If you are using CAD to make 2D, dimensioned plots of 3D parts, then you are working way too hard. If you are using plots to control fabrication, inspection and assembly, you are causing others to work way too hard.
Machines and computers are consistent. People are creative. Use the correct tool for the job: don't try to turn a person into a machine. While it may give results that are reasonably consistent, it will not be as consistent as the results obtained from the use of a machine.
DON'T BUILD TO PLOTS - BUILD TO FILES
Most fabrication shops, today, use CNC (Computer Numerical Control) machines to fabricate parts. The few shops that do not use CNC should be home shops, shops that cater to people who really don't know what they want (experimental) and need an experienced machinist to help in the design, or shops that really don't understand what is happening or don't have the money for the right tools. In most manufacturing and R&D environments, these shops should generally be avoided. CNC is the best method to get what you want and to get it consistently.
When a CNC fab shop gets a plotted drawing as input to fabricate a part, they must sit down at their computer and completely redraw the part geometry, using their CAD/CAM software.
Wait! Didn't you do that in order to make your dimensioned 2D detail drawing? Sending a plot to a vendor requires redundant effort -- more money and an opportunity for errors.
Send the .DWG file -- not the plot. Most vendors have AutoCAD and their CAD/CAM interface software accepts either .DWG of .DXF formats. If the vendors don't have the right software:
1 Find vendors that do, and use them
2 Tell the vendors to get the right software.
If you have gone to all of the trouble of dimensioning a drawing, and send the dimensioned .DWG file to a vendor; the vendor (or his software) will strip all of the dimensions, notes and format. They tend not to fabricate dimensions, notes and format on the part. This means that you have wasted however much time that it took to include all of these useless and redundant entities into your drawing file.
Dimensions are redundant? Yep! Remember that I said that CAD is a database? All of those dimensions exist for every entity in the database. If you draw a line from 1,3,5 to 6,8,10, the length of the line, position and orientation are all in the database. A LIST command will show that -- as well as DBLIST. Need further proof? Make a DXF output of the entity. It tells exactly what is in the database.
When you dimension this line, the software translates the database information into 2D distances according to the active coordinate system. This assumes that you have accurately picked the endpoints -- something that becomes increasingly difficult on increasingly large or complex parts.
One point that should be made is that, while most vendors are used to dealing with 2D orthographic projections, and can program the mills, lathes and punches from this data; there is nothing like an accurate, 1:1 scale, 3D part geometry to make questions go away. This is especially true since AutoCAD gave us AME with the capabilities to define material (another vendor question answered).
Working with solids might be considered, by some, to be cumbersome. I have been known to forget to change from 6061-T6 aluminum to 1010-1018 steel, when I designed a part. I remembered it after the file got to the vendor -- but a quick phone call took care of that. Another method to consider for defining or describing various of the requirements would be attributes. I give you more on this, later.
For those of you who have followed me, so far -- I know that you have been waiting to find out about tolerances. How does one define tolerances without dimensions?
You don't.
This means that you don't have to define tolerances. Consider that most CNC equipment is designed to work at tolerances that are 10 time closer than the majority of tolerances that you normally want held. A CNC mill usually works in the area of +/-.0005 or better. A machined part usually has a general 3 place tolerance of +/-.005, Now, a vendor can make the part that loose -- if you want to wait until the mill is near end of its useful life and has not been maintained.
It is sometimes required that some features be held to a tighter tolerance than +/-.005. If the CNC machine cannot hold the closer tolerance, then the vendor will use some other method to accomplish the closer tolerance. You can use this to your advantage: If you bought a part that needed a hole added, you would make some sort of part modification drawing, showing what size hole is required and where it is to be located. Do exactly the same thing if you need a closer tolerance. The vendor must do it by some other method, so give him another template (.DWG file). This is also helpful in weeding out the tighter tolerances. You may find that you are spending just a bit more time changing the design to use looser tolerances so that you don't have to generate another file.
There should be a written agreement with the vendor to describe how he should deal with the information that you give him. For example, a shop might get an agreement that says that aluminum parts are to have a finish of gold alodine per ..... This information would be essentially identical to the notes that you would normally put on a drawing. This might also include the conditions that let the vendor know what applies under what circumstances. These would be the same rules that you would follow to select notes for the drawing. There will always be exceptions. To accommodate these, again use attributes -- and, again, more on this, later.
DON'T INSPECT -- CONTROL THE PROCESSES
Back in the days when we were working on some of the early R&D projects (Rocks, dirt, etc.), we controlled processes to get consistent parts. The drawings spelled out exactly how the parts were to be made. In time we realized that the fabricators actually knew what they were doing. We didn't have to tell them to "drill" a hole. In fact, we didn't care if they removed the material with their teeth. We started just describing the result that we wanted. We really haven't learned to trust them, completely -- so we still inspect the parts. The vendors have come to depend on inspection to help them control their processes.
If we and the vendors are doing things correctly, we would stop doing their job and they would do it in a more productive manner. Normally, if the vendor controls his own processes, we would have no need to inspect.
Given a CNC mill, the vendor can use on-machine probes to verify a cut after it is made. This allows him to verify the accuracy of his process and gives him a running history on each tool.
When we are dealing with very small quantities, or feel a need (for whatever reason), a part can be inspected by CMM and compared to the 3D model file. In fact, the 3D model file can (and should) be used to program the CMM, just as it is used to program the CNC equipment that built the part.
FILE CONTROL vs. DOCUMENT CONTROL
Consider that controlling the plotted document (as most companies do) is controlling the result of the work. For what is is worth in the control process, the .DWG file could be erased from your hard drive and document control wouldn't even care. Some companies control both and both can differ. It is too easy to quickly make a change to a plotted file, send off a copy to a vendor, and forget to change the .DWG file.
The 2D dimensioned detail drawing is as redundant in the control process as it is in fabrication and inspection. This redundancy is prone to errors.
If the parts are being fabricated and inspected from the model file, the model file is what needs to be controlled.
Controlling electronic media might seem to be a major undertaking; but, it can be far simpler than it first appears.
The easiest and most flexible tool available is the attribute. An attribute can be inserted an modified by the user. It can also be read by an executable file. This gives it a degree of bi-directional control that can be very useful.
An attribute can be useful for indicating the file revision. It may not be the only way.
One can insert a revision block as an attribute. The information contained in the attribute or block of attributes could be more comprehensive than the normal revision block on a drawing -- or it could be only the revision. If an executable file is written to scan through the .DWG file and find the attribute, it can set the drawing to be relational in a database. This means that all of the required information does not need to be contained in the .DWG file. The rest of the information could be in another database such as Oricle or Paradox. The executable can rename the drawing based on the information contained in the attribute -- or it could make a subdirectory and store the drawing in that subdirectory according to that same information. The concept of storing the drawing according to the revision and the drawing number can be attractive. There may be additional information that one would want to be associated with the drawing. This may include FEA files, associated text files, test data and vendor histories.
When a copy of the .DWG file goes to a vendor for fab, it is important to control the copy. This can be accomplished by a single person, with password protection, inserting an INVISIBLE attribute in the file at some predetermined location. This attribute might include information such as "PO#". If this information is also CONSTANT, it is doubly difficult to inadvertently modify. The information can be read for confirmation by any text editor that has a search feature. This copy of the original goes to the vendor who will quote or fab the part. He can make a translation of the file, if he needs to; but, this controlled file takes precedence.
AUTOMATING BILLS OF MATERIALS
Assuming that all of the .DWG files on a project are 3D model files, it makes sense that the layout and assembly is a 2D file with all of the parts inserted as 3D models, with the block names (drawing names) being the drawing numbers.
It is then a simple matter to write a lisp routine to automatically extract the part numbers and quantities of each and output the information to a text file.
AUTOMATING THE ASSEMBLY PROCESS
When all of the parts are defined as 3D model files, it becomes a relatively straight forward process to describe the assembly of the parts using something like 3D Studio.
The parts are imported into 3D Studio and animated into their assembled positions. That animation is saved and used to estimate assembly time and to show assemblers how the parts are assembled.
As I said, earlier, using people as if they are machines is not very effective. The animation of the assembly, using 3D Studio, has inherent the motions of the parts in the assembly process. This information can be output from 3D Studio into a text file. That text file can be translated into a file to program a robot to assemble the parts.