Reciprocating surface grinding: grinding parameters
Speed ratio qs
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The speed ratio qs is an important indicator as to whether a grinding process runs optimally. It is the ratio between the surface speed of the grinding wheel and the speed of the workpiece movement. For example:
Grinding wheel surface speed: vc 30 m/s
Workpiece (table) speed: vw 21'000 mm/min
qs = Grinding wheel surface speed vc * 1000 * 60 / Workpiece (table) speed (mm/min)
qs = 30 (m/s) * 1000 * 60 / 21’000 (mm/min)
qs = 86
Aim for the following values:
Rough grinding: 60 to 80
Finish grinding: 80 - 120
qs < 50: danger of chatter marks
qs > 120: danger of burning (thermal damage) |
Depth of cut ae
It is very difficult to give general values for the depth of cut as there are enormous differences in machine tool design and, therefore, also rigidity. Additionally, interrupted cuts, particularly as found in blanking dies, allow only shallow depths of cut.
| Depth of cut ae |
0.003 - 0.005 mm (0.0001 – 0.0002 in.) per stroke
For wheel diameters 250 - 300 mm (10 – 12 in.)
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0.01 - 0.015 mm (0.0003 – 0.0005 in.) per stroke
For wheel diameter ³ 350 mm (³ 14 in.) |
| Surface speed vc: |
28 bis 35 (50) m/s (5,600 – 7,000 (10,000) sfpm) |
| Lateral wheel displacement: |
50 - 75% of wheel width per stroke |
| Workpiece or table speed vw: |
establish speed ratio qs
(roughing qs = 60 - 80, Finishing qs = 80 - 120)
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| Example: |
Finishing process; vc = 30 m/s; qs = 90
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qs = Vc * 1000 * 60 / vw
vw = Vc * 1000 * 60 / qs = 30 m/s * 1000 * 60 / 90 = 20’000 mm/min = 20 m/min
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Specific material removal rate Qw’ (Q-Prime)
The specific material removal rate Qw’ indicates how much material in cubic mm one (1) mm width of grinding wheel can remove in one (1) second. As it always refers to one (1) mm width of grinding wheel and the same unit of time, direct performance comparisons can be made between different types of grinding wheels and grinding processes.
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Guideline for Qw’
0.5 to 1. 0 mm3/mm/s Fine finishing
1.0 to 2.0 mm3/mm/s Finishing
3.0 to 5.0 mm3/mm/s Target value
>10.0 mm3/mm/s High performance grinding at ³ 80 m/s (³16,000 spfm)
Formula for the calculation of Qw’
Example: Depth of cut per stroke ae = 0.005 mm,
Qw’ = ae * vw / 60 = 0.005 mm * 25'000 mm/min / 60 = 2.1 mm3/mm/sec |
Creepfeed grinding : grinding parameters
Speed ratio qs
Grinding wheel surface speed: vc 25 m/s
Workpiece (table) speed: vw 500 mm/min
qs = Grinding wheel surface speed vc * 1000 * 60 / Workpiece (table) speed (mm/min)
qs = 25 (m/s) * 1000 * 60 / 500 (mm/min)
qs = 3,000
Use the following guidelines: qs > 1,000 to 10,000
qs < 1000: danger of thermal damage to workpiece
The following illustration shows the usable range of speed ratio qs: |
Depth of cut ae
It is very difficult to give general guidelines for the depth of cut as there are enormous differences in creepfeed machine tool design and, therefore, also in spindle drive power and overall machine rigidity. The actual width of the grinding wheel also bears a lot of influence on the depth of cut.
Depth of cut ae
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if possible do not go below 0.5 mm (0.020 in.)
depth of cut in creepfeed mode as this
may lead to burning (thermal damage)
depending on machine tool design
ae should be between 1 and 5 mm
(0.040 – 0.2 in.)
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| Surface speed vc: |
25 to 28 m/s (5,000 – 5,600 sfpm) |
Specific material removal rate Qw’
Guideline for Qw’
| 2.0 to 4.0 mm3/mm/s |
Limit on high alloy tool steel (D2 or DIN 1.2379)
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| 3.0 to 5.0 mm3/mm/s |
Target value for low alloy tool steel |
| >10.0 mm3/mm/s |
High performance grinding |
Workpiece (table) speed vw:
Determine Qw’
Machine tools up to 10 kW Qw’ » 5
Machine tools up to 20 kW Qw’ » 10
Machine tools around 50 kW Qw’ » 20
Example: 10 kW spindle power, Qw’ = 5, ae = 2 mm
Qw’ = ae * vw / 60
vw = Qw’ * 60 / ae = 5 * 60 / 2= 150 mm/min
Choice of dressing tools
Basically, stationary dressing tools cannot be recommended for CBN surface and creepfeed grinding. Use rotary dressing tools only:
· Diamond dressing rolls
· PCD form dressing disks
· Diamond form dressing disks
· Air-driven dressing turbines.
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