Question about splines in CAD

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daniellammers
daniellammers
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Joined: Sat Dec 17, 2016 1:22 pm

Question about splines in CAD

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If you dimension a spline like on the photo, do you get the same result in every CAD program (SolidWorks, AutoCAD, etc.)?

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coaster
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Joined: Sat Jun 30, 2012 4:10 am

Re: Question about splines in CAD

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I think of splines as a peice of string sown with a needle and thread, total length is the only dimension is all that it can be defined by. If you want obtain more you should convert it into arcs, this will give you polar dimensions which will be handy for creating a construction based on that spline.

I'm aware it's not as simple as i make it out to be, best knowledge is gained by skepticism and a search for truth.
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JMshapecreator
JMshapecreator
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Re: Question about splines in CAD

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I think the brief answer is that not all splines are created the same way.In some software there is an option to determine the type of mathematical function that creates them and unless you have determined the type in each program you may be unlucky.
Its quite interesting to delve into the origin of splines as they were originally pieces of wood used in the process of lofting boat hulls and later adopted by early aircraft builders.As you might expect,the type of spline used to generate the plan view of a large passenger liner wouldn't take the bending necessary for the hull sections of a small launch so clearly it was necessary to have quite a selection.The parallel splines would have had similar behavioural tendencies in spite of their scale but occasionally there would be a need for a tapered spline to follow all the points on something like a waterline ending.The loftsmen-thats what they were called-would have planed a spline to a taper that would pick up the points and add it to his collection.With a CAD options list we may just select a different type of derivation from the common cubic function.The end result is the same as the traditional woodworking way and illustrates what may happen if you assume that all splines are derived the same way.
There is a fascinating book by S.S. Rabl about lofting,which includes lots of worked examples of boat hulls and aircraft shapes.You may be amazed by the ability of the old time shipbuilders to develop hull plate expansions with rivet holes correctly located to line up with adjoining plates.All done with a piece of wood and a pencil.

Greg Locock
Greg Locock
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Re: Question about splines in CAD

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Your splines from two CAD programs won't generally agree unless they are using the same formulation for the spline equation, or you have a simple case.

Read up on NURBS splines here https://en.wikipedia.org/wiki/Non-unifo ... l_B-spline and then start asking the CAD vendors questions.

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coaster
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Joined: Sat Jun 30, 2012 4:10 am

Re: Question about splines in CAD

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Thank you JMshapecreator for putting S.S. Rabl on my radar, my purpose now for the rest of today is grab a copy from my nearest library to feed my feeble brain.
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