Mechanical design interview questions for robotics roles

Mechanical interviews for robotics roles are not about remembering formulas. Interviewers want to see whether you can take a rough requirement, choose a mechanism, defend a tolerance and describe how you would build and test it.

This guide lists the questions that come up repeatedly in Indian robotics and automation interviews, with answer notes that show the depth expected. It covers tolerances, materials, bearings, mechanisms, motor sizing, design for manufacture and the follow-up questions that separate a candidate who has built things from one who has only studied them.

What the interview is really testing

The pattern across all rounds is the same: they probe one level deeper than your answer. If you say aluminium because it is light, expect to be asked about stiffness, grade and anodising. Say it once, correctly, and the conversation moves on.

Tolerances, fits and GD&T

What is the difference between clearance, transition and interference fit? Answer in terms of the hole and shaft tolerance zones, then give one robotics example each: a bearing seat is usually an interference or transition fit, a pivot pin in a bushing is a clearance fit, and a dowel locating two plates is an interference fit.

Why not tolerance everything to plus or minus 0.01 mm? Because cost rises steeply as tolerance tightens, and most of the time a well-chosen fit does the job with a looser general tolerance. A good answer mentions the specific feature that needs the tight tolerance and why the rest does not.

• Know the meaning of H7 and h6 and be able to say which is the hole and which is the shaft.
• Explain position tolerance and why a bolt hole pattern uses it instead of plus or minus dimensions.
• Explain datum selection and how it affects inspection.
• Know that flatness, parallelism and perpendicularity behave differently from size tolerance.
• Be ready to describe how you would measure a feature on the shop floor with a micrometer or height gauge.

Materials and heat treatment

When would you harden a part? When wear or fatigue matters: shafts in bushings, cam surfaces, gear teeth. Give the process, not just the word: case hardening for a wear-resistant skin with a tough core, or through hardening for a small part that must be hard all the way through. Mention that hardening distorts, so grinding usually follows.

Bearings, shafts and power transmission

How do you choose between a ball bearing and a bushing? Ball bearings for low friction and higher speed, bushings for dirty environments, high load at low speed, or where space is tight and cost matters. A robot joint that moves slowly under a heavy arm is often better with a plain bearing and a hardened pin than a small ball bearing that brinells.

• Be able to explain radial versus axial load and which bearing takes which.
• Know why a shaft needs a shoulder or a retaining ring rather than relying on a press fit alone.
• Explain preload and why it reduces play but increases friction and heat.
• Know that belt tension affects bearing load and that an over-tensioned belt kills gearbox bearings.
• Know the difference between backlash in a gearbox and compliance in a belt drive.

Mechanisms

Why use a four-bar linkage instead of a direct drive? To move a tool along a useful path with one actuator, or to keep the motor away from a hot or dirty area. Classic example: a parallel jaw gripper where the jaws stay parallel through the stroke.

What is a toggle mechanism for? Multiplying force near the end of travel. A clamp that locks over centre needs almost no holding force from the actuator once closed, which is why simple manual fixtures use it.

• Explain degrees of freedom for a planar mechanism using Gruebler or Kutzbach, and be ready for a quick two-link example.
• Describe how a cam profile converts rotation into a defined motion, and one reason not to use one.
• Explain why a linear guide needs two rails or one rail plus a drive constraint to stop rotation.
• Know that every extra joint adds backlash and cost, and that good design removes joints rather than adding adjusters.

Motor and gearbox sizing

This is where many candidates lose the round. The expected chain is: estimate the load and its acceleration, compute the required torque at the joint, reflect it through the gearbox and efficiency, then check speed and inertia against the chosen motor.

# joint torque for a rotating arm segment
torque_static = mass * 9.81 * com_distance
torque_accel  = inertia * angular_acceleration
torque_joint  = torque_static + torque_accel

# motor side
torque_motor = torque_joint / (gear_ratio * efficiency)
speed_motor  = speed_joint * gear_ratio

• Mention inertia matching: a large ratio makes the load easy for the motor but adds reflected inertia and friction.
• Know that holding torque is not the same as continuous torque; a stepper loses torque as speed rises.
• Say that the gearbox efficiency matters, typically well below unity for a worm drive at high ratio.
• Include a safety factor and say why, rather than picking a round number.
• Be ready to describe how you would measure the actual torque on a built machine.

Design for manufacture and assembly

A bracket has six tapped holes and two dowel pins. How would you simplify it? A strong answer looks at function: which holes locate, which clamp, whether a single machined pocket replaces several parts, whether the dowels are needed at all if a shoulder does the job, and whether the part can be made from standard extrusion.

Expect follow-ups on minimum wall thickness for your process, corner radii on a milled pocket, tap drill sizes, and why a slot is easier to make than a precise hole pattern. In India, also expect questions about laser cutting sheet metal, local machining lead times and whether the design tolerates a hand-finished part.

• Know the difference between a design that is easy to model and one that is easy to assemble.
• Mention one-way assembly: parts that cannot be fitted in the wrong order.
• Explain how you would tolerance a stack of plates so the total error stays inside the requirement.
• Know that 3D printed threads are weak and that heat-set inserts are usually better.
• Explain how you would prototype a part in a day without a machine shop.

A worked design question

Design a bracket to hold a 2 kg camera at the end of a robot arm. The interviewer is not looking for a finished CAD model. They want to hear the sequence.

• Estimate the loads: weight plus acceleration, plus whatever the cable and connector impose.
• Choose a material for stiffness before strength, because a camera mount fails by wobble, not by breaking.
• Decide the interface: how it bolts to the flange, how the camera attaches, and how it is adjusted.
• Pick a thickness from a deflection estimate, then add a gusset rather than increasing thickness everywhere.
• Consider cable routing and strain relief before finalising the shape.
• Say how you would check it: deflection at the camera, vibration during motion, and a repeatability test on the image.

How to prepare in two weeks

• Pick your best project and redraw one part properly, with datums, fits and a tolerance that you can justify.
• Rehearse the motor sizing chain on that project with real numbers, out loud.
• Learn the names of five manufacturing processes and where each is cheapest.
• Prepare one failure story: what broke, what you measured, what you changed, what the result was.
• Practise explaining a mechanism in sixty seconds without jargon.

Interviewers remember the candidate who can say why a dimension is 6 mm instead of 8 mm. That answer proves the design was reasoned, and it is worth more than a long list of software on a resume.

Related: robotics engineer interview questions, embedded systems interview questions, robotics resume for freshers, robot arm CAD guide, 3D printed robot parts, robotics skills roadmap.

A documented mechanical build answers half the questions before they are asked; start with this portfolio method.

Sources and further reading

• ISO TC 213 — dimensional and geometrical product specification standards.
• ASME codes and standards — drawing practices and dimensioning references.
• Engineering Toolbox — material properties and mechanical formula references.