Monday, February 28, 2011

CATIA NC Machining Reverse Motion Fix

I am machining some parts in CATIA V5 R18 and if I zoom in on the vectors in wire-frame mode I am noticing areas where the tool is reversing a fraction of a millimeter, which has become an issue. I need to eliminate all of these reverse motions. My two major concerns are:

1. What is the easiest way to detect these reverse motions? Currently, I have to zoom in really close on the vectors, the only way I know how to see them in CATIA, which is a pain.
2. How do you avoid this reverse motion? What's the best way to fix them?

The answer to question number one is unfortunately there is no other way in CATIA to detect these tiny reverse motions. If you have some sort of post-processor that will usually pinpoint problem areas but if you don't have access to a post then you're out of luck. As far as concern number two, my guess is the surfaces are crap (especially if they are not native CATIA data and imported from some other system). You can try to rebuild them to get better results. Often times, I will delete the two axial moves using the toolpath editor. Then, I will select the icon to connect the path, select one of the high-lit points, and enter .000 as the value. This way, you will get a direct connection in machine feed. It's kind of weird, but if I want a Rapid connection, I will enter .1 as the value.

Also, sometimes, as in the example, you would end up with a small zig-zag motion. In that case, I will often delete a point if available or move a point along the direction before "connecting" it. As always, toolpath editor is last resort but I'll use it rather than spend a bunch of time fighting surfaces and machining operations. I always note in my programming log, any locked toolpaths, why, when, and what type of edit was made. It's good to keep a log using Excel for whenever you have to make "manual" edits of any sort.

Wednesday, February 23, 2011

How do you measure the length of a spline in Catia?

You may be wondering if it is possible to measure the length of a spline in sketcher or a 3D spline in Generative shape design. In other words, what is the length of the spline if it were straightened out. For example I have a part made utilizing the rib command and it is not made in Sheet metal design so I can't make use of the unfolded view in drafting and measure the length of that part. What do you do?

There are two methods of measuring the length of a spline which come to mind.

  1. You can create a parameter called "Length" and then you would add a formula. Write length() and place the cursor between the parentheses and double-click on the spline. For example: length(`Geometrical Set.1\Sketch.1` )
  2. In the sketch, pick the curve and hit measure item. If you don't get the length of the spline hit customize and set length to the edge group.

Now you know several methods of how to quickly and easily measure a 2D or 3D spline in CATIA.

Monday, February 7, 2011

What is a Mylar drawing?

Mylar is a trade name for bopet (biaxially-oriented polyethylene terephthalate) polyester film. Also known as Melinex or Hostaphan, this transparent, durable material is used in food storage, insulation and sailing, but it is perhaps best known for its applications in the graphic arts, most notably for CAD drawings. A few customary drawing techniques enhance its positive attributes.

Architects and engineers use Mylar in designing their work. The schematics are drawn up, usually by a computer program such as AutoCAD, and then printed. Schematics on Mylar reflect the depth and dimension of the finished project more easily than blueprints. Once the Mylar is drawn up, these illustrations become part of the legally binding agreement between contractors, the employer and the design firm. 

Many companies today are in the process of taking their un-dimensioned drawings on stable-based material (Mylars) and preserving them by fully integrating the legacy Mylars into the automated work environment through digitization and subsequent storage capacities.

One frequent problem often encountered in this transfer of data is projects may require a very precise tolerance of +/-.005" accuracy over any five- or ten-inch area for the digital scanned files. The drawings are on Mylar, which is a plastic sheeting material that may be affected over the years by changes in temperature. This means that the older drawings may have stretched or otherwise changed in size, affecting the accuracy of the drawing itself. The accuracy of the final digital file is of paramount importance. These digital files are stored in an electronic repository as a permanent archive for all of our drawings. If we need to refer to these digital drawings, or print out a new paper drawing, it is essential that the measurements be as accurate as, if not better than the original Mylar drawing when it was new.

  More information on un-dimensioned Mylars:

  • An un-dimensioned drawing presents the engineering definition graphically without the use of numerical dimensions.
  • Sizes and relationships of un-dimensioned features are determined by scaling the original drawing or by scaling a stable reproduction made from it.
  • Un-dimensioned drawing forms are identified by the letters PCM or "Photo Contact Master."
  • Effectivity notes indicate part replacement, upgrades and modifications for various models.
Un-dimensioned drawings are typically used to define parts with all or a majority of tolerances plus or minus .030" or greater. This includes sheet metal parts, extrusions, cable assemblies, and wiring harnesses.

What are the components of a Mylar drawing?

  • Title block
  • Bill of materials
What information can be found in a bill of material?

  • Part number
  • Description
  • Zone
  • Quantity
  • Material Specifications
  • Extrusion callouts
  • Thickness and size
  • Flag notes
  • Revision level
What are common abbreviations found on Mylar drawings?

  • ECN: Engineering Change Notice
  • ECO: Engineering Change Order
  • PCR: Project Change Request
  • PCM: Photo Contact Master
  • A.N.D: Army Navy Drawing
  • BDN: bend down
  • BUP: bend up
  • BR: bend radius
  • R: radius
  • TYP: typical
  • JOG: joggle
  • SHN: shown
  • OPP: opposite
  • FLG: flange
  • EOP: edge of part

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