Base Alignment vs. Start Alignment

[[La...]]

Hello, I am new to CMM programming and I am having trouble with understanding alignments. More specifically base and start alignments and the difference between the two. Could someone break this down for me?

Thanks!

[[Cl...]]

Start Alignment simply locates (tells) the software, where in the volume of the measuring space the part is.

So, now the software knows the parts XYZ location relative to MCS (machine coordinate system), and it can now measure the Base alignment.

[[Du...]]

start alignment are generally much simpler alignments than the base alignments, usually easier to reach features or features that are quicker to manually probe. in the attached screenshots my base alignment and start are similar but in the start alignment i use a single circle.

[[Ke...]]

Base Alignment: Foundation of your program, this tells the system exactly where the part is and how it is located, and is the starting point of every measurement in a program (depending on the programming style, some people use this more directly than others)

Start Alignment: A rougher locating, can be used for an approximate find in order to allow the base alignment to run more consistently if part locating on the cmm is a bit less precise. It is also useful to create a simple way to manually teach a part location manually on the cmm, particularly in the case of complex base alignments. 

These are also both part of the hierarchy of inheritance for alignments, with the order being CMM System->Start Alignment->Base Alignment->Secondary Alignment with Base as parent->Secondary alignment with Secondary as parent. Calypso will work its way up that chain referencing back until all degrees of freedom are fully defined

[[Sv...]]

If it’s still the way calypso works, while programming you have to define the base alignment fist (its the base of the calculation in the software) and after that all other alignments. But then in cnc mode he will proceed as mentioned above: start -> base -> ….

Edited August 7

[[Ma...]]

As mentioned create your base alignment first. Then you can select several features to roughly locate your part as your Start Alignment. Calypso will then display the distance from your start alignment is from the base alignment. It is important to note that your Start Alignment doesn’t not have to be the same features that make up the Base Alignment. That is the beauty of this method. Choose features that define X, Y, Z. It could be just three simple points. Also the Start does not necessarily need to have the Rotation in Space or the Planner filled out, only if applicable.

The intent is to quickly locate a part:

• if fixturing is not repeatable 
• you need to move the part around the table occasionally 

 

[[Je...]]

Base alignment should robustly define your part in the measurement volume (strive to achieve a robust definition for 6 DOF).  Start Alignment is a "locator" for the Base Alignment to start.  It can be as simple as a single point to define a single DOF.  IE - you have a fixture that is often moved laterally on your CMM but the Y and Z location is repeatable (as is the rotational constraints) as if it were slide against a rail.  A single point on the -X or +X side of a feature would be enough to inform CALYPSO on where to look for the part and conduct a Base Alignment.  Using a Start Alignment is a highly preferable method to reducing the quantity of "loops" and improved break conditions needed for Base Alignment calculation/repeatability.  
Important note: Start Alignment features are intended to be REAL features, not theoretical/calculations (with a few simple exceptions).  It is meant to be significantly simpler in its structure as to make the start-up task for an operator much easier.  Even in full CNC mode, this is still advised to improve your repeatability.  I STRONGLY advise the use of a Start Alignment for any part that will see any quantity of production type measurement.  Even with fantastic fixturing, you will see an improvement in Base Alignment break conditions (hint: I use a CNC Start Alignment and no Base Alignment looping/break condition in 99% of cases and still pass GRR without issue)

[[La...]]

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Correct me if I'm wrong, but any time a measurement plan is running, it will go through the start alignment first once I manually locate the part, then it will measure the base alignment to confirm where the part is? Sorry if I misunderstood.

[[La...]]

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Thank you for the reply! I was curious because I have ran programs where I have only had a base alignment, but no start alignment created. Does that mean if I only have a base alignment and no start it will just use the base to roughly locate the part and the begin the measurement plan? 

[[La...]]

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I appreciate the response. When programming, since it's ideal to create a base alignment, would it be best to create a start alignment as well? I was wondering since I have always just had a base alignment in my programs but I have never created a start alignment.

[[La...]]

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So, in a program, could the base/start alignment influence the calculations?

[[Ma...]]

If you have complex base alignment where you need specific place to measure or you are changing probes, then use more simple start alignment.

It will tell a machine where it can measure base alignment. You can also do loops for base alignment to refine precision of base alignment. Loops can not be set on start alignment.

For better imagination - if you have a part along machine axis, then your start alignment can contain only 3 points ( X,Y,Z ) rest will be taken from machine.
So you will have faster location of a part, then running 3 planes with many points.

In other words - use bigger features to have more stable alignment for START and for BASE use what drawings tells.
If you have RPS, then use start alignment with basic features, then use RPS in base alignment.

Start and base alignments are ONLY TELLING where is ideal part ( nominal features ) - if you want to measure on correct place a bended part, then you would need to make a new alignment from features and then set that new alignment for features on bended part.

[[Ch...]]

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It will only run the Start Alignment if you check the small box "automatic measurement" otherwise they may be skipped during CNC runs unless you start CNC run with a 'manual start alignment'

To answer your other question, yes, if you do not have start alignment it will only calculate /compute with the base alignment (which if not 'lopped' could lead to potential decrease in accuracy)

I strongly suggest also always using a START Alignment.

 

Good luck.

[[La...]]

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Thank you! This clears up some of my confusion. Have a nice day.

[[La...]]

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Thank you for the reply. I've been seeing others mention loops, I wasn't sure how to properly do them and was wondering if you'd be able to explain the basics of how to properly do a loop on a base alignment if you don't mind.

Edited August 14

[[Ma...]]

No problem.

What LOOPs do in base alignment is - machine runs base alignment from previous known origin and vectors ( start alignment and other methods ).
Now machine knows a new position of origin and vectors and is preforming new base alignment from that new position.

For enabling loops you go program alignment window and click "LOOP" button.

Basic formula for stopping loop prematurely is "baseSystem().valueA<0.05" where value "0.05" is maximum allowed movement.
So you can define 10 loops, but with this it can stop earlier.

For details look into manual for your version and look for "Iteratively adapting the base alignment". <- this works only for base alignment

One thing to note - if you want this iterative loop for alignment in a program ( not base or start align. ) then looping features won't bring this correction.
It runs on same place X times as defined in a loop.

Edited August 14

[[La...]]

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Okay, that makes sense. The alignment will continue to loop for a set amount of times, but can stop earlier if it gets within a certain tolerance. I will be sure to test this out! Thanks! Have a great day.

[[Ma...]]

It will run at least twice to get the delta between the measurements.