Concentricity Calculation

[DrBudman]

Just curious if some can help me understand the concentricity calculation. I have a part that I have to check the concentricity of two diameters. The first diameter (ID1) is also my origin and is measured with tip 90,-90, the second diameter (ID5) is measured with tip 90,90. My question is, when I dimesion these using ID1 as my first feature and ID5 as my second feature I get .0049, but when I dimension using ID5 as my first feature and ID1 as my second feature I get .0009 (which is the correct reading). I understand with other calculations like parallelism to make sure to pick them in the correct order so as to not magnify the error, but with concentricity I would think this would not mater.

Don't make fun now ... I'm the guy with ver 3.6 , I hope your not going to tell me thats the problem.... I hope I'm just not understanding how it calculates them.

Thanks

[BSchwark]

The second feature entered should be your Datum feature. The order is important. Do not know how the math is working.

BS

[Scott Appleyard]

When dimensioning Parallelism, Perpendicularity, Concentricity, Coaxiality, Angularity or Runout, alway pick the feature 1st and Datum 2nd. PC-DMIS uses the same basic principle for all these types of dimensions. When dimensioning concentricity with a callout of |CONC|0.5|B| for example, you will select your feature first and the datum second. PC-DMIS will create a Theoretical tolerance zone 0.5 in width that is perfectly concentric to the Datum feature. Pc-DMIS will try to 'contain' the measured feature within the tolerance zone. The actual width of zone needed to 'contain' your measured feature will be reported.
HTH!

[DrBudman]

Still out to lunch here

I'm understanding the reasoning of making sure to select the datum 2nd as required for other callouts. For example, parallelism - if I have a plane 10" in area and the second 2" in area, the callout order is critical as it will magnify the error depending on the size of the feature, but in concentricity, is it not using the center point of each circle to find the error? The center never changes in size and should not change the error no matter which one is first. And I am measuring both features aligned to the same plane feature.

Thanks... I'll get this sooner or later...... I've been programming CMM's for a long time, but I'm only about 1-1/2 years into PC-Dmis... the more I use it, the less I find out I know about it , but thanks to you guys in the forum I have learned a great deal. I do not post much, just read alot and print your posts out for later reference.

Thanks... some times things just don't make sense and then it will hit me in the head like a 24 ounce Budweiser......

[Matthew D. Hoedeman]

You might want to check the 'surface' direction vector for the 2 circles. I think it might use that 'surface' direction vector as an axis and then compares the center point for the feature back to that 3-D line. If the ACTUAL vector's are different, you would have a different 'axis', depending on which was used as the datum.

[DrBudman]

Vectors

The vectors are correct, 1,0,0 and -1,0,0

I'm beginning to suspect Ver. 3.6.......

Thanks all for the input.

These things are helping push my SMA (Hopefully)...... I show them your guys comments like.... "3.6 is junk" and they keep putting me back to the top of the pile....

Thanks

3 hours to Bud:30

[McGyver]

is your 1st circle (datum) inside your alignment procedure? If not that is the cause of your deviation.

[KBearden]

Stupid question here, but are you taking at least three sample hits on the second feature circle? I would assume that the datum circle has already been leveled with the origin plane Yes? Vectors are nothing without proper sample hits... Good Luck!

[DrBudman]

Reply

My first circle is inside my alignment, it is the origin in Y & Z. You might have to explain to me the sample hits. My understanding is that sample hits are when learning a hole with the joystick. I entered the holes by typing in the exact location and proper vectors not using joystick. The datum circle is leveled to my alignment leveling plane.

[tenndoc]

I'm sure there are others that can give a better example of this but - - - When using the auto circle feature you want to have PCDmis 'sample' three points around the hole (as long as the surface is perp to the hole). This will cause the probe run normal to the hole during the measure. In the auto circle pop up box there is an 'initial' and 'permanent' option. 'Initial will only sample the surface the first time the program is run while 'permanent' will sample the surface each time the program is run.

[\v/inston]

This will cause the probe run normal to the hole during the measure.

normal to the hole only if the hole is square to the surface.

[tenndoc]

Thanks winston -

[Matthew D. Hoedeman]

Well, in this case, the sample hits might not be the best idea. They can be used with several DCC Features, Surface, Circle, Slot, Edge, etc. The purpose of the sample hits will depends on how many you take. If you take a single sample hit, it will allow you to set the depth of touch, based on the current, actual 'height' of the surface of the part. If you use 3 or more, it will also give you the actual surface vector and that is what Pcdmis will use to measure the feature. For example, if you have a hole in a surface that should be flat (0,0,1 vector) and it is a thin surface, you might want 3 or more sample hits so that the depth the probe measures at is constant around the hole. The actual surface the hole is in may have a vector of 0.150,0.503,0.851. Now, that is WAY off angle (just for the example) and if you only used 1 sample hit, some of the hits will either shank real bad or the probe will scoot over the top of the part. If you have thick stock or you know that the surface angle will NOT be off too much, you can use just a single hit, in which case the touches will be in the 'plane' of the nominals and at a depth to the one sample hit. Single sample hits work real well with edge points, but circles generally work better with 3 or more. Myself, if I am unsure of the surface angle, I will use 4 sample this and 4 touches to measure the hole. This will give me sample hits at the same locations as the touches, thus giving me the best possible touches in the hole. Now, if it is something that I know will be flat and stay flat, I will use just a single sample.

So, the reason I say sample hits might not be the best idea is that if you take 3 or more, Pcdmis will be looking at a vector for the hole that is NOT nominal, no matter how small, and that could throw off the concentricity calculation. The depth of the 'actual' feature will be calculated along the actual surface vector that is calculated by the sample hits and can 'move' the center off center, if you know what I mean. An example of this would be measuring a cylinder. If you take a sample hit on the top to set a depth below the top to measure at, you had better only take a single hit. If the top of the cylinder is not square to the cylinder itself, then with 3 sample hits, the calculating vector line will NOT match that of the cylinder. When using samples and depths, you will notice that no matter what you use for a depth, it will give the same Z values for the feature, everytime, since it is based on the sample touch. So, if the top of the cylinder is not square to the sides, and you measure 3 hits on the top, it will project the circole touches along the actual vector of the top, back to the height of the top. Now, it has been skewed off to the side and you have bad data. A single sample would work well for this, not 3 or more.

[DrBudman]

Thinking Finally

Ok.... I think I might be onto this thanks to all your posts. I went back thru my pc-dmis 101 notes and I seen where they say to do a alignment level to the part right after I measure my DCC plane, I don't do this, I measure my part in manual, (plane, circle, line for ex.) align to it, then in DCC I remeasure all the features and then aling to it, I don't align to the plane and then measure the circle. Are my vectors coming from my manual plane measurement since I did not use sample hits and I did not level to the plane prior to measuring my DCC circle??? That would mean my second circle vectors are coming from my dcc plane and first from the manual plane.

I hope that made sense...... dcc plane then level, dcc circle and line and finsih my alignment.

Thanks