What is the difference between isocline and true draft 1

[man2007]

I read through the NX4 documentation about Isocline draft vs True draft. But I did not quite understand the concept of isocline draft.

The explaination about True draft can be easily understood and we can verify the same by taking section curves as told in the documentation.

Can somebody help me in understanding the isocline draft?

Is it possible to measure the draft angle of isocline as in case true draft?

 

[cowski]

The main difference is the isocline (iso = same, cline = angle) will ensure that the entire face you are tapering will have the same angle at any section you cut through. True draft might fudge it a little bit. The 1/2 cylinder in the help docs is a good example. Isocline taper would fail in this instance where true draft will apply, but will not apply the same angle that you specified across the entire surface.

Which one you use really depends on your requirements. I deal with a lot of molded parts so I prefer to use isocline, but I have had the odd occasion to use the true draft option.

 

[hudson888]

Man,

Cowski has it correct. He also alludes to my reading of the situation. Seems that we only use true draft when isocline won't work. I usually find myself drafting from a shut line of some description so I use the method from the edges most frequently.

What you want to watch for is where more than one face radiates from the edge in the drafted region, and sometimes adding draft propagates extra faces, then check for edge to edge tangency changes across the individual drafted faces. I hope this concept isn't lost on people as it is difficult to word clearly enough.

Best regards

Hudson

 

[man2007]

Thanks for your reply.

I have just created a simple extrude and experimented with both the options. Please see the file attached. I took section curves in both the cases. The curves with White color represent True Draft which gave the value exactly 10 deg, but the blue colored lines taken with isocline draft did not give me the exact 10 deg value, as opposed to your explaination.

 

[hudson888]

Man,

I have taken a few liberties in your file to help show the difference. It is hard to grasp at first but the Isocline method is usually preferred.

You should be able to see that the true draft is equivalent to an extrusion at a line angled to the draft vector lying on a plane normal to the edge. In this case the construction is simple and the results straightforward.

You'd be excused for thinking that was good for exactly 20 degrees of slope relative the the draft vector, and you'd be wrong. Use slope analysis of each result and by resetting the data range you'll see the accuracy of isocline is to be preferred.

Best Regards

Hudson

 

[man2007]

Well, what I understood from this is, the isocline surface will not have same draft angle all around. And it will produce a surface having range of draft angles, of course the values are very much nearer to the actual draft angle!!

As long as I am in the modeling application I can check it with Draft Analysis tool. But when you go to drafting, (ok we mentioned the draft angle value in the General Notes and never shown it on the actual drawing) if I take a section near the pink or red band shown in the Draft Analysis tool, I will get glaring deviation in the draft value. Now how do you justify this?

There must be some reason for adopting this method (of course complex one) of creation of draft angle, which at this point I am failing to understand. I think most of the people in this forum are feeling the same.

 

[hudson888]

Man,

Isocline method appears to give the most accurate result when the 3D model is concerned, whereas you observed that the true draft method generates a sectionally accurate shape. This is on the face of it far from easy to reconcile.

Take a look in plan view at the profile of the top edge that you are drafting from compared with that of the bottom edge that is the result of 20 degrees of draft. The top face is curved but in plan view the edge appears to be a straight line. Yet as a result of the top face curvature the bottom edge is a curve regardless of which method by which you undertake to apply draft. This is why I showed those two curves in my file to highlight the difference between the two methods.

Now establish three datum planes normal to the bottom edge of the solid using the plane on curve method and placing one at either end and one in the middle. Use the edge of the solid not my curves it will be better if things are associative. Create associative sections using the planes and measure the angles relative to the base. Edit the parameters of the draft from isocline to true draft and back should allow the sections to update associatively so that you can measure at the base how the angles change and observe where the sections fall on the solid etc. You can also use the deviation analysis to measure either "edge to edge" or "face to face" referencing the base of the solid and the bottom edge of the drafted face. All of this should observably fall in line with what the slope analysis was telling you.

What you are seeing is that the sections are radial to the base of the solid, and in fact at any point below the curved top face the drafted face proceeds to radiate from the edge that is being held. I used the base being normal to the draft as a reference for measuring it may also help to recognize that you could measure using the section curves that fall through the solid onto the rear face (on the XY plane of your model). You can see that they are both vertical (aligned with the draft in the Y axis), and parallel with one another. Do not be deceived into thinking that the sections are angled somehow because they appear to cut the corners at the ends of the drafted face.

Now I suppose if you cut sections exactly like that on your drawing then it should be possible to dimension it as such. I would use this example which was a very good case in point to show how and why isocline method is usually preferred for molding purposes in plastic parts and castings. It is also considered by most who regularly perform this kind of modeling infinitely more important to create accurate models than to facilitate sectional drafting techniques. The truth of the matter in those cases is that the model is often offered up with a general note on the drawing stating the the 3D math model is to be used as the master and applying a general tolerance for all but the few datums and/or otherwise critical dimensions that are shown on the drawings having their own specific tolerances. In that way few if any such drafted faces are ever sectioned to have an angular dimension applied.

One last point of interest is that if you were to take an intersection point between the two base curves that I supplied and then use it and either of the curves to establish a datum plane you could cut a section showing that at that point the two methods produce identical results.

Hoping you're able to undertake so experimentation of your own based on this and come to comprehend the results. I would attach an updated model but it won't work for me at present. Anyway I think you need to experiment more for yourself. Taking ownership of the analysis and going through a process will always help you to better understand.


Best Regards

Hudson