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Boolean Subtact Problem
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* April 22, 2018, 09:09:18 PM
I am using Turbocad Deluxe 2016 2D/3D, version 23.2 under Windows 7 64 bit.

I am drawing the wooden underframes of a railway wagon with a view to having it 3D printed. The basic objects in the drawing are either simple extrudes or boxes. Where two parts of the frames butt up against each other I am showing the joint as a small triangular groove. This is done by extruding a triangular prism of the required size for each groove that is then Boolean Subtracted from the main object. For the most part this works.

However I am often finding that the resultant subtraction can just leave a hole in the surface rather than forming the groove. The problem seems to occur when I am trying to add a new groove that joins to another, existing groove.

I have enclosed both a .TCW and .STL version of the drawing. Most of the grooves are nicely formed (and have done a good job of showing the joins in a previous model drawn using TurboCad 16).

There are four short grooves, one in each corner, that are causing trouble. The two on the left hand side have worked correctly. The two on the right hand side appear to have just deleted some of the original surface without adding the extra facets required to form the groove. This shows up quite well in MeshLab. All four attempts at making these grooves were done by subtacting copies of the same triangular prism. I suspect the problem occurs because one end of the new groove starts in the middle of an existing groove and TurboCad may have trouble in handling the intersections.

Once this problem occurs is seems that the solid object gets converted into a surface because it is no longer fully enclosed. Therefore normal solid editing tools can't be used to cut off the offending piece and and add a fresh piece in.

Can anyone shed any light on what is happening and offer advice on how to avoid this problem and to fix it when it occurs?

Thanks in advance.
Regards,
Michael Milway.

PS It appears that I cannot upload .STL files. I have appended a ".txt" to the file name. Please strip this off before attempting to open it with MeshLab.

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* April 24, 2018, 12:45:13 AM
#1
Not known if you have this sorted yet or not. 

This may to be a limitation in TC surfaces used in both pro and deluxe.  and seems to be caused by the bottom of the V, where the small V shapes meets the ones cut width-wise, , in v16 (not 2016) this throws up a degenerative error, but it seems nowadays a change in programming means the part is converted to surface/sheet instead of throwing up an error.

There are some workarounds, depending on what tolerances you have, one would be move the small v shapes up very slightly, say 1/100th, so the bottom of the V's are not coincident. and try that (may need more than that).  a second workaround could be to, cut a longer groove and then fill in excess afterwards , there is a sight chance of getting a degenerative error, but it works for me.

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* April 25, 2018, 11:04:33 PM
#2
Andy,
thanks for your reply.
I have done a bit more investigating and agree that the problem is caused as you have stated. To make things worse, it appears that the subtraction may work in one instance, but then set up the resultant solid to fail during the next subtraction; even where the original conditions don't occur. The instances of the problem also appear to be quite random.

I have been told that similar problems exist in Lightwave, Maya and Google Sketchup.

I have followed your suggestion and extended the groove slightly and then added a filler piece back in. A bit tedious, but it gave me no problems. I made sure that I carefully inspected each groove from a number of different viewpoints. I have also saved it as a .STL file and checked it with MeshLab. All the grooves now appear to be properly formed.

Incidentally I upgraded from version 16 to TurboCad 2016 (version 23.2) in an attempt to solve the degenerative errors. I was getting these when attempting to subtract two objects with co-incident surfaces.

Regards,
Michael Milway.

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