[itsme]

Mark,

I've just seen that it is possible to get a needle roller bearing that has an OD of 28mm, ID of 22mm and width of 12mm.

I've had a quick look at the plans and I think it would be possible to fit this into the rotor. The needles would still run on the eccentric shaft (so it would need to be hardened), but I think the diameter of the eccentric 'lobe' could be reduced from 24mm to 22mm without having to adjust the rest of the shaft. What do you think?

[CJL5585]

Itsme
------------------

Your project is looking good. I am following your progress. So far, it is looking like you are going to have a WINNER.

Jerry

[RotarySMP]

We're not worthy...
We're not worthy..

Fantastic progress you are making here Warren. Great to see someone who has the time doing something interesting with it.

From the photos you sent, that grinder looks like it gives a pretty nice finish.

I am not so sure you want something soft fo the seals. One classic pair is chrome and cast iron. I wonder if you can find some 1mm chromed steel? Cast iron piston rings are also used in cast iron bores, although the apex seals would be extremely fragile made of C.I.

You may find that that the cast iron Trochoid surface is too soft, only time will tell. I'd just use medium carbon steel and see what wears out first. Material compatiblity was THE devil in the details of early wankel development. Accept you are taking some shortcuts, and will likely have to redo some bits in service.

The main thing is you are making huge progress.

That bearing sounds like it is worth the try. I have some 2.5mm silver steel, and had a bit of a play with turning up some needles this weekend, but the steel measured 2.47mm, does not have a perfect polished finish (good but probably not good enough for 12000rpm), and is soft. Hardening and polishing 33 little needles seems like a mugs game.

It is a lot easier to make one perfect bearing surface and harden the e-shaft, than to try and make a perfect fitting e-shaft, and 33 neddles, and that recessed ID. Do you have an model engineering club or school machine shop where you live. It shouldn't be too hard to find someone who can case harden your e shaft for you.

A better option for you next engine will be a pressure lubrication system and plain bearings. (the two rotor and liquid cooled, and ....)

[LongRat]

Just like to add that I am following this progress intently too. This recent progress is very exciting, well done! I'm very impressed with the finish that the grinder gives, better than I expected.

[rboeser]

Add me to the list of people impressed. Very nice work indeed, It's good to see a project like this move forward. Great job!

[devincox]

Firstly, Thanks for posting your work Warren.

Does anyone have an example of the parametric equation, to plot out the epitrochoid, as it would be used in excel. I have found the equation:

x = (a + b)cos(t) + h cos((a/b + 1)t)
y = (a + b)sin(t) + h sin((a/b + 1)t)

online but do not know but do not know which values to plug in for a,b,t.
I assume a is the radius from the large circle and b is the radius from the small circle and h is the height between some point and the center of the small circle, but what is t? Tangent?

I have a 4 axis CNC sherline mill and 2 axis cnc sherline lathe and would love to be able to start cutting on a project like this.

Thanks for your help,
Devin

[ViperTX]

and this is an equation for what??....fist I thought it was for an ellipse....anyway t is an angle, h generally is the height of something, and a and b are generally major and minor axis lengths.....

[devincox]

ViperTx, the equation is to plot out an epitrochoid in excel so I can generate the gcode for my cnc machine.

Devin

[WayneHill]

http://www-groups.dcs.st-and.ac.uk/~...itrochoid.html

This will give you an idea of how the formula works.
What are you looking for this formula to do?

Something like this?
http://www.monito.com/wankel/j-epi.html

[devincox]

Wayne,
I am trying to do what Warren did:


"CJL5585, the 'Excel Process' is very simple. In the case of the epitrochoid, you have got the parametric equations for the shape, so it is easy to plot many different points along the curve.

I have set the spreadsheet up so that there are 721 rows. It starts at 0 degrees and then ends up back at 0 degrees going up in 0.5 degree steps (the spreadsheet uses radians, but it's easier to picture degrees). For each 'step', you get an X and Y coordinate. You need to set it up so that there is a column for the X values and a column for the Y values. In a new column, you can then generate the Gcode using the x and y coords that have been generated.

This is an example of what could be written in the Gcode column:

="G01 X "&ROUND(G2,4)&" Y "&ROUND(H2,4)&" F300"

This would write 'G01 X', followed by the the value in G2 (rounded to 4 decimal places), followed by 'Y', followed by the value in H2 (also rounded to 4 decimal places), followed by 'F300'.

If, for example, we assume that the value in G2 is 4 and the value in H2 is 6, then the code would be 'G01 X 4 Y 6 F300'.
"

You then take the resulting gcode and use it to cut the epitrochoid shape for the rotor housing for a wankel engine.

Thanks,
Devin

[RotarySMP]

Could one of you take a look at this spreadsheet and tell me where I have the error? If you put in an offset it skews the trochoid rather than enlarging it.
Column A is the rotation angle, B is the angle of tangency, C is the X coordinate and D is the Y coordinate.

Devin, T is the angle of rotation(theta). The formulas are used to give you the X and the Y locations for each unit of rotation. If you are doing it in degrees, you might use 0-360 or you can use 0- 2pi radians. The dsubdivisions are your choice.

[RotarySMP]

Check it out. An animated movie on the how the Wankel
goes together and works and comes apart

http://www.rotaryeng.net/

Under INTRODUCTION TO ROTARY ENGINE TECHNOLOGY

[itsme]

Hi there,

Mark, I have had a look at your spreadsheet. I found a problem with your angle of tangency (or obliquity). You had taken the square root of both the numerator and denominator, when you should only take the root of the denominator.

Yours:

=DEGREES(ACOS(SQRT((Radius+3*Eccentricity*COS(RADI ANS(2*Theta)))/(9*Eccentricity^2+Radius^2+6*Eccentricity*Radius*C OS(RADIANS(2*Theta))))))

How it should be:

=DEGREES(ACOS((Radius+3*Eccentricity*COS(RADIANS(2 *Theta)))/(SQRT(9*Eccentricity^2+Radius^2+6*Eccentricity*Rad ius*COS(RADIANS(2*Theta))))))

If you think about it, the angle of tangency when theta is 0 should also be 0. Yours was giving a value of 80-something degrees.

The problem above is NOT related to the 'squonky' epitrochoid that your spreadsheet is giving. I had the exact same problem with mine. As far as I know, your spreadsheet is giving the correct shape in the first and third quadrants, but not the second and fourth. I have not read this anywhere, but I think that the equations that we have used (for the offset) are only true for 0 <= theta <= 90 (or however you write this on a computer...). What I therefore did, was calculate the values for the first quadrant and then just copy them, changing the signs, for the other 3 quadrants (this can be done since it is symmetrical).

Once you've made these changes, you should have a spreadsheet that gives you a correct epitrochoid. The offset is not only useful for the offset in the engine design, but it can also be used for tool radius compensation - that's how I've been doing mine.

BTW, your spreadsheet is a lot neater and more user-friendly than mine...

I hope this helps.

Regards
Warren

[itsme]

Devin,

If you have a look at Mark's spreadsheet, you'll get an idea of how it is done. Mark's equations are all correct, besides the one that I pointed out in my last post.

If you are not sure what the purpose of the offset is, it is there because rotary engines don't use a 100% true epitrochoid. They use a shape that is parallel to the true epitrochoid and 'sits' a constant distance outside of the true epitrochoid (this distance is relatively small). This is done to make the apex seals have as little in-and-out movement as possible.

Regards
Warren

[WayneHill]

I can do it in Vector with my MTB Pro Macro option.

[CJL5585]

I can do it in Vector with my MTB Pro Macro option.

Wayne that is really impressive. I am not familiar with Vector. Could you give me some link or info. Would appreciate.

Jerry

[XRACERJB]

how do you make code with excel spreadsheet?

[WayneHill]

Wayne that is really impressive. I am not familiar with Vector. Could you give me some link or info. Would appreciate.

Jerry

http://www.imsrv.com/
http://www.imsrv.com/mtb/

[RotarySMP]

Hi Warren,

thanks for picking that up. I once programed these into a TI graphing calculator and didn't get the sqonky trochoid there. It gave clean ofsets. Unfortunately that calculator died, so I haven't been able to check why it worked differently to Excel.

I noted that you did your tool path in half degree steps, what is the rough linear distance between code lines? I wonder if that resolution would be fine enough for a larger trochoid?

Wayne, I have heard good things about Vector. That looks pretty cool. The trochoid you have drawn is too "waisted" for a wankel, but the process looks cool.

I looked at feeding the parametric equations into Catia, and just inputing the Offset, radius and eccentricity, but was told by Catia guru's that it is easy to do that in Excel and just have Catia draw a spline of the excel file, which was easy.

XracerJB,

To make code from an excel spreadsheet is pretty easy. There are many methods. If you know you will be be doing a lot of parametrically defined shapes, you might want to program a macro to automate but one way would be:
(fo the file I posted)
Cut and past the rows C and D (X and Y coordinates) into a new spread sheet as values only.
Add new column A and use Excels autofill to do line numbering (not really required)
A a new column B and fill it down with X
A a new column D and fill it down with Y.

Save this file as text, with tab separation.
Open the text file and add your G code header and footer, and any machine operations before and after the tool path.
The line before the excel code will need a G01 and Feedrate.

You can cut and paste this block of code adding Z axis plunges between blocks.

Obviously you can get a whole lot more sophisticated than this.

Since I only have a CNC lathe at present, and everything is therefore 2D, I find I do most coding by hand in a text editor. I started using the free version of Cutviewer lathe, so I see my tool pathes and edit them.

[itsme]

Hi Mark,

The linear distance between points for the SW84 ranges from approximately 0.17mm to 0.32mm. I also put in some values for one of the old NSU engines (I'm not sure on the model, but it is approximately 500cc). For this size epitrochoid, the distance ranges from about 0.67mm to 1.27mm.

I haven't done too much on my engine this week. I have found bearings which I will hopefully get in the next few days (provided the supplier can get that roller bearing from his supplier...). I have also ground a tool to cut my internal gear. Initially I started with the back-end of an old drill. This steel machined quite well with a HSS endmill. I then heated it to a cherry red colour and quenched it in oil, followed by tempering. I set it all up and gave it a test run on some aluminium and it cut it like butter - sadly it wasn't quite so effective on steel (in fact, it didn't work at all on steel...). I then decided to do it properly and pulled out a piece of HSS tool steel. 2 1/2 hours later, I had ground the tool on my mill with the grinding attachment. It worked well, but I'm not keen to do that again. Let's hope the tool lasts 21 cuts in steel. I should be getting some steel today for the rotor, so I should be able to start on that.

It's funny with the CNC software. Before my conversion was done (not that it's finished yet), I didn't think I would be able to get by without CAD/CAM, but since I've been using the mill, the first time that I've used the CAM was for the gear tool. Oh well.

Regards
Warren