Use of Offset Tool Results in Incorrect Dimensions


#1

So, I’m attempting to create an 18" wide column with a 30" footing. The footing consists of three consecutive discs, each one being 4" smaller in diameter than the one below it.

To do this, I:

  • Created a circle with a 15" radius using the Circle tool (15" radius = 30 dia.).
  • Then using the push/pull tool I raised the circle to create a 3" high disc.
  • Afterwards, I used the Offset Tool to create a second disc 2" smaller on top of the disc I just created and repeated the process outlined above.
  • After repeating this process four times I thought I would end up with a column 18" in diameter at the top of my structure. However, this is not the case. Instead I have ended up with a column that is 17 7/8" in diameter.
    Not only that, but after closer scrutiny, I discover that the two discs between the base and the column are off too (short by 1/16" each respectively).

I attempted to compensate for this 1/16" deduction by offsetting each subsequent disc by 1 15/16" instead of the 2" offset that I had previously used. However, this had the opposite effect (see screenshot attached: image on the left is the first attempted column and the one on the right is the second).

What gives?


#2

I suspect you are not being careful to click on one of the vertices of the circle when starting the offset. Because SketchUp represents circles using polygons, only the vertices are on the theoretical circle. I had no problem when I was careful:


#3

Hmmmmm, could be the problem. Thanks for the suggestion.


#4

Of course, you could also draw another circle each time instead of using the offset tool…


#5

Apparently I’m not holding my mouth right when I do it. How do I find these vertices that you mentioned?

I mean, I select the offset tool, I position my cursor on the plane that I wish to offset and click (as instructed by the instructor window). If I move my cursor over the edge of the object (the circular line) I get a red dot and text that reads “on edge”. Occasionally, I get a green dot and text that reads “endpoint” but, even then I get the same results (oh yes, and I am typing my dimensions into the "measurements space down at the lower right of the window) after measuring.


#6

@KlammSS, the dimension you input is the very distance between two corresponding segments of both circles, i.e. the parent and the child.
Since SketchUp uses segmented circles instead of true circles, this offset won’t result in subtracting or adding the input to the radius you started from.

The new radius even depends on the number of segments you started with.
Offsetting a 12 segmented circle, R6000mm by 1000mm inward results in R1964,723820mm
Offsetting a 24 segmented circle, R6000mm by 1000mm inward results in R1991,371039mm

The more segments you start with, the closer the new radius will be to when they were true circles.

The offset tool is correct in offsetting a bunch of connected planar edges as is the “circle”.

I may be missing what trick @slbaumgartner is using on vertices of his circle though.

p.s. you can adjust the incorrect radius in ‘Entity Info’ for the selected child circle before you continue manipulating its face with Push/Pull etc. Right after the offset operation.


#7

There is no trick. Of course, you knew that. :wink:

@KlammSS
The nice round dimension numbers are the misleading result of working with low precision Architectural units.
Entity Info will show a ~ before the radius while the Dimension tool will not.
If you want precision, work in 3-place decimals and keep Entity Info open as you model.


#8

HoHo George, not so sure about that. I was on SU14 at the time I wrote that.
It could have been that SU16 does have the ability to distinguish between pulling a segment or pulling a vertex with the ‘Offset’ tool. I couldn’t check.
It’s like the new polygon tool in SU16: you can decide to either appply its inner or its outer circle being used when you draw the polygon.

I only suspect that offset is still the same, i.e. always applied on segments of circles.


#9

Just done a quick experiment in SU 2016.

Even if I start an offset to a 24"R circle by clicking on a cardinal point (say on the red axis) the dotted line that tracks the mouse moves perpendicular to one of the edges, not along the red axis.

Even if I type 2" in the offset measurement box, the offset circle is reported by Entity Info as a circle with an approximate radius of 22", not an exact one. In earlier versions of SU, the offset wasn’t even a circle, just a bunch of edges in a curve, if I remember correctly.

Looks to me as if SU is offsetting perpendicular to an edge, not radially, in this instance, so the radius is off by the cosine of the angle subtended by an edge from the centre - or something like that.

[Edit] The upload looks fine in the post preview, but isn’t showing properly in the forum thread. What’s happened? Pasted in from a screenshot to the clipboard.

[Edit 2] Saved screenshot file first, then dragged and dropped in. That worked.


#10

@john_mcclenahan, just as I suspected, no extention in options to choose from with offsetting circles or arcs. Thank you for confirming.


#11

That would be an Eyeball inference.
The square red inference point pictured is an On Edge inference, not an Endpoint inference.


#12

It lit up briefly on an endoint inference, but I couldn’t get it to hold it.

Maybe I should try again.


#13

No matter what / (or how hard) you try, you will always end up with that black dotted line perpendicular to one of both (beside the vertex (green inference) you picked) edges of the original circle. It shows the distance for your input. No exact increment for the original radius is possible without using ‘Entity Info’. Maybe they will be implemented as a second option some day when offsetting some segmented curve that has a recognized radius (SketchUp arc and circle).


#14

The offset tool is unfortunately not “geometry-aware” so it doesn’t recognize circles or arcs. It just offsets edges, creating new edges that are parallel to the originals, with endpoints perpendicular ot the original. That is why an offset arc ceases to be an arc, as all its segments are not the same length, for instance.

Anssi