The Blacksmith's Craft
PART IV
The lessons which follow are on more advanced forging and welding, calling for a higher degree of skill. By applying the techniques he has already learned, the knowledge gained by experience and by using common sense the student should have no difficulty in following the exercises.
BENDS TO DIMENSIONS
In making bends to measurements, allowance has to be made for the amount of
metal used in actually forming the bend. The allowance will vary with the
type of bend and with the individual smith; no hard and fast rule can be laid
down. Experience will show what allowance to make, but the figures given below
are a good guide for a start.
(a) Radius Bends
Radius bends should have the inside radius equal to the thickness
of the metal. No preliminary upsetting is required.
To make the U-shaped piece from11/4" x 1/2"
shown in Lesson 26,
subtract once the thickness of the metal for each bend from 3" which
is the outside measurement over the bends. The marks will therefore be 2"
apart.
If the two dots are kept in the middle of each bend and the correct radii
maintained, the dots will finish up in line with the inside surfaces and the
given measurement will be correct.
The same allowances are made for the Z bend in 1" round,
also shown in Lesson
26, and again, if the correct inside radii are maintained the dots will
finish up in line with the inside edge of each leg.
(b) Plain Square Corner Bends
Plain square corner bends have the inside corner forged square and the outside
corner left rounded, and no preliminary upsetting is required. With
light section, square and flat metal the corner can be strengthened by upsetting
the material as the bend is made. The inner and outer corners must be forged
simultaneously; if the inner one is squared first, it will be galled in forming
the outer one.
To make the Z-shaped piece in 1/2"
square shown in
Lesson 28, subtract from the outside measurement half the thickness
of the metal for each bend. If the dots are kept on the diagonal line joining
the inner and outer corners of each bend, the finished measurement will be
correct.
Although the shape is different, the same allowances are made for bending
both the shaft staple in Lesson
27 and the U-bolt in Lesson 28. Again, the dots should
be kept on the diagonal line through the corners of each bend.
(c) Forged Square Corner Bends
Forged square corner bends are upset before bending starts. The extra metal
is then worked into the bend to form a corner square on the outside and with
a radius on the inside.
To make the Z-shaped piece in Lesson
29 (which has a square corner on the outside and the radius on the inside
of each bend) it is first necessary to upset the metal to 11/2
times its thickness where each bend will be formed. To the outside measurement,
add a quarter of the thickness of the material and mark the bar. Next, upset
the bar evenly around the mark using 11/2
times the thickness of the material for each upset. The overall length of
the bar will now be reduced by three times its thickness. The marks will be
11/4 times the thickness of the bar closer
together than the outside measurement of the finished bend. If the marks are
now kept on a diagonal line through the corner, the outside measurement will
again be correct.
