Hey,
I just had to put in my 2 cents here. I’ve had my unit up and running about 5 months, and doing daily runs on it about 3 months. Ted, thank you for a great design. My unit is 3x4 foot and is highly Modified. I would post my mods, but, I just don’t have time to.
First mod is I put mid way supports on the outer rails on both X and Y axis. But, I wasn’t happy that the gantry had too much flex. So, I made an Extreme mod of the Gantry. First, I put 2 pipes on both the X and Y (only on the gantry, not on the outer fixed rails). I kept the center rollers on the outer rails the same - only one pipe is attached to the center rollers, the 2nd pipe is screwed to the main pipe. On one axis I put the 2nd pipe on the top, on the other axis i put the 2nd pipe on the bottom. The pipes are screwed to each other, and i used 4 screws - one at each end and several along the length. The two pipes - one on top of the other, has a lot more stiffness. But, due to there now being 4 pipes going to the Z gantry, 2 on the X and 2 on the Y, i had to totally redesign the Z axis and gantry. I went with HEAVY Duty - using 3/4" thick oak plywood plates, one on top, one on the bottom. And redesigned the rollers to account for the 2 pipes on each axis. It is heavy and bulky, but, it is SOLID. With the dual pipes on both X and Y I still get flex in the gantry, but, far far less. Previously I just had to put a slight pressure on the gantry and get very noticeable flexing. Now I have to exert a fair amount of force to get a slight flex. Much more solid.
At the same time I switched to an 8mm lead screw rather than the 5/16 threaded rod. The Z is much faster - can run it at 2000 mm sec. BUT - with the lead screw there isn’t much resistance, so the weight of the motor would make the Z axis drop straight down when the stepper motor would turn off. I ruined a number of bits and have holes at my home position and all over the spoiler due to the motor just dropping straight down. Finally, my latest mod was to add a Counter Weight system to my Z axis. That adds another 6+ lbs to my gantry, but, so far i have not seen any issues with that. The counter weight added some bulk, but, it has taken all the weight off the Z axis motor. With power off I can literally spin the Z axis and just as easily move the Z up or down - freely. And, yet, it no longer drops on it’s own. (Like I said, my unit is highly mod-ed).
At one point I had updated the outer center rollers with the newer design that has built in braces to keep it from bending. But, I had only updated the Y axis (because I had damaged it at one point). The pressure of the timing belt on the older center roller caused the motor plate to bend after a while. But, like I said, i had only replaced them on the Y axis. After running the system for several months - all of a sudden it started messing up on my v-carve work. I finally found the X axis was skipping. It was binding. With power off I could freely push the Y axis, but the X was binding. In the end it wound up being the older Center roller, the X axis that i hadn’t updated - it was leaning over. That slight tilt was causing the belts to bind up enough to make it skip when it was carving. I then printed the newer center rollers and that took care of the binding.
But, before I figured out what was causing the X to skip, I looked into what I could do to up the current to my motors and was looking at wiring separate drivers for the X Y motors rather then running them in parallel. I am using 84oz torque NEMA 17’s that max at 2 amps. But, the drivers max at just over 2 amps as well. I had set the trim pot on the X Y drivers at 1.1 volts, so they would output 2.2 amps. While researching what i was going to do, i found this thread. Good thing I did.
It was so easy to change over to series on my design where the wires come out of the cable chain on the center roller, i just disconnected the wiring for both motors there and easily changed it from parallel to serial. WOW - finally, the 84 oz motors are running at their full potential. WOW - what a freakin difference. I can now plow through oak with ease. Hitting rock hard knots on pine is no longer an issue. I am now 3D carving pine, maple, and even red oak skipping the 1/4" end mill rough cut, and just directly carve with the 1/8 ball nose bit (at 45 degree angle) into maple at over 4000 mm/sec with no problem. But, just to be more sane, i try not to run it over 2500 mm/sec. I tried 3D carving in solid bamboo and skipping the rough cut, and it was cutting it very clean running at 4000+ mm/sec, but then the bit broke, so I think i was pushing it a bit too much. Now the issue is not the lack of power or solidity. I just have to learn the limitations of the bits and material. (I tried V carving in laminate flooring which has HDF backing, and that stuff is like milling steel. My carbide tip bits go dull before i can complete 1 piece. And, that was running them at only 900 mm/sec. I will try slower, but, i am thinking HDF may be too hard - not worth it).
As far as the main difference between parallel and serial - it makes sense. When the motors are in parallel the current is split between the 2 motors. For illustration purposes, say the motors are 2 amps at 4 volts on each coil. But, our drivers can only output 2 amps total. So, when it is wired in parallel the driver outputs 4 volts so that the current is 2 amps. But, those 2 amps are divided in parallel, 1 amp to each motor. Both motors see the 4 volts, but, each motor only gets 1/2 the current the driver is putting out.
In series, the driver is still set to output 2 amps, but now the load is higher resistance, so the drivers have to output more volts. Even though the driver is out putting double the volts, in series each motor only sees 1/2 the voltage, but, the full amperage passes through each motor. So, the full 2 amps now passes through each motor. The driver is now out putting 8 volts at 2 amps, and each motor only sees 4 volts each in series, but, at the full 2 amps. So, we just doubled the power to the motors, as you said, for Free. I haven’t turned my drivers down yet. I should, because they are still set to 1.1 volt, or the max of 2.2 amps. But, the motors are rated for it, it’s just the drivers are running on the hot side. I should be able to turn down to .8 with no major loss - but right now, i am very pleased with the added power. I noticed the X and Y run smoother.
I have added IR leds to my DW660, so I can make a Tach and view the RPM. Took off the hand tightner on the DW660 and printed a small holder for the IR leds, and painted a mark on the motor shaft just above the collet. I still yet have to program a separate arduino to view the RPM, but the IR sensors are out putting signals. The next mod is to read the RPMs and create a PWM controlled AC speed control to automatically control the RPM on the DW660 spindle. I got it designed, just need some spare time to build it.
