Tutorial AV: Rocker Arm

Source

Ming C. Leu, Wenjin Tao, Amir Ghazanfari and Krishna Kolan, NX 12 for Engineering Design (Missouri University of Science and Technology), exercise 4.10.1 (Rocker Arm).
You learn

arcs typed in the fields beside the pointer (centre, radius and start angle, sweep), Extrude, Block and Cylinder with Unite, Hole with a countersink and with a counterbore
Sizes

the drawing’s, in inches, typed in millimetres.

The book gives the rocker arm only as a drawing; its views are small and rotated, and the replay follows this reading of them:

  • The pivot is at the origin: a boss Ø1.00 × 1.00 along ZC. Its hole is Ø0.38 through, with a 0.75 × 82° countersink.

  • The bar is 0.31 wide (XC), runs from the pivot up to 2.50, and is 0.75 thick (ZC).

  • The arm is 0.38 thick: a band between R1.50 and R2.50 about the pivot, from 90° to 180°, with its end rounded R0.50.

  • A slot 0.50 wide runs on R2.00 from 105° (75° from the horizontal) to 180°, with R0.25 ends.

  • The top boss is Ø1.00 along XC on top of the bar, its centre level with the top, 2.50 up. It runs from 0.12 behind the bar to the bar’s front face. Its hole is Ø0.44 through, with a Ø0.75 × 0.25 counterbore.

Step by step

1. The arm

Create Rocker_arm and click Sketch on the XC-YC plane. Draw the band with Arc (A):

  • the outer arc: centre (0, 0), R2.50 from 90°, a 90° sweep;

  • the R0.50 end: centre (−2.00, 0), from 180°, a 180° sweep;

  • the inner arc: R1.50.

Close it with a Line (L) on the YC axis from 1.50 to 2.50. Draw the slot the same way: arcs R2.25 and R1.75 from 105° to 180°, and an R0.25 arc at each end.

Type every value in the fields beside the pointer. Where a typed point lands on an end point, the sketcher makes the two coincident, so both loops close. Ctrl+Q, then Extrude the sketch 0.38 along ZC.

2. The bar and the bosses

  • Block 0.31 × 2.50 × 0.75 at the origin, Unite: the bar.

  • Cylinder Ø1.00 × 1.00 along ZC at the origin, Unite: the pivot boss.

  • Cylinder Ø1.00 along XC from (−0.12, 2.50, 0.375), long enough to reach the bar’s front face, Unite: the top boss.

3. The holes

Select the pivot boss’s top face, click Hole and place the point at its centre. Choose Countersink: Ø0.38 through, countersink Ø0.75 × 82°. Select the top boss’s back face, click Hole at its centre and choose Counterbore: Ø0.44 through, counterbore Ø0.75 × 0.25.

The rocker arm: the band with its slot

Save the part.

The replay

# In the book (NX) In FreeCAD NX-Style Result Notes

AV1

Create a new file (Rocker_arm)

Ctrl+N, Model template

same

new document ['Unnamed3']

AV2

Units in the New dialog (the book picks inches)

metric part kept; book sizes typed in mm

differs

FreeCAD sets units in Preferences, not in the New dialog; NXStyle defaults to millimetres (document: Standard (mm, kg, s, °); 25.4 mm shows as 25.40 mm)

AV3

Save it as Rocker_arm

file name given once by script (the system file dialog is not driven); a change, then the Ctrl+S key

same

Rocker_arm.FCStd rewritten by Ctrl+S: the file holds the change and the part is unmodified afterwards (modified before the key: True); FreeCAD format, not .prt

AV4

Sketch the arm: the band R1.50 to R2.50 from 90° to 180° with its R0.50 end, and the slot on R2.00 from 105° (75°) to 180°

ribbon 'Home › Direct Sketch > Sketch' on 'XY-plane (Base plane)'; arcs (A) and a line (L), every point typed in the fields beside the pointer

same

8 curves in 2 loops, closed True; area 1309.0 mm², the drawing’s 1309.0; how: centre typed, start typed, sweep typed; line 2 typed

AV5

Extrude the arm 0.38 along ZC

ribbon 'Home › Feature > Extrude', a side picked (click; Curve Rule Inferred Curves), dialog opened True; Boolean 'None', 'lengthEdit' = 9.652 mm; OK clicked

same

same solid as the reference: 0 mm³ differ, box within 1.3e-07 mm, 10 faces (10 in the reference)

AV6

Block 0.31 x 2.50 x 0.75 at the pivot: the bar, Unite

ribbon 'Home › Feature', Type 'Origin and Edge Lengths', XC/YC/ZC 0/0/0 typed, length 0.31, width 2.5, height 0.75 (in) typed in mm, Boolean 'Unite', OK

same

same solid as the reference: 0 mm³ differ, box within 3.6e-15 mm, 14 faces (14 in the reference); origin (0.0, 0.0, 0.0); one undo step ['Block']; with Body

AV7

Cylinder Ø1.00 x 1.00 along ZC at the pivot, Unite

ribbon 'Home › Feature', Type 'Axis, Diameter, and Height', Specify Vector 'ZC', XC/YC/ZC 0/0/0 typed, diameter 1, height 1 (in) typed in mm, Boolean 'Unite', OK

same

same solid as the reference: 0 mm³ differ, box within 0 mm, 15 faces (15 in the reference); origin (0.0, 0.0, 0.0); one undo step ['Cylinder']; with Body

AV8

Cylinder Ø1.00 along XC on top of the bar, from 0.12 behind it, Unite

ribbon 'Home › Feature', Type 'Axis, Diameter, and Height', Specify Vector 'XC', XC/YC/ZC -3.048/63.5/9.525 typed, diameter 1, height 0.43 (in) typed in mm, Boolean 'Unite', OK

same

same solid as the reference: 0 mm³ differ, box within 1.4e-14 mm, 16 faces (16 in the reference); origin (-3.048, 63.5, 9.525); one undo step ['Cylinder']; with Body

AV9

Hole on the pivot boss’s top face, at its centre

Face14 selected + ribbon 'Home › Feature > Hole'; ribbon 'Sketch › Curve > Point', the point typed in the sketch on the face

same

1 point(s); at the centre True

AV10

Hole Ø0.38 through, countersink 0.75 x 82°

'Diameter' = 9.652 mm, Depth 'Through all', 'Countersink', 'HoleCutDiameter' = 19.05, 'HoleCutCountersinkAngle' = 82, MB2 = OK

same

same solid as the reference: 0 mm³ differ, box within 3.2e-14 mm, 18 faces (18 in the reference)

AV11

Hole on the top boss’s back face, at its centre

Face3 selected + ribbon 'Home › Feature > Hole'; ribbon 'Sketch › Curve > Point', the point typed in the sketch on the face

same

1 point(s); at the centre True

AV12

Hole Ø0.44 through, counterbore Ø0.75 x 0.25

'Diameter' = 11.176 mm, Depth 'Through all', 'Counterbore', 'HoleCutDiameter' = 19.05, 'HoleCutDepth' = 6.35, MB2 = OK

same

same solid as the reference: 0 mm³ differ, box within 4.8e-14 mm, 22 faces (22 in the reference)

AV13

The rocker arm

compared with the drawing’s dimensions built by the Part API

same

same solid as the reference: 0 mm³ differ, box within 4.8e-14 mm, 22 faces (22 in the reference)

AV14

Save the part

file name given once by script (the system file dialog is not driven); a change, then the Ctrl+S key

same

Rocker_arm.FCStd rewritten by Ctrl+S: the file holds the change and the part is unmodified afterwards (modified before the key: True); FreeCAD format, not .prt
Result Means

same

The book’s NX steps work as written.

differs

It works, but the FreeCAD way is different; the notes say how.

limit

The book’s way fails in FreeCAD itself; a workaround follows in the next steps.

missing

FreeCAD has no such function; a stand-in follows.

skipped

Not replayed; the notes give the reason.

FAILED

Did not work.

Each step is compared with FreeCAD’s Part API build of the drawing’s dimensions.

What differs from NX

  • In FreeCAD 1.1.4, values typed in the sketcher’s fields are stored rounded to six decimals, with angles in radians. Its autoconstraints also drop the coincidence a typed point lands on, so the arcs ended up to 3.3e-5 mm apart and the profile stayed open. NX-Style keeps the typed values exact and joins the ends (PRs #37 and #39); the outline now closes to 2e-14 mm.

  • Where two arcs meet tangentially, the join already holds the second arc’s radius, so its typed radius dimension is not kept as a constraint.