Thanks everyone for the information and suggestions.
1. I use the (DesignCAD 3D MAX) CAD program, and have been using it since 1988. I am very familiar with it and was an alpha and beta tester for decades. You can design in any Imperial and Metric units and switch between different units, with appropriate scaling. You can also scale designs from one size to another. We have used it at work for decades to design everything from huge oceanographic instruments to tiny molds for resin casting, and innumerable mechanical packaging jobs.
For example, my USS
Oklahoma City CLG-5 design was created in inches at 1:1 scale. The hull is 7320 inches long (610 feet). But for 2D plotting I change the scale to 1:96 to get full sized 1:96 prints.
But for 3D printing I first change the scale to 1:96 and then change measurement units to millimeters, with appropriate scaling. This changed 96 inches at 1:1 scale to 1 inch at 1:96 scale, and then changes 1 inch to 25.4 millimeters. I save this file as my working 3D print file. From this I generate STL or OBJ files. As I have posted earlier these STL files usually print the 1:96 scale parts with about 100% +/- 0.2% accuracy.
However, I am still using the 2018 version of DesignCAD and it almost always generates STL files that have errors. I have four different programs that report these errors (AccuTrans 3D, 3D Builder, Photon Builder and Chitubox). Microsoft's 3D Builder is the only program I have found that will correct these problems and generate a STL file that works with all the other programs.
What is weird is that the files all appear the correct size in AccuTrans and Builder. Some of the larger objects come in to Chitubox at the correct size. But the smaller objects always come in huge, larger than the print area. But up to now they have all been exactly 10x too large, so I have used the Scale function, changing scale from 100.0 to 10. This brings them back to the correct size.
Except for this tiny grating!
2. Here are photos to show the strange scaling problem I am seeing.
Attachment:
Vent and grills.jpg [ 124.26 KiB | Viewed 793 times ]
This is the bucket vent with the 10 grill sections - all created in the same file, 1:96 with millimeter units. It was saved as an STL, blessed by 3D Builder, and imported into Chitubox. Everything came in at the correct size in millimeters. The grid has 1 cm spacing, and the vent is about 14 mm wide. The grids import at the correct size (X = 3.7432 mm, Y = 0.4019 mm , Z = 2.1828 mm). So as long as the total object dimensions are larger than some magic number the parts import at the correct size with no scaling.
Unfortunately, there is no way to edit files in Chitubox to cut out parts that are not wanted - in this case the vent. The vent will print correctly at 0.05 mm vertical stepping, and that will be 5x times as fast as the 0.01 mm stepping that will be necessary to print the grills (if they print at all). I want to print the vents at 0.05 mm stepping and the grills at 0.01 mm stepping. This requires separate files for the vent and the grills.
This leads to the problem illustrated here:
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Three sizes.jpg [ 119.73 KiB | Viewed 793 times ]
After loading the vent and grills file shown above, I loaded two separate files of the grill. One was scaled in millimeters and the other in centimeters before creating the STL files. The grill was just cut out of the bucket vent file, pasted into a new blank file that was already using millimeters as the units. The size measures correctly in both the original and the new grill only file.
Then the middle STL file was generated for the grill with mm units. It came into Chitubox hugely oversized (X = 95.04 mm, Y = 10.19 mm, Z = 55.41 mm). This is actually 25.39 times the real dimensions. Scaling to 1/10, as I have to do with some otherr small objects, I get the size shown in the middle (units - mm) that is2.5 times oversized.
To test the idea that saving the files with centimeters as the units might solve the problem, I redefined the measurement units in the CAD file to centimeters, with appropriate scaling, created another STL file and imported it into Chitubox. It was even larger than the mm scaled file, and when scaled 1/10 came out as you see it on the right (units = cm).
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My problem probably has something to do with the original DesignCAD STL files. 3D Builder shows the size of the mm grill to be about what I get when I import it into Chitubox and scale it 1/10.
I guess I will have to use the Chitubox Scale function to check everything I try to print to be sure that it has imported at the correct size. I tried this in Photon Workshop (a version of Chitubox) and It won't allow me to input dimensions less than 1 mm! When I try to set the X dimension at 3.7432 mm it just uses 3 mm! However, this does seem to work in Chitubox.
Measuring the dimensions of an irregular object could be tricky. Fortunately I can ask DesignCAD what the maximum XYZ dimensions of an object are.
I certainly cannot use a simple import and scaling procedure for everything. PITA!
Of course the whole point in this exercise is to determine if such a tiny (~3x2 mm) object can be printed and if the extremely small openings in the grill will actually be open. The thickness of the grid bars is 0.05 mm, so if I print at 0.01 mm each horizontal bar will get five print layers. I have printed a few vertical objects that are only 0.05 mm thick, so it might be possible.
To be continued ...
Phil