The importance of Geometric Dimensioning & Tolerancing
(GD&T) has increased exponentially since its introduction to the workplace in the 1950s. It has grown to become a universally accepted system that is used by professionals in the production/engineering industry for technical drawings.
From the very first stage of designing a product to the very last of quality assurance, GD&T has a role at each step of the production cycle. For this very reason, a significant number of employers expect their employees to have relevant knowledge of Geometric Dimensioning & Tolerancing.
The question then is: how would you, someone who is not as familiar with GD&T but is well-experienced in the production business otherwise, meet these expectations?
Our GDandTBasics.com site is bombarded with queries about GD&T interview questions and job roles quite often. Keeping this in mind, we decided to address these concerns in this article and guide people on how to prepare for their interviews and ace them.
If you are worried about a job interview, you are in the right place! Bear with us as we list down the possibilities and point you to the right areas to focus on.
Step into the Interviewers’ Shoes
Before all, the very first thing to do is not to get carried away by thinking you have to know everything. If you are interested in a job whose advert mentions GD&T, it does not mean you need to be a specialist in it. All you need to do is to be clear about what your interviewer is expecting of you.
It is important to understand that GD&T is simply a skill set that complements the work of designers, machinists, inspectors, etc. They use GD&T on the job; it is one aspect of their work routine, not everything they do. This means that your primary function remains the same, but you will additionally be using GD&T to perform it.
So how much should you know for an interview? This depends entirely on the position that you are interviewing for. It is advised that you research the company, what the job entails, and what aspects of GD&T you might be using there. That specific segment is what you need to target. Table 1 below has some examples for your guidance.
|Job Role||Area of Knowledge|
|CMM Inspection||Interpretation of symbols & measurement setup|
|Machining||Machine limits and tolerance zone interpretation|
|Maintaining Design of Engine Fits||Interpretation and Application of Tolerance Zones, Datums Etc.|
|Mechanical/Design Engineer||Interpretation and/or Application of Tolerance Zones, Datums Etc.|
East vs. West: Know the Difference!
The next thing to keep in mind is the interview practices in your country. This may seem unnecessary, but from our experience of working with a diversity of international clients, we have identified some key differences in how interviews are conducted in different parts of the world.
In the West (US, Canada, Europe), interviews are all about the role and your ability to overcome its challenges. They are designed to see whether you can apply your knowledge of GD&T or not and how you derive from your experience in this field. So, focusing on how to tackle application-based questions is a good idea if you are living in these regions. Get ready for an actual drawing and direct questions about carrying out your task.
In the East (China, India, Pakistan, Bangladesh), however, GD&T interviews are more knowledge-based than application-based. Interviewers can ask you about specific definitions or conventions, or ask questions that target very specific portions of GD&T standards. You should, of course, be well-prepared for application-based questions, but if you live in these parts of the world, it is better to also brush up on your book-knowledge before an interview.
These are the fundamental concepts you will want to be clear about before your big day. With these confusions out of the way, we now dive deeper into what type of questions might be asked based on your profession. In our opinion, production jobs can be divided into three broad categories: designers, inspectors, and machinists – and so we consider some dummy questions for each of these.
Machinists play a central role in the production industry, acting as a link between designers and inspectors. Their use of geometric dimensioning and tolerancing is somewhat limited as compared to the other two categories, so we’ve considered them first.
As a machinist, you encounter GD&T in the engineering drawings you receive. The tolerance zones of each feature and the corresponding GD&T symbols are things you will be expected to know.
Your interviewers will be looking to test how well you can interpret these drawings. They will also be interested in your ability to understand the physical significance of these symbols from a manufacturability standpoint. Let us look at figure 1 as an example.
From this drawing, the interviewer might ask you about the feature control frame for the hole. The main elements are:
- 3 Datums: A, B, and C.
- Hole size dimension: Ø16
- Hole size tolerance: +/- 0.1
- Position Modifier
- Position tolerance: 0.2
- Basic Dimensions 15 x 15 to Datums B and C
You might be asked how you would machine this hole. Your answer should be that you would be careful enough to position your drill bit so that it is no more than 0.1 away at any point from its true center, which is 15 units from both datum B and C. You also may be asked how perpendicular the hole will be. This is built into the position tolerance – requiring you to also have perpendicularity to datum A of Ø0.2 since the tolerance zone must first be perpendicular to A. Moreover, you can add in that you might use an end mill and circular interpolate the hole to have better control over the diameter and the location.
Responding this way will show your employer that you are adept at reading GD&T off a drawing. This is enough to convince them you are the right person for the job.
A good machinist knows how to determine whether their part is acceptable or not. Employers value this skill since it leads to minimal wastage of resources down the line if a bad part is caught right after machining.
Referring to the hole in figure 1, you may also be presented with the scenario where the center is off due to machine inaccuracy. To determine whether it is inside the tolerance zone or not, you can use the simple formula shown in figure 2.
Let us demonstrate how to use this formula. Suppose you just finished machining this hole and decide to check whether it is in tolerance or not. You measure its actual position using calipers, which comes out to be 14.95 with respect to datum B and 15.08 with respect to datum C.
This obviously is not ideal since both readings vary slightly from their true values of 15. So, now you must plug your true and actual values into the formula to check your tolerance. The process is as follows:
The final position tolerance is Ø0.188, which is less than the allowed position tolerance of Ø0.2. Hence, your part is in tolerance and acceptable. You are lucky! Had this value been greater than Ø0.2, your hard work would have gone to waste.
Moving on to the second category of inspectors. They work at the very end of the chain of production and evaluate whether the finished part(s) are within specification. Owing to the nature of their job, inspectors should expect a bit more technical interviews than machinists when it comes to GD&T.
An inspector needs to demonstrate that they can not only interpret GD&T, but also use it to create suitable inspection strategies. Each part requires some preparations to be made and protocols to be followed, which vary for individual cases. Linking these up with GD&T is crucial for inspectors.
One popular type of interview activity for inspectors is to give a standard GD&T drawing and asking how to inspect the part. Referring back to the hole in figure 1, an impressive response would be as follows:
- Setting up and stabilizing the datums: A-B-C (in that order).
- Take sets of readings for the hole using a CMM probe, say 5 points around the circle at a few different depths.
- Using these measurement points to establish an axis for the hole.
- Determine the actual position of this axis with respect to datum B and C and its perpendicularity to datum A all at once.
- Using your 5-point readings to check the hole’s size and comparing it with its tolerance limit of Ø16±0.1.
This is a very simple example and certainly not the only interview scenario you may face, but it serves the purpose. There are many different types of inspection roles, from quality engineer, to setup tech, to CMM operator. However, you only know how to inspect a drawing when you know what every symbol and concept means. If your inspection procedure takes into account the datums, tolerance criterion, modifiers, etc. specified via GD&T, you are good to go.
Designers are the individuals who apply GD&T to an engineering drawing that is then used by machinist and inspectors. Therefore, during an interview for a design position, you will have to prove that you can accurately apply GD&T to a print without causing any confusion or issues during the production or inspection process.
One thing to know beforehand is that if a company explicitly states that they require proficiency in GD&T, it means that they are using it on a very regular basis. You cannot just skim through random GD&T resources and expect to perform well. You need to know your stuff well, right down to the specifics.
Moving on to the interview, there are many things designers need to demonstrate. First of all, they need to be aware of their toolset: symbols, conventions, modifiers, etc. For example, you might be given a blank drawing and asked to suggest the best material modifiers for a feature. If you know them all and how they differ, you can easily answer.
Another point of interest is your concern for functionality. Going back to figure 1 again, let us assume that a bolt has to go in the hole. Your interviewer might inquire about the tolerances you would assign for the bolt to function properly. Do not assume this, though. Ask what will be going into the hole. After all, we would be spitballing if we do not know what the true functional requirement is.
The main thing they would judge by your answer is your broad-mindedness as a designer. You cannot assign a tighter control than functionally required, because then you would unnecessarily be increasing machining and inspection time and costs. On the other hand, a poorly toleranced hole can lead to the bolt not fitting. Your answer should have some balance.
To sum it up, you can impress your interviewer by showing your practicality. The perfect answers would demonstrate your ability to use GD&T to ensure all aspects of designing are taken into consideration, and that you are making the production process as convenient as possible while maintaining functionality.
What’s Your Preparation Strategy?
Now that you know what to expect in your interview, you can start focusing on preparing for it. You can check out our blog/advice column for a bunch of free learning resources that you can use to brush up on basic concepts. Learning topic by topic is great when you are using things as a reference or figuring out how to get started, but in order to be proficient, you should take some sort of GD&T course. This is the best way to simplify things and understand how all of the individual topics come together to form one unified framework.
If you are interested in learning Geometric Dimensioning & Tolerancing from a professional trainer, Engineer Essentials, owner of GD&T Basics, has exceptional online courses to offer. As a member of our class, you would get full access to our helpful learning material, the chance to participate in forums and webinars, interact with peers, and also receive a certificate upon completion.
“An investment in knowledge pays the best interest.”
Best of luck with your interview!