CC command ??

[bcammack]

Geometry isn't my long suit, so perhaps this makes sense to everyone but me.

I have four nested ellipses to clear a rebate for an undermount sink. I use a 1/2" bit, so for each CC command, I reduce the diameter by 1", but I'm baffled by the necessity to reduce the Y-proportion by a small increment as well to preserve symmetry.

If I'm nesting the ellipses, reducing their primary diameter for each iteration, shouldn't the proportion stay the same? I mean, they're symmetrical copies of one another, just scaled down.

I'm sure this is another instance of logic being a way to go wrong with confidence...

[richards]

Brett,
What is the size of the cutout and the size of the additional Y-Axis correction?

[bcammack]

Here's the entire program:

[bcammack]

Found that the "Loop: &loopcount = &loopcount + 1" isn't proper syntax.

I do wish there was some sort of iteration structure in this language...

[richards]

That one got past me too. Is the syntax error because something other than a label is on the line? I use that same increment syntax all the time; however, it is on its own line, i.e.:

&count = 0
&maxcount = 10

LOOP:
&count = &count + 1
'
' do something here
'
IF &count < &maxcount THEN GOTO LOOP

So far, I haven't been able to find anything wrong with your code. I'm wondering whether the 'cutter flexing/gantry flexing gremlins' might be causing the small offset error? There are times that I get a 'hump' in a circle when I make an aggresive cut. The solution to the 'hump' problem has been to take shallower cuts and/or slow down the feed rate by 25% to 50%.

[bcammack]

I just was mystified that the Y axis proportion changed as I scaled down the ellipse. It is demonstrable in SB3 preview mode. Without changing the proportion the toolpaths of the ellipses are closer together at the top than the bottom.

[rob_bell]

It's odd that a cabinetmaker and I were discussing this exact problem yesterday regarding an oval tub.

I'm not completely sure that this is the root of your problem but in the end we determined that a scaled ellipse is an ellipse but the offset of an ellipse is not.


8631

[harold_weber]

Brett, the easiest way for me to explain what you are seeing is to look at your parameter &proportion. You want this to be the ratio of the maximum Y-dimension of the ellipse to the maximum X-dimension of the ellipse. Suppose the two values start out at:
max X dimension = 20
max Y dimension = 16
So your &proportion would be 16/20 = 0.8
Note that 16/20 is a fraction. From algebra, you keep the same value of a fraction if you MULTIPLY the top and bottom by the same number.
However, in your case, you want to SUBTRACT the same value from the top and bottom. If you subtract 1 from top and bottom, you get 15/19 = 0.7895, so your &proportion is now different. You can simply recalculate its required value this way for each subsequent tool path.

This all agrees with Robert's post above, I believe you really want to cut his left hand figure, which is not a true scaled ellipse.

One suggestion to make your life easier: If you use the ShopBot CP command instead of the CC command, you would not need to keep changing the value of &ypos, since the center of the ellipse is not changing.

[bcammack]

Thank you for the explanations.

I guess I wasn't expecting that an ellipse that is 1/2" inside the path of another ellipse would have a different aspect ratio than the first. It seems counter-intuitive to me.

I probably would end up with the CP command at some point. This is all a "voyage of discovery". I started out with a Part Wizard-generated program to clear the area within the outer ellipse and just truncated the code when it reached the inner point where the sink cutout began.

Since then I discovered that I could replace nearly three kilobytes of "M2" commands with essentially four "CC" commands. It suited because I could translate the same transition/start point from the thousands of "M2" commands.

Thanks again, everyone.

[bcammack]

Followup:

I seem to have hit upon the relationship between the proportion value and the diameter. If you subtract 1/4 the distance between the current ellipse and next ellipse from the current diameter and then divide the result by the current diameter, you get a factor that you can utilize to adjust the proportion to suit the smaller, nested ellipse.

The following code illustrates:
*********************************************** **
' Cut the rebate for the sink lip

J2,21.77,18.684
MZ,0.750
&increment = 0.92