Author Topic: Mechanical Design  (Read 31037 times)

Jim McMillan

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Re: Mechanical Design
« Reply #15 on: April 24, 2011, 03:49:23 PM »
Progress...

Well I've got my Arduino running. Wound up using the DS1307 clock after all, since the better one was backordered. I set the time and ran a test program on the Arduino and it seems to be keeping the correct time. Then I assembled and installed the Adafruit "motor shield" and tested it using some example programs. Was able to run both of my steppers together and separately. They were pretty weak running on 5V from my computer's USB port, but did fine with a 9V power supply plugged into the Arduino. So, all I need to do now is wire up some limit switches and the electronics are done.
I still haven't got the pinion gears made because I'm waiting for the tap to cut them with.  Had a little fiasco with shipping, I was in the middle of a move and it wound up in the wrong place. Should have it straightened out next week. Meanwhile I'll get the limit switches mounted and wired. Might have this thing together in a week or two if all goes well.
Then I'll be bugging you all for help with the software!
Jim


Gabriel

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Re: Mechanical Design
« Reply #16 on: April 25, 2011, 02:21:45 PM »
It sounds like it's coming together. Hopefully using the wormgears should make set up easier for you. I had a small test heliostat set up that used wormgears, and it worked on the first try. This was using a computer with a parallel port to control the heliostat not an Arduino, but it does go to show how many issues you should hopefully avoid using wormgears.

I should have a little bit more free time soon with the semester coming to a close in a couple of weeks, so I'll be around more to answer questions or work out any bugs if needed.


Jim McMillan

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Re: Mechanical Design
« Reply #17 on: May 04, 2011, 02:00:41 PM »
I agree that worm gears seem like a good choice. The key (for us DIY types anyway) is being able to make them economically. I chose ACME threaded rod for my prototype because I had a bunch of 1/2-10 ACME threaded rod in my junkbox. I think if you were trying to do it with hardware store parts that plain-old threaded rod would work OK too. I also considered using smaller rod, but the beauty of 1/2 inch is that there is room to drill a 1/4 inch hole in the end for direct coupling to my 1/4 inch stepper shafts. Also there is enough material so you can turn the other end down to fit a 1/4 inch bushing and have a nice shoulder remaining to take thrust loads.
Still waiting on that dog-gone ACME tap, but they tell me it has been shipped. Hopefully it will arrive by the weekend so I can get those gears cut.
Got the limit switches on and made some little cams to actuate them.
Gabriel, have you incorporated the changes in your software to allow use of the Adafruit "motor shield" ? Bernard posted his Dutch version, which I have downloaded, but I'd like to get your latest English version if possible. My current configuration is:
Arduino Uno
Adafruit motor shield to drive the steppers.
Only one machine at this time, no shift registers.
DS1307 clock
Motors are 6V, .85A, .9 deg per step. They seem to have pretty good torque running off of the Adafruit board with a 9V external supply. Some accel/decel ramping might be nice but probably not needed due to the high gear ratio and the small (12x12in) mirror that I'm using for testing.
Step resolution .025 degree per full step. May change slightly depending on how the gear cutting goes.
One limit switch on each axis.
This is getting exciting!
Jim

Gabriel

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Re: Mechanical Design
« Reply #18 on: May 07, 2011, 10:45:23 AM »
You seem to be getting pretty close. My exams for this semester are now finished, so I will finally have the chance to do some in depth work on the Arduino program, and the site in general.

I'm going to put together a simpler program designed specifically for worm gears. This way I can cut back on a lot of the features which will invariably just confuse people. It shouldn't take long, so, unless something comes up, I'll upload it here in a day or so.

I'll look into the motor shield. I'm planning on rearranging the program so that people can more easily drag and drop their own code for controlling stepper motors. Once that's done it should be a piece of cake to modify it for the Adafruit board.

I'm still trying to figure out how to get the acceleration to work. Like you said, it isn't really necessary for what you are doing, but it's something I'm wanted to add for awhile, so I'll go ahead and try to figure it out.

Have fun!
Gabriel


Jim McMillan

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Re: Mechanical Design
« Reply #19 on: May 07, 2011, 07:16:55 PM »
That sounds wonderful Gabriel.
Well the tap arrived today, so I'm running out of excuses for not finishing this thing! I'll be "making chips"  tonight as the machinists like to say. Hope it works! The main concern is that the pitch diameter comes out right since my design doesn't have provisions for adjusting the shaft spacing.
Here are a couple pics of the mount. The two pipe flanges with the springs between are to enable fine leveling adjustment. I still need to mount the Arduino board. It'll probably just be strapped to the square tube next to the steppers for now. On the next one I think I'll make provisions for it to live inside the square tube to keep everything nice and tidy.


Jim McMillan

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Re: Mechanical Design
« Reply #20 on: May 07, 2011, 07:18:16 PM »
The other pic...

Jim McMillan

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Re: Mechanical Design
« Reply #21 on: May 08, 2011, 01:05:48 PM »
Damn, I screwed up on a basic dimension and it looks like I need to re-make the aluminum parts, or at least modify them.
The good news is that the gear cutting went well. Pics attached.
« Last Edit: May 26, 2011, 11:27:24 AM by Jim McMillan »

Gabriel

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Re: Mechanical Design
« Reply #22 on: May 10, 2011, 06:11:04 PM »
Yeah it looks like the gear cutting went really well! They should be perfect for the job. Is that black stuff the threads are cut into just some sort of plastic?

I've spent the last few days rewriting and modifying large chunks of the Arduino Sun Tracking / Heliostat program. Everything I've thought of adding or improving over the 9 months since I first released it has finally been done. The program is a lot cleaner now too. I doubt that I've made it any easier to understand should someone want to look under the hood, but, for those who don't care, it should be a lot easier to set up.

I'm still double checking some things and am also still trying to figure out how to get that Accelstepper code to work with it, but it should hopefully be finished soon. and by finished, I mean finished. :)


Jim McMillan

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Re: Mechanical Design
« Reply #23 on: May 13, 2011, 11:17:48 AM »
Gabriel,

Lol. Is anything ever REALLY finished?

About the program, personally I'm not concerned about it being easy to understand. I just want something I can use to run my machine and that has some provisions for customizing the setup. My only special request would be to have provisions for plugging in code for the Adafruit motor shield and possibly some "step and direction" code to interface with common commercial stepper drives.

I've been meaning to ask, is there anything I can do to help support your efforts? Obviously you've put a lot of work into this project and, as a beneficiary of that work I would like to contribute. I noticed a link for contributions on your home page. Are cash donations preferred, or would some custom gears and/or mechanisms be more useful? My little Taig mill and lathe are pretty handy for that kind of stuff.

The pinion gear in the photo is made of Delrin (a.k.a. Acetal) plastic. Turns out that this sample was cut too deep so didn't work very well with my current shaft spacing. I think the key is to make the circumference of the blank evenly divisible by the number of teeth desired, and then advance the tap into the blank by a distance equal to the depth of the worm screw threads. Originally I thought that the "pitch diameter" would be less than the outside diameter, as it is with regular spur gears. In this case however, the first pass "locked in" the tooth count, then as I went deeper and the circumference decreased the bottom of the threads started overlapping. If you look closely at the pic, you can see a good tooth profile on the sides of the "groove", and the threads getting real thin at the bottom. The good news is that almost anything will work, but if you want accurate control over the tooth count and shaft spacing it takes a little planning.

Jim

Gabriel

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Re: Mechanical Design
« Reply #24 on: May 14, 2011, 07:14:32 PM »
Ha ha, no but I wish it would be really finished. At the very least it is finished enough that I can stop thinking about it so much. So far it looks like I finally figured out that Accelstepper bug. Or maybe I shouldn't say figured out so much as I made it disappear without being entirely sure why doing what I did made it suddenly work. I'm going to let it run awhile longer before I document and release it, but if you are ready for it now I can go ahead and post it here. I designed the program to be as adaptable as possible. Adding the Adafruit motor shield code should be a piece of cake I think, and changing it to "step and direction" code would be even easier. I have no way of testing the Adafruit code on my own though sense I don't have one. You'll be the first to try it in conjunction with the AccelStepper library.

If you really want to donate, PayPal is generally the easiest. The donations that people have given so far have all gone straight back into improving the heliostat project. Of course, another equally good contribution is just to help with the overall design, which is basically what you are doing now. 

At some point I'm going to end up building some more heliostats, and I'll try and copy what you did with the wormgears when making them. I don't have the sort of tools that you do for working with metal, but I have ideas for how to work around it. Basically what I'm striving for is something that is accessible to as many people as possible. I may try putting together a Sketchup model when I have the chance, and, depending on how this summer goes, maybe even try building one or two.

Take Care,
Gabriel

Jim McMillan

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Re: Mechanical Design
« Reply #25 on: May 14, 2011, 11:21:15 PM »
Hey Gabriel,
Sounds good. I'll kick in a little via the PayPal link and continue to post my progress here.
It would be pretty easy to set up a jig for cutting the gears. You just need a way to hold the blank as it spins while you rotate the tap against it while keeping the two rotating axes roughly perpendicular. A drill press with the drive belt removed could hold the blank while you spin the tap against it with a hand drill. Or instead of a drill press you could make something with a couple skate bearings and some hardwood blocks. I'm sure there are lots of low-tech ways it could be done by applying a little imagination. It mainly depends on what kind of tools and materials you have at hand I suppose.
I'm looking forward to the new software, though I'm not quite ready for it so you may as well hold off posting until you're ready. I got the machine cobbled together and moving using some really ugly gears since I trashed all my Delrin stock trying to make them come out perfect. I ordered some more Delrin, and now that I figured out the process I think the new ones will be a piece of cake. Unfortunately I won't be able to spend much time on it again till next weekend since I'm working a lot during the week. On the other hand it's fortunate that I am working because now I can afford to buy the parts I need!
Question on the limit switches: I gathered from some of your instructional posts that you are using only one input (pin 13) and that both switches are in series. I attached a little sketch of what I think you had in mind. Is this correct?
Jim

Gabriel

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Re: Mechanical Design
« Reply #26 on: May 15, 2011, 05:24:20 AM »
Thanks Jim,

I made a wormgear once before using my CNC machine. It was fairly low tech aside from the fact that I used a CNC machine to make it. I might go that route again.

Gah, no sorry that's a typo (which I already fixed). The limit switches should be wired in parallel not series. Sorry about the confusion.

Thanks again,
Gabriel

Jim McMillan

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Re: Mechanical Design
« Reply #27 on: May 15, 2011, 11:42:43 AM »
Like this?

Gabriel

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Re: Mechanical Design
« Reply #28 on: May 15, 2011, 01:39:26 PM »
That looks right.

Jim McMillan

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Re: Mechanical Design
« Reply #29 on: May 19, 2011, 11:00:49 PM »
Thanks Gabriel.
I've got her all wired up and running with a stepper test program. About all that's left is to mount the mirror. Should get that done this weekend. There will be some fine tuning to do on the mechanical bits. Right now I've got almost 2 degrees of backlash in the AZ axis, but close to zero in the EL. Feels like most of the slop is in the gear teeth. I think I'll swap the AZ and EL gears to see what happens. I can live with some backlash on the EL since the weight of the mirror will tend to bias it in one direction.
Looks like I have a little more work to do on the gear cutting. I made two new ones tonight. This time I started with a 1.146 diameter blank, hoping to get 36 teeth to mesh with the 1/2-10 acme screws. My reason for picking that number is because it gives about a 3.6 inch circumference so with a 10 pitch screw I would get 10 degrees of axis motion for one turn of the stepper. It just seemed like an easy ratio to work with.
Anyhow, I'm finding it's a little harder than I thought to get predictable results from the "tap and free-wheeling blank" method. I tried 3 times on the same size blank and got three different tooth counts (37, 38, and 39). The trouble seems to be that the first revolution after the tap touches the blank is what determines the tooth count, and from there you're locked into that count. If there is a little slippage on that first turn the tooth count will be off. Next time I think I'll try and "pre-slash" the blank with the right number of teeth and then do the free-wheel thing with the tap to finish up. I have a CNC rotary indexer for my mill so I'll probably use it to pre-cut the blanks with a tiny end mill or engraving v-bit, though I'm thinking there is probably a simpler way. For now I have two pretty decent looking gears; 37 teeth on the EL and 39 on the AZ. I guess I'll just work out the right numbers to plug into the software for now.
So, Gabriel, how's that software update coming along...? I might be ready to give it a whirl this weekend!
Jim