Fabricating Carpenter
Stainless Steels
June 2006
Machining and Abrasive Wheel Grinding
Machining
These three characteristics of stainless steel
exert the most influence on machinability:
- Relatively high mechanical properties (including yield strength)
- High work-hardening rate
- Ductility
These factors explain the material's tendency
to form a built-up edge during machining. For example, the chips removed in
machining exert high pressures on the nose of the tool and therefore tend to
weld fast, producing what machinists call a "bug." This causes the tool to run
hot, slows down the job and interferes seriously with the finish.
The austenitic stainless steels (300 Series)
are not only troublesome because of "bugging" and chip disposal, but they work
harden so that the tool, in passing over the work, will harden the surface and
thus interfere with the next cut. The only remedy for this is to reduce the
speed, increase the cut somewhat, if possible, and keep cutting. The tool must
not be allowed to dwell on the work.
The best mechanical method for chip control is
to grind the tools with a fairly steep top rake or lip angle. Tools with a 5º to
10º angle will generate less heat and be freer and cleaner cutting. Generous
chip curlers or chip breakers are also a decided advantage. It is also helpful
to stone the top of the tool smooth as an aid to skidding the chips. For
general-purpose drilling, twist drill makers produce a drill for drilling
stainless steel. It has a shorter flute and overall length than regular drills
and is therefore heavier and stronger. As sold from stock, this type of drill is
generally pointed with an included angle of 140º.
Where close tolerance and fine finish are
necessary, consider using a shave tool with a light cut and fast speed. This
tool should be sharply ground and stoned. Running at high speed while taking a
light cut (0.002/0.008" or 0.05/0.20 mm) produces an excellent finish and holds
to extremely close tolerance.
Sulfur-based cutting fluids have been
recognized for years for their ability to cool and prevent seizing. As a result,
properly blended sulfur-base fluids have become the standard cutting fluids for
machining all
types of stainless steels. Here is a handy
rule-of-thumb to use regarding the mixture: If the chips are welding to
the tool, keep adding sulfur-based oil. If tools are failing by rapid abrasion,
add more paraffin-base oil.
The real answer to machinability came with
Carpenter's development of free-machining stainless steel. Both sulfur and
selenium have been successfully added to stainless alloys to secure free-cutting
properties. Carpenter Stainless Type 416 was the first free-machining stainless
steel. Later, Carpenter uses selenium in the manufacture of Carpenter Stainless
Type 303 Se.
Since the 1970s, Carpenter has developed and
improved upon a line of enhanced machinability stainless machining bar grades.
The most recent enhancement is the Project 70+® stainless family.
Users of Project 70+ machining bar have reported faster machining speeds,
improved finishes and extended tool life. Access more than 200 alloy datasheets,
including machining data visit Carpenter's technical information database at
www.cartech.com.
More detailed information about machining
Carpenter alloys is available in the booklet, "Machining Carpenter Specialty
Alloys." Request a booklet in the
Product
Literature section.
Abrasive Wheel Grinding
Precision grinding is required on jobs in
which you desire excellent surface finish, exceptionally close dimensions and
geometric accuracy, or when heat-treated parts are too difficult to machine.
For this work, the grinding wheel is the heart
of the job. Wheels for precision grinding may contain either aluminum oxide or
silicon abrasives, which may be bonded by shellac, rubber, silicate, resinoid,
etc. Avoid the use of grinding wheels containing iron oxide. Contamination of
the stainless surface with iron oxide will cause rapid corrosion and rust
pitting. Selection of the right wheel for a job can often be made from
experience on previous work. On a new job, it is best to consult a wheel
manufacturer for guidance in your selection.
The method of holding or supporting the work
will vary with the type of machine used and job to be done. On special jobs,
various types of work holders, chucks or collets are available or may be
designed and produced in your own tool room.
Whether cylindrical, universal, surface,
internal, centerless, thread or special grinders, grinding machines should be
massive. Distortion and vibration cause many poor grinding jobs.
In general, the most efficient grinding speeds
are in the range 5500/9000 surface feet per minute. The optimum speed within
this range will depend upon the grade of stainless, type of grind, rigidity of
the machine, and wheel selection.
The 300 Series austenitic stainless grades,
being gummy, should be ground with a wheel having a porous bond to avoid early
loading of the wheel. The straight chrome steels in the 400 Series can be ground
with a harder wheel. It is seldom necessary to start with less than 60- to
70-grit wheels for the roughing cut. This should be followed with an 80- to 100-
grit wheel having a soft or porous bond to provide faster cutting and prevent
burning.
Note: When changing wheels from one size
grit to another, it is important that the work be cleaned and all "wild" grit be
removed. When the coarse grains are carried along to the finer grit wheels, deep
scoring or scratching may occur.
Troubleshooting grinding problems
Traverse marking:
Check the edges of your grinding wheel. They
may be too sharp and should be slightly rounded off to avoid a "dragging edge."
Such marking may also be caused by excessive spindle spring or too high a speed
on finishing cuts. Lastly, traverse may be too fast for the work speed. This
leaves a pattern on the work that can be corrected by slightly decreasing the
traverse speed.
Loading or Glazing: The wheel may be too hard or not dressed
often enough. Dressing may be too fine or dresser too dull.
Work "Out-of-Parallel": This condition is usually caused by
mechanical faults such as "sloppy ways," improper setting of tailstock, or
center not concentric with the work piece. Check accuracy of the dressing
operation. If wheel is dressed off-center, it will not conform with surface of
work part. After first cut is made, check for straightness, taper or chatter
marks. Proper adjustments in setup at start of job will reduce rejects and save
time in the long run.
Lubrication: Practically all grinding is done with
water-base coolants because of their ability to dissipate heat rapidly and thus
prevent spoiled work due to overheating. Exception: On thread grinding
that requires a highly finished and smooth surface, sulfur-base oils—either
straight or cut back with paraffin oil—may be used.
Lubrication serves to reduce friction between
work and wheel, and cuts down the resistance of the metal to the abrasive.
Further, it washes away the chips and abrasive particles that might otherwise
score the surface and spoil the part. A steady flow of coolant retards loading
of the wheel and prevents impregnation of particles into the metal.
Avoid highly alkalized lubricants, as they may
deteriorate the wheel bonds. This condition can be safeguarded against to some
degree by increasing the percentage of water in the mixture. This condition
should be checked carefully, as premature decision may put the fault with the
steel or wheels while the real problem lies elsewhere.
