Plotter

[Gordon]

A couple of folks have been using a Silhouette plotter for making gaskets. I recently purchased a Portrait plotter and tried it and it seemed to work well. They looked great. Then I tried to use the gaskets and discovered that they were about 5% undersize. I had imported DXF files from my CAD program. I contacted the company and they told me that DXF files never import true to size on any program. My CNC mill did not know that. They suggested that I had to export them in PNG format so I found an online conversion and that seemed to change it to 50% size.

What has been your experience? How have you solved the problem or did I do something wrong?

I am running out of material to experiment on.

Gordon

 

[GailInNM]

Gordon,
It sounds like the plotter software has a bug.
DXF files are well defined and I don't know of any CAD that outputs a distorted DXF file. But here is what you can try that won't cost you any stock.

Draw a circle in your CAD of known radius, say 1.000 inch, and a known center. While you are at it draw a line from 0,0 to 1,1. Generate a DXF file from this drawing.

You can now open the DXF file with your favorite text editor. You may have to rename the .dxf with txt in the file name to be able to open it. It will be a long file with all the first part defining layers, line types and other such things. Buf if you go all the way to the end you should see something like this . I have annotated the parts that should agree with the coordinates you used in your test drawing.
Gail in NM

SECTION
  2
ENTITIES
  0
CIRCLE
  8
3
 10
0.0    //x coordinte for circle center
 20
0.0    //y coordinate for circle center
 30
0.0    //z coord  not used for 2d CAD
 40
1.0    //radius of circle
  0
LINE
  8
3
 10
0.0    X/x coord line start point
 20
0.0    //Y coord line start point
 30
0.0    //Z
 11
1.0    //X coord line end point
 21
1.0    //Y coord line end point
 31
0.0
  0
ENDSEC
  0
EOF

 

[Gordon]

The CAD program is producing the correct size but the plotter is converting the DXF to the plotter language under size. I suspect that the plotter software is failing to compensate for the cutter width. It would be like a CNC mill following the center line instead of offsetting half of the cutter diameter. Customer support had a whole page which they read to me stating that the problem was with the DXF which I told her was nonsense but she had no technical knowledge and was just reading from the prepared script. She stated that they have been aware of the problem for at least three years. I suspect that for most of what the machine is used for it does not matter. At this point I found that I can rescale the part. The OD of the original part is 2.150 and it comes out at 2.062 when loaded so scaling to 104.27% makes it the correct size. I have not tried to actually cut pieces yet but it looks hopeful. I assume that rescaling will have to be done every time and I also assume that it will be a different scale each time a different piece is loaded.

 

[dnalot]

Hi

I have been using that plotter but have been using the machines built in drafting system to design my gaskets. It is a bit crude but you quickly learn to make it work.

If you know how much the imported drawings are off you can resize the drawing before you import it.

My machine makes great gaskets.

Mark T

 

[Gordon]

Mark: I just made the same gaskets and rescaled them with the plotter software. I imported the DXF and it was off by 4.3% so I rescaled it to 104.3% and it seems to be fine. I just picked out the OD and it told me what the machine thought it was and divided it by what it should be and then rescaled it to that. It does make nice gaskets but the rescaling is a PITA but worth it to get the good gaskets. I am curious as to whether it will always be off by 4.3% or if different sizes will result in different percentages.

 

[GailInNM]

Gordon,
I misread your first post and thought that the MFG was saying that your DXF file was the problem., so what I suggested was just to verify thet the DXF file was correct.

If you scale the part as you described the OD of the part will be correct but any bolt clearance holes will be over size and out of position.

I have a somewhat similar situation when cultting gaskets on my laser cutter/engraver. It has a beam width of 0.006 inches. It is treated as a printer so cuts directly from my DWG file and dows not go through a DXF translation. l

What I do with it is to created a new drawing and inport the gasket outline into it. Then on the new drawing I offset the paths by 0.003 ( 1/2 the beam width) and erase the old line. Of course this leaves a gap on aquare outsde corners and overcut on inside corners. These are taken care by ulsilng a FILLET command with a radius of ""0".

I do a similar thing when mechanical engraving on my CNC mill with a home built floating engraving head. There I use engraving cutters with a tip diameter of either 0.2 to 0.3 mm and compensate accordingly.

But, from the numbers you gave, 2.150 resulting in 2.062, it would indicate that you are using an engraving cutter with a tip diameter of 0.088 inch which seems very wide for an engraving cutter unless you are using something like a 3/3diameter end mill. But if you are, then an offsett of 0.044 from your numbers would be in order.
Gail in NM

 

[Gordon]

Gail: I resized the whole piece (actually the whole drawing since I have eight pieces on the sheet) It resizes the holes and the layout so everything seems to be OK. The results look good.

 

[GailInNM]

Glad you have good parts. That's always the goal and what ever gives us that result is the right way for us.
Gail in NM