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Question about the small arc method

constadin
Hello all!

Please let me know if I am doing something wrong following the small arc method.

Btw, I am using vector points and auto circles with NO CAD always in DCC mode. Let's also assume that I am measuring in Z plane.

1) I am measuring the small arc with a fixed radius auto circle.
2) I am setting my origin XY to this circle.
3) Just to be sure I am repeating with a new auto circle fixed radius and realign to it.
4) I am taking 3 vector points in polar coordinates to be precise.
5) I am reporting the PR of the points.

This morning I had a 80deg on an arc R2.8mm to measure. I used this method and I was getting repeatable results at 2.80. The issue is that the radius has been measured to be 2.75mm (verified with 2 other methods also).

I also tried to change the fixed rad value based on the R measurement I had performed before with least squares (around 2.75 non repeatable value) and I was getting results close to that value (the least square value).

With this method, does the fixed Radius value provided alter the centre that is being found? Doesn't this beat the purpose of using this method?

Thanks in advance!
Parents
  • : 

    A5 =ALIGNMENT/START,RECALL:A4,LIST=YES
ALIGNMENT/LEVEL,XPLUS,PLN_0
ALIGNMENT/ROTATE,YMINUS,TO,CYL_2_8,ABOUT,XPLUS
ALIGNMENT/TRANS,YAXIS,CER_7_68
ALIGNMENT/TRANS,ZAXIS,CER_7_68
ALIGNMENT/TRANS,XAXIS,PLN_0
ALIGNMENT/END
R2_8A =FEAT/CONTACT/CIRCLE/DEFAULT,CARTESIAN,IN,FIXED_RAD
THEO/<-4.96,-3.1825,0>,<0,1,0>,5.6
ACTL/<-4.965,-3.1825,-0.0043>,<0,1,0>,5.5215
TARG/<-4.96,-3.1825,0>,<0,1,0>
START ANG=35,END ANG=-35
ANGLE VEC=<1,0,0>
DIRECTION=CW
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
A8 =ALIGNMENT/START,RECALL:A5,LIST=YES
ALIGNMENT/TRANS,XAXIS,R2_8A
ALIGNMENT/TRANS,ZAXIS,R2_8A
ALIGNMENT/END
CER10 =FEAT/CONTACT/CIRCLE/DEFAULT,CARTESIAN,IN,FIXED_RAD
THEO/<0,-3.1825,0>,<0,1,0>,5.6
ACTL/<0.0015,-3.1825,-0.0013>,<0,1,0>,5.5215
TARG/<0,-3.1825,0>,<0,1,0>
START ANG=35,END ANG=-35
ANGLE VEC=<1,0,0>
DIRECTION=CW
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
A6 =ALIGNMENT/START,RECALL:A8,LIST=YES
ALIGNMENT/TRANS,XAXIS,CER10
ALIGNMENT/TRANS,ZAXIS,CER10
ALIGNMENT/END
PNT1 =FEAT/CONTACT/VECTOR POINT/DEFAULT,POLAR
THEO/<2.8,200,-3.1825>,<-0.9396926,0,0.3420201>
ACTL/<2.7627,199.9842,-3.1814>,<-0.9396926,0,0.3420201>
TARG/<2.8,200,-3.1825>,<-0.9396926,0,0.3420201>
SNAP=NO
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
PNT2 =FEAT/CONTACT/VECTOR POINT/DEFAULT,POLAR
THEO/<2.8,180,-3.1825>,<-1,0,0>
ACTL/<2.7614,180.1517,-3.1805>,<-1,0,0>
TARG/<2.8,180,-3.1825>,<-1,0,0>
SNAP=NO
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
PNT3 =FEAT/CONTACT/VECTOR POINT/DEFAULT,POLAR
THEO/<2.8,160,-3.1825>,<-0.9396926,0,-0.3420202>
ACTL/<2.7588,159.9775,-3.1817>,<-0.9396926,0,-0.3420202>
TARG/<2.8,160,-3.1825>,<-0.9396926,0,-0.3420202>
SNAP=NO
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO


    After that I am reporting the PR of the points. Btw this is for info only (leftover code after 1 week old testing), normally the ACTL/< was around the radius I had previously defined the fixed rad value. Is there anything wrong with the method I followed? ​
Reply
  • : 

    A5 =ALIGNMENT/START,RECALL:A4,LIST=YES
ALIGNMENT/LEVEL,XPLUS,PLN_0
ALIGNMENT/ROTATE,YMINUS,TO,CYL_2_8,ABOUT,XPLUS
ALIGNMENT/TRANS,YAXIS,CER_7_68
ALIGNMENT/TRANS,ZAXIS,CER_7_68
ALIGNMENT/TRANS,XAXIS,PLN_0
ALIGNMENT/END
R2_8A =FEAT/CONTACT/CIRCLE/DEFAULT,CARTESIAN,IN,FIXED_RAD
THEO/<-4.96,-3.1825,0>,<0,1,0>,5.6
ACTL/<-4.965,-3.1825,-0.0043>,<0,1,0>,5.5215
TARG/<-4.96,-3.1825,0>,<0,1,0>
START ANG=35,END ANG=-35
ANGLE VEC=<1,0,0>
DIRECTION=CW
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
A8 =ALIGNMENT/START,RECALL:A5,LIST=YES
ALIGNMENT/TRANS,XAXIS,R2_8A
ALIGNMENT/TRANS,ZAXIS,R2_8A
ALIGNMENT/END
CER10 =FEAT/CONTACT/CIRCLE/DEFAULT,CARTESIAN,IN,FIXED_RAD
THEO/<0,-3.1825,0>,<0,1,0>,5.6
ACTL/<0.0015,-3.1825,-0.0013>,<0,1,0>,5.5215
TARG/<0,-3.1825,0>,<0,1,0>
START ANG=35,END ANG=-35
ANGLE VEC=<1,0,0>
DIRECTION=CW
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
A6 =ALIGNMENT/START,RECALL:A8,LIST=YES
ALIGNMENT/TRANS,XAXIS,CER10
ALIGNMENT/TRANS,ZAXIS,CER10
ALIGNMENT/END
PNT1 =FEAT/CONTACT/VECTOR POINT/DEFAULT,POLAR
THEO/<2.8,200,-3.1825>,<-0.9396926,0,0.3420201>
ACTL/<2.7627,199.9842,-3.1814>,<-0.9396926,0,0.3420201>
TARG/<2.8,200,-3.1825>,<-0.9396926,0,0.3420201>
SNAP=NO
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
PNT2 =FEAT/CONTACT/VECTOR POINT/DEFAULT,POLAR
THEO/<2.8,180,-3.1825>,<-1,0,0>
ACTL/<2.7614,180.1517,-3.1805>,<-1,0,0>
TARG/<2.8,180,-3.1825>,<-1,0,0>
SNAP=NO
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO
PNT3 =FEAT/CONTACT/VECTOR POINT/DEFAULT,POLAR
THEO/<2.8,160,-3.1825>,<-0.9396926,0,-0.3420202>
ACTL/<2.7588,159.9775,-3.1817>,<-0.9396926,0,-0.3420202>
TARG/<2.8,160,-3.1825>,<-0.9396926,0,-0.3420202>
SNAP=NO
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO


    After that I am reporting the PR of the points. Btw this is for info only (leftover code after 1 week old testing), normally the ACTL/< was around the radius I had previously defined the fixed rad value. Is there anything wrong with the method I followed? ​
Children
  • in reply to constadin
    Why do 2 of your 3 points have a K vector value?
    Is your cylinder actually a cone?
    Is it non-perpendicular to your level plane?

    -.34 = ~40 degree of cosine error. This could absolutely produce half a mm of resultant error.
  • in reply to constadin
    The circles are at the J plane that's why (J remains 0) Slight smile
    Before giving the code I wanted to simplify the request and just focus on the method and identify errors that I potentially did. This is why I said " Let's also assume that I am measuring in Z plane"

    Also the part is really small and does not really have a plane normal to the cylinder. It is a hollow tube of a few millimeters of diameter that has a offaxis semi cylindrical area cut off from it normal to its main axis. Imagine an extruded donut, grabing 4 points every 90 degs and pulling the north and south point while pushing the other 2.

    In any case, the method is what is I am after for, for future references also Slight smile
  • in reply to constadin
    are you sure these vector values accurately represent being tangent to the circle, when it takes the hit? Your vector says you are measuring those two hits at 40 degrees from the middle hit, however your PA's are only spaced 20 degrees from the middle hit at 180. Vector error is what this method should be mitigating.

    You can reproduce the first and third points by doing a copy/paste with pattern with an angle of +20 then an angle of -20, of the middle hit.
    or you can rotate your alignment 20 degrees each way and keep middle point's coords and normal (zero) vector.
  • in reply to constadin
    I am not sure what you mean by PA being different in pnt1 and pnt3 than my vectors. ACOS 0.9396926 is 20 degs. I am taking the points at -20 degs, 0 degs and 20 degs. The method of doing that (If I remember correctly) was with c/p pattern as you suggested. AFAIAC, there is no error here Slight smile
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