| 1. Why do Diamond Wheels perform best while grinding Carbides & Ceramics ? | ||
Grinding
Cemented carbide is a much more severe operation than grinding ductile
steel. When the steel is ground the predominant material removal mechanism
is failure by shearing. Material is removed in the form of ductile chips,
with a relatively low expenditure of energy and a low rate of wheel wear.
When cemented carbides are ground the material removal mechanism is totally
different. The abrasive in the grinding wheel fractures the tungsten carbide
grains on the surface of the work piece. The grinding swarf consists of
fine powder instead of chips. Impact with the carbide destroys conventional
grinding abrasives quickly. The knoop indentation hardness of tungsten
carbide grains is about 1880 kg/sq/mm, while the knoop hardness of Silicon
carbide grains is only around 2480 kg/sq.mm. These grains wear fast when
pitted against carbide grains, which are nearly as hard. Diamond abrasive
grains have a knoop hardness of 7000-10000 so they are about three to
four times harder than the silicon carbde grains. The above also holds
good for Diamond wheels they are used to grind Ceramics. For this reason
Diamond wheels are almost universally used and perform best while grinding
Cemented Carbides and Ceramics. |
||
|
2. How can you ensure low grinding cost with Diamond Wheels ? |
||
There
is no clearly defined technique or method of increasing material removal
rate (mrr) that will guarantee the most effective utilization of the diamond
grinding wheel. Laboratory tests have indicated that both in wet and dry
grinding of cemented carbides, the life of the wheel is shortened in some
direct relationship to the increase in mrr, irrespective of the technique
employed for increasing the removal rate. There is some indication however,
that in case of wet grinding the use of heavy cross-feeds will take a
somewhat lesser toll in the wheel life than will downfeed or the table
speed. However a very important factor to consider is that the wheel is
only removing carbide while the wheel and the workpiece are in contact.
Any technique which increases the noncontact time between the wheel and
the workpiece diminishes the effective mrr and increases cost. |
||
|
3. What are the recommended wheel speeds for Diamond Wheels ? |
||
| Grinding
wheel performance is influenced by wheel speeds. Very low wheel speeds
reduce grinding efficiency. Very high speeds causes excess heating. The best recommended wheel speeds are : For Dry Tool and Cutter grinding - 3000-4000 SFPM (15-20 M/sec) For Wet surface grinding - 3500-7500 SFPM (18-38 M/sec) |
||
|
4. Why do CBN wheels have better efficiency whilst grinding hard ferrous metals ? |
||
Cubic
Boron Nitride (CBN) wheels are used for the grinding of hard, difficult
to machine ferrous metals. They remove material at lower temperatures
than those which occur with conventional abrasives under identical process
conditions. Because of the lower grinding heat generated by the CBN wheels,
there is little or no thermal damage occuring to the surface of the workpiece.
Therefore cutting tools and parts ground with CBN wheels generally have
a longer useful life. Material removal rates in many cases are so high
that they can even replace lathe, milling machine and conventional grinding
operations. |
||
|
5. What should be the ideal speed and feeds while using the CBN wheels ? |
||
| The
recomended speeds and feeds are as follows :
Surface Speeds: 5000-6500 SFPM (i.e.25-33
m/sec.) At speeds within this range, longest wheel life and finest surface
finishes are obtained.
Table Speeds: High table speeds up to the
maximum speed of the grinder, result in the best grinding performance.
At high table speeds mrr are high thus increasing the number of pieces
ground and hence reducing labour and overhead cost per piece.
Crossfeeds: Large crossfeed increments,
1/4th to 1/2 of the wheel width are recomended for the general purpose
grinding. Use smaller crossfeeds for finishing cuts.
Downfeeds: For easy to grind steels such
as M-2 & T-1 steels, use 0.001"-0.002" (0.02-0.05 mm)
per pass. For difficult to grind materials such as M-4 and T-15 use
0.0005"-0.001" (0.01-0.02 mm) per pass.
|
||
|
6. What is hard turning ? |
||
Hard
turning is generallly defined as turning Steel of Rockwell Hardness C-45
and above. The boundary between cutting and grinding is slowly diminishing.
Grinding however consumes at least five times more energy per metal removal
than cutting. Therefore cutting is more energy effecient than grinding,
but one can replace the other. Grinding and machining complement each
other.
Today hard turning is very widely applied
in the auto industry. PCBN tools can be very effectively utilized for
this purpose. Kindly consult your PCBN Tool manufacturer for further
details.
|
||
|
7. Can you list out some automotive parts where hard turning has already been successfully implemented ? |
||
Some
of the proven success stories are available for the following auto parts:
For more details you should consult your Tool manufacturer. |
||
|
8. Can you define the following terms ? a) Grinding Ratio b) Material Removal Rate and Specific material removal rate. |
||||
|
Material removal rate often written as MRR
is the actual material removed in the unit time and is expressed in
Cubic mm./min.
Specific Material Removal rate often written as SP. MRR is the material removal rate per unit wheel width, and is expressed in Cubic mm./min. It is important to note that when comparing different wheels for their MRR's it is only correct to compare their SP. MRR's and not their MRR's unless the wheels have identical dimensions (especially their widths) |
||||