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Engineering Graphics and CAD · Lesson 26 of 35

Degrees of freedom, interference, and clearance

Verify an assembly moves and fits correctly by reasoning about DOF and checking interference/clearance.

Readiness check

Learning objectives

By the end of this lesson you can:

Count the degrees of freedom of a joint and of an assembly.
Predict whether an assembly is under-, correctly, or over-constrained.
Run and interpret an interference check.
Distinguish intended interference (a press fit) from an error.
Verify clearances against the specified fits from L14.

Check your starting point

Five to ten minutes.

How many independent ways can a free, unconnected part move in 3D space?
If a jaw should only rotate about a pin, how many degrees of freedom should it keep after mating?
If two parts overlap in the model, is that always a mistake?

Interpretation.

Q1: Six: three translations and three rotations. Skill 26.1.
Q2: One (the rotation). The mates should remove the other five. Skill 26.1.
Q3: No; an intended press fit overlaps on purpose. Skill 26.4 distinguishes intended from error.

You need L25 (a mated assembly) and L14 (fits).

0 or 1 weak itemsContinue with this lesson.

2 weak itemsReview Lesson 25, then return.

3 or more weak itemsWork through the prerequisite examples before continuing.

The core idea

What it is. This lesson covers three checks on an assembly: counting degrees of freedom (the independent motions parts retain), detecting interference (parts occupying the same space), and verifying clearance (the gaps between parts) against the intended fits.

Why an engineer needs it. An assembly can look right yet be wrong: a mechanism with too many or too few degrees of freedom will not move as intended, and parts that interfere will not assemble (unless the interference is a deliberate press fit). These checks confirm the assembly is sound before it is built.

What problem it solves. It verifies that parts move correctly and fit correctly, catching motion and fit errors in the model rather than in hardware.

What goes wrong when it is ignored. An over-constrained mechanism jams; an under-constrained one flops; unintended interference means parts cannot be assembled; a clearance that does not match the intended fit gives the wrong joint behavior.

A simple mechanical example. The clamp's pivoting jaw should keep exactly one degree of freedom (rotation about the pivot). If the mates leave zero, it cannot move (over-constrained); if they leave more than one, it wobbles (under-constrained). Separately, the pivot pin may be an intended press fit (interference) while any other overlap is an error.

Degrees of freedom (DOF). A free rigid body has 6 DOF (3 translations, 3 rotations). Each mate removes some:

Fastened removes all 6 (0 remain).
Revolute leaves 1 rotation.
Slider leaves 1 translation.
Cylindrical leaves 2 (1 rotation, 1 translation).
Planar leaves 3.
Ball leaves 3 rotations.

A mechanism should retain exactly the DOF its function needs; the mates should remove the rest without over-constraining.

Interference and clearance. An interference check finds where parts overlap. Some overlap is intended (a press fit, from L14); the rest are errors. Clearance is the gap between parts, which should match the fit specified (a running clearance, a locating transition).

Part 5: Parts, assemblies, and drawings.

Check: explain the decision in your own words before using a CAD command.

The lesson map. Degrees of freedom, interference, and clearance becomes manageable when you move through the four checks in order and verify each result before continuing.

The skills, taught in order

Skill 26.1 - Count degrees of freedom

Concept. A free body has 6 DOF; each mate removes some, leaving the mechanism's intended motion. Terminology. Degree of freedom, translation, rotation, joint DOF. Procedure. Start from 6 per free part; subtract what each mate removes; the remainder is the retained motion. Reasoning. The retained DOF is what the mechanism can do; it should match the function. Failure mode. Losing track of DOF, so the mechanism is over- or under-constrained. Check. State the DOF a revolute mate leaves (one rotation).

Skill 26.2 - Judge under-, correct, or over-constraint

Concept. The assembly should retain exactly the intended DOF: no more (loose), no fewer (jammed). Terminology. Under-constrained, correctly constrained, over-constrained. Procedure. Compare the retained DOF to the intended motion. More means under-constrained; fewer means over-constrained; equal is correct. Reasoning. Correct constraint gives the intended, repeatable motion. Failure mode. Accepting a wobbly (under) or jammed (over) assembly. Check. For a jaw meant to pivot, state the correct retained DOF (one).

Skill 26.3 - Run and read an interference check

Concept. An interference check finds overlapping material between parts. Terminology. Interference, overlap, interference detection. Procedure. Run the check; it lists overlaps and their volumes; inspect each. Reasoning. Overlaps reveal parts that cannot coexist as modelled unless the interference is intended. Failure mode. Ignoring the check and shipping parts that will not assemble. Check. Run an interference check and read one reported overlap.

Skill 26.4 - Distinguish intended interference and verify clearance

Concept. Some interference is a deliberate press fit; other overlaps are errors. Clearances should match the specified fit. Terminology. Press-fit (intended) interference, error interference, clearance verification. Procedure. For each interference, decide if it is an intended press fit (per the L14 fit) or an error; keep the intended ones and fix the errors. Check clearances match the specified running or locating fit. Reasoning. Not all overlap is wrong, and clearances must match the intended joint. Failure mode. Deleting an intended press fit, or missing an error. Check. Given a flagged overlap at a press-fit pin, decide whether to keep or fix it (keep, if it matches the fit).

Worked example 1: count the clamp jaw's degrees of freedom

Problem. The clamp's pivoting jaw is connected to the grounded base by a revolute mate about the pivot pin. Confirm the jaw retains exactly one degree of freedom (rotation about the pivot) and that the mate scheme is neither under- nor over-constrained.

Planning. Start from 6 DOF for the free jaw, subtract what the revolute mate removes, and compare with the intended motion.

Solution.

Free jaw. On its own, the jaw has 6 DOF (3 translations, 3 rotations).
Revolute mate. A revolute mate about the pivot removes 5 of those: all 3 translations and 2 of the 3 rotations, leaving 1 rotation about the pivot axis.
Retained DOF. 6 minus 5 equals 1: the jaw retains exactly one rotational DOF, about the pivot, which is the intended motion.
Constraint judgement. The intended motion is one rotation; the assembly retains one; so it is correctly constrained (not jammed at zero, not loose with more than one).
Check by dragging. Dragging the jaw rotates it about the pivot and does nothing else, confirming the single DOF.

Result. The jaw retains exactly one degree of freedom (rotation about the pivot), so the revolute mate correctly constrains it, matching the clamp's function.

Why the method works. Counting from 6 and subtracting the mate's removed DOF gives the retained motion, which is then checked against the intended function.

How to verify independently. Try every motion: only rotation about the pivot is possible. One available motion confirms one DOF, as counted.

Worked example 2: interference, intended versus error

Problem. An interference check on the clamp flags two overlaps: (a) the pivot pin in its hole, specified as a press (interference) fit, and (b) the moving jaw overlapping the base at full closure by 0.4 mm. Decide which to keep and which to fix, tying the intended one to its L14 fit. The complication is telling a deliberate press fit from a design error.

Planning. Classify each overlap as intended (matches a specified interference fit) or an error (unintended collision).

Solution.

Overlap (a), pivot pin. The pin-in-hole overlap corresponds to the specified press (interference) fit from L14 (for example H7/p6). This interference is intended: it is how the pin is retained. Keep it. In the model, a press fit is often represented so it does not flag as an error, or it is annotated as intended.
Overlap (b), jaw into base. The moving jaw overlapping the base by 0.4 mm at closure is not a specified fit; it is two parts trying to occupy the same space during motion. This is an error (a collision): the clamp cannot close that far without the parts clashing.
Fix the error. Adjust the geometry or the motion limit so the jaw stops before colliding (add a stop, shorten the travel, or relieve the clashing geometry). Re-run the check to confirm the collision is gone.
Keep the press fit. Leave the pin interference; verify it matches the intended interference fit's magnitude.
Clearance check. Where a running clearance is specified (say the jaw sliding on a guide), verify the modelled gap matches the L14 clearance fit (positive, within the fit range).

Comparison. Overlap (a) is an intended press fit and is kept; overlap (b) is an unintended collision and is fixed. Not all flagged interference is wrong; classify each against the intended fit.

Result. Keep the pivot-pin interference (an intended press fit matching its L14 fit); fix the 0.4 mm jaw-into-base overlap (a motion collision) by limiting travel or relieving geometry.

Independent check. Re-run the interference check after the fix: the pin interference remains (intended), the jaw-base collision is gone. That result confirms the correct classification and repair.

Misconceptions and diagnostics

Misconception	Why it seems reasonable	Why it is wrong	Evidence that reveals it	Correction	Diagnostic question
"Any flagged interference is an error."	Overlaps look wrong.	Intended press fits overlap on purpose.	The flagged pin is a specified press fit.	Classify each overlap; keep intended press fits, fix errors.	"Is this overlap a specified press fit or an unintended collision?"
"More mates make an assembly better."	More seems more secure.	Extra mates over-constrain and can jam or conflict.	The mechanism will not move, or reports a conflict.	Use only the mates needed to leave the intended DOF.	"Does the assembly retain exactly the intended motion?"
"If it looks assembled, the parts fit."	It renders fine.	An interference check may reveal hidden overlaps.	The check flags a collision the eye missed.	Run the interference check.	"Did the interference check pass, except for intended fits?"

Practice ladder

Level A - Recognition

Task. For eight single joints, state the degrees of freedom each leaves. Deliverable. Eight DOF values. Success criteria. At least six correct. Answer guidance. Fastened 0, revolute 1, slider 1, cylindrical 2, planar 3, ball 3. Common errors. Confusing cylindrical (2) with revolute (1). Difficulty. Low.

Level B - Guided application

Task. Interpret a guided interference report, classifying each overlap as intended or error. Deliverable. A classified list. Success criteria. Press fits kept; collisions flagged for fix. Answer guidance. Compare each overlap to the specified fits. Common errors. Deleting an intended press fit. Difficulty. Medium.

Level C - Independent application

Task. Check the clamp's DOF and run its interference check, reporting results and any fixes. Deliverable. A DOF-and-interference report. Success criteria. Correct DOF count; overlaps correctly classified; errors fixed. Answer guidance. Count DOF from the mates; classify overlaps against the fits. Common errors. Missing an over-constraint or a collision. Difficulty. Medium to high. (Assembly task assessment evidence, Project P5.)

Level D - Transfer and design

Task. Diagnose a supplied over-constrained or interfering assembly and propose the fix, justifying it with DOF and fit reasoning. Deliverable. A diagnosis and proposed fix. Success criteria. Correct identification of over-constraint or interference; a sound fix tied to DOF and fits. Answer guidance. Count DOF; run interference; separate intended from error. Common errors. Fixing the wrong constraint. Difficulty. High.

Working with AI, and proving it yourself

Use AI as a tutor

Useful AI support:

Ask it to explain DOF for a joint type.
Ask it to help interpret an interference report you describe.
Ask it to suggest how to remove an over-constraint.

Limits:

A text assistant cannot see your assembly's DOF or overlaps.
It may call all interference an error.

Verify AI output against: the DOF table (fastened 0, revolute 1, and so on), the intended-versus-error interference distinction, and the L14 fits.

Prove it yourself

A plausible but incorrect AI answer, and how to catch it. You ask, "The interference check flags the pivot pin in its hole. Should I resize the pin to remove all overlap?" and the assistant replies: "Yes, remove every interference so no parts overlap."

This would ruin an intended press fit. Detect it with the intended-interference principle: the pivot pin is specified as a press (interference) fit, so its overlap is deliberate and retains the pin. The evidence is the fit spec (an interference fit from L14). Correct conclusion: keep the intended press-fit overlap; remove only unintended collisions.

Retrieval and spaced review

How many degrees of freedom does a free rigid body have?
How many DOF does a revolute mate leave? A slider? A fastened mate?
What does an interference check find?
How do you tell intended interference from an error?
What is an over-constrained assembly?
How do you verify a clearance?
Cumulative (L14): How does an intended press-fit interference relate to the fits you specified in L14?
Reconstruction task: From memory, count the clamp jaw's DOF from its revolute mate.

Answers. 1: six (3 translations, 3 rotations). 2: revolute leaves 1, slider leaves 1, fastened leaves 0. 3: places where parts overlap (occupy the same space). 4: intended interference matches a specified press fit; errors are unintended collisions. 5: one with fewer DOF than its function needs, so it jams or conflicts. 6: compare the modelled gap to the specified clearance fit. 7: a press fit is an interference fit (shaft larger than hole), so the modelled overlap should match the L14 interference.

Suggested review intervals. 1 day, 3 days, 7 days.

Reference mapping and next step

Read further

Onshape docs (interference)
links to L14 fits.

Standards details must be checked against the current official edition used by your institution or employer.

Finish the lesson

You can now: count degrees of freedom; judge under-, correct, and over-constraint; run and read an interference check; distinguish intended press fits from errors; and verify clearances against fits.

Self-assessment checklist.

I can count the DOF a mate scheme leaves.
I check the assembly retains exactly the intended motion.
I run interference checks.
I keep intended press fits and fix collisions.
I verify clearances against the specified fits.

Next lesson: L27 - Exploded views and bills of materials. Why it follows: with a correctly constrained, interference-checked assembly, you can now communicate it: an exploded view shows how it goes together and a bill of materials lists its parts, the documentation an assembly needs.

Required files or submissions: submit your Level C DOF-and-interference report (Project P5). Optional extension: in Onshape, run an interference check on your clamp and classify every flagged overlap.

Part V continues in 17-part5-lessons-cont.md with L27 (Exploded views and bills of materials), L28 (Drawings from CAD), and L29 (Revisions, versions, and engineering change).

# Engineering Graphics and CAD - Phase 4: Full Lesson Content, Part V (continued), L27-L29

Continues 16-part5-lessons.md. Holds L27 (exploded views and bills of materials), L28 (drawings from CAD), and L29 (revisions, versions, and engineering change). CAD actions given for Onshape with the software-neutral principle first. L28 integrates all of Part III. No em dashes.