Best Fit

[AuRules]

Just wanted to ask for a little help on Best Fit Alignments. 30 years of programming and never used Best Fit at all. Always the old 321 alignment or Iterative alignment. So I going to use the part I have to see if I can understand how this works. I have a part 7 inches in diameter, 1 inch thick aluminum plate. I have 2 patterns of .250 holes one at 3 inch radius and the other at 2 inch radius. 18 holes per pattern at 20 degrees apart. I did a Best fit alignment on the inner 18 hole pattern and looked at my results and not quit sure how does it clock/align to check the outer pattern. Please forgive my typing and spelling skills. I just picked the inner 18 hole and clicked on best fit and I think there's more to this like level, clocking and origin. Thanks for any help or comments.

[tking]

Did you use sample hits (3) when probing the holes? Use least squares option?

[AuRules]

Did you use sample hits (3) when probing the holes? Use least squares option?

No I did not. Will the sample hits determine the level?

[Matthew D. Hoedeman]

No I did not. Will the sample hits determine the level?

no, BUT, you have to pay attention to the best fit options. Make sure that only ROTATE is selected, at least I ASSume you can only rotate, that the face (or base) is Z level/origin and that either a center bore out outer diameter is XY zero. ROTATE only, you will have to un-mark things to get it to do this. Then it SHOULD rotate around to minimize all the radial deviations as it were.

[AuRules]

no, BUT, you have to pay attention to the best fit options. Make sure that only ROTATE is selected, at least I ASSume you can only rotate, that the face (or base) is Z level/origin and that either a center bore out outer diameter is XY zero. ROTATE only, you will have to un-mark things to get it to do this. Then it SHOULD rotate around to minimize all the radial deviations as it were.

They do have a option of a trihedral to choose these options, so if I chose the 18 holes how does it know which hole to clock to.

[Matthew D. Hoedeman]

They do have a option of a trihedral to choose these options, so if I chose the 18 holes how does it know which hole to clock to.

it doesn't clock to a single hole, but to the averages of all the holes. If you have a specific hole that needs to be clocked, then you shouldn't be using a best-fit.

[AuRules]

it doesn't clock to a single hole, but to the averages of all the holes. If you have a specific hole that needs to be clocked, then you shouldn't be using a best-fit.

Been clicking different option and noticed it will not clock to a single hole as you are saying. But I will ASSume if I choose the 18 hole pattern its only for the origin.

[Matthew D. Hoedeman]

Been clicking different option and noticed it will not clock to a single hole as you are saying. But I will ASSume if I choose the 18 hole pattern its only for the origin.

that will depend on the options you select. translation (XY origin), rotation (spinning around your current XY origin) or even level. You can make it so it uses those holes for level, rotate & origin, but I'm thinking you only want rotation. but, if you are just testing, you can make it do what you want. But, to rotate only, Rz only with none of the others active (Rotate about Z). If you want it to also center the XY origin on them, you want the Tx and Ty as well.

[louisd]

So bestfit alignments are great, once you get the hang of them...

Be cautious however. If you have two sets of 18 holes and they are called out to the same datums (with the tertiary open), they need to be simultaneously evaluated.
You can't bestfit rotate-align each pattern to itself, you'd have to construct a bestfit alignment to all 36 holes, THEN report your locations.

[AuRules]

I will try a few options, learned a bit just from the comments above thanks to all.

[AuRules]

Does this look half way correct

A4 =ALIGNMENT/START,RECALL:A3,LIST=YES
ALIGNMENT/LEVEL,ZPLUS,PLN1
ALIGNMENT/TRANS,ZAXIS,PLN1
ALIGNMENT/ROTATE_CIRCLE,XPLUS,TO,CIR10,AND,CIR1,ABOUT,ZPLUS
ALIGNMENT/BF2D,ZPLUS,LEAST_SQR,CREATE WEIGHTS=NO,TRANSONLY,USE SCALING=NO,0.0032,-0.0006,0,0
ITERATEANDREPIERCECAD=NO
Deviation Threshold=0.0005
SHOWALLINPUTS=NO,SHOWALLPARAMS=NO
ALIGNMENT/END

[Matthew D. Hoedeman]

Does this look half way correct

A4 =ALIGNMENT/START,RECALL:A3,LIST=YES
ALIGNMENT/LEVEL,ZPLUS,PLN1
ALIGNMENT/TRANS,ZAXIS,PLN1
ALIGNMENT/ROTATE_CIRCLE,XPLUS,TO,CIR10,AND,CIR1,ABOUT,ZPLUS
ALIGNMENT/BF2D,ZPLUS,LEAST_SQR,CREATE WEIGHTS=NO,TRANSONLY,USE SCALING=NO,0.0032,-0.0006,0,0
ITERATEANDREPIERCECAD=NO
Deviation Threshold=0.0005
SHOWALLINPUTS=NO,SHOWALLPARAMS=NO
ALIGNMENT/END

I would do TWO alignments, your 3-2-1 (and done) then a new alignment just doing the best fit.

[Roberto]

What is the GD&T callout on the drawing?

[AuRules]

There's no callout out on the blueprint, there's actually no blueprint. It's a part that our setup engineer ran to check the rotation of a Mazak CNC. I ran the part and in the reports I was playing with the alignments and notice a difference in the dimensions depending on which alignment I use. So I'm stumped on that a little but trying to understand the Bestfit versus 321 and which one is correct.

[AuRules]

I would do TWO alignments, your 3-2-1 (and done) then a new alignment just doing the best fit.

Will do, Thanks Matt

[tking]

What is the GD&T callout on the drawing?

I'd ask the setup gent how he wants it. What origin, what rotational alignment (what features) and which one to start the clocking. I don't think a best fit is desired for his purpose. A solid 3-2-1 with intentionally selected features would more likely be most informative. You're not trying to make a part look as good as possible (best fit) you're evaluating a machining center, 3-2-1 only I think.

[AuRules]

That's kinda of what I told him but that's what he wanted.