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Tool Selection in Turning Machining
Modern CNC turning centres machine a wide variety of shapes and materials, so it is important for CNC machine operators to understand the appropriate cutting tool for each job.
Selecting and using the right turning tool can have a critical impact on the outcome of turning operations, especially finish turning operations. How do you select a turning tool?
This article provides a detailed introduction to the selection of turning tools and the use of precautions in fine turning. We hope that it will help practitioners in the machining industry and improve machining efficiency and quality.
Catalogue:
- Ⅰ.What are the types of cutting tools？
- Ⅱ.How to choose the appropriate turning tool？
- Ⅲ.Commonly used turning tool combinations.
- Ⅳ.Additions.
- Ⅴ.Summary.
Ⅰ.What are the types of cutting tools？
All lathe-cutting instruments are single-point ones and are held by tool holders. The shaping outcomes depend on the angle they feed to a workpiece, cutting speed, feed rate, and depth.
One of the ways of categorising lathe cutting tools is to divide them into two main groups: the first group of tools, which feed at an angle in certain operations, can be either right-handed or left-handed tools; the others belong to the neutral group of tools, which feed perpendicular to the workpiece;
Of course, some tools can belong to both groups.
Here are 11 common types of turning tools:
Their function is to cut the outer material of the workpiece to approximately the correct size.
Due to the difference in accuracy, turning and finishing tools are required to remove material directly from the workpiece, thus obtaining surfaces with lower or higher accuracy, respectively.
They are used to cut flat surfaces on workpieces.
These instruments are used for cutting external or internal grooves and for other machining operations such as cutting slots.
They are designed to be flat or round and are used to form shapes with desired dimensions.
They are essential for machining threads on workpieces. It passes through the workpiece in a linear fashion, removing chips from the workpiece with each pass.
This type of instrument creates a recessed surface that is inaccessible using linear tools.
Their purpose is to create holes by separating the material or pushing it to one side, like a needle, rather than removing it.
These instruments are the most commonly seen and are used to eliminate sharp edges on inclined surfaces.
They can be used to enlarge existing holes to obtain new sizes and shapes if required.
These narrow-edged tools are used to turn workpieces in half.
Ⅱ.How to choose the appropriate turning tool？
First, determine the main offset angle of the tool
The main offset angle of conventional tools is generally between 90° and 95° for finish machining, with 91°, 93° and 95° being the most commonly used.
A main offset angle close to 90° allows the cutting force to be directed in the direction of the workpiece axis, without applying a perpendicular force to the workpiece, thus avoiding vibrations and deformations (especially important for machining thin-walled parts). The main offset angle is generally guaranteed on the toolholder.
Secondly, determine the tip angle (choose the shape of the tool)
Generally, the main forms of turning inserts we commonly use are:
- Round blades (beginning of R, ISO standard code, the same below)
- 90 degree square blade (S beginning)
- 80 degree diamond shaped inserts (C)
- 80 degree convex triangular inserts (W)
- 60 degree triangular blades (T)
- 55 Degree Rhombic Blades (Starts with D)
- 35-degree diamond-shaped inserts (beginning of V).
These tool shapes basically cover more than 90% of conventional turning inserts.
The above mentioned several kinds of inserts from the tool strength in the order of ranking: R, S, C (W), D, V.
Among them, R inserts have the highest strength and V the lowest. However, the higher the strength, the greater the resistance induced and the greater the risk of vibration. So, often for rough turning we use R, S, C(W). For precision machining, we choose D and V. Of course, the profiling ability needed for finishing is also best with V and worst with R.
Third, to determine the front angle of the turning tool back angle
We should be familiar with the characteristics of each tool. For example, negative front angle tool rigidity is good, not easy to collapse edge, suitable for roughing turning. The positive front angle insert cutting light and fast, is more suitable for precision turning.
Back angle and front angle have a matching relationship, general negative front angle inserts are basically 0 degrees back angle. Positive rake angle inserts have 5° (B, ISO code, the same below), 7° (C), 11° (P) three commonly used rear angle with (only refers to turning).
Ⅲ.Commonly used turning tool combinations:
- CC/CP (C-type positive rake inserts 7°/11° back angle)
- DC (D-type positive rake inserts 7° back angle)
- VC/VB (V-type positive rake inserts 7°/5° back angle)
- WB/WC (W-type positive rake inserts 5°/7° back angle)
These combinations are also the insert shapes used for conventional precision machining (front and rear tip angles are defined).
The combination of the three results in the following tool form for conventional precision machining:
95° main offset toolholder + CC inserts
93° main offset toolholder + WB/WC/DC/VC/VB inserts
91° main offset toolholder + DC/VC/VB inserts
The 93° main offset toolholder + DC is personally recommended as the best finishing solution (for turning radials only). With the right cutting parameters, this solution should be one of the best in turning at the moment.
Of course, the above choice is not the only one. In the case of medium and large diameter solid materials, where high linear speeds can be achieved, inserts with a 0° back angle (ISO code N) can also be turned with a relatively good roughness.
However, in this case the roughness is achieved by ‘grinding’ the thick aluminium oxide CVD coated inserts at high line speeds. The difference between this roughness and the roughness produced by the tool selected above is basically about one grade. In other words, what I can turn to 1.6 with CN inserts, I can turn to 0.8 with DC inserts.
Ⅳ.Additions:
Other inserts commonly used in bore finishing are TC/TP (T-type positive front-angle inserts 7°/11° rear angle)
Corresponding toolholder combinations:
91° main offset toolholder + TC/TP inserts
For fine turning of bores with a diameter of more than 16, you can basically use DC inserts, but there are two kinds of DC, 07 and 11, 07 inserts can process bores of about 16 to 20; 25 and above can use DC11 inserts;
And for fine turning with bores around 10 to 16, you can use CC inserts or TC/TP inserts;
For bores smaller than 10, only CC inserts are generally more suitable.
V. Summary.
Cutting tools for CNC turning centers are vital parts intended for exact shaping of workpieces. Each cutting tool has its specific purpose, which can be classified by the angle they are fed to a workpiece.
To select a sufficient cutting tool is vital to consider machining parameters and materials both a tool and workpieces are made of.
If you have the needs or questions in this regard, please feel free to contact us, ALLIED has many years of cnc machining and its surface treatment experience and exquisite technology, with perfect pre-sale and after-sale service system, we will give the fastest speed to reply, look forward to your consultation.
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