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378 Forging - S tamping - Heat Tieating
in which the work has moved past the tool (in turning)
or the tool has moved over the work (in shaping)
as indicated in Fig. 5. These small gaps are caused
by the edge of the tool pushing the layer of steel before
it until such time as the steel can move no further
without breaking. At the spot where the steel,
distorted by the moving tool, thus breaks, one of the
small gaps is formed. This accounts for the gaps being
periodic; the size and spacing of them depends
on the cut and feed. On a turned axle the gaps lie
along the axle and across the direction in which the
axle will be stressed in service; they may not, therefore,
do much harm. It is easy, however, to see
that the gaps in other objects may lie in a direction
which under service conditions would facilitate the
extension of the gap into a serious crack or a complete
fracture.
An example of the general effect of rough-machined
surface is shown in Fig. 6, which represents bent
test pieces made from the same bar but finished with
different degrees of smoothness. The depth of groove,
amount of distortion, and intensity of gaps on the
surface decrease from right to left. When deflected
under a hydraulic press the bars broke after bending
through angles which vary according to the condition
of the machined surface. These tests show the advisability
of grinding the machined surfaces of highlystressed
structural parts. The grinding allowance
should be sufficient to remove not only the peaks of
the grooves, but also to thoroughly bottom the gaps
or tears and the distorted surface. Smoothly ground
surfaces as compared with roughly ground surfaces
are also important. Rough grinding on a coarsegrained
wheel can produce considerable surface distortion
and also other evil effects. In this respect
rough grinding is hardly preferable to finished turning.
Non-Destructive Test for Wire Rope
A test which could be applied to steel hoisting rope
to show whether it is in safe condition or not, and
which would not require the cutting of a sample from
the rope, would be of great value. Every industry
and operation which depends on wire rope for hoisting
and haulage purposes is anxious to learn of some
method by which the condition of ropes can be determined
in service.
For some time the Bureau of Standards, Depart
ment of Commerce, has been investigating the possibility
of applying some form of magnetic test to
wire rope to determine its condition, as it is known
that breaks in the individual wires, worn places, etc.,
as well as the stress of the rope, affect its magneticpermeability.
The development of any practicable
test is a difficult matter, because of the many variables
which must be considered.
In order to design intelligently apparatus for the
non-destructive testing of wire rope it is necessarv
to know the nature and magnitude of the effects involved.
One of the causes of deterioration of rope is
wear, and the Bureau has recently- completed an investigation
of the effect of wear on the magnetic properties
and tensile strength of steel wire such as is used
in the manufacture of wire rope.
The Bureau found that wear increases the magnetic
permeability- for low magnetizing force, and decreases
it for higher values; in other words, opposite
October. 1925
readings are secured, depending on the magnetizing
force employed. A load on the wire produces a similar
effect, though it is much less in magnitude, and is
probably caused by a redistribution of the internal
stress in the wire. This change in magnetic properties
is accompanied by an increase in the tensile
strength.
The complete results of this investigation are given
in Scientific Paper No. 510 of the Bureau of Standards,
copies of which can be obtained from the Superintendent
of Documents, Government Printing Office,
Washington, D. C, at five cents each.
Testing Automobile Steels by Sparks
A novel method of inspecting automobile steels to
determine quickly the chemical composition is the
spark test, which, although by no means new, has
only recently been adopted as a routine method of
inspection in large scale production in the Buick
plant, said J. C. Ross, general superintendent of the
Buick Motor Company, in an address at the production
meeting of the Society of Automotive Engineers
in Cleveland recently.
The spark test depends on the fact that minute
quantities of certain elements in steel alter the appearance
of the sparks emitted when the material is
ground on an abrasive wheel. It is performed with
a small portable grinding machine that can be carried
from place to place and plugged into a light-socket by
a long cord. A binful of stock can be sparked on the
ends of the bars without removing or handling the
material.
Carbon, the most important element in steel, excepting
iron, has a very pronounced influence on the
spark. In testing medium and low-carbon steels by
this method, the carbon can be determined within
2/100th or 3/100th per cent by using a suitable standard
of known carbon content. Nickel in steel also
gives a characteristic spark, as do chromium, tungsten
and vanadium. One-half of 1 per cent of nickel
can be readily detected, which allows easy differentiation
to be made between standard steels.
In sorting mixed stock, the spark test is of great
help. Individual bars can be rapidly and cheaply
"sparked" and placed in their proper' classifications.
In the Buick plant, all piston-pin tubing stock is spark
tested as a routine method of inspection. Stock for
certain other parts, such as shackle bolts, is handled
in the same way. It is essential that the cores of these
parts, which are case hardened, should have a carbon
content of not more than y4 of 1 per cent, for a higher
carbon induces brittleness.
Gas Need Not Fear Competition
With gas appliance manufacturers engaged in intensive
research to improve gas-fired installations in
industrial plants, the future of the manufactured gas
industry in the heating field is assured, says the United
States Investor, an authority on public utility securities.
This publication also says that, in any competition
with electricity for heating, the gas interests have
little, if anything, to fear.
"Where proper attention has been given to the
factors of design and efficiency, gas heating appliances
have replaced electrical' appliances," the Investor
says.