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Pendulum Bob: a Lead Sphere Cast with Its Eye at the Top Centre
Youblob · CC0
Forge

Created by

Forge

18. October 2026NO
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Pendulum Bob: a Lead Sphere Cast with Its Eye at the Top Centre

A long pendulum needs a heavy, symmetrical bob: Foucault's of 1851 was a 28 kg brass-coated lead bob on a 67 m wire. This one is a solid lead sphere 12 cm across, about 10 kg, cast in sand with an eye bolt set in its exact top centre so the wire hangs through its axis. Size and method are ours. The picture: our drawing.
Intermediate
1 day

Instructions

1

Turn the pattern

Turn a hardwood ball 12 cm across on the lathe, checking it with the caliper in several directions, and saw it in half through its centre for a split pattern.

Tools needed:

Hardwood BlockHardwood Block
Wood LatheWood Lathe
Digital Caliper 6-InchDigital Caliper 6-Inch
HacksawHacksaw
2

Ram the mould

Ram one half in each part of the flask, draw them, and cut a sprue into the cavity to one side of the top. Set the eye bolt upside down in the sand of the top so its threaded shank hangs into the cavity exactly on the sphere's axis.

Materials for this step:

Eye BoltEye Bolt1 piece

Tools needed:

Casting SandCasting Sand
Moulding FlaskMoulding Flask
Hand TrowelHand Trowel
3

Melt and pour

Melt about 11 kg of lead in an iron pot on the forge. Skim it and pour into the sprue in one go until the sprue stands full. Let it cool in the sand.

Materials for this step:

Lead IngotLead Ingot11 kg

Tools needed:

Cast Iron Melting PotCast Iron Melting Pot
Blacksmith ForgeBlacksmith Forge
Crucible TongsCrucible Tongs
Digital Kitchen ScaleDigital Kitchen Scale
Welding GlovesWelding Gloves
Leather ApronLeather Apron
Face ShieldFace Shield
4

Trim and check

Shake it out, saw off the sprue and file the seam. Hang it by the eye: it must hang still with the eye straight above the centre. Weigh it.

Tools needed:

HacksawHacksaw
File SetFile Set
Digital Kitchen ScaleDigital Kitchen Scale
Welding GlovesWelding Gloves

Materials

2

Tools Required

15

This blueprint makes

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Pendulum Bob

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Sand Casting

Sand Casting

by ForgeNO

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CC0 Public Domain

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