T-value calculations

[dlewis]

How does the T-value do its calculation? I notice that some T-values don't always trig out (pythagorean theorem) for the part axis effected. Is the T-value usually always a right angle trig. calculation of the X & Y values or can this value be affected by negatives on the surface of the part? *(eg.. deviation values for a point are: T = 1.130, X = .069, Y = -.959, Z = -1.399).

[Matthew D. Hoedeman]

If you use SNAP POINTS for your vector points, they will work out absolutely perfectly (sqrt(xdev^2+ydev^2+zdev^2)). If you do NOT use snap points, then the drift that EVERY CMM has, will throw this calculation off. Same thing, kind of, for edge points. If you are using a surface sample (like you should), then the values will not trig out UNLESS you use the GAP ONLY option for dimensioning the edge point, this will take out the surface deviation of the part, simulating the SNAP POINT option of the vector point. If you are reporting the "S" value as well on the edge points, you can NOT use GAP ONLY.

[Scott Appleyard]

I've always used the (Xdev*I+Ydev*J+Zdev*K) calculation for manually findind the 'T' value. This is the method used by the Canadian B&S distributer gurus that was passed down to me when I worked for them. I've never had a problem with incorrect calculations.

[\v/inston]

If you use SNAP POINTS for your vector points, they will work out absolutely perfectly (sqrt(xdev^2+ydev^2+zdev^2)). If you do NOT use snap points, then the drift that EVERY CMM has, will throw this calculation off. Same thing, kind of, for edge points. If you are using a surface sample (like you should), then the values will not trig out UNLESS you use the GAP ONLY option for dimensioning the edge point, this will take out the surface deviation of the part, simulating the SNAP POINT option of the vector point. If you are reporting the "S" value as well on the edge points, you can NOT use GAP ONLY.

they should make it default that way then give you the option to turn it off.

[Matthew D. Hoedeman]

Both are correct formula's for the radius of a sphere (which is what the T value is). One requires you to use SQRT, the other requires you to know the vector of each axis. However, BOTH formula's will give the exact same answer when used on SNAP POINTS.

The benefit to using SNAP POINTS is really very basic, ALL CMM's have some amount of drift, meaning HOW CLOSE does the CMM actually stay on the exact approach vector of the point. For a real simple example, use a point with a vector of 0,0,1. The nominals are X100, Y200, and Z300. With SNAP POINT on, the X and Y will NOT show any deviaiton, and they should NOT since they have NO vector. However, without SNAP POINT, your machine may actually touch at X100.012, Y199.990 and Z whatever. This will show deviations in the X and Y axis and there should NOT be any, it is the 'fault' of the machine. However, using the method that uses the vectors, since the X and Y vectors are ZERO, anything you multiply by those vectors will be zero and you are left with ONLY the Z deviation (1 x ZDEV) for the T answer.

Now, the BEST reason for using the SNAP POINT is your customer. What do you tell him when he comes in and asks you WHY there is an X and Y deviation for a point that is square to the Z axis? SNAP POINT eliminates this problem. The point is 'snapped' onto the perfect vector line, normal to the vector, then dimensioned. It WILL be directly on the perfect vector line of the point and the deviations will then work in BOTH equations.

[Matthew D. Hoedeman]

It used to be default (PRE V3.7), but NOW it is a major pain in the rump, all these years, it has been default and those of us who always used it now have to try to remember to turn it BACK ON when we are trying to program under a deadline and are in a HUGE hurry. It is even worse than if they changed which side the steering wheel is on in cars. It is NOT something HUGE and VISABLE, you have to look for something that you never looked for before. it SUX!

[dlewis]

snap enable

Is there a quick way to turn all of the snap points on in a program, so that I don't have to F8 each individual form? Kind of like replace all in the editor?

[Matthew D. Hoedeman]

Is there a quick way to turn all of the snap points on in a program, so that I don't have to F8 each individual form? Kind of like replace all in the editor?

I don't think so, at least I have never found a way to do it.

Kind of like the snap point option, eh? I have always liked it, at least since it was added to the software.

[JoeRZ]

snap = yes

if program's already written...
you can try REPLACE under edit in edit window and replace snap = no with snap = yes. Should replace all matches.

[Matthew D. Hoedeman]

I don't know if that will work, but you can try it, it usually will not work with any part of the code that is un-changable, like the "SNAP =" part.

[craiger_ny]

This is something I don't use so I know nothing about. What if your surface vectors in all 3 axis (not 0 or 1 on I, J, or K), does snap have an algorythm for this? Or would it come from the cad (find nom) for the area where the hit took place? Is snap workplane dependent?

Craig

[JoeRZ]

Matthew - you're right... can't use replace because it can only replace one field at a time and snap = yes uses two - "snap =" is one and "yes" is another.
guess no quick fix but will look.

[rangerboat72]

if program's already written...
you can try REPLACE under edit in edit window and replace snap = no with snap = yes. Should replace all matches.

Global Update will change all of them to SNAP=YES automatically. I use it quite frequently but I've forgot where I got it from. I'll look for it and let ya' all know where to get it from. It is awesome for something like this especially when you have a couple hundred features to update or change.

[rangerboat72]

Well, I thought I would be able to provide you with a link to the website where I got the GLOBAL UPDATE script from but I was unable to locate it. Hopefully the attachment will work. If not please let me know. The GLOBAL UPDATE script is very handy for several PC-DMIS features. You'll love it once you use it a couple of times... It has saved me a ton of time...

[Matthew D. Hoedeman]

What SNAP does is really quite simple:

1) Measures the point
2) Constructs a plane, perp. to vector, through the ACTUAL point
3) Intersects the vector and the perp. plane and reports THAT XYZ.

This give you a point exactly on the nominal vector line, thus giving you a result that WILL have the correct, exact XYZ deviations simulating a machine that can stay perfectly on the correct, absolute approach vector. The actual vector does not matter, 0,0,1 or 0.102, .923, 0.356 or what ever.

IT is NOT workplane dependant.

It does NOT do a find-noms, the noms stay the same as the original programmed feature.