I have a turbocharger compressor wheel that I am trying to reverse engineer. Using my CMM, I have generated a 3D curve, representing the edge of one of the blades. I need to figure out the 2D profile that was used to turn this shape on a lathe.
I thought I'd be able to revolve the curve around my center axis and then intersect it with a plane. I find that I can not revolve a 3D sketch.
Ultimately, I am trying to define the bore that this wheel would operate in. I do not have access to any 2D or 3D data for that part at this time.
If you have the curve points in an appropriate format, you could import them into Excel, convert the cartesian coordinates to cylindrical, discard the angle coordinate, then import back into SW using curve through free points.
"TOP" wrote in news:1121968261.088485.203590 @g43g2000cwa.googlegroups.com:
That did the job. I didn't think to try that when the revolve failed. I aldready had all the geometry I needed to make this work, I just had to click in the right places.
Projecting a 3D sketch to a 2D plane and revolving does NOT give an accurate result
Imagine a simplified case: revolving a single point to produce a circle.
Imagine opening a new model, and creating a vertical axis through the origin. Create two points, one lying on one of the vertical construction planes, and one lying on a plane which has been offset (parallel) from the first plane. Imagine the second point projects coincident onto the first.
When you revolve the two points about the axis, you will get two different circles. For very small offsets, the difference may not be noticeable, and the same will be true for a 3D sketch which almost lies on a single plane through the axis.
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Assuming the 3D curve extends up and down, a safe way to simultate "revolving" it would be to create a horizontal circle at the top endpoint of the curve, representing the revolution of that point around the (say vertical) axis. Now sweep that circle vertically downwards, with the centerpoint travelling along the vertical axis as a path, and using the 3D sketch as a guide curve to vary the diameter.
The radius of the circle will need to be defined, within the circle sketch, by piercing a sketch point to the 3D curve, and making the under-defined circular periphery coincident with that point. Might have to use selection filter when picking the sketch point to ensure the curve endpoint is not accidentally picked instead.
Haven't time to test-drive this just now for a 3D curve (works fine with an off-axis 2D sketch), good luck to anyone who does.
I think the methods described are not giving the accurate result. Instead you neeed to make a swept surface this way (replacing a rotation with a sweep): The path curve is the axis of rotation. The guide curve is the 3D curve itself. The profile to be swept is an arc with a point on it constrained "pierce" to the 3D curve, and the centre of the arc is to the axis. Every other method is a fake, and not giving accurate results. I've used the above mentioned method and it works.
Otherwise I can send you a file illustrating this if you provide your email address.
Hopefully in the future SW and co. introduces sweeps using not only sketch profiles but solids (for example it is useful for simulating material removal of cutting tools of NC machines, etc).
You are correct of course. I didn't think it all the way through! I'm surprised to find that you (or I) can't revolve a 3D sketch or a projected curve.
I compared your method to just sweeping the 3D curve on an arc and the results seem to be the same with the difference being that the simpler sweep gives a copy of the 3D curve at the end of the surface. The three sketch method ends up with what appears to be identical surfaces to the sweep on an arc but does so with some funky artifacts in the case I tried.
Do you have a case where the three step method gets a different result to the two step?
Your "two step method" is probably going to work better if you want to simulate a revolution through less than 360 degrees. The start and end boundaries of the surface will be defined by the 3D curve and its transform, and one of the other boundaries is defined by the arc of the path.
The "three step method" is better for the circular case. There's much less computation, and a purer surface should result, because the path is a straight line, the closed sweep profile avoids the software having to work out how to close or twist the sweep, and the sweep profile is a geometrically primitive circle rather than a complex algorithmic 3D spline (which can only be numerically approximated).
That is an interesting analysis. However, I tried it and started changing the 3D spline. At some point the two surfaces started to diverge as evidenced by the rendering tesselations becoming smooth on certain portions of the surfaces. I then added a number of points to the 3D curve. Not spline points, just points. Then I used the measure tool to measure the distance from the points to the surfaces. I couldn't find a point of divergence to 8 places for the two step, but the 3 step showed divergence in some cases for the full circle as well as not capturing the full 3D curve. If the 3D curved back on itself in the 3 step method the sweep along a straight axis will not "back up" and go the end of the 3D curve.
Paul, Thanks for that, thought provoking as always.
The "backing up" limitation is interesting, and understandable. In the particular case of the impeller edge which set off this line of enquiry, I guess it's not an issue, but it's good to know that the "two sketch" sweep surmounts this, as it should. There's no way the three sketch method should be able to cope, given how a sweep with guide curves works, "under the hood".
Turning though to your quality check, my understanding is that you measured how the (quasi) "surface of revolution" conforms LOCALLY to its source spline. Hardly surprising, for the "2 sketch" sweep, that it conforms exactly. That spline, being the starting profile for the sweep, forms a boundary of the surface. Surfaces *have* to conform exactly to boundaries.
Presumably what MHill is seeking to to do is "rotate" the spline, then extract an "in-axis" profile as a tool path, so deviations in the accuracy of the rotation would matter. It matters therefore whether the local conformation you verified is carried to all other points around the periphery: in other words, are infinitely thin slices exactly circular and exactly concentric? If they are, to eight decimals, I take my hat off to the geometers and codesmiths. (In all humbleness, I probably should then eat it).
It's not surprising to me that the "3 sketch" method demonstrates the local divergence you describe (I presume it's confined to the last few decimal places?) because it relies on calculating the distance from an analytical straight line to an algorithmic element, and then using that distance to vary the diameter of an analytical circle. My understanding is that whenever analytical elements are driven by algorithmic ones, there is an inevitable mismatch of this order. However, I would expect that divergence would not be compounded elsewhere around the surface, in other words, the surface would be a perfect surface of revolution.
Re this query, it seems to me that conformity to the source data becomes academic once it passes beneath the accuracy horizon of the CMM, or you could even meaningfully relax it to that of the NC lathe (maybe 4 decimals?), whereas much higher levels of accuracy are relevant to whether a surface will continue to behave with decorum under future indignities-- trim, extend, patch, intersection curve, translate to different platforms, and such. My interest, right or wrong, was directed to the quality of the surface as a whole, and to a lesser extent the computational overhead.
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