How to choose correct insert in boring
To achieve good chip control, it is important to select the correct insert size, insert shape, geometry and insert nose radius.
- Select the largest possible nose angle on the insert for strength and economy
- Select the largest possible nose radius for insert strength
- Select a smaller nose radius if there is a tendency for vibration
l = cutting edge length (insert size)
RE = nose radius
See Rough boring and Finish boring for more information.
Boring insert shape
The insert shape and point angle for boring vary considerably, from triangular-shaped inserts with small point angles to round inserts.
Each shape has unique properties. Some provide the highest roughing strength while others give the best accessibility for fine boring.
Each shape also has its unique limitations. For example, high edge accessibility during machining leads to a weaker cutting edge.
Cutting edge strength (large point angle)
- Stronger cutting edge
- Higher feed rates
- Increased cutting forces
- Increased vibration
Less vibration tendency (small point angle)
- Weaker cutting edge
- Increased accessibility
- Decreased cutting forces
- Decreased vibration
Round inserts have stronger cutting edges and are hence more resistant to large depth of cuts than triangular-shaped inserts.
Round inserts are a productive solution for machining in cladded materials, e.g. boring Inconel 625 weld in oil and gas applications.
Insert nose radius and depth of cut
The nose radius, RE, is a key factor in boring operations. The selection of nose radius depends on depth of cut and feed, and influences the surface finish, chip breaking and insert strength.
Small nose radius
- Ideal for small cutting depth
- Reduces vibration
- Weak cutting edge
Large nose radius
- Heavy feed rates
- Large depths of cut
- Strong edge security
- Increased radial pressures
The relationship between nose radius and depth of cut affects vibration tendencies. It is often an advantage to choose a nose radius that is smaller than the depth of cut.
At small depths of cut, the resultant cutting force is radial, trying to push the insert away from the bore surface. When depth of cut is increased, the resultant cutting force is changed to an axial force.
As a general rule of thumb, the depth of cut should be no less than 2/3 of the nose radius. Avoid depths of cut smaller than 1/3 of the nose radius when finishing at small depths of cut.
With a small nose radius, the radial cutting forces can be kept to a minimum, while utilizing the advantages of a larger nose radius leads to a stronger cutting edge, better surface texture and more even pressure on the cutting edge.
The generated surface finish will be directly influenced by the combination of nose radius and feed rate. Read more about how to achieve good surface finish.
Use wiper inserts for improved surface finish with standard cutting data, or maintained surface finish at a substantially higher feed rate.
Wiper inserts are not recommended for unstable conditions and long overhangs.
Cutting fluid and coolant in boring
Chip evacuation, cooling and lubrication between the tool and the workpiece material are primary functions of cutting fluid. This affects the hole quality and tool life. Use an internal cutting fluid supply to get the fluid as close as possible to the cutting edge.
- Use a mixture of 5–8% soluble oil
- Higher pressure and volume improves chip evacuation
- Mist cutting fluid or minimal lubrication can be used, especially in aluminum
- Dry boring can be performed in short-chipping materials, preferably in horizontal or through hole applications
- Tool life will be reduced
- Compressed air will improve chip evacuation greatly
- Dry machining is never recommended in stainless steel (ISO M) and HRSA (ISO S) materials
- External cutting fluid supply is acceptable in short-chipping materials, but must continuously be directed to the cutting zone. If this is not possible, try dry boring
Assemble a boring tool
Assembled boring tools often have to be built from several items to suit an operation:
- Slides or cartridges
- Boring adapters
- Basic holders
It is always vital that recommended torque values are followed for the assembly and that a suitable fixture and a good tool pre-setter are used.
To get the best performance out of the boring tools, a torque wrench should be used to get the correct tightening of the assembled boring tool and insert. Torque that is too high will affect the performance of the tool negatively and cause insert, washer, cup spring and screw breakage. Torque that is too low will cause slide or insert movement, vibration and degradation of the cutting result.
See the rotating tools catalog for the recommended tightening torque for your boring tool.
Check insert seats regularly to ensure that they have not been damaged during machining or handling. Make sure that insert seats are free from dirt or metal chips from machining.
Replace worn or exhausted screws, washers and cup springs. Ensure that you have a torque wrench to obtain correct screw-tightening.
To get the best performance, we recommend cleaning all male and female parts and lubricating them with oil at least once a year. Lubricant should be applied when needed to the screw thread as well as to the screw head face.
- Clean all assembly items before assembly
- Use Molykote for screws
- Lubricate all assembly items with oil at least once a year
- Clean and lubricate fine boring heads and face grooving heads with oil. At a minimum, once a year; if in permanent use, once a month
- Use acid-free light machine oil type
- Mobil Vactra Oil No. 2
- BP Energol HLP-D32
- Klueber Isoflex PDP 94
Lubrication is carried out by depressing the spring-loaded ball and adding a few drops of light oil. Due to the centrifugal force, the oil is forced outwards, which stops dirt from getting into the adapter.
Lubrication of fine boring heads
Boring in different materials
See general turning for information about turning in different materials; the same recommendations are also valid for boring.