Gears (spur, helical, internal, rack)

Generate real involute gears — spur, helical, herringbone, internal ring, rack — from module and tooth count, with the trochoidal root, profile shift, backlash and blank features a made gear actually needs.

Gear generates a true involute gear — spur, helical, herringbone, an internal ring, or a rack. You give it a module and a tooth count; everything else — pitch, base, tip and root diameters, the tooth thickness, the span measurement — is derived and shown live in the panel. It lives on the Tools tab beside Thread, because a gear is a generator driven by a specification table rather than a shape you draw.

Steps

1. Draw a circle where the gear centre goes (Sketching basics). Only its centre is used — the diameter comes from module × teeth, not from the circle.
2. Select the sketch → Gear on the Tools tab.
3. Set module and number of teeth. Add face width, and a bore/hub if you need one.
4. ✔ OK commits; the gear appears in the tree like any other feature and stays editable.

Sizing: module, not diameter

A gear's size is set by its module mn (mm of pitch diameter per tooth), so:

• Pitch diameter d = module × teeth. A module-2, 20-tooth gear has d = 40 mm.
• Two gears mesh when they share a module and pressure angle. Their centre distance is (z1 + z2) × m / 2 — the panel computes it for you under Mating gear.

Working in inches? Switch Specify by to Diametral pitch; hammer8 converts and stores the module (m = 25.4 / Pd), so the file stays metric no matter how you typed it.

The Standard size dropdown offers the ISO 54 preferred modules (and the DIN 780 fine-pitch range below 1 mm, which is where most printed gears live). Free entry always works.

Pressure angle

20° is the ISO 53 / DIN 867 standard and the right default. 14.5° is legacy, 25° carries more load.

hammer8 asks for the normal pressure angle αn — the angle of the basic rack — and shows the derived transverse αt read-only. On a helical gear the two genuinely differ (tan αt = tan αn / cos β), and mixing them up is the classic way to make a gear that will not mesh, so the panel is explicit.

Helical, herringbone, internal, rack

• Helical — set the helix angle β (10–30° is typical) and the hand. A parallel-axis helical pair needs opposite hands and gains quieter, overlapping tooth contact.
• Herringbone — a chevron: the helix reverses at mid-face, cancelling the axial thrust a single helix produces.
• Internal ring — teeth point inward; set the rim Ø for the outer boundary. A pinion runs inside it at centre distance (z2 − z1) × m / 2.
• Rack — the straight bar a pinion rolls on; set its length, height and width.

The tooth profile

• Basic rack (ISO 53) — forms A–D differ only in bottom clearance and root fillet radius. A (full 0.38·m fillet) is the default and the strongest root. Pick Custom to type the four coefficients yourself, within the DIN 867 range.
• Root form — Trochoidal is the real generated shape, the curve a hob's rounded tip actually traces. Circular is a simple tangent arc: faster to read, common in 3D-printing generators. Sharp leaves no fillet at all (avoid it in anything loaded).
• Backlash j — circumferential play, taken half off each flank. For printed gears 0.15 mm per gear (0.3 mm across the pair) is a sound starting point.

Profile shift and undercut

Below about 17 teeth at 20°, the cutter bites into the flank as it forms the root — the tooth is undercut, which weakens it and spoils the contact. hammer8 does not hide this: it generates the undercut flank as it really would be cut, and warns on the feature.

The fix is a positive profile shift x, which moves the cutter outward. Click Auto and the panel sets the minimum shift that clears the undercut (x ≥ (17 − z)/17 at 20°). A shift also raises the tip and root diameters by 2x·m and thickens the tooth — all reflected in the readout.

Blank features

Everything a made gear needs, in the same panel:

• Bore — a through hole on the axis.
• Keyway — tick it and hammer8 offers the DIN 6885-1 key for that bore; both dimensions stay editable. A keyway needs a bore to sit in.
• Hub — a boss on one or both faces. The bore is drilled after the hub, so it runs right through.
• Lighten — Web thins the disc between hub and rim; Spokes cuts it away and leaves N radial bars. They are alternatives, not a stack.
• Accuracy grade (ISO 1328) — recorded for the drawing. It never changes the modelled geometry; hammer8 always models the nominal tooth.

For a chamfer on the gear faces, apply the ordinary Chamfer feature to those edges after the gear is built.

Reading the panel

The Derived block uses ISO 21771 symbols, so the numbers are the ones on a gear drawing: pitch d, base db, tip da, root df, transverse angle αt, tooth thickness sn, and the base tangent length Wk over k teeth — the span a micrometer actually measures.

Mating gear takes a partner tooth count and reports centre distance, ratio and contact ratio εα. Keep εα above 1.2; below 1.0 the pair loses contact between teeth and will not run.

Meshing two gears

1. Build the first gear.
2. Build the second on its own sketch, with its centre circle at the computed centre distance.
3. Give the second gear a tooth clocking of half a pitch — 180 / z degrees — so its teeth sit in the first one's spaces. Without this the two meet tooth-on-tooth and interfere.

Check the result with Interference Check on the Tools tab.

When it fails

• "the bore leaves no material under the teeth" — the bore (or a relief diameter) has eaten into the rim. Shrink it, or use a bigger module.
• "a gear blank is either webbed or spoked" — clear one of the two.
• "the helix needs N profile sections" — a wide face at a steep helix angle on a fine-pitch gear needs more geometry than the kernel can sweep responsively; reduce the helix angle, the face width, or the tooth count.
• "bevel/worm gears land in a later phase" — those two are listed in the panel but disabled.
• The gear builds but is flagged undercut — see the profile-shift section above. It is a warning, not an error: the gear is real, it is just a poor one.
• For anything else, Fix a rebuild error covers how hammer8 reports feature failures.

Open this in the app →