| 摘要 |
A plastics beam compass 1 has a met a pivot point 2 carried by a slidable pivot member 3 lockable to a thin beam 4 by means of clamping ring 8. A graduated scale is provided on beam 4. A pencil can be received in a hole 6 in thickened, angled end portion 5. Clamping ring 7 on end portion 5 clamps the pencil in hole 6 on rotation of said ring. Pivot member 3 is provided with a handle H for moving said member 3 along the beam 4 to a selected position where it can be locked to the beam by rotation of flange 8a of clamping ring 8. Both clamping rings 7 and 8 include helical cam surfaces S (Fig. 8) which cooperate with matching cam surfaces on the end portion 5 and the pivot member 3 so that the rings can be locked through a single revolution or less. <IMAGE> |
