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Knurling – easy-grip profiles to make components easier to handle
Knurling enables an easy-grip profile to be added to the smooth surface of a round (cylindrical) component. It ensures that tools and components lie securely in your hand and do not move. The advantage of this is that the knurled workpiece can be used more effectively and precisely without it slipping in your hand. There are different knurling patterns to choose from, meaning components can also be given an attractive look.
Although knurling is not one of the most commonly used production methods, it is very important in a number of industries. For example, many tools used in manual trades have a knurled surface to ensure the tradesperson can optimally grip and use them. Knurled devices can also be found in the medical technology sector, such as on operating tables – doctors can work much more precisely and reliably if the surface of their instruments is knurled.
But returning to the production method: knurling is one of the methods performed on a lathe. The initial part is a cylindrical workpiece with a smooth surface. There are two ways to give this workpiece a profile or pattern: form knurling and cut knurling. The two methods differ in how the desired pattern is applied to the surface.
Form knurling
Form knurling is also known as knurl pressing. As the name indicates, the profile is “pressed” into the workpiece. In practice, this is done using a hardened tool that already has the profile you are adding to the workpiece – the knurl. This knurl is pressed onto the surface of the workpiece, compressing and deforming the material on the surface to apply the desired pattern. In other words, this constitutes cold forming of a metallic material without removing any material (no chips). However, form knurling does increase the diameter as the displaced material “flows to the outside”. The following formula provides a simple way to calculate the resulting diameter:
Final diameter = starting diameter + (X*t)
t is the pitch, i.e. the distance between two peaks in the profile. X is a value that depends on the knurled shape, i.e. the contour of the knurl. This value can be taken from a table.
Example: a workpiece has a starting diameter of 50 mm. The pitch is t = 0.8 mm and the X-value of the knurled shape is 0.5 (left-hand knurl, shape BL). The final diameter should then be calculated as follows:
Final diameter = 50 mm + (0.5 mm * 0.8) = 50.4 mm
| Knurled shape | Value X |
|---|---|
| Axially parallel, shape AA | 0.5 |
| Left-hand knurl, shape BL | 0.5 |
| Right-hand knurl, shape BR | 0.5 |
| Cross-hatched knurl, points raised, shape KE | 0.67 |
| Cross-hatched knurl, points indented, shape KV | 0.33 |
| Left-right knurl, points raised, shape GE | 0.67 |
| Left-right knurl, points indented, shape GV | 0.33 |
Step-by-step instructions for form knurling
The technical parameters and specifications for knurling are set out in the DIN 403 standard – we won’t discuss them further here, but instead provide some handy practical tips.
1. Turn the outside diameter of the workpiece: As the subsequent knurling step increases the diameter, the workpiece first has to be turned to a smaller diameter. This value can be calculated using the formula above. However, in practice you often obtain sufficiently accurate results if you turn the workpiece 0.3 to 0.5 mm thinner than the desired diameter.
2. Clamp the knurling tool: Now install the knurling tool in the toolholder – if the knurling tool has one wheel, it should be positioned centrally. Practical tip: position the knurling tool at a slight angle to prevent a bead forming in front of the knurl profile.
3. Press on the knurling tool: Now press the tool against the workpiece. Tip: to obtain a neat knurl profile at the edges, only press on half the wheel to begin with.
4.Switch on the lathe and test the knurl profile: Now switch on the machine. The machining parameters can be found in the relevant tables. Tip: to begin with, only press on the wheel with half the force as the full width of the wheel has not been applied yet. Then visually inspect the result to see whether the profile points are well-formed. If necessary, repeat the knurling process until you achieve the desired result.
5. Let the machine run with the feed: according to textbooks, the machine should run with a feed of “half the wheel pitch” – i.e. a feed rate of 0.5 mm if the pitch is 1 mm. However, lower values can often be used in practice. Work slowly, step by step.
6. Lubricate and cool: knurling produces a lot of friction because the tool is pressed onto the workpiece. Use lubricant to ensure the tool glides evenly over the workpiece and produces the desired surface profile. This will also reduce the build-up of heat. When working on brass and aluminium, a cooling emulsion can be used to lubricate/cool the knurling process. When working on steel, cutting oil is recommended. However, this must be removed from the knurl profile afterwards.
7. Switch off the machine: at the end of the knurling process, switch off the machine and feed and only then move the knurling tool away. Once the cutting oil has been removed, the knurled workpiece will then be ready in all its beauty.
Cut knurling
With cut knurling, the clue is in the name again: the desired pattern is milled, i.e. cut, into the surface of the workpiece. Like with knurl pressing, a knurl is also used here. However, it has sharp-edged teeth that scrape material off the surface to give the workpiece the desired profile. This, therefore, is a metal-cutting method.
It produces a particularly precise profile with a very high-quality surface. As a result, cut knurling is used in applications that are subject to high requirements regarding the quality of the profile produced.
Step-by-step instructions for cut knurling
Cut knurling uses different knurling tools depending on the profile you want to achieve. If you are aiming for a flat or helical profile you will need tools with one knurl cutter; if you want a cross-cut or fish scale pattern you will need two knurl cutters.
| Knurl type | DIN 82 | Tool | Knurl cutter | ||
|---|---|---|---|---|---|
| Helical knurl profile |  | RBL / RBR | A/FL | As necessary |  |
| Flat knurl profile |  | RAA | A/FL | 30° L / 30° R | |
| Cross-cut knurl profile |  | RKE | A/FK | 15°L + 15° R |  |
| Fish scale knurl profile |  | RGE | A/KF | 1 x 90° |
1. Preliminary work: Clamp the workpiece in the lathe. At the end of the knurl profile it should start with a 30° or 45° chamfer to make it easier to introduce the knurl cutter. The out-of-roundness of the turned part should be a maximum of 0.03 mm.
2. Select the appropriate knurl cutter: The knurl has a certain toothing and inclination angle depending on which profile you want to achieve. For example, for a flat knurl profile, you can use a knurl cutter that is inclined by 30 degrees. Its toothing may be in the form of either a left-hand or right-hand helix. Tip: when selecting a knurl cutter, consult the manufacturer.
3. Install the knurl cutter: Now mount the cutter on the toolholder. It should be able to turn without any play once installed.
4. Adjust the tool: On conventional lathes, now adjust the tool shank so that it is exactly at the centre height of the turned part.
5. Align the cutter on the workpiece: The cutter should be aligned parallel to the workpiece. Similarly to form knurling, please note that a slight initial cut of 1 to 2 degrees will produce a better result. Furthermore, do not apply the entire surface of the cutter to begin with. It can either be fed in at the chamfer produced, or initially make contact across a width of up to 1.5 mm – in this case you should quickly feed in the tool to the desired profile depth across this width. A simple formula for calculating the profile depth is provided below.
6. Knurl the workpiece: Knurl the edge of the workpiece for 2 to 3 seconds, then switch on the feed. Please note that the coarser the pitch, the smaller the feed rate should be. If you have not already done so, now also apply coolant or lubricating oil. Tips:
- long workpieces can be supported with a revolving lathe centre.
- Check the impression of the profile on the pre-turning diameter (⅓ of the wheel width).
- The starting point in the X-direction should always be on the material, not in front of the component.
7. Measure the workpiece: You can use a setting gauge to check the result. Mount it on the cutter holder instead of the knurl cutter. Tip: if the knurled path that was produced does not run completely parallel to the workpiece axis, the tool will need to be corrected accordingly. For this purpose, the tool has a spindle for adjusting the cutting head.
Tip: in practice, the desired result is often not achieved until after several passes with the tool. Ensure that the knurl cutters are always fed in at the chamfer/the initial knurled path at the end of the workpiece.
Formula for calculating the profile depth:
Profile depth = tooth depth +0.1 mm +- 0.05 mm
Example: if the pitch is 1 mm and the flank angle is 90 degrees, the tooth depth will be 0.5 mm. The profile depth will then be
0.5 mm + 0.1 mm +-0.05 mm
=0.6 mm +- 0.05 mm
Form knurling vs cut knurling – when should I use which method?
Form knurling has proven itself as an affordable method that can be used in many applications with moderate requirements regarding the surface quality. It is simple to perform, achieves good results and has a positive effect on certain material properties. However, it can only be used on robust materials because high forces are applied. Knurl pressing is not a good option for pipes, for example.
Cut knurling precisely removes material and delivers outstanding results when it comes to surface quality and accuracy. In addition, the method is suitable for use on thin-walled workpieces such as pipes as it does not apply high levels of pressure. Cut knurling also achieves a very good surface quality on materials such as aluminium and other soft materials.
We’d like to leave you with a final, important tip: to successfully knurl workpieces on conventional lathes, you need a lot of practice and skill. So don’t be frustrated if the result isn’t quite right the first time. With a little practice, your knurling results will get better and better.