It is the operation of producing straight lined, diamond shaped pattern or cross lined pattern on a cylindrical external surface by pressing a tool called knurling tool. Knurling is not a cutting operation but it is a forming operation. Knurling is done at a slow spindle speed (1/3 the turning speed). However speed & feed given for knurling is to be divided according to the job material and the finish required.
Purpose of knurling
- A good grip and make for positive handling.
- Good appearance
- For raising the diameter to a small range for assembly to get a press fit.
Types of knurls and knurling patterns
- Straight Knurling
- Diamond Knurling
- Cross Knurling
- Concave Knurling
- Convex Knurling
1. Straight Knurling
Straight Knurling is a knurling of straight lined pattern. This is done by using either a single roller or a double roller with straight teeth.
2. Diamond knurling
It is a knurling of diamond shaped pattern. It is done by using a set of rolls. One roller has got right hand helical teeth and the other has left hand helical teeth.
3. Cross knurling
It is a knurling having a square shaped pattern. It is done by a set of rollers, one having straight teeth the other having teeth at right angles to the axis of knurl.
3. Concave knurling
This is done by a convex knurl on a concave surface. This is done only by plunging the tool. The tool should not be moved longitudinally. The length of the knurling is limited to the width of the roller.
4. Convex Knurling
This is done by using a concave knurl on a convex surface. This is also done by plunging the tool.
Grades of knurling
Knurling can be done in three grades.
- Coarse knurling
- Medium knurling and
- Fine knurling
Coarse knurling is done by using coarse pitched knurls of 1.75 mm pitch. (14 TPI)
Medium knurling is done by using medium pitched knurls of 1.25 mm pitch. (21 TPI)
Fine knurling is done by using fine pitched knurls of 0.75 mm pitch. (33 TPI)
Types of knurling tool-holders
The different types of knurling tool-holders are:
- Single roller knurling tool-holders (parallel knurling tool-holders)
- Knuckle joint type knurling tool-holders
- Revolving type knurling tool-holders (universal knurling tool-holders).
A knurling tool-holder has a heat-treated steel shank and hardened tool steel knurls. The knurls rotate freely on hardened steel pins.
1. Single roller knurling tool-holder
It has only one single roller which produces a straight lined pattern.
2. Knuckle joint type knurling tool-holders
This tool holder has a set of two rollers of the same knurling pitch. The rollers may be of straight teeth or helical teeth. It is self-centering.
3. Revolving head knurling tool
This tool-holder is also called a universal knurling tool-holder. It is fitted with 3 pairs of rollers having coarse, medium and fine pitches. These are mounted on a revolving head which pivots on a hardened steel pin. It is also self-centering