# Help me understand the "Follow Me" tool better

I need a “Circular Cutoff Tool” (for a automatic lathe) that I’m designing in Sketchup (Pro). Here’s a view of a similar design:

The face that’s colored yellow is critical to properly machining the part and is the controlling surface for the tool. The face is 1/4" below the centerline of the hole (which is 5/16" diameter). The radial dimension from the center of the hole to the outermost tip of the colored face is 1.5". In order for a machinist to make the tool on (a different) lathe, I need to provide dimensions of the profile that is defined by a section that is positioned about the center of the hole as shown below:

Obviously (or maybe not!), the geometry of the yellow face of the first image above (the “working” portion of the cutoff tool) is slightly different from that of the section cut.

I could work through all of the messy geometry/trig to evaluate the dimensions of the section cut but I think sketchup can do the hard work more easily.

Here’s an animate GIF of my attempt to do so:

I first created a properly dimensioned face for the working portion of the tool and then create a circle whose center is properly oriented for mounting into the machine.

I select the circle that represents the hole (bore) of the tool and attempt to use the “Follow Me” tool using the face. You’ll see that although the surface is extruded around the circle, its dimensions also change (thus making the result unusable).

The SKP file is attached below.

Any ideas on how I can extrude the face about the circle while maintaining the dimensions and without changing the geometry of the cutting surface?

Example.skp (85.7 KB)

The key thing is that the profile must be positioned perpendicular to the first segment in the path and it’ll end perpendicular to the last segment. If the profile isn’t perpendicular to the first segment, it’ll be projected (not rotated) which is why you get the distortion.

If you know what the profile dimensions are exactly, I’d just draw it and not try too hard to get Follow Me to make the 3D version exactly. SketchUp’s really not the right tool for the job if you need to extract the exact profile after running Follow Me. As you know, SketchUp approximates circles with multi-sided polygons. You can get very close by using a very large number of segments in the circle but it’ll never be exact. How close does it have to be?

Surprisingly, the tolerance on the dimensions aren’t too critical for the part I’m making. The current method used for the machine that will use this tool is not analytical – it’s a table lookup of less than 20 values (interpolation points). In this case, Sketchup is probably able to do a bit better than using the “old school” method.

Using Sketchup also allows me to provide some pretty nice detailed drawings (via Layout) of not only the part I’m making but all of the tooling necessary to make it.

Using your description, I drew some circular arcs from the center line of the hole that span from the various points on the working surface to a plane that is perpendicular to the path. I then connected these points to form a new face to extrude. Here’s the result: