Hi Mal

> Some of these cylinders will work with Boolean-Subtract
> but others either don't subtract at all or the base object
> is deleted rather than the cylinders.
>
> Is this a limitation of the geometry library or am I likely to
> be doing something wrong?

It's hard to say without examining the actual model file, but if the cylinder/tube type objects are crossing over each other then you're probably running into a limitation of the geometry library where it does not handle the intersection between those kind of shapes that cross at a kind of "crown point".

If you can use a circle of slightly larger or smaller radius for the tubes that cross over another one, that can help solve the problem.

Also another area of difficulty is if the "seam edge" of the tube runs right along the surface to be booleaned. One possible way to work around that problem is to actually rotate your circle around a bit so that the seam edge of the tube is not grazing right along the other surface.

Things like edges that barely coast along another surface or barely graze each other to meet at single points, stuff like that tends to be a lot more difficult calculations for the booleans to process.

If it is possible for you to post the model file that would help a lot in giving some more specific information.

- Michael