The tool selection and cutting parameters determination in NC machining are completed in the state of human-computer interaction, which is in sharp contrast to ordinary machine tool machining.
At the same time, it also requires programmers to master the basic principles of tool selection and cutting parameters determination, fully consider the characteristics of digital machining in programming, and be able to correctly select tools and cutting parameters.
CNC machining tools must adapt to the characteristics of high speed, high efficiency and high degree of automation of CNC machine tools.
Generally, they should include general tools, general connecting tool handles and a small number of special tool handles.
The tool handle should be connected with the tool and installed on the power head of the machine tool, so it has been gradually standardized and serialized.
There are many ways to classify NC cutting tools.
1. Structure of NC machine cutting tools
1. Integral type;
2. Inlay type, which adopts welding or machine clamp connection. Machine clamp type can be divided into non indexable and indexable;
3. Special types, such as compound cutting tools, damping cutting tools, etc.
2. Materials used for manufacturing tools
1. High speed steel cutting tools;
2. Cemented carbide cutting tools;
3. Diamond cutting tools;
4. Other material cutting tools, such as cubic boron nitride cutting tools, ceramic cutting tools, etc.
3. Classification of cutting process
1. Turning tools, including outer circle, inner hole, thread, cutting tool and so on;
2. Drilling tools, including drill bits, reamers, taps, etc;
3. Boring tools;
4. Milling tools, etc.
In order to meet the requirements of CNC machine tools, the number of adjustable and durable tools has reached 40% ~ 90% of the total number of CNC machine tools in recent years.
4. Characteristics of NC tools
1. Good rigidity (especially rough machining tools), high precision, low vibration resistance and thermal deformation;
2. Good interchangeability, convenient for quick tool change;
3. High service life, stable and reliable cutting performance;
4. The size of the tool is easy to adjust to reduce the adjustment time of tool change;
5. The cutting tool shall be able to reliably break chips or roll chips, so as to facilitate the removal of chips;
6. Serialization and standardization to facilitate programming and tool management.
5. Tool selection
The tool selection is carried out in the human-computer interaction state of NC programming.
The cutter and shank shall be selected correctly according to the processing capacity of the machine tool, the performance of workpiece materials, processing procedures, cutting parameters and other relevant factors.
The general principle of tool selection is: convenient installation and adjustment, good rigidity, high durability and precision.
On the premise of meeting the processing requirements, try to choose a shorter tool handle to improve the rigidity of tool processing.
When selecting the tool, make the size of the tool adapt to the surface size of the workpiece to be processed.
In production, end milling cutter is often used to process the peripheral contour of plane parts;
When milling the plane, carbide blade milling cutter should be selected;
During high-speed milling, select the convex cutter and groove;
When machining rough surface or rough machining hole, corn milling cutter with cemented carbide blade can be selected;
Ball end milling cutter, ring milling cutter, conical milling cutter and disc milling cutter are often used for the machining of some three-dimensional profiles and variable angle profiles.
When machining free-form surface (mold), because the cutting speed of the end of the ball head tool is zero, in order to ensure the machining accuracy, the cutting row spacing generally adopts the top dense distance, so the ball head is often used for surface finishing.
The flat head cutter is superior to the ball head cutter in terms of surface machining quality and cutting efficiency.
Therefore, as long as it can not cut, flat head cutter should be preferred in both rough machining and finish machining of curved surface.
In addition, the durability and accuracy of cutting tools have a great relationship with the price of cutting tools.
It must be noted that in most cases, although the selection of good cutting tools increases the cost of cutting tools, the improvement of processing quality and processing efficiency can greatly reduce the whole processing cost.
On the machining center, various cutting tools are installed on the tool magazine respectively, and the tool selection and tool change actions are carried out at any time according to the program regulations.
Therefore, the standard tool holder must be adopted so that the standard tools used in drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine.
Programmers should understand the structural size, adjustment method and adjustment range of the tool handle used on the machine tool, so as to determine the radial and axial dimensions of the tool during programming.
6. The principle of tool arrangement shall be followed
In the machining process of economical NC machine tools, because the grinding, measurement and replacement of cutting tools are mostly carried out manually, which takes up a long auxiliary time, the arrangement order of cutting tools must be arranged reasonably.
Generally, the following principles shall be followed:
1. Minimize the number of tools;
2. After a tool is clamped, all machining steps it can carry out shall be completed;
3. The tools for rough and finish machining shall be used separately, even those with the same size and specification;
4. Milling before drilling;
5. Finish the surface first, and then finish the two-dimensional contour;
6. If possible, the automatic tool change function of NC machine tool should be used as much as possible to improve production efficiency.
7. Principle of reasonable selection of cutting parameters
During rough machining, productivity is generally improved, but economy and processing cost should also be considered;
In semi finishing and finishing, the cutting efficiency, economy and processing cost should be considered on the premise of ensuring the processing quality.
The specific value shall be determined according to the machine tool manual, cutting parameter manual and experience.
The following factors should be considered:
1. Cutting depth t.
When the rigidity of machine tool, workpiece and tool allows, t is equal to machining allowance, which is an effective measure to improve productivity.
In order to ensure the machining accuracy and surface roughness of parts, a certain allowance shall be reserved for finishing machining.
The finishing allowance of NC machine tools can be slightly less than that of ordinary machine tools.
2. Cutting width L.
Generally, l is directly proportional to the tool diameter D and inversely proportional to the cutting depth.
In the machining process of economical NC machine tools, the value range of L is generally L = (0.6 ~ 0.9) d.
3. Cutting speed v.
Increasing V is also a measure to improve productivity, but V is closely related to tool durability.
With the increase of V, the tool durability decreases sharply, so the choice of V mainly depends on the tool durability.
In addition, the cutting speed is also closely related to the processing materials.
For example, when milling alloy 30crni2mova with an end milling cutter, V can be about 8m / min;
When milling aluminum alloy with the same end milling cutter, V it can be more than 200m / min.
4. Spindle speed n (R / min).
The spindle speed is generally selected according to the cutting speed v.
The calculation formula is: v = Πnd / 1000.
The control panel of the NC machine tool is generally equipped with a spindle speed adjustment (magnification) switch, which can adjust the spindle speed by an integral multiple during the machining process.
5. Feed speed vF.
vF shall be selected according to the machining accuracy and surface roughness requirements of parts and the materials of cutting tools and workpieces.
The increase of vF can also improve production efficiency.
When the requirement of machining surface roughness is low, vF can be larger.
During processing, vF can also be adjusted manually through the repair switch on the machine tool control panel, but the maximum feed speed is limited by the equipment stiffness and feed system performance.