Anomalous Circle Trim

If I draw a circle, then a diameter, then try to trim away one half - it mostly doesn't work.

I solve the problem by adding a chord in the half to be trimmed, trim the circle to that, then trim the small arcs left to the diameter.

My problem is, I frequently get the required half erased when I attempt to remove one of the short arcs.
It is as though the machine remembers what I was trying to do, and tries to help out.

I say anomalous because I am not sure it always happens as described, just often enough to be annoying.
I frequently get some sense by undo, change the reference point and zoom in a bit.
I always get there in the end.

But, is there a sure-fire way to do this sort of activity ?

John.

Today, I cannot reproduce the problem.

But, it is real and has caused numerous circuitous approaches to achieve the desired result in real drawings being developed.
There has to be some subtle state or condition that is sometimes present.
Still looking for that anomalous condition.
Trying combinations of grid size, zoom and snap.

One aspect is that sometimes, in real drawings as compared to simple tests, I get already erased arcs re-appearing.
Does this prompt any suggestions ?

John

Well, some quick tests gets some cases reproducing the anomalous behaviour.

I think it gets down to symmetry.
If you try to trim a half-circle and select a point virtually at the mid-point of the retained arc, the machine refuses.
If select close to the trim diameter, works as expected.

But, if then go to the other end of the arc and select again, the erased half-circle re-appears !

So, my plan for new drawings is:
 - do not trim circles with diameters, only chords,
 - when trimming, select the retained arc close to the trimming chord,
 - avoid repetitive trim steps, always ESC, ESC, then do the next requirement,

Still open to alternative suggestions,
John

John,

I can confirm your observations, from time to time I'm affected by this issues too.
Sadly, the impelling power to battle this is narrowed by another workaround that works well for me.

Normally, when I trim circles, I already have other entities or it's easy to construct lines through the trimming points. Then I use the divide tool with intersection snap to split the circle/arc and delete the parts I don't need.

Armin

I have played around a bit and may have found an explanation.
I found no difference in behaviour between diameter and chord. I discovered that when you select the circle to be trimmed and nothing seems to happen, in fact something has happened, the circle has been cut at one point, and now is an arc (properties!), with start point same as endpoint, still looking like a circle. I suspected it might have something to do with the drawing direction of the circle. Further testing revealed that if you click the to be retained part of the circle at the counterclockwise end near the limiting entity, you get the desired result. Clicking at the clockwise end just cuts up the circle.  

Armon,
Thanks for the tip on divide (and conquer), a handy way to do it.

Dellus,
Having lots of fun experimenting with multiple chords, get all sorts of interesting results, the CW and CCW aspects, the circle splitting up into an arc, and then re-joining, all fascinating.
Apparent memory of previous actions is especially interesting, and will be investigated when I get some 'play' time.

But, I now know it is not just me or my machine, the situation can be managed, and I can get on with my drawings.
Thanks,
John