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reverse engineering, need help with spline

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Message 1 of 11
chad38
773 Views, 10 Replies

reverse engineering, need help with spline

Well, I may have my work cut out for me. I'm reverse engineering an assembly that is composed mainly of two weldments made mainly of sheet metal. There is one area that is giving me trouble, there is an area that is basically a spline, and I'm at a loss as to how to best come up with a final shape. I've gone about measure this a few different ways, but none of the measurements lead to anything that matches the measurements that I pull off of elsewhere in the part. Because of the shape of this part, it is not an option to place on our 3d scanner because of the protrusions welded to it.  So I have been trying to use a spline to draw this variable radius as best I can, but boy is it not only difficult to fully constrain the spline, but also it's pretty hard to decide where I should be concerned with whatsoever. And boy is my work on this area of the part UGLY.

 

Have any of you come up with a way to get dims you are confident in a variable radius like this, and if so, please share. Attached is a snapshot of where I am with this part thus far. The spline area is on the left.

 

Capture.JPG

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10 REPLIES 10
Message 2 of 11
JDMather
in reply to: chad38

Designers don't generally use splines for something like that (unless they want to make the shop floor workers/inspection mad).

 

I'll wager that you could replace the spline with an arc or combination of tangent arcs to simplify.


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Message 3 of 11
chad38
in reply to: JDMather

I understand, however I feel that there must be a real reason this is the shape it is for a reason. Any suggestions as to how to get reliable dims off of the part in question so as to closely replicate it?

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Message 4 of 11
JDMather
in reply to: chad38

Do you know the function of the part?

Is it some sort of cam action.


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Message 5 of 11
chad38
in reply to: JDMather

I don't know the specific function, it seems to be a clamp of some sort. I don't think there is any cam action going on with it because there is no serious wear on that edge that would indicate it was used as one. My concern is that the shape may be important for clearance of other parts that will be near it. Otherwise I don't see why it would have such a complex shape on that end. You can't see from my snapshot because of how different the shape I currently have is, but that end of this part looks almost like it is one side of a french curve.

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Message 6 of 11
swalton
in reply to: chad38

For something like this I would use a series of arcs that looks close.  Once I got something that looks ok on the screen, I would put in an idw and print it out full scale.  Put the part on the paper and see how close you are.  Repeat as necessary until you are confident it is correct.  I would bias my curves to standard dims: 1.5" radius, not 1.507".

 

If you have access to a manual bridgport or similar tool, you can chuck up a known diameter rod (drill or mill as well) and use it to touch off the part at several points.  Write down your data and use it to model your curves.

 

EDIT

If it is a clearance curve, just make your approximation a bit smaller and you should be ok.

Steve Walton
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Message 7 of 11
rmerlob
in reply to: chad38

First of all, if you must use splines, be sure to use control vertex type, interpolation type is really hard to control, try to copy the shape on a sheet of paper, scan it, constrain the picture in position making sure you keep it 1:1, and draw the spline over it.

 

Only constrain the end points and start out with a small number of vertex (vertexi , vertexes) and add as needed, attached is a random example.

 

The only time I dont try to use fully constrained sketches is with splines, I mostly use geometrical constraints to keep the lines conecting the vertex tangent, parallel or perpendicular to other geometry, and maybe add some points or construction lines with dimensions if I want to keep some design intent, again the attached file shoes some of that

 

As JDmather says, if it is not necessary dont over complicate it though, I use this when splines are absolutely critical, hydraulic profiles on turbomachinery for example.

 

Regards,

 

RM

 

PS It may take a while to get used to controling the splines in control vertex mode, but it is really powerful when you get the hang of it, keep in mind that this is what the surfacing pros use when modeling in software like Alias.

Message 8 of 11
pball
in reply to: swalton

For a couple of odd parts I've drawn up before, I traced out the part and scanned it. Then imported the image into a sketch and after getting it positioned/scaled properly just traced it. Then I printed a 1:1 drawing and checked it like swalton suggested.
Message 9 of 11
JDMather
in reply to: rmerlob


@rmerlob wrote:

Only constrain the end points and start out with a small number of vertex (vertexi , vertexes) and add as needed, ...

 

The only time I dont try to use fully constrained sketches is with splines, I mostly use geometrical constraints to keep the lines conecting the vertex tangent, parallel or perpendicular to other geometry, and maybe add some points or construction lines with dimensions if I want to keep some design intent,


I will elaborate on what I do if I cannot simplify with arcs -

 

As rmerlob indicates - use as few spline points as possible - I start out with only two.

Turn on the Handles and curvature and adjust (to scanned image) as closely as possible.

Use construction geometry to constrain the handles and/or tangent the spline ends.

Add additional nodes to the curve one-at-a-time if needed.

For most curves you should not have more than 5 or 6 nodes - the biggest mistake I see people make when working with splines is making the assumption that more points = more precision.

 

When you are finished adjusting the handles right click and turn them off and your spline will show as fully constrained even if you didn't dimension/constraint the handles (assuming you do dimension the location of the nodes).  Do not miss this critical step.


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Message 10 of 11
chad38
in reply to: JDMather

When you guys say nodes, you are talking about the circles at the end of the dotted line intersections? Or is that what you consider handles? Also, I tried right clicking like you said, but I found no options like the ones mentioned. I have an option to check or uncheck polygon visibility, but I see no noticable difference when I do.

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Message 11 of 11
JDMather
in reply to: chad38

I think we are talking about two different types of splines - I was referring the Interpolation Spline method.


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