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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES
FUNDAMENTALS OF
DRILLING & TAPPING
FUNDAMENTALS
OF THE LATHE
FUNDAMENTALS
OF MILLING
FUNDAMENTALS
OF SAWING
METALWORKING
SHOP TERMS |
STAINLESS STEEL ABRASIVE WHEEL GRINDING
In the production of parts from stainless bars, precision
grinding is required on jobs where you want perfect surface finishes,
exceptionally close dimensions and geometric accuracy, or when heat treated
parts are too hard to machine. For this
work, the grinding wheel is the heart of the job, and selection should never be
made by guessing. The choice of wheel must be determined for each type of job
and machine. 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
the 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 shop. Grinding machines, whether cylindrical, universal, surface,
internal, centerless, thread or special grinders, should be massive since
distortion and vibration are the causes for many poor grinding jobs. In general,
the most efficient grinding speeds are in the range of 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. Here is a
check list for correcting trouble spots in grinding stainless: If traverse
marking is occurring, check the edges of your grinding wheel. They may be
too sharp and should be slightly rounded off to avoid a “dragging edge.”
Suck marking may also be caused by excessive spindle spring or too high a speed
on finishing cuts. And last, but not least, traverse may be too fast for the
work speed. This leaves a pattern on the work which can be corrected by slightly
decreasing the traverse speed. Loading or Glazing: Wheel may be too hard, or not
dressed often enough. Dressing may be too fine or dresser too dull. Work “Out-of Parallel”: Usually caused by
mechanical faults such as “sloppy ways,” improper setting of tailstock,
center not concentric with work piece. Check accuracy of 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
adjustment is setup at the star of the 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. An exception to this is on thread
grinding requiring a highly finished and smooth surface. For this work, use
sulfur-base oils, either straight or cut back with paraffin oil. 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 which might otherwise score the surface and spoil the apart. 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 safeguards against to some degree by increasing the percentage
of water in the mixture. This condition should be check carefully, as premature
decision may put the fault with the steel or wheels, while the real problem lies
elsewhere. |
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Copyright © 2002 QUICK FACT GUIDES FOR MACHINISTS
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