SPRING 2024

166
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS HOW FASTENERS ARE MADE - PART 2: HOT HEADING AND SCREW MACHINING from page 162
FIGURE 9: EXAMPLE OF CROSS SLIDE
Screw machining was the original method of screw
manufacturing but quickly supplanted by cold heading
when that technology became available. Cold heading
became the favored method for its radical improvement in
productivity and efficiency. Recall that cold heading is fast
with average cycle speeds of about two hundred parts per
minute and does not generate much, if any, waste. Screw
machining, on the other hand, is slow, with the average
cycle on very simple parts falling in the five to ten parts
per minute range. Additionally, since screw machining is
a cutting operation, it generates waste. The greater the
change in diameters the more waste generated. Other
limitations include inability to create drive recesses in the
primary screw machining operations and parts that are not
as strong as cold or hot headed parts.
Although that list of limitations seems pretty
significant, there are some instances where screw
machining fasteners is very advantageous. These include
the following instances.
¤ Precision – Cold headed parts do not have
the dimensional precision that can be achieved in screw
machining. Although the typical screw machining process
does not produce parts that are super precise, it will be
capable of providing dimensional results that are several
times better than normal cold headed parts. Swiss screw
machines can sometimes hold tolerances as tight as
0.0002.”
¤ Complexity – Screw machining allows round
features like rings, grooves, undercuts, and multiple
diameter steps to be easily developed through the screw
machining process.
¤ Small Order Quantities – If a customer only
needs a small quantity of parts, it is exceedingly difficult
to support this cost effectively with cold heading. Screw
machining, however, is well suited to production of small
runs without having to carry the same burden that cold
heading would.
¤ Exotic Materials – This actually has two
facets. Some exotic materials are exceedingly difficult to
form but can be cut. In these instances, it proves less of
a headache to screw machine the parts than to attempt
to cold head them. Secondly, many of these materials
are extremely expensive. Cold heading requires, in
minimum cases, at least several hundred pounds of
continuously coiled metal. If the material is extremely
expensive per pound and only a portion of the minimum
coil weight is required to support the customer’s order,
a significant investment in material may be left over.
Someone must pay for this. As such, screw machining
material is purchased in shorter bars. If a little of this
material ends up left over the monetary impact is not
that significant.
Summary
Even though there are a couple of esoteric
manufacturing methods that very occasionally get
employed to produce a threaded fastener, the three
methods covered in this series represent almost the
entirety of manufacturing methods for fasteners. It is
important for fastener suppliers, especially distributors,
to have at least a rudimentary understanding of these
methods so that they know how to best serve their
customers when non-standard requests come in.
Knowledge of these processes will go a long way
to providing the customer with the best and most
appropriate solution for their needs.
Credits
¤ Figures 1 and 2 Courtesy of Hayden Bolt
¤ Figures 3 and 4 Courtesy of Unytite Inc.
LAURENCE CLAUS