstock in the factory configuration.

Well, off to bed-I'll update this again ASAP...

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