The peel turning operation serves the purpose of giving the bars a smooth even surface.
The peeler bars are cut into billet lengths. The lengths of the billets vary according to size and the final lenfth of the tubes.
After cutting the billets are deep bored. Large size billets are normally bored with a small pilot hole which will be enlarged in the expansion press, i.e. to avoid loss of material
Front end radius
The front end of the billet is turned to obtain a rounded outer edge. This is done in order to reduce the deformation resistance at the start of the extrusion. Billets for expansion will have a tapered bore machined so that the expansion mandrel can be forced through more easily
Each billet is inspected. Any surface defects are ground off.
As a last operation before extrusion the billets are ground off.
Heating before expansion/extrusion
The billets are heated to approx 1200 degrees C before expansion and extrusion. The heating is carried out in induction furnaces -- a very rapid process.
Lubrication before expansion
Glass is used as a lubricant. The outside of the billets is coated with finely ground glass which melts and protects the surfaces of the billets during expansion. To protect the expansion mandrel a conical glass plug is placed on top of the billet. The plug melts and lubricates the inner surface of the billet during expansion.
The expansion operation is carried out in a vertical press. A conical mandrel is pushed down into the pilot hole in the billet and explains it to a larger diameter.
Lubrication before extrusion
Glass is used as a lubricant. The operator lubricates the inner surface of the billet by inserting crushed glass into the hole of the billet. This is done with a spoon as long as the billet bore. The billet then rolls over a table with glass powder lubricating the outer surface.
The tubes have to be cooler after extrusion. As a rule, they are quenched in water. Small sizes with thin walls may be cooled in air on a cooling bed.