# Precise Subtraction of Solids in SolidWorks

I need to draw a 120mm diameter cylinder, 30mm thick, with 10mm diameter holes protruding 50mm from the periphery into the bulk (normal or at an angle to the periphery).

This task I’m trying to accomplish by subtracting small solid cylinders from the main cylinder. However, I’m finding great difficulty in, first, creating a part drawing consisting of all these 12 small cylinders (to be subtracted from the main cylinder) and, second, to position them precisely when overlaying them over the main cylinder for subtraction. The way I understand it I have to first draw a part consisting of all these 12 small cylinders but each one of them is at a different angle which doesn’t work with extrude. How can this be done? Then, even if I manage to draw this part, consisting of the 12 small cylinders, when I invoke it to have it subtracted from the main cylinder the positioning appears arbitrary and I can’t seem to be able to place it precisely co-axially. Do you have any suggestion how this cylindrical part can be drawn? Thanks in advance for your help.
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Author Commented:
Now I’ve found the solution and want to describe it for future reference and for others who may be interested in it.

First, how one draws cylinders lying in one plane at angles smaller that 90deg is still unclear to me, so I dropped that idea. Therefore, instead of subtracting smaller cylinders from the large cylinder, I drew a 12 sided Polygon:

File->New->Part->Extruded Boss/Base (never mind we’re in Sketch and not in Features)->Choose Front Plane->Polygon (w/ 12 sides)->click on the check mark

That was Step1 of the project.

It is easy to make holes on each of the 12 sides after extruding it (Exit Sketch->Features) by using the tool Assembly Features->Simple Hole (if you don’t see it go to Tools->Customize->Commands and drag it to the leftmost strip).

More complicated is to draw holes which are not normal to the curved surface of the large cylinder. For that purpose, after drawing the Polygon, before extruding it, a polysided figure has to be drawn. This is Step2 of the project:

Line (from every vertex (corner))->CTRL-click on that line and on side (edge)->Add Relation->Perpendicular->Click on line to choose length->Line (from previous vertex to end of line)->Click on the side (edge)->delete

Repeat this 11 more times (maybe there are easier ways to do it but that’s for the future). The intermediate clicking on the check marks to confirm operation is omitted because it’s evident.

Step3 is the extrusion of this more complex polygon. After extrusion it will form plain surfaces at an angle to the normal to these surfaces drawn from the axis:

Exit Sketch->Click on check mark to confirm Extrude (after defining the thickness, of course)->Click on the face of the obtained Polyhedron where you wan th the hole->Chose Normal To tool (to have the face parallel to the monitor plane)->Sketch->Line (draw a diagonal)->OK->Right-Click on diagonal->Split Entities->Point->Features-Click again on Point at the middle of diagonal->Assembly Features->Simple Hole->Choose diameter and depth->OK

Repeat this 11 more times.

Here we use the main property of the tool Assembly Features ->Simple Hole which is that such hole can only be drawn from flat surface, at that only normal to that surface. Therefore, if that surface is at an angle to another surface the hole through it, being normal to the former, will also be at an angle to the latter surface. Once this trick is understood everything else is trivial, although slightly time consuming, as seen.

After all the holes are “drilled” we may subtract form that complex part a ring part with internal diameter just right to have finally a cylinder (instead of the body with that complex form). That task is trivial so it will not be described it here in detail.
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