Author Topic: Elmers #46 Comber Rotary Engine-How to layout cam  (Read 2979 times)

Offline ChipMaker

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Elmers #46 Comber Rotary Engine-How to layout cam
« on: December 30, 2016, 10:00:21 PM »
I recently became interested in Elmers #46 build of the Comber Rotary Engine.  In his build instructions, I am a bit confused on just how to layout the cam "circle".  I would, otherwise, build to the dimensions that he has given.  I'll bet there has to be a easy way to do it!  So to the guys that have built this engine; how did you do the cam layout?   I appreciate the information, Karl

Offline GailinNM

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #1 on: December 30, 2016, 11:03:54 PM »
Karl,
I can he;[.  A few years ago I did the cams for the Comber engines for an HMEM team build for a few of the cams on CNC.  I will dig out the links a little later in the day.  While doing this I developed an Excel spread sheet that can simplify the making of the cam on a manual machine also.  If you have a DRO on a mill it makes is quite easy compared to Elmer's method.  Not too bad with out a DRO using dials however.

Dou you have Excel.  If so I can give you things in that format.  If not I can send a set of output coordinates that is the output from the spread sheet. It may take a day to find this as I will be busy for the next few hours.

By the way.  When Elmer originally published this engine design they made a mistake in the title and that has continued on forever.  The engines proper name is COOMBER, after the inventor.  Note the two "O"s in the name.  The only reason this is important is if you want to fill in your knowledge of the history of the engine with an internet search.
Gail in NM


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Offline GailinNM

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #2 on: December 30, 2016, 11:57:03 PM »
Karl,
Here is the spread sheet and some notes. Let me knof if you need assistance using it.  Without change it will give the XY coordinates in 1 degree increments.  You can start using 10 degree increments, or 20 and see what it looks like.Just depends on how much you want to file. IMPORTANT: For this spread sheet to work the  milling cutter diameter must be the same as the roller diameter.
Gail in NM
« Last Edit: December 31, 2016, 12:05:06 AM by GailinNM »
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Offline GailinNM

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #3 on: December 31, 2016, 01:01:51 AM »
And for those who are interested here is a JPG of the drawing used to generate the math used for both the spread sheet and the CNC parametric code.
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline ChipMaker

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #4 on: December 31, 2016, 06:31:28 PM »
Wow Gail!  Many thanks for all of your information on the "Coomber" ring cam, and fast too!  I do have Excel, but my machinery here in MI is quite basic.  I have a Sherline lathe and Mill, Unimat and a Atlas 6" lathe.  I have made an assortment of engines, by taking a lot of time doing it, using files too!  I was wondering, how close I would come to cutting the cam shape if I would paste the paper outline of the cam drawing on metal and cut/file the shape out?  Hmmm.  I know this is not a machining technique, but it might just work for my one engine build.  Again Gail, many thanks for the posts here, I'll look them over, and see if I can incorporate that information in my build.  Seasons Greetings to you and yours, Karl in MI

Offline kvom

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #5 on: December 31, 2016, 09:53:23 PM »
If you want the points on the cam then set the diameter of the tool to 0 in the spreadsheet.  Then you need some method of converting the point list to a drawing that can be printed 1:1.  Otherwise you'd need to use the drill points to chain drill and then smooth the cam.  Not an easy task.


Offline kvom

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #6 on: January 01, 2017, 02:17:46 AM »
I decided to try to get my engine running today.  It was an HMEM group build from 2008, and Gail supplied the group with the cam.  I didn't receive all the parts, but about 18 months ago I finished it.  It never ran very well.  Since Cabin Fever is dedicated to Elmer's Engines, I decided to give it another go.  Got it running tonight, although it wanted 40 psi to do so.


Offline ChipMaker

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #7 on: January 01, 2017, 06:10:33 PM »
Thanks KV for the reply; happy to see that you just got your engine running too!  Like all engines, they have to be loose, no tightness anywhere, as you know.  I would suspect, then, yours will run on a lot less air pressure.  Again, because this engine is so unique, in my opinion, I want to build one and add it to my Elmers engine design, collection.  Happy New Year 2017.  Karl in MI

Offline GailinNM

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #8 on: January 02, 2017, 12:30:09 AM »
Great going  KVOM.  Glad you finally got it going right.

Karl:  Here are a few of things that you may find helpful in making the cam on your equipment if you use the spread sheet.

First make two new columns for the XY coordinates. These columns will contain the XY coordinates that the existing XY columns have but have 2.000 added to them.  This way yu don't have to do any calculations to deal with the negative values of the coordinates as all the coordinates will  be positive values.  Makes working with the dials much easier.  Just remember to always remove the backlash.  All you are doing with this method is moving the reference to the chankshaft center to x=-2 and Y=-2 instead of 0,0.

Use a 1/4 inch center cutting end mill to Poke the holes in.   After the first hole, move around the cam in a counter clockwise direction.  This places the cutting load on the endmill on the edge that deflects it toward the center of the cam.  After it is through the stock it will spring back to the correct position to finish off the edge. This prevents any gouges in the cam face from tool deflection.

Although the spread sheet is in one degree increments there is no purpose in usling this fine an increment.  At one degree increments the cusp left by the clutter overlap on adjacent holes is less that 0.001 inch so it is less than the dial setting error when you are cranking handles.
If you use 2 degree increments the cusp height is about 0,.002 on the average and a wipe around the cam face with a bit of 320 or so abrasive paper with your finger  and it will be as close to perfect as needed.
If you use 3 degree steps then the average cusp height will be a bit over 0.004 inch and this will require a little bit more finishing.  But a bit of 320 paper wrapped around something an inch and a half diameter or so will remove the cusps to an acceptable level in 5 minutes or so.

It's still a few hours to do 120 or 180 sets of coordinates but should not take you more than a long evening.

Last hints.  Make the slots on the forks a little bit wider.  Elmer called out 0,130 wide riding on the 1/8 inch thick cam.  Any misalignment and the fork rubs on the cam.  0.140 or even a bit wider is a more realistic number.  Something else I did was to replace the rollers with 1/4 diameter ball bearings.  Had to put a bushing in the ball bearing to get the shaft to match.  I made mine run a bit smoother.  Any hobby hhop that has RC Car parts will have them.  Certainly not necessary but it did make mine run smoother. That 1/4 inch roller runs about 15 times  faster than the engine rpm so at 2000 RPM engine speed the roller is at 30,000 RPM minus slipage of course.

Have fun and remember we like pictures.
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline ChipMaker

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #9 on: January 03, 2017, 06:33:19 PM »
Thanks Gail for the additional info on cutting the cam for the Coomber Rotary Engine.  I am going to put a ball bearing at the flywheel end of the crankshaft; I use the ones that are cheap and are used on various wood router bits.  I did not think of adding ball bearings to the connecting rod ends, but that is a good idea; I was just a little concerned about the "pounding" they will take.  I'll try and digest what you sent on cutting the cam, but for now, I decided to start the build by cutting the brass for the cylinder and flywheel.  I am thinking about leaving enough material on the cylinder as I turn it, for the heads, which I will part off later.  As with all machine work that I do, it takes a lot of "noodling" for me, as I detest making scrap which is a waste of material and time.  I attached a photo hereto to show my work so far; the brass I cut for the cylinder/crankshaft and flywheel.  Regards, Karl in MI.

Online b.lindsey

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #10 on: January 04, 2017, 02:39:52 AM »
Every journey begins with the first step Karl. You are off an running now!!

Bill

Offline ChipMaker

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #11 on: January 25, 2017, 07:03:51 PM »
With the help from those that posted here offering to help me with my build of the CAM for this engine; thank you very much!  Yesterday, I assembled all of the parts, added air, and it ran!  It ran, but took 30 ~ 40 lbs of air to do it; the engine had a "tight" spot. Once I worked that tight spot out, it now will run on less than 5 lbs of air; if I crank the air up, it will about fly off of my bench!

GailinNM, helped me the most with the JPG that he sent along in his post here to help me with the layout of the CAM.  Again, my shop has only lathes, a 6" Atlas, Unimat and Sherline.  I do not have the technical knowledge to operate anything more.  So, using this information, I chucked up a portion of 1/8" brass sheet in my 4 jaw chuck on my Atlas lathe for the CAM, and turned the I.D., to a perfect hole.  Then, I offset this setup up by 0.030", using two opposite chuck jaws and turned again.  Then, I used these same two chuck jaws, and did a 0.060" offset, and turned once more.  This gave me a total CAM shape 0.060" out of round, at the 9:00 and 3:00 o'clock positions, if you will.  Now I had the CAM shape, I hoped this would work!  The CAM was about the last piece that I built for this engine.  I used a ball bearing from a old computer printer for the end of the crankshaft, near the flywheel, why not, as suggested here? 

The two attached photos will show the engine as it is now; assembled and running just beautifully!  It is totally free, and will self-start if the crankshaft is in the correct position, with very little air pressure, and even just "tick" over which is fun to watch too.  Since it is running so well, I don't think that I am going to take it apart and do some reshaping of the two uprights or even polish it up.  I think I will just leave it alone so I don't have any aggravation.

I feel that this engine of Elmers, maybe more challenging than one would first think.  The reason being, everything has to be "just right".  This means, a very close fit where required, like the valve, and trueness just about very place with everything else.  This build takes a lot of "fiddling around" to get it to run on very low air pressure and run rather quietly, without pounding of the connecting rods rollers against the cam.

I have built Elmers "Geared Engine" (I bought the two gears) and "Wobble Plate" engine, and I believe this Coomber Rotary Engine was the toughest! 

In one of my next posts, I will attempt to send along a short video of it running, but I am attaching here two photos of it.  The flywheel end of the crankshaft, were the ball bearing is, is short, because this was the size of the thick brass that I had in stock.  As a mater of fact, I cut it from the same round bar that I made the flywheel from!

Thanks again guys for your help and encouragement.  What of Elmers engines might be the most challenging would you guess?

Karl in MI

Offline GailinNM

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #12 on: January 25, 2017, 07:46:09 PM »
Congratulations on another runner Karl.
Gail in NM

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Offline ChipMaker

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Re: Elmers #46 Comber Rotary Engine-How to layout cam
« Reply #13 on: January 25, 2017, 08:45:53 PM »
My second photo in the previous email, did not post, so here it is here.

Karl