Cylinder measurements

[djayne]

Can anyone tell me the significance for auto cylinders alternating clockwise to counterclockwise on the next row? Should I apply the same measurement method when manually measuring a cylinder? Or is the just the alogrithm that the software uses? I know that our LK CMM measures in the same method so maybe I should be measuring the same manually?

[craiger_ny]

I can't belive it matters. I edit hits on nealy every feature I measure to keep moves between features to a minimum.

Craig

[#2#]

The software just does this. It doesn't matter.

On your manual hit cylinders... if after you create a cylinder put your cursor on the cylinder feature (top couple of lines) and press f9 it will bring up a feature edit window. If you change the number of hits in the appropriate window and then check the "regenerate hits box" and click ok it will ask you if it is ok to equally space the hits. If you click yes it will regenerate the hits equally spaced at the levels (only 2 with v 3.7 but user input for v4.0 I believe) you took your manual hits. Then when you run this feature it will run as described for an auto cylinder.

[Wes Cisco]

What the instructor said

Actually it does matter a little. The thing is that when pc-dmis evaluates the hit data and tries to construct a cylinder out of it, it often overlooks the obvious/correct possibility and gives you something out of the 8th dimension instead. (See the note in the help files about cylinders & number of hits). To help the software not do this, the autocylinder alternates between cw & ccw. When I took the basic class about a year ago the instructor told us we should take manual cylinders in the same way for this reason and also try to use more than 5 hits per level. He also said odd numbers of hits per level work better than even. You can disregard all of these guidelines and still get the correct cylinder sometimes. Sometimes you can follow all the guidlines and still not get the cylinder you need. Do you think the pc-dmis gods care? HTH

[djayne]

I'll have to experiment with this. For some reason I've been trying to mock the CMM during manual measurements anyway just because I wasn't sure why it does it.

[Bill **********]

Think about it in a physical sense. Why add move points and go back to the other side when you can drop up/down, take a row and repeat? It's just more effective programming.

Bill

[#2#]

I am guessing that Wes is refering to the problem of PCDMIS guessing that the vector for the cylinder is 90º to the actual vector. To solve this when taking manual hits try taking hits on more than two levels. Then there is no doubt as to the vector.

When making a manual cylinder I always take only 8 hits unless the vector comes out wrong and then I do 12 (4 on 3 levels). Then I f9 and enter the number of hits I want (odd number per level) and regenerate hits as described above. Using f9 also allows you to tweak the vector if the cylinder is supposed to be oriented in a perfect manner but the manual measured cylinder isn't quite perfect.

[cmmguy]

[IMG]http://www.*******.com/misc/bsflag.gif[/IMG] I think that I have to call BS on the explanations for the CW vs CCW.

The answer is more obvious when you measure a partial cylinder. If you didn't use a CW and then a CCW, you would be potentially colliding with obsticals getting back to the starting point for each point level. The problem gets worse for OD cylinders. So to guarantee that you don't hit anything, you go back the same way that you came from...

[craiger_ny]

......Using f9 also allows you to tweak the vector if the cylinder is supposed to be oriented in a perfect manner but the manual measured cylinder isn't quite perfect.

If you measure manually and want to edit F9 is great but if you are like me and do the bulk of it in the command window then you will get a familiar anoiance that Goodluck is talking about. To get around it edit your vectors first or you will find yourself going back and re-editing your noms.

Craig

[djayne]

That makes pretty simple sense of it. I could save myself alot of headaches wondering why if I'd just ask someone!

[Don Ruggieri]

Speed, and safety

PC-DMIS measures in alternating directions for speed and safety. Its faster not to have to go around to to the start point, especially if they are only partial cylinder walls. It also improves the odds that you won't hit anything on the way back, if you didn't hit something the first time around.

As far as fitting, the worst case is when you measure two levels with 4 equally spaced hits. There are multiple solutions. There are numerous ways to avoid this; more levels, more hits, offset levels, etc. The direction (CCW vs CW) does not directly improve nor effect the solution.

If you are learning a feature for the first time (Learn mode, guess mode, etc.) it is best to seed the cylinder with 3 hits at one level, then at least two more at one or more added levels. If it is already in the edit window and you are just executing it manually, as long as you spread the hits to accurately reflect the feature surface, it matters less what direction, order, or pattern they are taken in.

[#2#]

My best guess as to why it reverses direction... Someone thought it would look cool and programmed it like this to impress someone.

[djayne]

Thanks Don - That makes alot of sense.