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Wednesday, January 23, 2019

What’s the “CMM runout error”?

What’s the “CMM runout error”?

When you measure the runout on CMM, the CMM induces a lot of error into the measurements. If you have 2 axes in your CMM program for the runout, for example, the axis of a feature, a circle, and the axis of the datum -A-, a cylinder, each axis of movement during the measurement is going to add an element of error. The axis of the features and datum -A- always are off to the axes of CMM. That “off” is a very little angle, A1/A2 angles, but that “little angle” end up magnifying error substantially when , for example, it is about a .0005 tolerance.

You can simply see it if you examine the difference in the A1/A2 angles between the features. You cannot change the A1/A2 angles on CMM. On a Roundness tester you make A1/A2= zero before measuring the runout. A roundness tester allow us to correct the A1/A2 angles, to center the part axis on the spindle so that the runout will be in the part in which the axis feature-datum have 0° angles to the axes of the machine.

The A1/A2 angles are the root cause of the CMM runout error, and of the deviation between 2 different fixturing on 2 different CMM’s, between 2 different CMM’s, between the CMM and the Roundness tester, and between the CMM and the Dial indicator.

As a conclusion for the CMM runout error: No problems when the part is perfect .When the parts are not perfect the CMM may reject good parts, parts in which the runout is on the high side of tolerance.

Wednesday, December 5, 2018

There are written these words;

"piho" , "gsis" , "dve", " gon", " a-e ", " han ", " rouu-rro ", and" sis "

Albanian language has :

'Kon" word I think comes from the phonetic transformation of gon=gojn(mouth)n=kon when giving something to the baby's mouth ", where "g' has been transformed to a " k ". The root of words I think comes from the Albanian language;

1) "piho". 2) "gsis". 3) "dve" 4) " gon ". 5) " a-e ". 6) " han". 7 )" rouu", and 8) " sis ".


1. piho=(Albanian) "pi"=drink

2. gsis=Albanians words; "gji" and "sis'= women breast

3. "dve"+(Alabanian) '"e dy"=together

5. "gon"= (Albanian) "kon"= (Provides food for children in their mouth).

6) han= (Albanian) "ha"= eat
7. 'rouu"=(Albanian) "ro"=live

The sentence make sense only from the Albanian language= 

" Me pi sis te dy per te rrojtur", in English= "Drinking milk together from the breasts o to live", or in  Italian+ "Bere latte insieme dal seno a vivere".

Friday, April 20, 2018

CMM Fixturing Measurement Techniques

Does CMM correct a bad fixturing of a workpiece?

Does CMM correct the impact that a bad fixturing has on GD&T's, lengths, diameters and angles ?

I think, the part fixturing and part alignment into the fixture is the main factor in CMM incorrect measurements. If we define the part fixturing and alignment incorrectly as a concept the CMM will be used incorrectly. 

If the CMM result of a trueposition of a feature to datum -A- is out of the tolerance can the CMM correct the true position if it the datum A is measured as a cylinder and the part is 3 degrees skewed from being parallel to the CMM axis. I never have assumed that the CMM corrects the errors of fixturing, and the part can be measured without a precise fixturing/alignment.

First question I will ask is what’s our benefit if the CMM does that?

We do not need a precise fixturing-alignment of the part into a fixture and consequently we save some time/cost on this.

Practically, how it is going to work when we measure parts on CMM and we see a red CMM result?

If we take for granted what we are being told that CMM corrects the errors of fixturing-alignment of the part, we need to know the answer of some very simple questions.

First, I would like to know how much error of fixturing in angles or .xxx” (in numbers) the CMM corrects? 

Secondly, the CMM corrects that for all kind of dimensions and all GD&T’s or only for some of them? 
As we know, there are 2-d and 3-d dimensions, simple features and sphere features, and the roundness is not the same as the perpendicularity, or the parallelism as the true position. The CMM may correct the perpendicularity, or parallelism when there are errors of fixturing but may not correct the fixturing errors for the the roundness or the true position of cylindrical features.

On the other end, if there is not an “error correcting” number, for example up to 3° from being parallel to Z axis, or .005” in Z direction, or a list of GD&T’s how can we accept assumption that CMM corrects the errors of fixturing?

When we use a CMM program how we are going to understand the borders that the CMM corrects or does not correct the errors of fixturing-alignment of the part and to react, to fix it in order to get e good measurement? 

We want to know that number with the simple purpose to apply that knowledge and having accuracy and precision in measurements.

What CMM does if the work piece is aligned wrongly  -0.2” in Z direction and 3° skewed from the Z axis?

Calypso knows the old position of the workpiece from the base alignment when is defined the workpiece alignment, when the coordinate system is established by probing physically the part, if you used the standard method to define the workpiece alignment, the 3 known steps of the part alignment, level-rotation-origin.

First time Calypso will scan the first feature of the base alignment according to the old data position. For example, if the first feature is a bore with the axis positioned perpendicular to Z axis, Calypso on the first time will touch only half of the bore and you can see how the probe will scan the air for the other half. After Calypso gets the new points Calypso will scan the same feature again, but this time correctly because the new features position now is recognized and being corrected. The old position is overwritten. So, Calypso corrects this kind of fixturing errors.

Practically, there will 
always be an error of fixturing, it does not matter what kind of base alignment you are using, automatic, or manual alignment or start alignment, Calypso will always correct the old alignment to a new alignment.

Calypso corrects the workpiece to a new position, but Calypso cannot correct that 3° angle of a bad fixturing. The angle will still be 3° to the CMM axis and it will have an impact, for example, on the roundness of a bore which could be as bad as .002” or more when the actual Roundness is .0003. 

Without a good fixturing it is not possible of forming datum planes or axes from their high point contact, right angle, distance, circular form, orientation, location, center axis of the feature. No good fixturing may form incorrectly datum frames. 
No good fixturing will also cause the probe measurements will not be taken enough perpendicular or parallel to the probe body which will produce errors and give results that are less repeatable and accurate.

Use always 3-jaws-clamping devices, vises, magnet and tall pins to set the part when it is being programmed on CMM. A good fixturing allows the datums to be repeatable.

From my experience a bad fixturing has a huge impact on GD&T's, a medium impact on lengths and diameters.

Tuesday, November 14, 2017

GR&R Studies

It’s all about the measurement variations and correcting the factors that contribute on it.

Measurement variations can come from three main sources: the person using the gage, person to person and part to part. It is also the gage itself a factor.

MSA studies exist to discover and quantify the amount of variation coming from these different sources, so that corrective action may be taken if necessary.

There are too many formulas of Gr&R studies. It depends from our intention:

1) To discover the amount of variation coming from three main sources.

We need the data of one dimension-measurements(a) of three persons(b) for 10 parts(c), taken from Measurement Systems Analysis Reference Manual, 3rd edition (Chrysler, Ford, General Motors Supplier Quality Requirements Task Force). Ten parts were selected that represent the expected range of the process variation.

Three operators measure the ten parts, three times per part, in a random order.

2) To examine the accuracy of a gauge, how accurate is a gauge when compared to a reference value and if it has the same accuracy across all reference values.

Gage repeatability and reproducibility studies determine how much of your observed process variation is due to measurement system variation.

Minitab provides two main methods for Gage R&R Crossed: X and R, and ANOVA.

The X and R method breaks down the overall variation into three categories: part-to-part, repeatability, and reproducibility.

The ANOVA method goes one step further and breaks down reproducibility into its operator, and operator by part components whereas the X and R method does not.

How to interpret the the Gr&R results.

The % Study/Var column shows the Total Gage R&R. Less than 10% -the measurement system is acceptable.

Saturday, June 18, 2016

Minor diameter of spline as the spline datum

Is it possible to use the minor diameter of a spline as  a  datum when the print requires to check the GD&T’s of other features to a spline datum?

In this days, it is known that the Spline Gauges are the best inspection tools to check the splines. Spline gauges have an advantage over old methods of checking spline dimensions because the use of spline gauges not only ensure the quickest process for acceptance, which means also less timing cost, but also eliminates the need to inspect the profiles with old ways such as three pins method, pin+dial indicator methods,etc.

 By using ‘GO’ and ‘NO GO’ spline gauges and plain ‘GO’ and ‘NO GO’  we ensure:

1    1)  ‘GO’ spline plug gauge checks the profile flank , the equally spiced angles  for all teeth, the extension of the root radius to the pitch circle diameter, pitch circle diameter and the runout of pitch circle diameter to major diameter of spline.

     2)  ‘NO GO’ spline plug checks the major diameter and space tooth width.

3   3)    GO’ and ‘NO GO’ plain gauges check the minor diameter of spline.

So, all gauges  ensure all dimension and fitting of the spline and among other dimensions also that the position of 3 min-pitch-max diameters to each other are within their tolerances. 

As a conclusion, when the GO/NO-GO spline gauges and plain plug gauges accept the spline it is possible to use the minor diameter of spline as a datum when the print requires to check the GD&T’s of other features to a spline datum. 

Wednesday, October 7, 2015

Pre-Plate pitch diameters, major diameters, and minor diameters of thread rings, thread plugs, and plug gauges.

The rules for determining Pre-Plate pitch diameters, major diameters, and minor diameters of thread rings, thread plugs, and plug  gauges are as follows:

To determine Pre-Plating dimensions for external threaded parts: 

For external threads subtract the max plating thickness from the high limit Pitch Diameter, and then subtract the minimum plating from the low limit Pitch Diameter  

For the minor and major diameters reduce the maximum diameters by half the maximum plating and reduce minimum diameter by half the minimum plating.   

To determine Pre-Plating dimensions for internal threaded parts: 

For internal threads add the max plating thickness to the low limit of the Pitch Diameter, and then add the minimum plating thickness to the high limit of the Pitch Diameter.  For minor and major diameters increase the minimum minor diameters by half the maximum plating and increase the maximum minor diameter by half the minimum plating. 

To determine Pre-Plating dimensions for thread ring gauges: 

For thread ring gauges subtract the max plating thickness from the Pitch Diameter of the Go thread ring gauge. Then subtract the minimum plating from the Pitch Diameter of the No-Go thread ring gauge. For the minor diameter reduce the Go ring minor diameter by half the maximum plating and reduce the No Go ring minor diameter by half the minimum plating. 

To determine pre plating dimensions for thread plug gages: 

For thread plug gauges add the max plating thickness to the Pitch Diameter of the Go thread plug gauge.  Add the minimum plating thickness to the Pitch Diameter of the No-Go thread plug gauge. Increase the major diameter of the Go thread plug by half the maximum plating and increase the No Go thread plug major diameter by half the minimum plating. 

Example for Thread Plug Gage:

Plating of .0002” - .0003” allowance per side multiple x 4
.0003 x 4 = .0012” Max
.0002 x 4 = .0008” Min

½ - 20 UNF 2B Pre-Plate

Basic standard Go Pitch Diameter = .4675” + .0012” = .4687” Go  Pre-Plate Pitch diameter.
Basic standard No Go Pitch Diameter = .4731” + .0008” = .4739” No Go Pre-Plate Pitch Diameter.
Basic standard Go Major Diameter = .5000” + .0006” = .5006” Go Pre-Plate Major Diameter.
Basic standard No Go Major Diameter = .4948” + .0004” = .4952” No Go Pre-Plate Major Diameter.

Example for Thread Ring gages:

Plating of .0002” - .0003” allowance per side multiple x 4

.0003 x 4 = .0012” Max
.0002 x 4 = .0008” Min
.0003 x 4 = .0012” Max
.0002 x 4 = .0008” Min

½ - 20 UNF 2A Pre-Plate

Basic standard Go Pitch Diameter = .4662” - .0012” = .4650” Go Pre-Plate Pitch Diameter
Basic standard No Go Pitch Diameter = .4619” - .0008” = .4611” No Go Pre-Plate Pitch Diameter
Basic standard Go Minor Diameter = .4446” - .0006” = .4440” Go  Pre Plate Minor Diameter.
Basic standard No Go Minor Diameter = .4511” - .0004” = .4507” No Go Pre-Plate Minor Diameter.

If no minimum and maximum plating thickness is given, then the given plating thickness is considered nominal or minimum plus 50% to determine maximum plating.

Monday, February 9, 2015

Number of turns on the thread gauge

Calculating the number of turns on the thread gauge for the thread Size M10 x 1-6H.

The number of turns on thread gauge:

Length of threads: 0.402 ± .015: A length without threads is 0.158- Not given on the picture, and the diameter of countersink is 0.435 / 0.437 x 90°

Thread Size: M10 x 1-6H

I. Minimum of turns
a) Convert the pitch: 1 / 25.4= 0.03937”
b) Minimum of turns:
1) Calculating first the threaded material length when the 0.402” dimension is on the minimum of its tolerance and the 0.158” dimension is in on the maximum of its tolerance:

L1 = (0402 - 0.015) - (0.158+.004)=0.225”

2) Calculating the length without threads.

From the dimension of countersink 0.435 / 0.439. Nominal diameter of countersink is 0.437
From the dimension of the M10-1, the max diameter for internal threads is 10 mm-converted in inch 0.393”.

3) Calculating:

Minimum full length thread=L1-H=0.225 - 0.022= 0.203
Number of turns for the minimum full threads length: 0.203 / 0.0393=5.1653 turns

c) Maximum of turns

Calculating the opposite case when the 0.402 is on the maximum of its tolerance and the 0.158 dimension is on the min of its tolerance:

L 2= (0.402+0.015)-(0.158- 0.004)=0.263”

L max full thread=L2-H= 0.263 - 0.022= .241

Number of turns for the maximum full threads length: 0.241 / 0.0393=6.13 turns

Number of turns: 5.16 turns minimum / 6.13 turns maximum