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28 September 2001

Bending Glass Tubes

Excepted from Procedures in Experimental Physics by John Strong

 

Fig. 9.

A tube to be bent is heated in the cross-fires with continued rotation until it is quite soft along a length equal to several diameters. It is then removed from the flame and bent to the desired angle with the apex down as shown in Fig. 9. As large tubes are difficult to heat uniformly, imperfections often occur. They are also present in small tubes, particularly in small, thin-walled tubes, that have been bent to a sharp angle. Imperfections are worked out in every case by local heating with a pointed flame. When one portion of the tubing wall is heated until it is soft, the general form of the bend is maintained by the portion on the opposite side of the axis of the tube. If the outside tends to flatten as shown in Fig. 9(b), it is corrected by' blowing while the glass is soft. If the inside surface folds' as shown at (c), it is locally heated with a sharp pointed flame and worked by alternating shrinking with blowing until it is uniform. These corrections are followed by a general heating to anneal the whole bend.

Fig. 10.

A glass coil is made on a mandrel. The mandrel is usually either a steel or brass tube covered with asbestos paper. The paper is applied wet, the ends being lapped and cemented with sodium silicate. After the paper is dry, this lap joint is sandpapered. One or more coats of stove polish or some other | form of carbon will prevent the glass from adhering to the asbestos. Notches in the end of the tube secure the coil to the mandrel. The procedure is illustrated by Fig. 10.