BITS U103 · Lesson 5 · ~50 min

From drawing to model: cracking tutorial-grade parts

The tutorial sheets have jumped from plates to full machine parts — clevises, yokes, brackets bristling with radii. This page is the missing skill: a five-question method that turns a scary isometric drawing into a short, ordered feature list before you touch the mouse.

Start here — count, don't panic

Put the tutorial sheet's part next to this list: a block, a cylindrical boss, a bore, two identical rounded arms, one angled arm, a small side boss, and blend radii. Every one of those is a command you already know from the features lesson — extrude, hole, mirror, fillet. Nothing on that sheet needs a tool you haven't used.

What the sheet doesn't tell you is the order, and that's the actual assignment. Two facts worth holding onto: first-years everywhere stall on exactly this genre of drawing (search "clevis bracket CAD exercise" — it's the standard rite of passage, not a BITS speciality); and the classmates who finish fast aren't seeing the answer at a glance — they're doing a version of the planning below, quickly, often on rough paper. The method is learnable in one evening. That's this page.

1 · Reading a dimensioned isometric

The drawings lesson taught you first-angle multiview: three flat views, and the work is reconstructing the 3D shape. A tutorial-sheet isometric is the opposite deal — the shape is handed to you; the work is extracting sizes and directions. Three reading rules cover almost everything:

Every ⌀ is a cylinder, and the face it's written on tells you its axis. A ⌀ on a face you see as an ellipse means the cylinder's axis is perpendicular to that face. Group the ⌀s by axis direction — a part usually has only two or three directions, and each group will share a sketch orientation.
Every R is an arc lying flat in the face where it's drawn. A big R on the end of an arm (R15, R33) is the arm's rounded end — it belongs in the arm's profile sketch. A small R where two lumps meet (R4, R5) is a blend fillet — it's a Fillet feature at the very end, not sketch geometry. Telling these two apart is half the reading skill.
Linear dimensions run along the three axis directions. An isometric drawing squashes angles but the numbers are true lengths along each axis. If a size you need isn't given directly, it's derivable from ones that are — derive it on paper, never eyeball it off the picture.

One more thing the picture can't show: the back of the part. Standard convention is that undimensioned hidden geometry is symmetric with what you can see. If a dimension is genuinely ambiguous or illegible, write your assumption on the submission and ask the instructor — stating assumptions is normal engineering practice, not an admission of weakness.

Classic trap

Trying to model the picture. The isometric view is the answer, not the method. Beginners try to sketch the outline they can see — a huge, unconstrainable mega-sketch — and get nowhere. You never model what a part looks like; you model the flat cross-sections it's made from, one small fully-constrained sketch per feature. If any sketch of yours has more than about eight lines, you've merged two features.

2 · The five questions

Ask these in order, on paper, before opening Fusion. Ten minutes here routinely saves an hour of un-modelling.

#	Question	What it gives you
1	Which way do the cylinders point? Sort every ⌀ by axis direction.	Your sketch-plane orientations. Each direction is one family of sketches.
2	What's the anchor? The biggest plain lump that everything else attaches to.	Feature 1 — sketch it on an origin plane, centred, so the whole part hangs off geometry that can never break.
3	What repeats or mirrors? Twin arms, hole patterns, symmetric halves.	Work you don't do. A two-armed fork is one lug plus a Mirror.
4	What are the remaining lumps called? Name each with an idiom from the table below.	A known one-sketch recipe per lump — no invention required.
5	What comes last? Blend fillets, chamfers, cosmetic rounding.	The tail of the tree. Geometry first, cosmetics last — and your volume checkpoints stay checkable on the way.

The output of the five questions is a numbered feature list — the part's recipe. Writing it down is not optional ceremony: it's the difference between modelling and flailing, and in a lab exam it's also your recovery plan when something breaks at minute forty.

The idiom table

Machine parts are built from a tiny vocabulary of standard lumps. Learn to name them on the drawing and each one collapses into a recipe you already know:

Idiom	On the drawing	The recipe
Boss	A raised cylinder, usually with a hole down its middle (⌀36 outer / ⌀20 bore).	Circle sketched on a face → Extrude, Join → then the hole.
Lug	A flat arm with a rounded end and a hole (the R15-end, ⌀-holed arms everywhere on tutorial sheets).	One stadium profile — two parallel lines closed by an arc — extruded to the arm's thickness, then the hole. One sketch, always.
Fork / clevis	Two identical parallel lugs with a gap between.	Model one lug offset from the mid-plane → Mirror across it.
Rib / web	A thin triangular or flat stiffener bridging two lumps.	Triangle on the mid-plane → Extrude symmetric, Join.
Arch / slot cut	An arched window, tunnel or U-slot cut through a wall or block.	The lug's stadium profile again — extruded as Cut instead of Join. Same sketch, opposite operation.
Blend fillet	Small R (3–6 mm) written where lumps meet.	Fillet feature, last. Never sketch it.

Check yourself: on the tutorial sheet's clevis, how many cylinder-axis directions are there? (Look at the ⌀36/⌀20 boss on top, the fork-arm holes, and the angled arm's ⌀16.)

Three. The boss and bore point vertically; the fork-arm holes share one horizontal direction; the angled arm's hole points along a third direction perpendicular to that arm's face. So you'll need sketches in three orientations — which is precisely why it can't be one clever mega-sketch.

3 · Worked example: planning a hoist clevis

Here's a part of the same species as the tutorial sheet's, with clean numbers so every step can be shown exactly. Specification, exactly as a sheet would give it:

Rectangular body 50 × 40, height 45.
On top, centred: boss ⌀36, 10 tall, with a ⌀20 bore right through boss and body.
Off one 40-wide face: a fork of two lugs, each 10 thick with a 20 gap between them. Each lug: 36 tall (flush with the base), reaching 40 out to a rounded R18 end with a ⌀16 hole.
Off the opposite face: a single angled lug, 15 thick on the part's centre plane, 30 wide with an R15 end and a ⌀16 hole, its centreline leaving the face 27 above the base and sloping down at 30°, hole centre 45 along the centreline from the face.

Run the five questions and the whole part falls into five features. Watch it assemble — new feature in blue, everything already built in grey:

1 · body

2 · + boss

3 · − bore

4 · + fork (mirror)

5 · + angled lug

And the same thing as the written plan — this is the artefact the five questions exist to produce:

#	Feature	Sketch plane	Profile	Operation
1	Body	XY origin plane	Rectangle 50 × 40, centred on origin	Extrude 45
2	Boss	Top face of body	Circle ⌀36 at centre	Extrude 10, Join
3	Bore	Top face of boss	Circle ⌀20 at centre	Extrude, Cut, extent All
4	Fork	XZ origin plane	Lug stadium + ⌀16 hole (sketch below)	Extrude from offset 10, thickness 10, Join → Mirror across XZ
5	Angled lug	XZ origin plane	Angled stadium + ⌀16 hole (sketch below)	Extrude symmetric 15, Join

The two sketches worth seeing

Features 1–3 are drill-1 material. The fork lug and the angled lug are the two sketches that make people freeze, and both are the same object: a stadium — two parallel lines closed by an arc — plus a concentric circle for the hole.

The fork-lug sketch. Two horizontal lines, one tangent arc, one circle — fully constrained by 40, 36 (= 2 × R18), R18 tangent to both lines, ⌀16 concentric. The dashed left edge lies on the body's face; the extrude's Join fuses them.

The angled-lug sketch, drawn here from the same numbers the volume key uses. The whole trick is the dashed construction line (45 long): start it on the face, 27 above the base, angled 30° below horizontal — then hang the profile off it: edges parallel at 15 each side, R15 arc centred on its far end, ⌀16 concentric. Angled features are easy when the angle lives on one construction line and everything else is parallel or perpendicular to it.

Classic trap

Filleting early. On the sheet, small blends (R4, R5) visually dominate — everything looks melted together, so beginners reach for Fillet in the first ten minutes. Then every later sketch lands on a curved face, dimensions have nothing square to attach to, and the timeline turns to glass. Blends are the last features, always. A useful mental trick: imagine the part with every small radius sharpened to a corner — that is what you model; the fillets are paint.

4 · Build it — with checkpoints

Now build the clevis from the plan above. This is the lesson's real exercise, and it marks itself the same way the drills do — but at every stage, not just the end. After each feature, read Inspect → Physical Properties:

After feature	Volume must read
1 · Body	90 000.00 mm³
2 · Boss	100 178.76 mm³
3 · Bore	82 900.00 mm³
4 · Fork (both lugs, holes in)	117 857.52 mm³
5 · Angled lug (hole in)	140 393.03 mm³

Checkpoint discipline is the professional habit hiding in this exercise: on a five-feature part a mismatch found at the end could be anywhere, but a mismatch found at a checkpoint is in the feature you just made. In the lab exam, that's the difference between a ten-second fix and starting over.

Unpack this step — where the checkpoint numbers come from

Each is plain solid arithmetic: 50 × 40 × 45 = 90 000; the boss adds π × 18² × 10; the bore removes π × 10² × 55 (through boss and body — height 55). Each lug's profile is stadium area 40 × 36 + ½π × 18², minus the π × 8² hole, times 10 thick, twice. The angled lug: 30 × 45 + ½π × 15² − π × 8², times 15. Nothing beyond mensuration — the part only looks like it needs harder maths.

Check yourself: your volume after feature 4 reads 119 868.14 instead of 117 857.52 — about 2 011 too much. What single mistake causes exactly that?

Exactly one lug's hole is missing: π × 8² × 10 ≈ 2 010.62. The usual cause: the Mirror was given the lug's extrude but not its hole feature, so the second arm came out solid. Select all of a lug's features when mirroring — or cut the hole once with extent All after both arms exist, so one cut drills both.

5 · Back to the actual tutorial sheet

Now decode the assigned part with the same five questions. Reading its drawing with the idiom table in hand:

The tutorial sheet: a clevis bracket with a 36 boss and 20 bore on top, two R15 fork arms, an angled arm with an R33 end and 16 hole, and small blend radii

Anchor: the central body the arms grow from.
Boss + bore: ⌀36 raised ring with the ⌀20 bore, on top — features 2 and 3, identical shape to the worked example.
Fork: the two parallel R15-ended arms with their common hole — one lug stadium, mirrored. Their shared hole axis is your second cylinder direction.
Angled lug: the long arm sweeping down to the R33 end with its ⌀16 hole — one construction line at the drawing's stated angle, one stadium, one hole. Same sketch as the worked example, different numbers.
Side boss: the small ⌀26 pad with the ⌀12 hole — a boss idiom on that arm's face.
Blends last: the R4/R5 radii where everything meets — one Fillet feature at the end, several edges selected together.

That's roughly eight features. Two honest cautions: some dimensions on a photographed sheet will be barely legible — derive what you can from overall sizes, state any assumption in your submission, and confirm with the instructor rather than guessing silently. And since the sheet gives no volume key, use the sharp-cornered checkpoint habit: get all geometry matching your own arithmetic before the final fillet pass, because after fillets the volume is no longer hand-checkable.

6 · Hints for the assignment batch

The exercise sheets handed out after the clevis (August 2026) are all the same species — five parts, and not one of them needs a command beyond this page. They're listed here in a sensible working order, easiest first: doing them in this order means each part reuses a decision from the one before.

These are pointers, not plans. Run the five questions on paper first, write your feature list, then open the hint and compare. And since these sheets give no volume key: after each simple stage, check Fusion's volume against your own arithmetic (a base plate is one multiplication) — the sharp-corner checkpoint habit from section 5.

1 · The stepped block — all flat faces, a U-slot, one half-round notch

The busiest-looking drawing in the batch and the simplest part in it: there is not a single boss or lug on this thing.