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CNC translation

ramgoat

New member
Hi, I am having some worries about sending out a design to be CNC cut on a end-mil. I have done classes on CADCAM before, but I have never actually had a part made. This is my first time sending out a part.

The part I am sending out has a section like a kidney basically. It has e holes in the middle at fixed radius from the center, these holes a equally spaced. The actuall coordinates of the holes have no discrete value, they are just based on where I want them to be. The "kidney" is also arbitrarily oriented, there is no true upright or anything.
Now, my worry is, when I send out the part will the machine sort out the origin and all the arc lengths and hole positions for itself? My whole point here is, is it necessary for me to send a fully dimensioned drawing or is the solidworks file enough? I even have some parts to send out that have tapped holes in them, can the CNC program read those holes or should I still have to send out a dimension drawing for the operator to put those in into whatever program be it mastercam etc?

Thank you
 
Hi, The answer is yes. It is best practice to send a fully dimensioned drawing along with your model. The only software that knows that a hole has thread is the native software that the model was created with.


I get the impression from your post that you expect to send out a model and get the exact same thing back. That is not going to happen. Everything has a tolerance. If you have not acounted for tolerance and manufacturing error in your design you will definately have problems when it comes time put your assembly together.


If you are going to spend the money to make a prototype you better make sure all of the pasrts will fit together or you will end up with a very expensive paper weight.
 
Thanks. I didn't expect to get the same thing so that is why is asked here.

What If I have no specific tolerances? Does the CNC machine have a default? I don't remember my tolerances.
smiley11.gif


I will post an image of the parts. I have a stainless steel arm with a kidney shaped slot that slides on a Teflon tongue. The arm carries a 30 kg weight, and is supposed to rotate around a certain point. What tolerance is best for things with guide slots (to avoid looseness)? The parts are about 12 inches by 12 inches in size.

What do you think about +- 0.25mm?
 
If you are providing the CNC machinist a 3d cad file or wireframe cad file (eg, .iges, .dxf, .sat), then the dimensioned print should be sent as well to provide a reference for the machinist to use to check the part dimensions after it has been produced.
The CNC Program can be produced from the CAD file that you send. The programmer would most likely not need the dimensioned print to program the part unless he/she wants to verify an entity in the CAD file.
As far as tolerances go, you should probably build the tolerance into the 3D part you create in SW. For example, if you want to be able to put a 6mm pin though a hole in the part, you should make the hole slightly larger than 6mm (unless its a press fit). The CNC machinist will make the hole to whatever size the hole is in the CAD file or the print. You should specify to the CNC machinist that the print prevails over the CAD file or vice-versa.
Edited by: MadPickinSkills
 
ramgoat,


A typical blanket tolerance for CNC is +/-.005 for feature location, and perhaps +/- .010 on a surface callout(GD&T). Different machinists will agree to different tolerances, but the cost is adjusted accordingly.


For non-orthogonal geometry in a CNC, I will provide a 3D CAD file and a 2D drawing which is partially dimensioned. The drawing will have a blanket tolerance (in the dwg format), the material/finish callout, and callout all holes if applicable, especially threaded. Overall dimensions are provided as reference, or not if you want. If I'm using GD&T for my holes, then I'll call out my A, B, and C datums as well. The machinist typically does not pull the tolerance from the CAD model (despite what the OEM CAD application engineers fantasize about) and unlike a 3D CAD file, an engineering print is a legally binding document, along with the Purchase Orderit is stapled to. Sending a "package" to your machinist which consists of a P.O., 3D CAD file, and 2D print ensures you get exactly what you want at the price you've agreed to.


Jim
http://www.linkedin.com/in/shawengineering
 

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