Technical Tips - Designing in AutoCAD

This month Peter Rodgers gives us some insight into preparing CAD data from Autocad. This process talks us though the features and methods to accomplish a clean cad design that will work for your photomask or photoplot films. Peter is one of our CAD engineers based in in Oldham. For further information, you can email him at pete.rodgers@jdphoto.co.uk   "One of the biggest problems with AutoCAD when using it as a design tool is that the design can be layed out in many different ways on screen – some of which look correct on screen but do not export correctly. This is because quite often the element used is a display element – one that will display the drawn element on screen, but does not hold through export ."

"Below i'll outline the various features to use in Autocad to get the cleanest data for your design. Remember that clean data not only means faster processing times for us, but also reduces the possibility of extra CAD charges - so it's a win-win situation ! "

Zero Width Closed Polylines
This property should be used for virtually all of the elements on the design layer that represent a boundary. A boundary consists of a series of straight segments forming a closed area. The first and last vertex must be the same. A boundary may not overlap on itself nor self-intersect, however it may touch itself The entire boundary entity should be drawn by using a closed, continuous zero width polyline. This represents the external boundary of the element, and these boundaries will be automatically filled when processed. It’s important that these boundaries are closed correctly, are sequentially continuous, and contain no duplicate items. You may use the ‘close’ command in AutoCAD to completely close the polyline.

Examples of errors....

For display purposes, you may choose to use the AutoCAD command ‘solid’. This will solid fill the closed polyline whilst being displayed, and it a good visual reference as to how the finished photopositive will look. However, this is not necessary because all closed zero width polylines will be automatically filled solid when processing into a photopositive. Be carefuly NEVER to use HAtching, even if the hatching is set to Solid !

Constant Width Polylines
This, in effect, is a line with a width applied to it and should NOT be closed. The width may vary from vertex to vertex. If a user needs to draw a frame around an area using a constant width polyline, do this with two separate polylines.

Lines and Arcs
These may be used to form closed boundaries, but the designer must take great care to make sure that the endpoints snap together and that there are no hidden lines under other lines. When using lines and arcs, the correct ‘line weight’ must be used to represent the artwork correctly.

Islands and Isolated Areas
Islands are clear field areas surrounded by a dark field on the mask (think of the hole in a donut as being an isolated area). They cannot be drawn in the same manner as you would for normal boundaries because AutoCAD has no way to determine that a particular polygon interior should be clear. In the drawing below, the designer drew one boundary inside of another [a]. Instead of getting the image he desired [b], he got the solid image shown in [c]. This is because when processing the DXF into a photopositive, each boundary will be filled solid independent of its relationship to other boundaries.

There are 3 different ways to overcome this.....

1) Two mating boundaries You can use two separate boundaries that butt up against each other to form the frame. Only use a closed polyline - if you use lines, most likely they will be linked incorrectly to form a self intersecting boundary. These actually touch but are shown separated for clarity.

A Mating Boundary

2) Re-entrant Boundary Another method to realize an island is to go into the interior of your polygon and out again via the same path. This is called a re-entrant boundary. This is best done using a closed zero width polyline.

A re-entrant boundary

3) Layer Drawing "Islands and "Holes" - These are areas of clear film surrounded by dark film. The procedure is simple. Create one or more additional AutoCAD layers to hold the inner polygons. Then draw the "holes" on this layer. When the photopositive is processed, two files are produced: one for the outer polygons and a separate one for the inner polygons. We then tell the operator to subtract the hole layer. Example -- Create the basic AutoCAD layers adding one for inner polygons: After drawing the outer metal figures on the first layer , draw the inner polygons on the second layer. When these files are sent for imaging to us, be sure to indicate that they should be merged; have layer 2 subtracted from layer 1.

Layer Level Boundary

Text and Fonts
AutoCAD supports three types of text fonts: .shx, .ps, and TrueType. However, shx fonts are the original vector font format supported by AutoCAD and the only one recognized by DXF conversion software. In effect, no fonts are embedded into the DXF file when it is exported; the font file is simply referenced. Because of this, it is important that SHX files are not modified in any way otherwise the font will not be displayed correctly. If you need to change the style or format of the SHX font, rename the font to something new that will not have been used anywhere before. There are two types of SHX fonts: stroke fonts such as txt.shx and romans.shx and outline fonts such as helv.shx. Stroke fonts use one or more strokes that need to be assigned a width to properly convert. Outline fonts consist of outer and inner outlines defining the area to image. Text, or course, can also be built up from a series of lines, arcs and boundaries to mimic a font face.

"I hope that this basic information of use to you. Getting Autocad designs right first time can be a pain, but if you follow the guidelines above then i'm sure that time will be saved in the long run. If you need any more information on the above features, don't hesitate to contact me ! "

See the Archives for other Tech Tips...