Wobbler Air Engine project for CCBCMD machine tools class CMM112

[PStechPaul]

I started the second semester of Machine Tools class at the CCBCMD Community College in Catonsville, MD, and our shop project will be a wobbler air engine. Here are the plans:














I have PDFs and other material for the class here.

Has anyone seen or built an engine similar to this? About the closest I could find is this:

<a href="https://www.youtube.com/watch?v=ootr4HGAg-8" target="_blank">http://www.youtube.com/watch?v=ootr4HGAg-8</a>

Some questions I have, that I will also discuss with the instructor, are:

1. Would it be better to make the cylinder from steel, to match the material of the piston and manifold/valve?

2. Similarly, perhaps the crankshaft should be steel, or perhaps brass. I think steel would be best, to match the thermal expansion of the steel dowel pins.

3. The flywheel is aluminum, but I think it would be better if made from steel, for greater inertia. Actually, I think it would look nice made from brass, and I have a 6" piece of 2" 360 Brass stock I could use. The aluminum was probably chosen for cost reasons.

[philjoe5]

Hi Paul,

I recently documented my building of a Wobbler here:

http://www.modelenginemaker.com/index.php?topic=4652.0

My build is related to your project, though I think my engine is simpler than yours.  I built mine to act as a standard project for our local Makers Group.  It would be an introduction to making a machine that requires some precision and accuracy.

As far as materials go, just avoid having identical materials in contact that will be moving at high velocities.  These engines won't normally see a load applied to them so an aluminum cylinder with a brass piston will provide hundreds of hours of running time.

A steel crankshaft is probably best but I built one of these with a brass one and it chugging along quite nicely.  The more mass in a flywheel, the better, so I'd use brass, steel, cast iron for that.

Good luck with your project.  I'm interested to see how yours turns out

Cheers,
Phil

[b.lindsey]

That should be a good project Paul. I agree with Phil, alum. cylinder, brass piston, steel crankshaft, and brass flywheel.

Bill

[Ian S C]

The little wobblers that I have built, I used brass, or one with bronze for the cylinder, and stainless steel for the piston.  steel for the crankshaft and pin, brass for the crank web, brass for the bearing, flywheel, and bearing. Here's the smallest one, 1/8" bore, 1/4" stroke.
Ian S C

[ChipMaker]

What a great school project!  Students get to learn how to read drawings, identify various materials, and get to use some really neat shop machines!  I hope that they know just how lucky they are, and the will end up with a engine they built themselves and can share with others, that may lead to a career.  Great!  Regards, Karl, CM

[ausdier]

Here is a 3D PDF of the parts as drawn with a little licence on some parts that weren’t there and a couple of washers on the crankshaft.
Hope this is ok.

[PStechPaul]

That's a very good rendering. Do you have this in a CAD format that I could import to TurboCAD? I think the instructor uses Solidworks for the drawings, and I can import at least some of those models, although I don't know if I can edit them. I had thought about drawing the parts in TurboCAD but that would take some time, and if you have already done so, no sense re-doing it.

I see that this PDF is a 3-D view which can be rotated, but I can't take a snapshot or extract the image. I was able to take a screenshot:



I have a tendency to analyze a design and look for better, or more unique, ways to build it, but I think the instructor has designed it this way as an exercise to learn and gauge various techniques and skills. It seems to me that the large mating surfaces of the cylinder and manifold would be a source of a lot of friction, and may require lubrication, so perhaps something like a Teflon or silicone gasket might be beneficial.

Thanks!

[philjoe5]

Paul,
It would seem that there is a lot of friction between those 2 flat surfaces.  I thought that as well before putting mine together.  If the spring pressure pulling the cylinder against the manifold is set right, the counter effect of air pressure pushing the cylinder away presents a nice equilibrium.  Now add a film of oil between those 2 surfaces and you've got a pretty smooth motion.  Of course, the bearing surfaces of the cylinder and manifold should be highly polished.  I used a flycutter on mine followed by successive polishing up to #800 grit.  Then I got lazy and quit.

Ausdier,
Great job on the 3D rotatable drawing

Cheers,
Phil

[b.lindsey]

Ditto on Phil's comments..wobblers are intended to have that metal to metal fit but they do need to be smooth and with some oil for lubrication. I think an intermediate piece like Teflon film or any gasket arrangement would be more complicated than beneficial.

Bill

[ausdier]

Hey Paul.

Send me a PM with your email and I will send a few formats for you to try.

Cheers.

(Render done with Keyshot 5)

[PStechPaul]

[edit] Just noticed that it looks like the flywheel is not centered on the shaft. But it might be an illusion due to the mirror finish. Otherwise the rendering is really nice!

[from HSM] Last night I got started on the project by making the flywheel, but I used a piece of 2.062" diameter x 1.56" round 1117 steel. I turned it to 2.000" with a 0.875" neck. I made the neck slightly oversize and about 0.02" longer than specified, and I faced the opposite end so that the flywheel is about 1.50" long rather than 1.125" as specified. I figure that, for a flywheel, more (mass) is better, and this should be a bout 4 times that of the specified 6061 aluminum. Here are pictures and links to some videos:
 
 
 
 
 
 
 
 
 
 http://enginuitysystems.com/files/CA...wheel_1721.AVI
 
 http://enginuitysystems.com/files/CA...wheel_1723.AVI

[PStechPaul]

I have made a few more parts, at least "partially". My problems with parting the piston is documented in another thread, but here is the (almost) finished result, with the repaired part:
 

 
Here is the crank, made of 6061 aluminum, surronded by a coil of swarf from a successful parting operation:

 
And with a 0.375" dowel pin inserted. It should be a press fit, but I was able to push it in by hand (further than as shown), so some Loc-Tite may be necessary:
 

 
I also bored the cylinder to 0.625" using a boring bar on the Clausing lathe toolpost at school:
 

The finish is a little rough because I bored it by hand rather than power feed, since the carriage stop clutch is not reliable. I could have used it and disengaged when it got close to the stop and finished by hand, but this should be good enough.

 
The bore might be 0.001 larger than tolerance, but the piston goes in smoothly with some air compression resistance, and it makes a nice "POP" when I pull it out quickly. A machined part's way of saying "Who's your Daddy!" 

 
I may modify the flywheel something like this:
 

 
I made it thicker than specified on the drawing and from steel rather than aluminum, so it is quite heavy. This modification will make it much lighter while still maintaining considerable angular momentum. I would also like to machine a groove in the flywheel for a belt to connect to a DC motor as a generator, and it may also then be able to work in reverse as a pump. But I need to concentrate on getting these parts finished, and the other parts as well. I have all the materials I need, now, and I bought some reamers and other tooling so I can do much of it in my home shop.
 

[b.lindsey]

You are making some good progress Paul. I think the steel flywheel should serve you well. Looks like you should be finished long before the end of the semester

Bill

[PStechPaul]

I am hoping to get this finished and running in time for Cabin Fever, just six weeks away. The semester ends May 21. I'll be having cervical spine surgery on my neck April 16, so I won't be able to do much for awhile, although in a month I should be able to drive and resume somewhat normal activity in time to take the final exam. That's why I need to stay concentrated on this project, and do some work at home. It seems to take much longer in the school shop, finding the tools needed, getting set up, distractions, and leaving time for cleanup. About all I did last Thursday was cut a piece of 1" square steel for the manifold, and then bore the hole in the cylinder. We have about three hours in the shop, which seems like a lot, but time passes quickly, and I work slowly.
 
I was using only 200 RPM, and the instructor said I should use 1000. I also went slower because I forgot that the Clausing cross-slide is calibrated in diameter, rather than radius, and I was wondering why the hole didn't open up as I had expected. I thought it might be due to the backlash, and also I think I may have sometimes bumped the handwheel as I was moving the carriage. There is probably a way to lock the crossfeed, but it was not apparent, and I didn't want to mess around with adjusting the gibs or otherwise attempting to secure it better.

[b.lindsey]

I know all too well about the school shop thing. Of course by the time the kids check their cell phones 50 times, ask questions (which is fine), and do a half baked job cleaning up after themselves, they don't manage to get a lot done. Then (back when I was managing the shop), come the end of the semester they wanted me to hang around till 6-7 PM so they could catch up. Adults are so much easier to work with 

Bill

[philjoe5]

Good progress Paul.  Be sure to bring it to CF.  Hope to see you there

Cheers,
Phil

[BadSpellar]

Why is the crank shafts designed the way it is. I don't think I have ever seen one with the heavy side on the connecting rod side. Won't that really mess up the balance?

[vascon2196]

I have my students each build a small wobbler out of clear acrylic, aluminum, brass, and steel.

At the end of the 10-week quarter they have learned blueprint reading, setup/layout, measuring, mill & lathe basics, tolerancing, and assembly/fasteners.

Air engines are perfect for those types of students.

I'm happy to see you doing this...it is extremely important that this generation of students know how to apply what they are learning.

Great job!

[philjoe5]

I'm setting up a workshop for the Local Makers group, Make717, so anyone age 12 and up can make one of these simple but quite robust engines.  I'm with Chris, not many young folks at model engineering shows but I think it's from lack of exposure not lack of interest.

Besides, who am I going to sell my tools to when I get (more?) senile?

Cheers,
Phil

[PStechPaul]

I may be the oldest student at 65, but many others are 40-60, and only a few of typical college age (18-25). A few of them may have home shops, but AFAIK I am the only one doing any work at home. This week it will be warmer (above freezing) so I may be able to do more work in my unheated shop.
 
I made a document listing my proposed steps for making each of the pieces:
 
http://enginuitysystems.com/files/CAMM112/CCBCMD_CAMM112_Air_Motor_Project.pdf
 
http://enginuitysystems.com/files/CAMM112/CCBCMD_CAMM112_Air_Motor_Project.odt
 
I sent copies to the instructor. If you have any suggestions or better ways to perform these operations, please let me know.

[PStechPaul]

I've made more progress on the engine. At school we took an hour to go over stuff that will be on our mid-term exam this Thursday, and then we went to the shop. We got a demo of knurling, and then one other guy and I got ready to set up the cylinder in a special jig:
 

 

 

 
But there were problems getting it aligned, and I got on another machine to flycut the side of the cylinder:
 

 

 
I took a video clip of the flycutting operation:
http://enginuitysystems.com/files/CAMM112/Cylinder_Flycutting_1798.AVI
 
Next I set up the mill to locate and drill the 3/16" hole for the crankshaft pin. My part already had the 3/8" shaft installed so I tried to locate it from the edge. Without a DRO, I used an edge finder and the dials, but I made a mistake and located it about 0.14" too close to the center shaft. Then it was time to clean up and go home. Here is the crankshaft and some of the parts to complete the engine:
 

 
I never liked the idea of the aluminum crank. I was anxious to get the engine together and running, so I decided to make one from 1/4" steel in my home shop:
 

 

 
I used a piece of O-1 drill rod instead of the dowel pin. It's too long, because I copied a 1-1/2" dowel that was in the bag of parts, rather than the black 3/4" I should use. But I put it together, and after a bit of "running in" and 3-in-1 oil, it turns smoothly and should run if I connect the air hose.
 

 
 

 

 
 

[PStechPaul]

I got it running! I also attached a small 24 VDC electric motor with an O-ring that I spliced together with Cyanoacrylate (Super Glue), and I was able to run it as a pump by applying voltage (24 VDC and about 100 mA) to the motor. So that's 2.4 watts or 1/300 HP. Here are some raw video clips:
 
http://enginuitysystems.com/files/CAMM112/Air_Engine_DC_Motor_1839.AVI (23 MB)
http://enginuitysystems.com/files/CAMM112/Air_Engine_DC_Motor_1840.AVI (80 MB)
 
Ignore the TV noise in the background. 
 
Then I connected it to my newly repaired compressor (rebuilt the check valve about a year ago), and it runs pretty well. The monster steel flywheel lets it run fairly slow. I'll be adding a proximity sensor and the TigTac circuit to show the actual RPM. I connected an incandescent lamp (#47 6V @ 150mA) and it lights quite brightly when the motor is running at full tilt. Here are more raw video clips:
 
http://enginuitysystems.com/files/CAMM112/Air_Engine_Running_1837.AVI (42 MB) - Very first run - before adding motor.
http://enginuitysystems.com/files/CAMM112/Air_Engine_Running_1841.AVI (47 MB)
http://enginuitysystems.com/files/CAMM112/Air_Engine_Running_1842.AVI (42 MB)
http://enginuitysystems.com/files/CAMM112/Air_Engine_Running_1844.AVI (70 MB)
http://enginuitysystems.com/files/CAMM112/Air_Engine_Running_1845.AVI (20 MB)

[b.lindsey]

Its a runner for sure Paul, always satisfying seeing them tick over for the first time. So now what are you going to do for the rest of the semester

Bill

[PStechPaul]

Last evening's class was mid-term exams. I think I did pretty well. Most of the class left after they had finished, but four of us went into the shop about 6:30 to work on our projects. I decided to complete the cylinder, which needed the rounded profile cut on the special jig. First was to mill the rounded end with a straight end mill:
 

 

 
Then I used the round corner cutter to mill from the square end to the center of the hole, then using the rotary table turned the piece 180 degrees, and then continued the other side:
 

 
I had some problems with the rotary table so it seems it was not perfectly set at 180 degrees. On the final pass I fudged it a bit and ended up with a ridge, but it can be cleaned up by using the straight mill again:
 

 

 

[90LX_Notch]

Paul-

Congratulations on your first runner.

-Bob

[Roger B]

Excellent work    The first runner is always good   

[philjoe5]

Nice runner Paul.   I like the generator setup.  Hopefully I can get some details about it at CF.

Cheers,
Phil

[PStechPaul]

Thanks, everyone. It is a good feeling to see a creation run for the first time. I will be adding a valve and pressure gauge so I can easily regulate the speed and see what it takes to run. I have 1/8" NPT fittings and some 1/4". I put this together, but I'll use shorter or close nipples, or I might even make my own fittings. I might even try my hand at making a valve, but I have other priorities and that would just be for the learning experience and perhaps "bragging rights". 
 

 

 

 
As for the generator, I'm using a 24 VDC "hobby motor" that I bought two of from SurplusShed for $0.75 each and 50% off that. They seem to be well made and little 24 volt motors are somewhat rare and very useful as generators. I will probably order maybe 10-20 more, hopefully if they have another sale, and I'll bring them to CF in case anyone wants one. It has a resistance of about 75 ohms so the maximum power at rated voltage would be into a 75 ohm load with 12V or 2 watts.
 
I happened to have a small pulley that fit the 2 mm motor shaft. I might order some drive belts like these, but they come from Asia and delivery is very slow. It may be better to order O-ring type belts from SDI. A 3/32" thick and 3" diameter (9.4" long) neoprene belt is only $0.85. It looks like the combination of a grooved pulley and the flat flywheel works quite well, and no additional groove is required. They don't seem to have pulleys small enough to use on a 2 mm shaft. But they should be easy to make.
 
I found some from a UK supplier:
http://www.ebay.com/itm/Qty-10-18mm-Miniature-Model-Belt-Pulley-Wheel-for-2mm-Motor-Shafts-/120894972581
http://www.ebay.com/itm/Qty-5-8mm-Diameter-Miniature-Model-Belt-Pulley-Wheel-for-2mm-Motor-Shaft-/120848374323
 
Here are some that are from a Chinese supplier:
http://www.ebay.com/itm/5pcs-Small-Orange-Belt-Fixed-Pulley-3mm-2mm-for-Toy-Robotic-Module-Car-Model-/391085408796
http://www.ebay.com/itm/10pcs-plastic-sheave-belt-pulley-6mm-white-small-2mm-hole-timing-pulley-for-DIY-/231374536151
 
You can get an O-ring splicing kit with five sizes for $26:
http://www.use-enco.com/CGI/INSRIT?PMAKA=505-3883&PMPXNO=948270&PARTPG=INSRHI
 
Here is 3mm Viton O-ring cord $7/meter:
http://www.ebay.com/itm/3mm-Diameter-Viton-O-ring-cord-75A-Max-temperature-260-c-price-for-1-meter-/330920156954
 
Probably easier and cheaper to get it from McMaster. They have Buna-N, Viton, silicone, neoprene, and EPDM. Ranges from $0.21/foot to $1.67/foot for 1/16". Various grades of hardness, chemical resistance, temperature, et.
http://www.mcmaster.com/#o-ring-cord-stock/=we8wgs
 

[PStechPaul]

Beautiful warm evening yesterday for our shop class. I decided to finish the base, which required cutting the edge profile with a 1/2" ball end mill, and then flycutting the top surface. The mill I chose to use did not have a working DRO, and I had some problem with backlash on the X and Y dials, and also I neglected to account for the edge finder radius when locating the back left corner from which I started. I used a depth of 0.230", leaving 20 thou for finishing, and also about 0.020" extra for the edges. It cut pretty well, conventional milling in all four directions, but it messed up a bit on the second cut left to right in the X axis. I did not have the Y axis locked and it may have shifted in about 20 thou. But then I got the "hang" of it and continued the rough cut and then got within about 5 thou using climb milling for a nice finish:
 

 

 
The corners look like they are cut correctly, although I think they need to be cut deeper, and cleaned up. The edges definitely need to be milled clean and bright, but I can do that in my home shop:
 

 

 
I especially like the finish obtained by flycutting. My first attempt with a cutter from the tool drawer had many fairly deep gouges, even when I went painfully slow with only about 5 thousandths DOC at about 250 RPM for a 3" diameter cutter. Looking carefully at the cutting edge, it was apparent that it was not correctly ground for this process. So I touched it up on the grinder and did it again, with much better results. Then the instructor recommended that I use the high speed setting with a light DOC and slow feed, which definitely produced a nice finish:
 

[Don1966]

Looks good Paul and doing the edges gives it a nice touch. 

Don

[PStechPaul]

I mounted the generator more solidly, and also made a post where I mounted the miniature bayonet base lamp (in an old military style lampholder), some binding posts to read the generator output voltage, and an inductive proximity sensor. I also connected it to the TigTac prototype and ran into some problems, as discussed in a HSM thread, but it now works reasonably well. Here is a copy of the latest post in that thread, showing the (almost) final set-up, with some video clips of the engine running on compressed air and lighting the 12V lamp, and running as a compressor by applying 20-25 VDC to the motor:
 
I added a 1k resistor from the sensor input to the PIC pin being used, and a 220 nF capacitor to ground. This seems to have eliminated the noise that previously caused erratic readings. The original code used an interrupt-on-change (IOC) method to detect contact close and open events, but that was very sensitive to noise, and difficult to implement a software filter. Now I am polling the contact state every 1 mSec in an ISR and requiring 3 consecutive stable readings to determine status. Since there is equal delay on sensing proper open and closed conditions, the accuracy of the dwell and RPM (based on time between closings) is only affected by the 1 mSec uncertainty of the sample. Thus a modest 600 RPM corresponds to 100 mSec/rev and accuracy is about 1%. Here are some pictures of the setup:









 Some video clips of the operation and waveforms:


http://enginuitysystems.com/files/TigTac/TigTac_1908.AVI (111 MB)

http://enginuitysystems.com/files/TigTac/TigTac_1909.AVI (147 MB)

http://enginuitysystems.com/files/TigTac/TigTac_1910.AVI (160 MB)

 and a video clip of the motor/generator running on compressed air (warning - loud compressor noise!):

http://enginuitysystems.com/files/CAMM112/Air_Motor_Generator_1900.AVI (170 MB)

[b.lindsey]

Paul, interesting setup but the files are painfully slow to view. Any chance of another format? AVI's seem to be bloated in my experience. Uploading to Youtube might be another option.

Bill

[Don1966]

I think Bill's has a good point here I can't open them in windows explorer it just sits there waiting for it to upload and takes forever. the other option is to save target as and that takes forever. Youtube would differently make life easier to see the video.

don

[PStechPaul]

OK, here it is:
 
<a href="https://www.youtube.com/watch?v=GUFMYTKVAv0" target="_blank">http://www.youtube.com/watch?v=GUFMYTKVAv0</a>
 
It seems that YouTube has made more "improvements" and now I cannot find how to choose a frame to use as the sample image (thumbnail) for the video. I waited for the choices to appear when it was done processing, but they never showed up. Then, I tried the "video manager" and editing options and there are no options to add the image. Previously you could select one of three from the video, or add your own. Does anyone know how to do this?
 
Somehow it "fixed itself". There were some new buttons that magically appeared along with the selected thumbnail choices. Weird...