17 Replies Latest reply on May 31, 2016 7:53 AM by Thomas Bernert

# WHAT IS DATUM AND HOW TO USE IT?

HELLO FRNDS

I NEED AN URGENT HELP FROM U ACTUALLY I M SO MUCH CONFUSED BY DATUM,I HAVE LOT OF QUESTION ABOUT DATUM

Q1. WHAT IS DATUM IS IT JUST FOR REFRENCE ONLY?

Q2 DATUM IS LINKED TO PLANE,EDGE OR PROJECTION VIEW?

Q3HOW DO WE KNOW THIS IS A,B OR C

Q4 CAN SOLIDWORKS AUTOMATICLY USE THIS FEATURE I MEAN CAN SW AUTOSELECT A,B OR C

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Ankush,

I'm sure someone will come along that is more familiar with geometric dimensioning than me, but this may help to get you going.

Q1: A datum is basically a common reference feature (line, point, axis, etc.) that you define, in order to be referenced by dimensions.

Q2: Not sure I'm following what you are asking here.

Q3: Datums are labelled however you/your company wants to label them. I think there are industry standards out there to control this, but not 100% sure. Try looking for GD&T standards.

Q4: I don't know if SW can automatically put the datums in, but I've included a youtube link for a quick tutorial for you.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Ankush,

A1: A datum is a reliable reference from which measurements are taken.  Calling out your datums in a drawing is one of the steps towards making sure that a part is manufactured and inspected in such a way that it can be replaced with another part made from the the same drawing and fit correctly.

A2: The datums together indicate how the part rests when it's mounted on specific surfaces.  Surfaces are generall easier to inspect than egdges but you can extrapolate the intersection of two surfaces or the extension of a cone to a theoretical point that can be used as a reference.

A3: It doesn't matter what you call your Datums.  What matters is the order inwhich they appear in your feature control frames.  Feature control frames are those little blocks of symbols and numbers that usually have "A|B|C" at the end.  Reading the feature control frame from left to right, the first datum is the primary mounting feature, the middle is the secondary reference and the third is the tertiary reference.  The order tells you how to set up your part in order to machine or inspect it a specific feature.  The symbol tells you what characteristics of the feature are important and and the shape of the tolerance zone and the numbers tell you the size of the tolerance zone

A4:  SolidWorks includes a tool called DimXpert that will progressively show you how well-defined your dimensions and tolerances are for creating the part.  Part of the process of using that tool is to identify the part datums early on.  There is an automatic dimensioning Wizard where SolidWorks will try to guess what the dimension scheme of the part is.  YOu can try that out and see for yourself what it choses for Datums.

All of this is part of ANSIY14.5-2009-a standard for Dimensioning and tolerancing parts generally called GD&T for Geometric Dimensioning and Tolerancing.  SolidWorks supports the annotations that are used in the standard, but the underlying knowlege of the standard isn't covered in the SolidWorks help and requires a good ammount of study and training to use effectively.

Usually the starting point for learning GD&T is a training class offered by a company certified to teach it.  You can also learn about it from books published by ASME (ASME.org) or other publishers like the IHS Drawing Requirements Manual.

I remember the first time someone sandbagged me with this subject.  I was working for a company that made fibre extrusion machines and I was way out of my depth  in terms of machine design.  My boss told me my hole patterns had too much tolerance stack-up and I needed redimension the drawing using GD&T.  They had a library of ANSI Y14 books and I struggled all night one night to simultaneously absorb and understand the material and get this project done and I wound up punting it back to my boss and looking pretty stupid when I told him, I don't know how to do this.  It sucked, but I learned that there are some things you can't just figure out on your own, where you really need an expert to guide you.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

GD&T = Geometric Dimensioning and Tolerancing.

Find some good sources on this topic.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Everyone has their favorites, but I worked my way through Geometric Dimensioning and Tolerancing in 2007 by James D. Meadows, and gained a pretty good foundational understanding of GD&T and it's application.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

but i m confuse why we need this

and one thing more i want to ask does datum allways need minimum 3 refrence A,B,C

and pls also tell me if i have a drawing in which datum is included does i start my drawing from 1st ref A

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Ankush Gupta wrote:

and one thing more i want to ask does datum allways need minimum 3 refrence A,B,C

Have you ever made a part on a milling machine and/or a Lathe?

If I have a vice set up on a milling machine I need to establish 3 datums.

The first datum I would establish is the machine table (or parallels in the vice).  This would be my A datum as it does not change.  I can take the vice off of the table and put it back on and the table is still the table.

Then I would indictate (find) the "exact" location of the non-moving jaw of the vice.  This jaw does not move relative to the vice it is where it is (relative to the rest of the vice assembly), but if I take the vice off of the table - I need to re-establish the location relative to the spindle of the machine.  This would be my B datum.

Then I would set up a stop to the left or right side of the opening between the vice jaws.  This position is the least robust of the three.  It is entirely dependent on the other 2 and needs to be established relative to the other two.  This would be my C datum.

Now that I have 3 datums defined I can mill my dimensions with confidence.  I can even take a part out of the vice and measure - put it back into position (relative to the 3 datums I have established) and continue milling.

With a lathe, perhaps only 2 datums are needed.

The obvious Datum A is the axis of revolution.

The Datum B would be established relative to some fixed point like perhaps the jaw of the chuck or some stop on the ways.

1. That is how the machinist uses datums to establish control.

2. The inspector should use similar datums in determining if the dimensions match the drawing specifications.

Working backwards to the original design - the designer should establish datums that make logical sense for 1 & 2.

This helps ensure quality, consistency and reduces cost.

There is a lot more involved in this (look for terms like MMC LMC and others in your research), but this is a pretty good basic description.

When you are designing a part in SolidWorks you should fully define your sketches.
This is similar to what the machinist will have to do in making the part, but you might use slightly different techniques to make efficient use of symmetry about the Origin (datum) planes.  If the machinist has to do this, the designer has no excuse....

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

This is a broad question. Do you mean DATUM in the GD&T world or in the Solidworks world?

In SolidWorks, you are using your three base planes, the origin, or axes involving a combination of the previous. An axis at the intersection of your front and right planes is an example of a  DATUM. Dimensions to these are DATUM dimensions and, except under special circumstances, do not change when you move another feature. If you dimension to a feature of size (a surface), the new feature moves as the feature it's dimensioned to does.

The GD&T descriptions above are fairly good, just remember the good practice of using solid features as DATA. All you need is a plane, a line and a point to define your references...No "Air Datums", if you please...They drive inspectors nuts...

Good luck.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

THANKS TO ALL OF U FOR KIND HELP

PLZ CLEAR MY ONE MORE DOUBT

LET I HAVE A DRAWING TO MADE

IN THAT DRAWING I HAVE DATUM A AND B

A IS AXES AND B IS SOME FACE

SHOULD IT IS CONVINENT TO START  MY PART IN SOLIDWORKS FROM THAT AXES ON WHICH DATUM A IS SHOWN

OR DATUM IS ONLY FOR MACHINING(LATHE,MILLING,VMC,BMC ETC)

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Use a Revolution feature with axis constrained to the Origin center point.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

no sir i want to know that is datum totaly linked to machining or it is also use to build model from drawing i mean let in a drawing datum is shown & i need to made model from that drawing should i start my drawing from datum or datum is not needed for model

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Ankush Gupta wrote:

... should i start my drawing from datum

Yes you should use datums.

The Origin center point is a obvious datum.

The Top, Front and Right planes are other datums (data?) that you should use in building models.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Hi Ankush

An axis can be a datum? i hope axis is an imaginary line

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

>>>i hope axis is an imaginary line.

The earth rotates about its axis. Can you lay your hands on it? This is a good example of an air datum...

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

This is why ASME does not support the use of axis directly.  A datum has to be attached to actual part geometry in some way or another.

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Which is another reason I said in 2 places to avoid using it. Perhaps if you'd read the whole thread you'd have noticed that...

• ###### Re: WHAT IS DATUM AND HOW TO USE IT?

Ankush,

You should take a look at this http://www.aidt.edu/course_documents/Manufacturing_Skills/GD&T/Fundamentals_of_GDandT.pdf

Also see about getting that caps-lock key fixed.