TIPS

Computer Aided Design for Making Model Makers

by Joseph Harris

What it is.

CAD is short for "Computer Aided Design". The key word is "Aided". Some one asked me "'how does the computer know what to do?". It doesn’t. The concept and design is in your head as it has always been since the first man had an idea. The computer is just a tool that can make your drawings better. It replaces the use of conventional drawing instruments with electronic ones. Additionally, it has many features that help you shorten the time required to make good drawings. This is good news if you are not a good draftsman. Personally, I'm a terrible draftsman. The computer allows me, despite considerable artistic ineptitude, to make better drawings. Below are the things that I find most useful.

Accuracy

I can take measurements far better than I can draw. Autocad is capable of accuracy to the 16th decimal (.0000000000000000"). You may select the decimal you want to work to. You can design a full size ship if you want or if you are making drawings from a set of plans, you can use the drawings as is without converting the dimensions from one scale to another. The generation of your drawing to the size you require is performed after your drawing is complete. Scale conversions always cause some problems due to the large number of calculations involved. It Invites error. Computers do not make calculation errors of that kind. Dimensions will convert exactly. I made a complete set of frame drawings in 1:48 scale and an identical set at 1:96 scale from the same drawing. Another application for CAD that I found useful is in drawing small objects, in scale, with a high degree of detail. I made full size scale drawings of cannon, carriage brackets, axle trees and component parts of my capstans. The quality of these drawings is far better than anything I could have drawn by hand. I should add that for all of its speed and accuracy, the computer is still subject to the "garbage in, garbage out" rule.

Alignment

Your drawing alignment will be perfect.. Frames will truly be straight and will not cant to one side or the other because you just can’t seem to fold that piece of paper exactly down the center. This almost invariably happens to me no matter how careful I am. There is no parallax error caused by looking though a plastic scale or other visual distortions or just bad eyesight. They also align perfectly to each other. When you can lay all your frames out and look at them all at once, frames that are out of line or out of proportion stick out like large nose warts and are much easier to find and correct.

Flexibility

The system provides you with some amazing tools. You can zoom in on any part of your drawing, literally to any level you want to go down to. Have you ever wished you could blow up some part of a drawing so you could put in all the detail and then put it back to its real size? You can with CAD. Sometimes I have a great new idea and I need to modify my drawing. Sometimes (OK, often) my great now idea turns out to be doodoo. Unfortunately my original drawing is now also gone. I now have a hacked drawing and no original. With CAD I can modify it in the computer, give it another name and keep both the original and the modification. I'll make up my mind which one to use later. One of its best features is its ability to eliminate complex, repetitive objects. A good example would be drawing an overhead view of a gun deck. If you wanted to put in the guns, you would have to draw 26 to'35 separate guns. In CAD, you draw one gun. You give it a name (gun sounds OK) When you need a gun, you simply go get "gun" and stick a copy of it wherever you want it. Another one would for drawing deadeyes for layout. Would you rather draw 40 of them or just one or two? This can be used for almost anything you need a bunch of.

Making Your Drawing

Inputting to the computer is not as complex as you might think. You are going to do electronically, what you have done with a pencil for years. The difference will be that you will be far faster and more accurate. Probably you will use a mouse to do your drawing. The computer provides you with a number of standard commands and shapes to make your drawing easier. For instance, segmented lines, circles, arcs, octagons, squares, rectangles etc. There are a lot of variations depending on the software you use. You can also type in dimensions. This is good for insuring that your start point is the same. For instance, you want your frame to join the keel at the top of the rabbet. This is essentially a reference point. Once you've established that reference, you can ensure all your frames will start at the top of the rabbet. Initially, you must "set up" your drawing board before you start. The information you give it will include the dimensions of your drawing surface(your electronic piece of paper), The dimensional units you want to work in, feet, inches, meters, hectares, acres, cubits or what ever. You can also put a grid on your drawing (which you can turn on and off as conditions demand) as well as have it automatically snap to the nearest increment you want to use( the nearest .005" for instance, in 1/48 scale, will give you a scale accuracy to within 1/4"). You also establish a reference point at the same time. For instance, in a frame drawing it is better to have your reference as the center of the drawing (vertically) and the top of the rabbet line(horizontally). Frames are always symmetrical so the one side you do will mirror perfectly. Another quick point is that you are not blindly number crunching at a keyboard. The system is graphic in addition to providing a digital XY axis display. Once you have established one outside frame line, you have all the information you need to produce the rest of the drawing. This is where all the computer functions start to become useful. I can type in a single command and cause the computer to take all my points and produce a curve between them. If that looks OK, I can give it the thickness of the frame I'm doing and cause it to produce a line that is exactly parallel to my original frame line. I can also taper the frame if I want. A line to include the top of the frame and the middle pieces and I have a half of a frame. I can now use a "mirror" command to exactly mirror the side that I drew. The commands themselves are usually single word commands. You tell it what you want to do, point out the items you want it done to and the computer will do the rest.

Getting Your Drawing Out

Getting your drawing out is also pretty straight forward. All of this software has some sort of output, usually to a plotter but could be to a printer. The computer and the plotter have to be set up. This is where you establish what size your output drawing will be. If your drawing was drawn in 1/4"/I' and you want 1/8"/l', you can plot at 50% of the original size. The drawings generated will be perfect 50% representations of your original drawing. The computer can provide you with near perfect artwork. Unfortunately it is not without a fair cost if you have to pay to have the plotting done. However, I had no problem finding, among my friends and acquaintances, some who worked at engineering companies having plotters and were willing to do my plotting for me. Even so the cost of copying 12 "E" size sheets(36x48) was in excess of $100.00 at Kinko's. If your scale is small enough to fit on an 81/2x 11 sheet of paper, it’s as cheap as copying anything else. Software (the drawing program) varies a great deal in both sophistication and cost. The really high powered systems, such as Autocad, Pro Engineer or Anvil can be very expensive. Others, such as Corel Draw or Autocad light are a lot less money. There are bound to be many others that I am totally unaware of. I am not really a computer person. Those who are heavily involved can probably enlighten us even more.Anyone wanting to discuss this technique further, feel free to contact me.