Though the process of creating a custom bicycle varies from customer to customer, the basic components are the same, allowing the use of dedicated tooling to be created. Often this tooling is designed with a range of variability so that minor changes in design can be accommodated.
Production work, however, necessitates that dedicated, fixed position equipment be used to ensure accurate, repeatable results from piece to piece.
Today, I want to talk about the importance of "soft jaws". Soft jaws are replaceable tooling that hold a part for machining, often not heat treated, allowing the operator two distinct advantages; the ability to shape the jaws to perfectly fit the piece being machined. Secondly, the shaped clamping surface provides greater surface contact allowing for a less robust clamping surface, leaving no impressions on the finished part.
The example we are going to look at here is a set of jaws that were shaped to accept the spider interface for the Hot Rod cranks. In this case, the part is water jet cut to the rough dimensions and the center hole needs to be opened up to 24.02 mm and then face relieved a skoosh so that it is a tight slip fit over the bottom bracket spindle.
 |
| The base part, before machining |
To place this part into a standard set of pointed hard jaws in a lathe makes concentricity difficult to attain part to part, as there is minimal surface contact between the jaws and the part to be machined. I used a tool post grinder, mounted to the cross slide, to create a shaped opening in the face of the jaws, allowing me to have a surface that holds the part perfectly flat and concentric to the spindle for machining.
 |
| The lathe jaws, ground to accept the part |
This shaping of the jaws permits a consistent origin point from which to begin the machining process, and contacts a majority of the outer surface leaving no clamping impression, therefore meeting our goals.
 |
| The part clamped and machined in the soft jaw |
Whether in a vice, lathe, or in a fixture, precision soft jaws created for the operation are an asset to the fabricator and should be considered for any production piece.
cheers,
rody
