Irregular shape cam grinding 2

[erosnicolau]

Hi, guys,

I have a cam with an irregular shape, about 10mm thick and about 46mm in the longer diameter. I need this to be cut and precisely ground to micron tolerances, if possible. There are three CNC ways of achieving this, in my book: wire-cutting (really not preferred because of the surface decarburization occurring, reducing the sliding properties), milling (but I can't find any CNC mill able to work with sub 0.01mm tolerances) and irregular shape grinding (which should really do the trick).

However, I do live in Romania, and apparently we're a technologically stuck behind country. I scoured far and wide but couldn't find a single CNC grinder able of more than just flat/round grinding. So, I'm coming to you guys with my questions:

1. Is there any other way of achieving this kind of surface, other than the ones I enumerated? Should I look for any different method?

2. Can you recommend anybody able to help me with my project?

Thank you so much,
Eros

 

[MikeHalloran]

I have no idea what you mean by "irregular shape grinding".

The simplest way to get advice about making your part is to make a proper drawing of it, and send it out for quotes. Be honest and say that you only need one, if that is the case.

Note that a proper drawing outlines what you want, and specifies exactly what you will not accept. If you send out a sketch with a verbal tolerance, no one will take you seriously, which may explain the reactions you report from local suppliers.

However, your location matters now less than ever.
Sign up at mfg.com and similar marketplaces.





Mike Halloran
Pembroke Pines, FL, USA

 

[erosnicolau]

@MikeHalloran The shape is something like the marriage between an oval and a hexagon, or an oval with 6 slightly protruding diameters. Not on oval, not a circle, you know.

About the technical drawing: of course I included it in the quote requests, and it DOES show the required tolerance (even if I explained to the locals that I would be willing to lower the standards and to accept a +0.01mm tolerance instead of the +/-0.0001mm). And of course I also sent out any file format they requested (dwg, dxf, step, stl etc.) And of course I mentioned it's about a unique part, for now. Your advice comes to confirm that I've been doing the right thing with my research so far. I even went ahead and started asking the local CNC distributors for references about any of their clients who might be able to help. Still no news so far.

Thanks for the mfg.com idea - I'll post a RFQ there as well...

 

[ornerynorsk]

Jig Grinding. Do you have a gage or instrument manufacturer that you could inquire with?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[erosnicolau]

@ornerynorsk YES, MAN, that's what I've been looking for. Unfortunately, none of that in my country (((
That looks exactly like a tool capable of delivering high tolerance irregular shape cutting...

Any specific recommendation?
Thanks,
Eros

 

[ornerynorsk]

I'm sorry that I don't have a specific company to recommend, it's been 20 years since I've done business in Eastern Europe.

Because your part is very small, it should be possible to contract with a company almost anywhere in the world.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[erosnicolau]

Well, I've lost hope of doing these parts in Eastern Europe anyways...

After almost two weeks of "no, we haven't even heard of such idiotic tolerances" everywhere I ask, I'm now turning my searches around towards any corner of the world that will help out... so I'm not looking for Eastern European companies alone anymore.

 

[BrianE22]

Did you search for "camshaft grinding"? Even with your reduced tolerance of .01 mm (.0004") the part would still be risky. I'd plan on making several blanks in the hope that one piece would be within specifications after grinding.

I also think you probably need centers on the part (if done by turning) and some sort of call out on the profile tolerance and the concentricity requirements between the outer profile and the inner hub. You probably also should specify the temperature at which the measurements are made.

 

[MikeHalloran]

Your drawing does not specify anything about the shape of the cam between the four tangent points, and it doesn't really locate them relative to any useful datum.

A cam grinder would expect to see an equation, like
R = f(theta),
or a table of R,theta
where R is the radius relative to a specified datum
and theta is an angle, measured from an index datum like a notch or hole.

Or, if it's supposed to be four orthogonal flats with radiused fillets between, you need to specify the radius of each fillet.

The actual cam profile is also affected by the nature of the follower. I.e., if you are using a flat tappet sort of follower, the shape will be different than that of a roller cam for the same motion. Cam drawings often include a view of the cam and follower assembly, with an equation or table defining the follower motion, not the cam profile per se.

In this case, because of what appear to be relatively small slopes, the follower shape (or the shape of the measuring instrument's tip) has only a second order affect on the difference between cam shape and follower path, but since you are specifying third order tolerances, you do need to tell the cam grinder exactly what the follower system looks like, or specify precisely what the measuring gage tip looks like.



Mike Halloran
Pembroke Pines, FL, USA

 

[erosnicolau]

Hi again, guys,

@BrianE22: yep, the parts have concentric surfaces - including an inner surface - that of the 10mm hole - fit for grinding. Also, when I sent the parts for grinding (everything but the external shape) I specified the tightest concentricity tolerance possible along with the press-fit tolerance for the 2 side bushings. About the temperatures - read on, below

@MikeHalloran: I only call this part a "cam" because of its general shape. However, it's not an actual cam (it doesn't lead a follower in the classic sense), as it is rather a "symmetric variable radius pusher" - nothing to do with the classic geometry of a cam. A normal cam shape is easily defined with mathematics similar to what you described. However, my "cam" shape is defined by some pretty complex trigonometry, so complex in fact that I let SolidWorks do all the math here. Impossible to be translated to anything closer to what a regular cam grinder would be able to use.

What I took from this thread so far was the idea of using grinding (thanks, @ornerynorsk) to shape this complex part's outer shape. However, the cheapest jig grinding service I could find was a German company which asked 180EUR(225USD)/part to grind.

So we've come up with another crazy idea: to build our own lever-type 1:10x pantograph, and to grind the cam's outer surface on a regular flat grinding machine, by reducing 10x a 10x bigger template cam. This way, any machining error on the big/model cam's shape would be reduced 10x on the small, actual cam. The mathematical model was verified in SolidWorks and proved viable. In order to eliminate the temperature-to-size variance dependence, we round-ground the cam parts' outer surface to an outer diameter equal to the max. diameter of the actual theoretical shape, to micron tolerance, in a controlled environment, in another dedicated shop (a very common and thus affordable operation). We then used a permanent marker to paint the cam's outer shape, and mounted the cam on the pantograph we built. By keeping the two cams (the regular one and the 10x bigger one) in perfect sync via a timing belt and pulleys, and by rotating them while gradually and precisely bringing the grinding wheel down against the cam, we can grind away the required shape, until we're left with only a thin line of paint on the cam. The beauty of the principle is that this way we're not depending on the room temperature anymore - you can see why.

Here I am handling this device mounted on the flat grinding machine, in the YouTube link below (please excuse the mess and crappy footage):

https://www.youtube.com/watch?v=fttjC3CLcmY

The current problem with this design is that the elastic variations induced by my hand manually rotating the big cam do translate into irregular small cam - grinding wheel contact, leading to unacceptable inaccuracies. The next step is to retrofit the pantograph with a slow speed, high torque motor, in order to achieve a constant, smooth rotation of the cams and to eliminate any variances induced by the manual handling. Keeping my fingers crossed that this will actually work as needed!

Anyway, I'll keep you posted with news.

Cheers,
Eros

 

[MikeHalloran]

IMHO, you were feeding the workpiece way too fast.
Try running the mechanism with a very slow gearmotor.

Nevertheless, congratulations on finding a simple solution.



Mike Halloran
Pembroke Pines, FL, USA

 

[erosnicolau]

Thank, Mike
Yep, as I was saying, I already have a high torque, slow speed geared motor, which I'm linking to the big disk shaft with a further 1:3 bike pinions & chain reduction. I will try to feed as slow as the motor will permit, this time. Will keep you guys posted, anyway
Eros

 

[ornerynorsk]

Ingenuity at work, that's clever! In grinding, rigidity of set-up is everything. Dressing the wheel and adding coolant will help tremendously, as well.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[erosnicolau]

@ornerynorsk Thank you too, man. I'm seriously considering giving up grinding on a classic vertical (horizontal axis) flat grinder, and using instead a small, horizontal (vertical axis), well-balanced, well-dressed stone, attached to a vertical axis high speed broach, mounted on a vertical slide. Smaller setup, eliminates all the positioning calculations (extremely important, extremely difficult to measure) and should hopefully work out just as well. As I was saying, I'll keep you guys posted.
Eros

 

[erosnicolau]

Another question for you guys... But first, the context:

As I mentioned, I plan on switching from horizontal spindle grinding to vertical spindle grinding. I only used a horizontal spindle flat grinder so far because that's what I had access to. However, for this application, a vertical spindle (with a cup wheel) is better suited (it eliminates all the aligning guesswork). However, I don't have neither one, neither access to one. So my plan is to build one out of a motor free of axial play, mounted vertically on a lathe vertical slide. This way, I would still have fine control over the height of the grinding wheel, which is essential.

The question: I know that not working with expensive, dedicated machines, you run into various problems related to vibration. What advice would you have for me, to eliminate as much as possible from the potential vibrations on such a rig?

Thank you so much,
Eros

 

[MikeHalloran]

- Move your shop to the wilderness.

- Do all your precision grinding when everyone within a mile is asleep.

- Mount your entire rig on a concrete or other heavy slab that is levitated by, e.g., truck tire inner tubes. You might get fancy and put a level control valve at each corner to control the pressure within a subset of the tubes, or you might adjust their pressures manually.

- Mount your entire rig on a concrete or other heavy slab that is in turn supported by a boat, floating in a pool of water. You may have to add random bits of plastic to the water to damp out oscillations.





Mike Halloran
Pembroke Pines, FL, USA

 

[erosnicolau]

Mike, you put a much needed smile to my face I was of course talking about reducing spindle vibrations but your lines were encouraging nonetheless )

So far, the only idea that comes up is to separate the grinding cup axle from the motor axle - see the image below (where I also figured the slow turning motor for the wheels, mounted on the lever; I didn't include the motor to spindle axis pulleys and the belts)