Mr. B. C. Tilghman has recently
discovered another and very interesting application of the
sand blast to industrial purposes. He has found that by
subjecting worn files to the action of the jet, the cutting
edges are rapidly renewed, and the file is made sharper than
The process is as follows: A stream
of fine sand impelled at a high velocity by a jet of steam is
applied to a file in the manner shown in the sketch, Fig. 1, at
an angle of from 10 deg. to 15 deg. from its face; the file
being moved about so that all parts may be acted on. The sand
for the purpose is very fine grit prepared by washing and
settling. It is used in the state of very soft slime drawn from
a receiver, as shown in the sketch.
The effect upon the teeth of a file
which has become dull by wear, is to grind away some of the
metal from the inclined Bides of the teeth Bo as to reproduce a
cutting edge, as shown by Fig. 2.
In this sketch the lines a a show
the flat surfaces produced by wear on the points of the teeth;
the dotted lines b b show the new surfaces produced by the
grinding action of the sand, and the new cutting edges produced
where this surface meets the vertical sides of the teeth.
The bottom diagram, Fig. 3, shows
the action of the blast on a new file. In this case it removes
the burr on each tooth, bringing it to a much sharper edge.
A comparative trial of the cutting
power of the sharpened files was lately made with the following
results: A piece of soft wrought iron was filed clean and
weighed; 1200 strokes were made by a skilled workman with one
side of a new 10 in. bastard file, the iron was again weighed,
and the loss noted.
The other side of this file was
then subjected to the sand blast for five seconds, and 1200
strokes were made with this sand-blasted side on the same piece
of iron, great care being taken to give strokes of equal length
and pressure in both cases. The iron was then weighed, and the
loss found to be double as much as in the first case.
These operations were repeated many
times, counting the strokes and weighing the metal each time,
and the quantity cut was found to gradually become less for both
sides as these became worn.
When the weight of metal cut away
by 1200 strokes of the sand-blasted side was found to be no
greater than had been cut by the first 1200 strokes of the
ordinary side when quite new, a second sand blasting was applied
to it for 10 seconds, and in the next 1200 strokes its rate of
cutting rose to nearly its first figure.
When the cut made by the ordinary
side of the file fell to about four-tenths of its cut when new,
it was considered by the workman as worn out, and a new file of
the same size and maker was used to continue the comparison with
the one sand-blasted side; 83 sets of 1200 strokes each, and 13
sand blastings were made on the same side of this file, and in
that time it cut as much metal as six ordinary sides. In 99,600
strokes it cut away 14 oz. avoirdupois of wrought iron, and 16.4
oz. of steel.
With an equal number of strokes its
average rate of cutting was, on wrought iron, 50 per cent,
greater than the average of the ordinary sides, and on steel 20
per cent, greater. As the teeth became more worn, the time of
the application of the sand blast was lengthened up to one
minute. After the thirteenth re-sharpening its rate of cutting
was nine-tenths that of the ordinary aide when quite new.
When the teeth become so much worn
that the sand blast ceases to sharpen them effectively, the file
can be re-cut in the usual way, and each set of teeth can be
made to do six times as much work as an ordinary file, and to do
it with less time and labour, because it is done with edges
constantly kept sharp.
The time required to sharpen a
worn out 14-in. bastard file is about four minutes, or
proportionately less if sharpened before being entirely worn
out. Smooth files require much less time. About 4 horse power of
60 lb. steam used during four minutes, and one pint per minute
of sand (passed through a No. 120 sieve), and the time of a boy
are the elements of cost of the operation.