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Prior to knurling, all necessary working tools and materials are to be properly made available.
Setting up of the lathe basically involves the following steps:
Clamping/chucking of the workpiece for knurling
The parts to be knurled are mainly chucked in hard chuck jaws since the screws, knobs, etc. are mostly to be provided with a thread, i.e. they will be subjected to further machining.
Moreover, the gripping surfaces need not absolutely run true. If, however, true running is necessary or machined surfaces are to be protected, the parts are chucked in soft chuck jaws which are to be turned internally for this purpose.
Checking the diameter to be knurled for size.
By pressing the teeth into the workpiece, the diameter of the knurled portion will increase by approximately half the tooth pitch since there is no chip removal.
Therefore, the diameter of the workpiece must be turned smaller by 1 to 1/2 tooth pitch depending on the material.
(d1 = d - t/2)
Figure 12 Increase in diameter
1 root circle
2 pitch circle
3 top circle
4 height
5 pitch t
Example: A steel part with a grip of d = 30 mm shall be spiral-knurled over a width of 40 mm. According to table 1 in section 2 the spiral-knurling pitch is 1 mm. By experience it is known that after knurling the diameter of the workpiece will be approx. t/2 bigger than before.
Therefore the grip is to be turned to
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d1 = d - t/2 |
d1 = 30 mm - 0.5 mm |
d = 29.5 mm |
A grip of d = 26 mm shall be straight-knurled over a width of 20 mm.
What is the straight-knurling pitch and to which size is the
nominal diameter to be
turned?
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Clamping of the tool for knurling
All straight and spiral knurling holders are generally to be clamped at right angles to the axis of rotation and to be positioned slightly below centre (approx. 1 mm).
Figure 13 Clamping of the straight
and spiral knurling holder at right angle to the axis of rotation
1 axis of rotation, 2 workpiece, 3 spiral knurling holder, 4 direction of tool pressure, 5 feed direction of tool, 6 direction of rotation of workpiece, 7 live centre of tailstock
Figure 14 Clamping of straight
knurling holder below centre
1 straight knurling wheel, 2 workpiece, 3 knurling holder, 4 tool support, 5 centre of workpiece, 6 centre of tool (below centre of workpiece)
Since the spiral knurling holder must exert double the pressing force compared to the straight knurling holder, it is more rigid and, moreover, provided with an edge to match the tool carrier (see Fig. 8).
All knurling wheels are fixed by unhardened steel pins.
Setting of the cutting values
Like with longitudinal turning and facing, the values to be set are determined by means of tables of recommended values depending on the kind of operation and on the lathe.
The rotational speeds of the workpieces should be a little lower than for turning with the same tool material (tool steel v = 6...10 m/min).
Example: A steel part (St 36) with a grip of d = 30 mm shall be straight-knurled over a width of 50 mm.
What is the rotational speed?
Cutting speed for tools of tool steel:
v = 6 - 10 m/min. Since St 36 has no special strength,
v = 10 m/min is selected.
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Given: |
d = 30 mm |
Required: n =? |
r.p.m. |
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p = 3.14 | |
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v=10 m/min | |
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If possible, the cutting depth should be produced in one feed setting.
Feed in axial direction will be necessary for longer spiral knurlings.
The longitudinal feed of the spiral knurling tools must always comply with the spiral-knurling pitch to avoid overlapping.
Making available coolant and lubricant
Cooling is absolutely necessary (normally by soluble oil or cutting oil) since the heavy pressing force produces heavy friction.
The following rules are to be observed:
- The knurling wheel must always be clean.- All holders are to be clamped at right angles to the axis of rotation.
- The spiral knurling holder’s matching edge bearing must be located at the upper edge of the tool slide to prevent the holder from being forced away.
- The straight knurling technique is used for all metals.
- The spiral knurling technique is used mainly for steel.
- The cross knurling technique is used for hard rubber, plastic material, etc.
What is the rotational speed for cross knurling of knobs of hard rubber with a gripping surface of d = 24 mm and 20 mm width?
Formula: 
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Given: |
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Required: |
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Calculation: |
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What is the purpose of knurled
portions?
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Why is the diameter becoming bigger by
knurling?
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How are knurling tools to be
serviced?
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Why is a low cutting speed selected for
knurling?
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