My first wankel engine 'bits'!

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    Thumbs up My first wankel engine 'bits'!

    Hi there,

    Well, I finally got my mill working in 2 axes and after a day of drawing pretty pictures with a pencil in the chuck, I decided that I would try some proper machining. Seeing that my z-axis isn't ready yet, I was pretty much restricted to 2D profiles only. The obvious choice then, was to have a go at some wankel parts in aluminium.

    I made a small housing as well as the rotor to fit it. I used my Excel spreadsheet to generate the Gcode for the housing and CAD/CAM to do the rotor. I have now modified my spreadsheet to take into account the tool radius when it generates the Gcode, so machining is easy. It can also generate code for a housing that is parallel to a true epitrochoid, which is what a running wankel needs. I needed to include the radius compensation, as I am using TurboCNC which can't do this on its own.

    The big test to see if my spreadsheet actually works was whether or not the rotor would fit properly inside the housing. I kind of shocked myself a bit with the results. It is spot-on!

    This particular 'engine' has an eccentricity of 2.86mm, a rotor radius of 20mm and the epitrochoid is 0.4mm larger all round than a true epitrochoid. The capacity is 4.12cc and it was cut using a 6mm endmill.

    I'm now one step closer to building a running engine! Once the rotor and housing can be made (which they can now), the rest of the engine is relatively simple to do.

    I've included photos of the rotor and housing as well as my mill.

    Regards
    Warren

    Similar Threads:
    Attached Thumbnails Attached Thumbnails My first wankel engine 'bits'!-machine-2-jpg   My first wankel engine 'bits'!-rotor-housing-1-jpg   My first wankel engine 'bits'!-rotor-housing-2-jpg   My first wankel engine 'bits'!-rotor-housing-3-jpg  

    Have a nice day...


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    Cool...any plans....what are you using for rotor tip seals?



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    Hi there,

    If you mean engine plans - no. I'll pretty much design the whole thing.

    I'm still looking at the metallurgy side of things. There are a few combinations of apex seal and housing material that minimise wear and chatter marks. Unfortunately, a lot of these processes are not really available to Jimmy-Pop in his home workshop...a lot of them involve 'fancy' plating processes and things like carbon seals. I'm thinking about maybe making the housing out of steel and having it hard chrome plated and then making the apex seals out of cast iron (something like piston ring material). Luckily, small engines like this don't tend to be used very much so the metallurgy is not as critical as it is in some applications (like cars).

    The only other part that I'm a bit weary of, is the internal gear that's used to phase the rotor. I've never made any gears, yet alone an internal gear. Sadly, no one seems to make these things in the right sizes, so off-the-shelf parts are out. That's the next big challenge!!!

    Regards
    Warren

    Have a nice day...


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    Nice work!

    BUT - the mill and surrounding area is entirely TOO clean....'you milling, or cleaning? Get back to milling!!



    Keep us up to date! Hopefully you can get a mini Wankel running! I am sure that would be a first.

    Scott

    Consistency is a good thing....unless you're consistently an idiot.


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    Hi Scott,

    If you look at the photo, you'll see that the chuck still has a pencil in it. The photo was taken before any metal was cut - it won't always be that clean, honest...

    My plan is to have an engine working in the next 3 months (since I've got nothing else to do for the next 3 months...). I'm fairly sure (know) it can (and will) be done. It won't be the first though. I've got a 5cc OS Engines wankel, which runs pretty damn well. There have also been a few others that people have made, but not many at all. I've only ever heard of 2 others.

    Regards
    Warren

    Have a nice day...


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    The SW92 wankel form VTH.DE is made completely out of medium carbon steel, with the rotor and housing chromed. Don't loose too much sleep over the materials. The really critical thing will be getting the rotor to side housing clearances tight enough for a good gas seal.

    Regards,
    Mark


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    yeah, i agree. with the small pressures involved, and affordabilty of such a small rotor... i don't think apex seals are terribly important, even if the tips of the rotor wear out over time, pop a new one in. I'm still trying to get my mill going, and i plan on doing a lot of IC projects. so i'm watching your project with great interest.



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    itsme
    these home built cnc's are cool as **** ain't they
    but what about your z axis , it's the least hassle to build.
    mike



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    Hi Mike,

    I think that this CNC mill is the coolest thing I've ever had. After the first time I used it, I couldn't believe how relaxing machining could be and how clean my hands were at the end!!! It's great!!!

    I guess the z-axis could be looked at as the easiest axis to convert, but I decided to do things differently. I thought the standard leadscrew looked a bit 'flimsy', and I don't think it was ever designed to be used for accurate positioning, as in CNC machines (although MANY people have used it successfully with CNC). For this reason, I have decided to scrap the standard leadscrew and replace it with a 12 x 3mm trapezoidal screw. The reason that it is taking me longer to do, is that I have machined a new aluminium bearing block to take angular contact bearings and I also have to replace that entire arm that connects the leadscrew to the head (yes, that big steel lump...). I'm hoping to have the lump rough machined where my dad works, but sadly, it's not their first priority, so I have to wait. I'm also going to use a counter-weight on the head.

    Once the z-axis is done though, I'm pretty sure I'll be making some awesome engine parts. I also have a few new ideas for a rotary engine, but I'm just trying to get them down on paper to see exactly what they all involve. I think a bit of 'design time' is in order - then the fun can begin!

    I love CNC!!!

    Regards
    Warren

    Have a nice day...


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    Quote Originally Posted by itsme
    and how clean my hands were at the end!!!
    Ok, I must be doing somethign wrong!

    Dustin B.
    ================
    I hear and I forget.
    I see and I remember.
    I do and I understand.


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    Itsme:
    I used my Excel spreadsheet to generate the Gcode for the housing and CAD/CAM to do the rotor.
    --------------------------------

    Love your wankel project. You are looking good. I am interested in using Excel to generate the g-code. Would you, if you have time, give me some more information on the Excel process.

    Appreciate it.
    Jerry



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    Hi itsme,

    I disagree on the apex seals. You will need them. There is no way you are going to get a metal to metal sliding fit on the three rotor apexs all around the trochoid. One leaky apex is two chambers without compression. You will never get it started.

    It is the side seals you can do without on a small motor, as you have a large surface area slowing the leak rate, but even then you need to control the clearance very carefully. There is a good article on the web of a guy who had to lap the rotor housings of his OS/ Graupner, which should give you a good target for this clearance. Couldn't find that article just now, sorry.

    The SW92 has simple one peice apex seals with a simple U shaped spring preloading them. I think they are also just hardened steel.

    Regards,
    Mark


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    Hi Mark,

    About those apex seals - do you not mean pleasuretool? I think if I left out apex seals, then I may as well give up before I've even started, because there's not even a chance that it will work.

    I've also been thinking about side seals and how I could include them on a small engine. I'm sure there must be advantages in having them, but it just adds a lot of extra complexity when making the engine. I'll have to see.

    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'.

    It really is very simple, but you must remember that your zero point will generally be right on the centre of the shape (assuming it is symmtrical) or wherever the (0,0) point is if you were to plot the shape on graph paper.

    I hope this helps.

    Warren

    Have a nice day...


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    Hi Warren,

    Sorry, you are right I was responding to Pleasuretools post.

    I have some contact with the guys at Dimond engines in Wiener Neustadt, and can't help wondering what a Mazda style wankel scaled to about 60 HP would be like. I really don't think the air cooled roller rotor bearing of the Fichtel and Sachs KM 914 family was a good direction for small wankels to take. The rotor bearing stresses end up setting RPM limits which limit the whole concept. I would love to see a small wankel with pressure fed plain bearings, oil cooled rotor and liquid cool housings. I bet you could rev the heck out of the thing.

    Have you tried mounting a dremel or such die grinder on your mill and grinding the trochoid to get a better finish?

    Regards,
    Mark


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    Hi Mark,

    I haven't tried anything yet to get a better finish, but I have thought about it. At the moment my mill is only capable of doing about 2000 RPM. This speed is really the limiting factor on the mill. It has also got those horrible nylon/plastic/rubbish gears in it, which I really don't like too much. I'm therefore planning on getting rid of those gears and replacing them with a nice, smooth, v-belt drive. While I'm at it, I can change the ratios to speed it up a bit. There are guys that have had this mill running at 8000 RPM with the standard motor using a belt drive. I'm not planning on taking the speed that high, but I'd like to try grinding the housing with this setup. I'd use a larger diameter wheel (>40mm), to try and get the tip speed (or whatever it's called on a grinding wheel) up a bit. I have no experience grinding, so I might find that the finish wouldn't be suitable if I did this, but I still need a higher spindle speed (I think I'll aim for 5000-6000 RPM). Does anyone know what kind of finish I could expect with a 40mm grinding wheel at, say, 5000RPM???

    Mounting a Dremel type tool on the mill also sounds like an excellent idea. I'd never thought of this. Any ideas on how I could attach one onto a Sieg X1??? I'm quite keen to try this before I do any (more) major mods to my machine.

    As for rotor bearings, I've never really paid too much attention to them. At this stage, my goal is to just get an engine running. Once I have got past that (rather large) barrier, then I can pay more attention to things like bearings and how performance can generally be improved.

    Mark, just out of curiosity, do you know how they make the housings at Diamond. What sort of machinery are they using?

    Regards
    Warren

    Have a nice day...


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    Warren,
    Appreciate the info on the Excel spreadsheet. Thanks.

    I will be following your progress, and am wishing you the best of luck in your endeavour.

    Keep us posted WITH some photos.

    Jerry



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    Hi Warren,

    Try the grinding wheel and see. In my experience the dremel grinding wheels are not very round, so you will want to dress it with a diamond before use.

    How about bolt a plate to the side of the head housing, and make a bracket off the plate like the CNC wood routers use. Your work envelope is so small that you don't need the spindle in the middle of travel.

    The SW92 uses short lengths or drill rod as home made needle bearings between e shaft and rotor.

    The rotor housings are cast, and used to be machined on the huge (like 40 ton) trochoid grinder that was originally built for Norton. That grinder was left in England. Now they are gound on a CNC, but it is not done in house. The reports I have had are that the CNC grounds trochoids are the most acurate ever.

    They have some sort of coating (probably something like Nikisil) on the Al.

    Regards,
    Mark


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    Hi Mark,

    Thanks for that info. I was thinking about trying a cast iron housing and rotor to start off with - no chrome or any other coatings. I'm thinking about then using aluminium end plates and steel apex seals. I'm fully aware that this combination probably wouldn't be too durable, but if I can just get something to run, I can then put more time into the materials on the next engines. At least with CNC, I can make lots of the same part without boring myself to death.

    Regards
    Warren

    Have a nice day...


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    Hello again,

    I have just ordered a new rotary tool with a flexible extension to modify for my mill. I am planning on taking the flexible extension apart and building the end of it into a blank 2MT arbor that will then fit in the spindle on my mill with a modified drawbar. I think this will give me greater accuracy and it also gets the (relatively) chunky motor out of the way. Hopefully I'll be machining super-accurate bores in the near future (assuming the stuff arrives soon...).

    Just as a matter of interest, does anyone know anything about the cooling system on the DKM 54 engine (the first running wankel). I know that the fuel/air mix passing through the rotor and its shaft did a lot of the cooling, but I'm not too sure on how the trochoidal housing was cooled. I have seen photos of the housing, and it doesn't appear to have any cooling channels in it and it would also seem rather complex to try and get coolant to this moving part. Any ideas?

    Regards
    Warren

    Have a nice day...


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    I don't know about the DFM 54, but all the KM914 derivatives use a total loss oil system and air cooling of the rotor. The Wankel GmbH engines of about the same size used a wierd system where they ported the induction charge through the rotor to cool it and lube the bearings, and then back out and around and into the intake. The advantage was that the premix system was the only lubrication system. The down side would be throughing away the wankels inherent superior breathing, and the RPM limits which roller element rotor bearing impose.

    For an engine the size you are making, an air cooled housing should be fine. The SW92 and OS/Graupner with their roller bearings have no rotor bearing cooling or lubriction other than blow down premix oil caused by the lack of side seals.

    Cast iron housings with spring steel apex seals would probably a good place to start. Look forward to hearing your results.

    I am starting to look at buying a house (not easy here in Vienna Austria - A city of around 2 million, but there are only about 50 houses on the market). One I have a garage I'll be able to bring my mill home and CNC it.

    Regards,
    Mark


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