Larry Cunningham · Dec 05, 2004 10:17 PM
#0 sourceI'm using AutoCAD 2000 LT, which is essentially a lightweight 2D version of the full blown AutoCAD. Nevertheless I find it quite capable for my needs.
The first thing I found out about is that laser cutters are fed material which is not completely uniform in areas. For that reason, you don't want to try use absolutely all the available material. You need to leave a little cutter clearance and reserve around the edges. For example, the 3 x 24 inch stab sheet for the Mo'Best got resized to 2-7/8 x 23-7/8, which allows a 1/16" border around it for the laser cutter.
In conjunction with this, I draw a rectangle box, which has its lower left corner at 0,0. This is not on Layer 0 (the laser cutter layer), but on a Text layer, which can easily be turned off. I use the OFFSET command to draw another box inside this, with a 1/16" offset. This smaller box is 1/8" less in both dimensions than my "native" material.
And I locate all the parts within the smaller box, which assures my 1/16" clearance on all sides.
I'm told to plan on a laser cutter width of about .005", although in most cases it can be ignored. Still, if I want a nice crisp fit for an 1/8" sheet in a notch, I draw the notch .120" instead of .125" (1/8"), and the laser cutter width will make it slightly wider.
Also, the laser cutter width is a function of thickness of the material, laser power, speed of motion, etc.
In terms of material usage, obviously it is better to use fairly standard sizes, in 6" intervals of length.
Another thing is to allow some breaks in the laser cutter paths, so the parts don't fall out of the sheet after they are cut. It doesn't take much, for example a 1/8" break in a laser cutter line can hold the material together in a sheet, and yet be easily cut with a #11 Exacto blade to free it. In consideration of this, I like to locate these interruptions on a straight line if possible, and a line which is usually aligned with the wood grain (for easy cutting).
An easy way to accurately make the interruptions is to simply draw a small circle (1/8" diameter) at the point, which is then selected as a TRIM object and trim out its center, then erase it, leaving a 1/8" gap.
I have found that 1/8" is not necessarily the optimum gap size for the interruption on laser cuts. It depends on material. For example, the 3/32" hard birch plywood I use for nose doublers and engine crutch laminations cuts beautifully, but a 1/16" long gap is adequate to hold the part. Similar comment for 1/64" plywood cuttings.
Another thing I want to do for laser cutter drawings is to eliminate redundant lines. These easily crop up in normal drawings and often we just leave them. However, for a laser cutter, these lines may force the laser to make an unecessary cut.
In the normal course of drawing, I like to generate blocks, which allow me to "re-use" drawing pieces easily. For example, I encapsulate and entire rib template in a block, which can then easily be inserted within a drawing. Or copied to the clipboard and taken to a different drawing.
There are some gotchas to deal with in AutoCAD with blocks. For example, suppose I create a block R1 which is a rib template in drawing A. And I also have a (different) block R1 in drawing B, which is also a rib template. If I use the clipboard to copy and paste R1 from drawing A to drawing B, AutoCAD will happily ignore the new block and substitute the existing block R1 in the second drawing. This can cause subtle problems.
Associated with blocks is the EXPLODE command, which breaks up a block. Note that you can have several "layers" of heirarchy in a block too. For example, I made drawings for several pieces of a trailing edge assembly, named TE1, TE2, TE3, and TE4. It is very convenient to deal with them as blocks, and I can quickly draw up the entire trailing edge assembly simply by placing copies of the blocks TE1 through TE4. Then I can make a convenient block for the entire assembly, which I might call TE.
This all works great, but as I said, you often would like to eliminate redundant lines that border two joined pieces. So a laser cutter drawing would like to EXPLODE the blocks down to primitives (lines, arcs, points, circles, polylines, etc.) and eliminate redundant traces.
When you use the EXPLODE, it may need to be used repeatedly if there are "nested" levels of blocks.
If you want to eliminate redundant lines, I have no doubt there is some fabulously good automatic way to do it, perhaps with a macro or something. But what I end up doing on parts which share a line border is to use ERASE, and select lines one by one, erasing until the last one goes, and then using UNDO to put that one back.
With some laser cutter software, it may be capable of doing this sort of cleanup all by itself, I don't know. But it is nice to start with an efficient, clean drawing if possible, right?
Another entity I like to exploit is the polyline. This allows you to join multiple line segments and line objects into a single entity. The laser cutter can smoothly trace a continuous path of this joined group of lines.
To do the drawing cleanup process efficiently, I normally add the laser cutter interruptions first. Then I select the entire drawing and use EXPLODE multiple times to get down to the lowest level, no blocks left. Then I use the PEDIT command (polyline edit) selecting multiple lines, and I select the entire drawing. At this point, the PEDIT command will ask to convert any non-polyline entities into polylines, and I answer Y to let it do this. I complete the command by using the JOIN function, and it creates several polylines, depending on the line interruptions. You can "see" these lines by selecting and highlighting them.
Finally, I try to clean up the drawing by using the PURGE command, which eliminate unused entities, including layers, line types, blocks, etc. This can reduce the size of the drawing considerably.
One other thing. The Brodak laser cutter lets me include a CYAN colored line on the laser cutter layer (layer 0, a GREEN line is used for the cutter mark). The CYAN line can be used to do a low power cut for marking the parts. Obviously, for this function, you want to chose a very simple stroke font for the part names, I use ROMANS.SHX for the Standard font. Also, although you can use long words for names, it is not efficient use of the laser time. Why say Left (Inboard Flap), when you can name it FL1? So, the rudder pieces are RD1, RD2, RD3, etc.
When the operator gets ready to do the laser cut, he can turn off all but Layer 0 and have the laser cuts (polylines, with interruptions), and the annotations. And the lower left corner of the drawing is at 0,0, which may or may not make it easier for his material alignment with respect to the drawing.
These are "tips" that I can pass on from using AutoCAD. I know various people are using Corel and other CAD packages with good success, so some of these things may not apply.
In addition, I suspect the more experienced laser cutters out there can provide some corrections, improvements, etc. over what I have described here. I'd like to open this thread to solicit all such information as you are willing to share.
(I feel like Columbo here: "Just one more thing, Sir--"). I should also mention using layers. The laser operator told me to use the default Layer 0 for the laser cut information. Depending on what else you have in your drawing, you may have some more. For example, I add a TEXT layer and on it I put - TEXT! and also some dimensions of the main sheet. The laser operator will turn off all but the appropriate layer(s) when he cuts, so you need to make absolutely sure the laser cutter lines are on that layer!
Another CAD information parameter is often line COLOR. Although I always used a white background and simple black/white lines in the past, I have discovered that most of the serious AutoCAD jocks set up to use something with a lot of contrast and good visibility, say a black or dark gray background, with high visibility green lines. Obviously, if your laser cutter keys on a color as well as a layer, pay mind to that as well.
While it can be helpful to use many layers in a CAD drawing, for the laser cutter drawing, I keep it to a minimum - 3 layers. Layer 0, the cutter layer, layer TEXT for text and sheet dimension text, and layer DEFPOINTS, which is automatically generated when you add a dimension annotation.
Again, the PURGE command can be used to eliminate unused layers.
Thanks in advance for any help and input!
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