Monday, April 05, 2010


Hydra Takes Its First Steps!

So I have been way behind on the blog posts over the past week, but we just hit a pretty major milestone with the Hydra multi-headed manufacturing machine project. The mechanical construction for the cartesian positioning system was finished at the end of last week and all of the major parts we were waiting on had also arrived by the weekend. Many many hours were spent over the weekend to get all the motors, couplers, electronics, and software working as desired. I'll go into more detail below, but for the impatient ones, here are some pictures of our progress

Close-up of the Bosch Colt router mounted on our Z-axis assembly. The router fits very snuggly in it's custom mount and appears to have some very impressive runout figures. You can also see that a second support was added to the Z-axis to help prevent it from wobbling. The additional of 2 more sleeve bearings definitely made the assembly more rigid.

Overview of the whole machine with all the motors mounted and electronics wired up. Taking a piece of advice from Tony Stark, we decided to run before we could walk and attempted some simple milling. A small piece of low-density wood was mounted to the table and cut using a general purpose dremel cutter. The Gcode was written by hand and tested using the Hydra-MMM host software file previewer before being sent to the machine. The cutting path was very simple, but it proved that the machine worked and that the spindle was more than capable of cutting these low strength materials. We then moved on to bigger fish...

Attempting some PCB milling using PCB-Gcode and some Eagle board layouts. A piece of cheap double sided copper clad from radio shack was mounted on the table and the Drewtronics 45 degree bit was used for the cutting.

If you want to see a video of Hydra doing PCB milling, I posted our first attempt on YouTube so you can see how this thing performs:

The final result of the PCB milling. The PCB drill bits from Drill Bit City have not arrived yet so no holes were drilled, but the accuracy of the traces was very promising! By the way, in case anyone was wondering, the board is a PIC stepper motor driver that I designed similar to the ones I posted about a week ago.

So now to the details...

There were a couple big setbacks that kept the project from doing any cutting earlier in the weekend. The first, and most dangerous was the fact that the couplers we had made to connect the motors to the ACME screws were slipping. The set screws we had used weren't holding well enough and the screws were simply tearing up the surface of the ACME screw shafts. To fix this we made the couplers have double set screws (one on each side) and also filed a D-face into the ACME screw shafts so that one of the set screws could grip that instead of the round surface. This seemed to solve the problem, however, we have also added some loctite to our shopping list as I have a feeling the vibrations from the motors are the culprits here.

The second hurdle was figuring out how to mount different materials to the work table. The intention had always been to use some type of clamp set with a wide array of tapped 1/4-20 holes, but we hadn't really thought much about how long this would take. We ended up drilling and tapping 49 holes (7x7 with 2" spacing) in the 0.25" thick aluminum table. This was no small task and took a pretty long time especially for the tapping. The end result looks very professional and has more than enough strength to hold the parts we intend to machine.

Finally, there were some minor issues with the electronics as well that will need to be addressed before further testing is performed. The stepper motors we are using have a 2.5 Ohm resistance per phase. If they are being run in a unipolar configuration, this means there is only 1.25 Ohm resistance from the coil end to the center tap. We are running the motors from a 5V supply which means we are pushing about 4 Amps per motor. The motors are only rated for 3 Amps so extended operation at these conditions may very well damage the motors. However, the larger concern is the fact that we have 5 motors that are each drawing 4 Amps. That is 20 Amps just for the motors! This is causing some serious heat issues within the electronics. The plan is to add some TO220 power resistors to decrease the current draw to around 2 Amps and add some heatsinks to boot. This will have to work for the time being as I am still having trouble getting my a3982 motor drivers working. I actually got one to work for about 30 seconds, but then I started to increase the current and am pretty sure I ended up blowing the chip again. No idea how I am doing this as I am using the exact values the reprap stepper driver v2.3 circuit is using and I have checked them against the a3982 datasheet and it shouldn't be possible to exceed 2 Amps current output. Not sure how it's happening, but the chips keep dying so I may have to look at some other alternatives. The Pololu a4983 stepper drivers with microstepping seem very nice at inexpensive as well. If I can't get my own circuit working, I will likely end up buying a few of those.

It's been a long day, so that is all for now, but we are hoping to have the extruder working by the middle of the week so keep an eye out for some more updates!

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