Knurling on the lathe
To give cylindrical workpieces a non-slip surface, they are machined using knurling wheels or knurling tongs. Knurling is a non-cutting process, as the profile of the knurling wheels is impressed into the surface of the workpiece by pressure and no material is removed.
Knurling requires some practice. Please note that there is no one perfect solution for producing an optimal knurling. In the following guide, we present our approach, with which we have achieved uniform knurling on aluminium. This is therefore only one possible way of achieving a beautiful end result.
Important parameters for knurling
The following factors play an important role in knurling and may need to be changed/adjusted slightly depending on the situation in order to achieve uniform knurling.
- Speed
- Infeed
- Feed rate
- Division of the workpiece circumference based on the pitch of the knurling tool
- Use of drilling and cutting oil
Determine the correct diameter
To ensure that the knurling is even and does not "overlap" on the workpiece surface, first determine the pitch of the knurling tool and the diameter of the workpiece. Pitch" refers to the distance between the grooves on the knurling wheel. Make sure that the pitch is "in line" with the circumference of the workpiece. A clean pitch extends the service life of your knurling tool.
To calculate the circumference, use the following formula:
Diameter of the workpiece x 3.14 (π)
Then divide the result by the pitch of the knurling tool.
In our example, the knurling wheel has a pitch of 1, which means that the circumference must be an integer so that it is optimally aligned with the pitch and a uniform knurl is created.
We have therefore turned the diameter of the workpiece to approx. 29 mm, resulting in a circumference of approx. 91 mm. The end result is therefore an integer without decimal places and works perfectly with the division of 1.
Setting the knurling tool correctly
Prior longitudinal and face turning of the workpiece ensures optimum conditions for the subsequent knurling process. The knurling tool is initially set to the centre of the work spindle.
As soon as the knurling tool is correctly set, a generous amount of drilling and cutting oil is sprayed onto the knurling tool and the area to be knurled.
Set the speed and feed rate on the machine. In our example, the feed rate corresponds to approx. half the pitch (0.5 mm/rev). Slowly move the knurling tool to the workpiece. Switch on the machine and feed. Make sure that the infeed is slightly less than the pitch. In our example, the infeed is 0.9 mm. To check the knurling, switch the machine off again. When you are satisfied with the knurling, switch the machine on and activate the feed after three to ten revolutions.
Knurling tool with one wheel
Knurling tools with a wheel are often used, for example, to produce axis-parallel grooves. However, if the wheel is set at exactly 90° to the workpiece, the radial pressure acting on the axis is greatest. This puts more strain on the lathe than when using two knurling wheels. For this reason, the knurling wheel should be set at an angle of 1° to 2° for axial and radial machining. The following instructions must be observed:
- The knurling wheel should be set at an angle in the same direction as the feed movement. This means that the left-hand tip of the knurling wheel should be moulding the workpiece. This is very important, as only this combination with the feed in the Z direction (to the left) will reduce the pressure and thus relieve the machine.
- The tip of the knurling wheel therefore embosses a smaller proportion of the workpiece per revolution, which reduces the radial force on the axis.
- This setting means that the knurling is applied to the entire depth of the knurling wheel right from the start.
Tips & tricks
- Clamp the workpiece so that only the surface to be machined protrudes from the jaw chuck.
- Use a travelling centre punch to support long workpieces.
- By using knurling pliers (Fig. 2), less pressure is exerted on the axis of the work spindle, as the two knurling wheels grip the workpiece from above and below, thus "cancelling out" the pressure.
- Please note that knurling increases the diameter of the workpiece by approx. 0.3 to 0.5 mm, as the material is pressed out of the grooves into the centres by the knurling tool.
- Knurling is not easy and requires a lot of practice and experience. Practise as much as possible to find the right settings for the parameters.
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