The materials used for jigs and fixtures are varied, and most are processed to give them resistance to wear. Non-ferrous metals such as phosphor bronze may be used in locations in order to avoid serious wear on mating parts. Hektelon, nylons and other types of fibre may be used in locations in order to prevent damage to the workpiece. A few typical materials which find application in jigs, fixtures, press tool and collets are given below.
1. High-Speed Steels (HSS): These steels are
normally considered to be either 18% (or 22%) tungsten, which contributes
toughness and cutting strength, 4.3% chromium, which enhances hardenability and
wear resistance, and approximately 1% vanadium, which provides high-temperature
hardness (red hardness) and impact resistance. The steel can be air- or
oil-hardened to Rockwell C64-65 and can be used to manufacture cutting tools
(drills, reamers, cutters, etc.).
2. Die Steels: These steels, also referred to as
high carbon (1.5-2.3%) high chromium (12%) cold working steels (HCHC), are
primarily used for manufacturing cutting press tools and thread-forming rolls.
For applications involving high temperatures, such as forging, casting, and
extrusion, hot die steels with lower carbon content (0.35%), reduced chromium
(5%), and alloyed with molybdenum (1%) and vanadium (0.31%) are employed. These
additives help maintain hardness at elevated temperatures.
3. Carbon Steels: These steels contain carbon in
the range of 0.85-1.18%. When oil-hardened, the steels attain Rockwell C 62-63.
As a general rule, these steels are used for applications in general tools to
cut softer materials such as wood or for hand tools like files, chisels,
razors, and others as used in the production of parts for jigs and fixtures
which wear out but are oil-hardened in order to further reduce wear.
4. Collet Steels: These are spring steels
containing about 1% carbon and 0.5% manganese, and they are usually tempered to
about ~Rockwell C 47 hardness. Collet steels are used in those applications
where only minor adjustments are made and resiliency toughness is wanted.
5. Oil-Hardening Non-Shrinking Tool Steels
(OHNS): These are steels containing 0.9-1.1% carbon, 0.5-2% tungsten, and 0.45%
vanadium. They are suitable for intricate workpieces of high precision, such as
taps, hand reamers, milling cutters, engraving tools, and complicated press
tools that cannot be ground after hardening, but which need a hardness as high
as Rockwell C 62.
6. Case Hardening Steels: These steels may be
carburized and case-hardened to give a hard surface layer (Rockwell C 59-63) of
0.6-1.5 mm thick. Typical of these is the 17Mn1Cr95 steel, which has the basic
composition of 1% manganese and 0.95% chromium. A modified version,
15Ni2Cr1Mo15 steel, has an addition of 2% nickel to lower thermal expansion up
to 100°C. Because case hardening steels are suited for parts that require only
localized hardness on small wear surfaces, they are an economical alternative
to fully hardened tool steels.
7. High Tensile Steels: These include medium
carbon steels, 0.45-0.65% carbon such as En89, and alloy steels, for example,
40Ni2Cr1Mo28, En24. By tempering their tensile strength can be increased up to
a maximum of 125 kg/mm², Rockwell C 40. Medium carbon steels find applications
for fasteners and structural while alloy steels are used where high stresses
are involved as in press rams.
8. Mild Steel: This generally contains less than
0.3% carbon and is the cheapest type available; it finds extensive application
in jig and fixture construction. Because of its lower hardness, it should be
used only in those components which are subjected to least wear and stress.
9. Cast Iron: Cast iron contains 2-5% carbon and
is noted for its very good resistance to vibration and hence it is used in the
fixtures. Its self-lubricating nature makes it very appropriate as a material
for machine slides and guideways. It has a strength that is about half compared
to mild steel but there are several grades of cast iron. Nodular cast iron has
the same strength as mild steel, while meehanite castings provide grades which
are heat-resistant, wear-resistant, and corrosion-resistant.
10. Steel
Casting: These represent a balance between the strengths of steel and the
versatility of castings by offering the best attributes of strength and mold
ability.
11. Nylon
and Fiber: Often used as soft liners for clamps, nylon and fiber prevent damage
or denting of the workpiece under high clamping forces. These materials are
usually secured to mild steel clamps with screws or adhesives.
12. Phosphor
Bronze: It finds extensive use in the making of replaceable nuts in the
screw-operated feeding and clamping systems because screw manufacturing is very
time-consuming and costly. So, the use of softer and shorter phosphor bronze
nuts minimizes the wear and this allows periodic replacement. Besides these
applications, phosphor bronze also finds its usage in those areas where
resistance to corrosion is required, for example in the boiler valves.