It is surprising some times the little foibles you run into with CAD. For instance the other day I want to create an accurate depiction of a threaded shaft. So I plug in the 1″ x 12 TPI thread size and SE says shaft size will be changed to meet the thread pitch. Thats what I want.
However that is not what I get. It still measures 1″ OD. Well ok I am thinking to myself, I know the real manufacturing OD is .993 because I have the physical sample in hand and the Machinery Manual also and neither say a 1″ OD for the major thread OD size. So instead I decide I will assign the correct OD and apply the thread pitch to that. Now I get a message that the shaft size is wrong and no thread can be created. I am looking at a huge list of thread types and pitches now I am wondering how full of errors it really is. Afterall is not the thread table supposed to reflect reality?
It turns out according to SE forum posters that this is the way the program was set up and that it is functioning as designed according to the developers. And that they seem to be satisfied with that is the general flavor of the comments by a poster who has raised that issue.
So I ponder how does MCAD software that is used ultimately to design something for manufacture not reflect the reality of manufacturing? To me what does the shop floor have to deal with, what actual parameters are needed to produce parts should trump every day over a developers idea or concept of sufficiency. What I do in SE goes direct to my shop floor and I have to deal with it accordingly.
Most CAM programs today work off of shapes. I take the 3D part and create cam plans off of it. Now when I can’t do that because of inherent program created inaccuracies there are problems.
The work arounds rely on you remembering to fudge numbers or create two sets of parts, one for design so you can do things like interference checks and another so you can cut off of correctly sized geometry. I think it all goes downhill from here and the inevitable scrapped parts from software induced confusion are guaranteed.
Every CAD program has these problems. Now the poor shmuck after wading through this CAD mire arrives at his CNC whatever to make a part where he knows that he has to now deal with toolpath problems for additional excitement and parts scrapping. I read the forums and I know from NX CAM to the cheapest cam program out there there are problems that are not trivial and the solution is quite often, after you break cutters or parts or machines, you learn where your expensive software will or won’t work and then it’s time to turn in your complaints and hope something gets done. I have had complaints in about toolpath problems on spiral in cut paths for up to three years with VX for instance that have never been fixed. I am not singling them out as particularly bad, it’s just the program I am using so I have personal knowledge of this. You can insert the name of your cam program into the same type of complaint I am certain.
So how is it that software for manufacturing does not reflect the realities of manufacturing? I don’t have a clue how this concept eludes developers but it does. The true manufacturing specifications for things like threads should just be in there and not something we users should have to fudge and fiddle with to get our goal of an accurately finished part accomplished.
Dave Ault
Machined Audio Horn Lens 