Author Topic: Comber engine question  (Read 5791 times)

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Comber engine question
« on: May 03, 2016, 03:53:54 PM »
Hey all. I'm thinking of builbing a Comber engine, Larger than Elmer's design. It would be of my oun design.
I've looked at Elmer's cam ring, read his layout technique, but can't really understand why the ring can't be
round, with the cylinder center offset half the distance of the piston travel, to the cam ring center??

That is a mouth full!. Basically, why isn't the cam ring simply be round?

Has anyone else tackeled this question?

Thanks, Sid
« Last Edit: May 03, 2016, 03:57:46 PM by sid pileski »

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #1 on: May 03, 2016, 04:28:09 PM »
The length of the piston rod is fixed and the cam followers must contact the cam ring at both ends.
If you offset the pivot bearing then the distance across the circle varies as the cylinder is rotated.

You can see this easily by drawing a circle and then offsetting a point down from the center.  Now draw lines both hooriz and vertical throulghl this point.  You can see that the length of these lines between the points where they intersect the circle are different lengths.  It gets more complicated when you add in the cam follower rollers as then the points are between the points where the cam followers touch the cam and these are not in line with the piston rod.

I have a different layout method that I used to cut the cam on CNC which would also work on a manual machine, especially if you have a DRO.  I can dig it out of the archives if you need it.
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #2 on: May 03, 2016, 04:38:42 PM »
BTW, If you want to research the engine a bit on the internet, search for COOMBER engine.
 Note the two "O"s in the name.  When the original article by Elmer was published the name was misspelled and the error has continued ever since...
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #3 on: May 03, 2016, 04:38:42 PM »
Gail- Thanks. That makes simple sence, when explained that way.

Yes I will be using my CNC to cut the cam ring.
Did you lay out the ring in CAD?
I would be interested to see your method.

Thanks, Sid

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #4 on: May 03, 2016, 04:48:55 PM »
I will dig out my notes later today..   Can your CNC do parametric programing, that is feed it an equatiion for the curve. The programing gets vary easy if it does and you use a cutter the same diameter as as your cam followers.  I will post a drawing showing the equations that you can use for either parametric or CAD.  It gets a little more messy if you cus a cutter of a different diameter than the follower but you can do it with a bit of effort.
Gail in NM 
I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #5 on: May 03, 2016, 05:04:53 PM »
Gail- My mill is a Prototrak. Conversational programing. (but G/M code behind that)
I'd like to do the laout of the ring in CAD. ProE/Creo for me.

Anything you have would be helpful I'm sure, and time saving too.

Thanks, Sid

PS, would it be easier to correcpond to each other via email?


Sid


Sid, I have removed your email from the public forum. Various spiders, etc. can see it in the open forum and use it to spam your email. You can send it to Gail via PM of he can see it in your profile if it is shown there. Have to be careful these days.

Bill
« Last Edit: May 03, 2016, 05:42:00 PM by b.lindsey »

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #6 on: May 03, 2016, 06:13:39 PM »
Sid,
Attache is a PDF of my setup sheet for HMEM team build of the cams.
To generate a CAD drawing of the cam I think the easiest way  would be to plug the equations into a spreadsheet such as Exxcel using your cam follower radius and let it generate a coordinate list for the path of the center of the cam follower at 1/2 degree or so increments.  You can probably pull that into your cad program as a polyline using a script file or equivelent.  Spline the polyline to get a smooth curve and then offset it by the radius of your cam follower to get the true cam outline. Yuu can then machine to that with any convenient cutter.  That lets the  CAD do all the messy calculations.

If you have any problems with that, I can do it here with your parameters and send you la DWG/DXF file.
Just let me know if I can help in any way.
Gail in NM

« Last Edit: May 03, 2016, 06:18:36 PM by GailinNM »
I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #7 on: May 03, 2016, 06:41:24 PM »
Thank you for your generous help!

I'll see what I can come up with.
What Cad system do you use?

Sid

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #8 on: May 03, 2016, 07:24:49 PM »
I use  several CAD setups.
Don't laugh too hard but my main battle CAD is ACAD12 running under DOS.  It is installed on a 3 gHz machine running native DOS. Really screams along.  I have a homegrown CAM system that I wrote in LISP set up with it sothat it drives an equally old Bridgepoort CNC milling machine and an again equally old Dyna CNC lathe.  I use Turbo Pascal to generate script files for it for complex curves. DOS talks well with the outside world for control purposes

Next is ACAD 2000 and ACAD 2004 that I mostly use for generating files for the laser lengraver/cutter although I frequently ilmport them from  ACAD12. ACAD 12 will not drive my Laser. These reside on a dedicated Windows XP machine as my more modern 64 bit Windows 7 machine will not run them very well and no 64 bit drivers were ever written for my old Laser.

Last is Rhino 3D that I generate files for the 3D printer running on Windows 7x64.
 
All set up with KVM switch so one key stroke switches to what ever one I want at the moment and the XP and Win7 xomputers are networked together so they can share files. DOS not so much so I use sneaker net with USB stick to share files with it.

Yes, I am an old geezer past 3/4 of a century old and stuck in the past in many ways.

Gail in NM



I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #9 on: May 03, 2016, 07:46:42 PM »
Well- while I use computers all day to do my design work, I only know the basics of them.
To me they are a tool, but I don't know the inner workings of them. When they break, I have a "guy".
Much like people driving cars. They drive for millions of miles every day. But,they don't know how they work, nor can they fix them!

Can you import a 3D step file? I'm doing my design now, but still may take you up on the offer to layout the cam ring.

Let me know.

Thanks, Sid

Offline Zephyrin

  • Full Member
  • ****
  • Posts: 766
  • near Paris, France
Re: Comber engine question
« Reply #10 on: May 03, 2016, 08:31:32 PM »
I wrote a few years ago a Excel spreadsheet (in french) to calculate the machining of this cam.
Problems related to the tangential position of the roller, really minimal and complicated, are totally eliminated by the use of a milling cutter of an identical diameter to the roller. The route thereof is determined by trig, it is a "curve" centered on the rotation of the cylinder center, the radius of which varies according to the angle of the cylinder. it is not a true circle halas !

the roller track is in black,and the theoretical circle in yellow... some builders prefer files and scrapers to adjust this cam !

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #11 on: May 04, 2016, 12:57:29 AM »
Zephyrin,
That is the same approach that I used, that is using a cutter diameter the same as the roller.  Only difference is that I used a parametric program in the CNC control and let it do the trig instead of using a spreadsheet. Only took about 20 lines of G code that way.

Sid,
I have never used step files.  I don't do much 3D work.  If you want me to send you a DXF file I only need 3 numbers.  It will only take about 15 minutes I think.
1. The center to center distance of the rollers (cam followers)
2. The radius or diameter of the rollers, but tell me if it is diameter or radius.
3. The offset of the centers which is 1/2 the stroke.

If the file is small enough I can attach it to a post by using a .txt suffix to a DXF file.  Then you would have to change the suffix to DXF to read into your CAD.  I don't think this forum software permits DWG or DXF files to be directly attached.  Or I can ZIP the files but that's usually a pain for me. Or PM me  your email and I can send them direct.  I prefer to keep everything on the forum where possible so others can see the information.
Gail in NM

I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #12 on: May 04, 2016, 01:29:22 AM »
Gail- I did pm you.

The rollers are .375 dia.
Half stroke at this point is .60
Center to center distance is 5.550.

Thanks again!!

Sid

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #13 on: May 04, 2016, 01:58:29 AM »
Sid,
I will be busy the rest of this evening.  I should be able to send you the file in the morning.
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #14 on: May 04, 2016, 01:59:18 AM »
Thank you.

Sid

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #15 on: May 04, 2016, 05:03:01 AM »
I finished up what I had to do early. I attached the DWG and DXF files to this post and the forum software has not complained yet so we try it.
Have fun.
Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #16 on: May 04, 2016, 02:25:43 PM »
Gail- I brought this into CamBam. It looks like it might work.
I'll give it a try and let you know.

Thanks, Sid

Offline GailinNM

  • Full Member
  • ****
  • Posts: 588
  • Albuquerque, NM, USA
Re: Comber engine question
« Reply #17 on: May 04, 2016, 08:14:01 PM »
Sid,
Glad to be of assistance.
A couple of notes.  As you probably guessed, the diagonal lines were just to check the center to center distance at a couple of random points. 

In my original instructions I said to spline the polyline curve. I did not do so on the files I sent you as the file size becomes a monster.  This morning I did a quick check to make sure that the error from not doing so was OK.  The error is much less than 0.0001 inch so there is nothing to be gained by splining the curve.

If you haven't already, I suggest that you use ball bearings for the cam follower rollers.  The rollers will be rotating at about 15 times the engine RPM so will eat up lots of power and need lubrication if plain rollers are used. I use ball bearings on almost all rotating parts any more.  AvidRC  has bearings at a good price and provides excellent service in the US. Chinese bearings of course but I have used over 100 of them with no problems.
http://avidrc.com/product/9/standard-bearings/

Gail in NM
I would like to be the kind of person my dog thinks I am.

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #18 on: May 05, 2016, 03:51:57 PM »
Gail- Yes, my design is using bearings in all locations, with the exception of the valve body.
I'm almost done with the design at this point.

Yes the file with all of the 720 little segments was too much for CamBam to handle for whatever reason.
I had one of the guys at work proccess it through BobCad. That worked. Took the file to my mill. It ran OK their, but have mot cut
metal yet.


Sid

Offline sid pileski

  • Full Member
  • ****
  • Posts: 162
Re: Comber engine question
« Reply #19 on: May 05, 2016, 05:15:15 PM »
Here is a PDF of the design.

Hope the attachment works.

Sid

 

SimplePortal 2.3.5 © 2008-2012, SimplePortal