In the forming and manufacturing of modern molds, with the improvement of the aesthetics and functional requirements of new products in the industries of machinery and electronics, automobiles, household appliances and so on, more and more complex parts make the mold surface more and more complex, and the proportion of free-form surfaces is increasing, which puts forward higher requirements for mold processing technology.
Due to the complex structure and high precision requirements of the die, the profile features and materials of different parts are quite different, and the die milling cutters used are also different.
Fig. 4-82 shows several milling cutters commonly used in die processing.
Fig. 4-82 several common milling cutters for mold processing
（1） Selection of die milling cutter with different machining methods
1. Select tools for mold NC Milling
In mold NC machining, flat end mills are commonly used to mill the inner and outer contours of parts and milling planes.
For the machining of some three-dimensional surfaces and variable angle contours, spherical milling cutters, ring milling cutters, drum milling cutters, conical milling cutters and disk milling cutters are commonly used.
The die milling cutter is used to process the forming surface of the die cavity, and the processing of the cavity mainly depends on various end milling cutters.
Die milling cutter is evolved from end milling cutter.
According to the shape of working part, it can be divided into three types: conical flat head, cylindrical ball head and conical ball head.
According to the material, it can be divided into carbide die milling cutter, high-speed steel die milling cutter, etc.
Cemented carbide die milling cutters are widely used.
In addition to milling various die cavities, they can also replace hand files and grinding wheel heads to clean the flash of cast, forged and welded parts, and finish some formed surfaces.
Table 4-17 shows the selection of die milling cutters with different processing methods.
Cemented carbide die milling cutters are widely used. In addition to milling various die cavities, they can also replace hand files and grinding wheel heads to clean the flash of cast, forged and welded parts, and finish some formed surfaces.
Table 4-17 shows the selection of die milling cutters with different processing methods.
|Category item||Indexable willow leaf ball end milling cutter||Indexable circular milling cutter||Large feed high speed milling cutter||Integral welding milling cutter|
|Deep cavity machining||3||4||5||3|
|Cutter bar and disc diameter /mm||20~950||Medium 10~p160||Medium 20~0100||91~ medium 40|
|Maximum back cutting amount /mm||Maximum blade length||Blade radius||12~2||5|
|Number of blades used||2||Round blade||3||2（4）|
|Scope of application||Versatile tool for die processing, easy to use, most suitable for stamping die processing||It is most suitable for contour, cavity and contour processing, and is best at plastic mold, die-casting mold, forging and stamping mold||Suitable for long and deep cavity milling||It has a complete variety and can be reground. Different tool materials and structure types are used|
Note: 5 → 1 indicates excellent → inferior.
2. Selection of die milling cutter
Reasonable tool life is generally divided into two types: the highest productivity tool life and the lowest cost tool life.
The former is determined according to the goal of minimum man hours per piece, and the latter is determined according to the goal of minimum process cost.
Compared with ordinary machine tool machining methods, NC machining puts forward higher requirements for cutting tools.
It requires not only good rigidity and high precision, but also stable size, high tool life, and convenient installation and adjustment to meet the requirements of high efficiency of NC machine tools.
The cutting tools used in CNC machine tools often adopt cutting tool materials suitable for high-speed cutting, and use indexable inserts.
For multi tool machine tools and modular machine tools with complex tool loading, tool changing and tool adjustment, the standard for tool life should be higher and the tool reliability should be ensured.
When the productivity of a certain process in the workshop limits the improvement of the productivity of the whole workshop, or when the cost of the whole plant shared by a certain process per unit time is large, the tool life should also be selected lower.
Factors such as tool manufacturing, grinding cost and complexity can be considered when formulating tool life.
In order to improve the production efficiency and give full play to its cutting performance, the tool life of the machine clamp indexable tool with short tool change time can be selected to be lower.
The service life of complex and high precision tools should be higher than that of single edge tools.
During finishing of large parts, in order to ensure that the tool path is completed at least once and avoid changing the tool in the middle of cutting, the tool life shall be determined according to the part accuracy and surface roughness.
（2） Selection of die milling cutter structure for machining different die surfaces
The tool structures selected in different processing stages of mold processing are also different. The structure of tools available for rough and finish machining of mold is shown in fig. 4-83.
Fig. 4-83 tool structure applicable to different processing stages
- a) Tools for rough machining of moulds
- b) Tools for die finishing
Advanced cutting tools can be used for the processing of small and medium-sized blanks, which improves the processing quality and production efficiency.
According to different die sizes, different rough and finish machining requirements and different machining positions, the corresponding end mills shall be used for large dies.
Small molds such as mobile phone molds (see Fig. 4-84a) are usually processed with integral end mills, while large molds such as bumper injection cavity (see Fig. 4-84b) are usually processed with integral end mills due to the consideration of economy and improving processing efficiency.
Fig. 4-84 mold processing
- a) Small mold, mobile phone mold processing
- b) Large mold, automobile bumper injection cavity processing
Table 4-18 summarizes the die milling cutters with different structures applicable to different die surfaces.
According to the characteristics of different die surfaces, correct and reasonable selection can greatly improve the efficiency of die milling, reduce the waste of cutting tools and reduce the production cost.
Table 4-18 die milling cutters with different structures applicable to different die surfaces
The main goal of die rough machining is to pursue the metal removal rate per unit time and prepare the geometric contour of the workpiece for semi finishing.
The main goal of die semi finishing is to make the workpiece contour shape flat and the surface finishing allowance uniform.
In the rough machining and semi precision machining of dies, it is mainly used for high-efficiency and economic machining with large feed.
Indexable blade milling cutters and high-speed milling cutters with large feed can be used.
Among them, the high-speed milling cutter with large feed can cut under very high cutting parameters.
Its workbench feed is very high, but the cutting thickness is small.
It belongs to large feed but small back cut.
The cutting force is mainly generated in the axial direction, which can reduce the vibration trend and obtain a very high metal removal rate.
(1) Selection of milling cutter for rough machining and semi finish machining of different profiles
1) For rough machining of large profile (plane, inclined plane, etc.), indexable blade end mills, face mills and large feed high-speed milling cutters shall be selected.
2) End milling cutter with round blade shall be selected for rough machining and semi-fine machining of small surfaces.
The circular blade has large arc radius and high blade strength.
3) For roughing and semi finishing smaller profile, the indexable blade ball end mill shall be selected.
The blade shape can be willow shaped, with small cutting force and high processing efficiency.
Ball end milling cutter shall be used for surface finishing, and the ball radius of ball end milling cutter shall be selected as large as possible to increase tool stiffness, increase heat dissipation and reduce surface roughness.
In general, the curvature radius of the finish machined surface shall be greater than 1.5 times of the tool radius to avoid sudden change of the feed direction.
However, when machining concave arc, the radius of the ball end of the milling cutter must be less than the minimum radius of curvature of the machined surface;
Small diameter ball end milling cutter can finish milling small chamfers on steep surfaces / straight walls.
However, when the ball end milling cutter is used to improve the machining efficiency by increasing the back feed, obvious cutting residues will be left on the workpiece after machining, which will increase the processing load of subsequent finishing tools.
Although the rough machining efficiency is very high, it will reduce the machining efficiency of subsequent processes.
(2) Selection of milling cutter for finishing different profiles
1) The end milling cutter with indexable blade type ball head can be selected for finishing large profile to realize high-precision machining;
The ball end mill can be used to finish the small profile to achieve high precision machining.
2) High precision ball end milling cutter (see Fig. 4-85) can be selected for finishing of micro arc r part to achieve high precision machining.
Fig. 4-85 high precision ball end milling cutter
3) For deep grooves and corners with small size and width, small-diameter integral cemented carbide tools can be used for back chipping and corner cleaning of each workpiece (see fig. 4-86).
Fig. 4-86 small diameter solid carbide milling cutter
When the mold cavity is a complex three-dimensional surface and it is difficult to be decomposed into several simple surfaces for machining, profiling milling should be used.
Profiling milling has high machining efficiency and is especially suitable for rough machining of the cavity.
（3） Selection of copying cutting tools in mold processing
Profile milling covers multi axis milling of two-dimensional and three-dimensional convex and concave shapes.
The larger the part and the more complex the shape to be machined, the more important the process planning becomes.
Typical profiling milling is shown in fig. 4-87.
Fig. 4-87 schematic diagram of typical profiling milling
Profiling milling mainly refers to the milling of convex cavities and curved surfaces, which is often used to process the contour of mold manufacturing.
The profile milling cutter mostly adopts arc edge.
Compared with ordinary milling, it can obtain smaller machining residual area under the same feeding conditions, so as to improve the surface machining quality.
1. Features of profiling milling cutter
Profile milling cutter is a tool equipped with indexable inserts with circular cutting edges.
All the milling cutters are equipped with circular cutting edges (indexable inserts for torus milling cutters or ball end milling cutters) or some are equipped with circular cutting edges.
The classification of profiling milling cutters includes sleeve milling cutters, milling cutters with spiral shanks, modular (spiral) milling cutters, etc.
The profiling milling cutter uses a circular blade, which makes the profiling milling cutter have a variety of advantages.
It can achieve small back feed and large feed, which is a supplement to the development trend of high-speed machining.
The profiling milling cutter has the following advantages:
1) Strong feeding capacity.
Some profiling milling cutters can feed directly into the workpiece like drill bits.
2) Spiral interpolation.
Large diameter holes can be easily and quickly machined by using profiling milling cutter and spiral interpolation.
3) Cutting edge strength.
Because there is no sharp angle, the circular blade can withstand greater tool deflection and vibration, allowing to increase the speed and feed during machining, while reducing the risk of edge collapse.
4) Number of cutting edges.
The round blade has more cutting edges available.
According to the size of the blade and the amount of back cutting, the circular blade can have 4~8 effective transpositions, and the material removal amount is at least twice that of the ordinary diamond and square blades.
This advantage can reduce the number of blade changes by the operator, with high efficiency and good economy.
5) Efficient cutting.
The use of round blades does not require a high machine power to have a high metal cutting rate.
Because of the high strength of the circular blade, it can be processed with a larger feed rate than the right angle milling cutter, and even rough machining with a larger load on a light machine tool.
6) The surface precision of workpiece after rough machining is high.
The surface milled with round blade is not like the rough machined surface with right angle cutter, which has obvious unevenness and low surface residual height.
After rough machining with round blade, the workpiece surface precision is high, and it can be directly semi finished, or even directly finished.
2. Selection of profiling milling cutter
1) Classification and selection of profile milling cutter blades.
Profiling milling cutters mainly include round blade milling cutters and ball end milling cutters.
The milling cutter with round blade is shown in fig. 4-88.
Ball type milling cutter and blade are shown in fig. 4-89.
Fig. 4-88 milling cutter with round blade
Fig. 4-89 ball end milling cutter and blade
2) Selection of cutter body and blade.
The cutter body selection of profiling milling cutter can be determined according to the application conditions (see table 4-19).
Table 4-19 cutter body selection under different working conditions
|The round blade tool is selected for this machining:|
1. The small blade has good versatility and low cutting force, which is used for finishing and unstable working conditions;
2. The large blade has high strength and high metal removal rate, which is used for rough machining and stable working conditions.
|Under this working condition, it is recommended to select ball head cutter.According to the design features of the part, it is necessary to select the appropriate tool body according to the fillet limit of the machining part and the limit of the tool diameter.|
|Split head and cutter bar are designed separately. Different types and lengths of cutter bars can be used for the same cutter head according to actual needs, and different types of cutter heads can also be used for the same cutter bar.|
|Integrated tool bar type, the tool bar diameter can be thinner.|
3) Selection of processing stage.
Generally, round blade cutters are used for rough machining, and solid carbide ball end mills and end mills with interchangeable solid carbide cutting heads are used for finish machining (see fig. 4-90).
Fig. 4-90 selection of profiling milling cutter in different processing stages
a) Milling cutter for profiling rough machining
b) Milling cutter for profiling finishing
4) In profile milling, there are four kinds of commonly used cutters: round blade milling cutter, ball end milling cutter, replaceable ball end milling cutter and solid carbide ball end milling cutter.
Table 4-20 compares the four commonly used profiling milling cutters in terms of cutting stability, processing cost and productivity.
Table 4-20 selection of different types of milling cutters in copying machining
|Milling cutter type|
|Considerations||Circular blade milling cutter||Indexable ball end milling cutter||Replaceable milling head type milling cutter||Solid carbide ball end milling cutter|
|Rough machining||Very good||good||Acceptable||Acceptable|
|Rough machining||Acceptable||Acceptable||Very good||Very good|
|Back cutting depth||middle||middle||middle||little|
|productivity||Very good||Very good||Very good||good|
5) Calculation of cutting speed (see Fig. 4-91 and Fig. 4-92 for schematic diagram).
When calculating the cutting speed of ball end milling cutter or round blade cutter, the actual cutting speed vc will be lower (if ap is smaller), and the table feed and production efficiency will be seriously limited.
Fig. 4-91 ball end milling cutter
Fig. 4-92 round blade
According to the actual or effective cutting diameter Dcap has:
For calculation of effective cutting diameter Dcap of ball end milling cutter:
- N — rotating speed, unit: r/min;
- Vc — cutting speed, unit: m/min;
- Dcap – effective tool diameter, unit: mm;
- DC – nominal diameter of tool, unit: mm;
- aP — axial back cutting amount, unit: mm.
Calculation of effective diameter of round blade tool:
- Dc – nominal diameter of tool, unit: mm;
- ic– nominal diameter of blade, unit: mm;
- ap — axial back cutting depth, unit: mm.
（4） Problems and Countermeasures in practical application of die milling cutter
The main faults of the tool blades used in mold manufacturing are hot cracking, then the wear around the cracks, chip buildup, and finally the edge collapse.
In the process of die milling, various machining problems will occur due to improper tool selection or parameter setting.
Table 4-21 shows the analysis and summary of tool problems in mold processing.
The above problems may occur during mold processing, including mold milling cutter problems and other reasons.
According to the characteristics of the problems, timely and accurate determination of the causes of the problems and reasonable and standardized correction can avoid production accidents, so as to reduce production costs and improve production efficiency.
Table 4-21 cutter problems and Countermeasures in mold processing
|Problem points and phenomena||Reason||Countermeasure|
|Tool breakage||1. The feed speed is too fast;|
2. Too much penetration;
3. The overhang is too long;
4. The blade is severely worn;
5. The cutting edge length is too long.
|1. Reduce the feed speed;|
2. Reduce the cut in volume;
3. Reduce the overhang;
4. Carry out regrinding in time;
5. Minimize the blade length.
|Excessive wear and hot cracking||1. The cutting speed is too fast;|
2. The inclination angle of side cutting is too small;
3. Workpiece hardness is too high.
|1. Reduce cutting speed and supply cutting fluid;|
2. Properly modify the inclination angle of the side edge;
3. The cutting fluid shall be supplied in the sequence of dry-type, water-soluble and non-water-soluble, and the surface treatment shall be carried out.
|Vibration in cutting||1. The cutting conditions are not met;|
2. unstable clamping of machined parts;
3. The overhang is too long;
4. The angle after side cutting is too large.
|1 Adjust cutting parameters;|
2 Optimize clamping;
3. Minimize the blade length and overhang;
4. Reduce the back angle of the side edge.
|Poor machining dimensional accuracy||1. Few blades;|
2. The blade length is too long.
|1. Use a milling cutter with a large number of edges;|
2. Modify the blade length appropriately.
|Burr on surface||1. The feed speed is too fast;|
2. The blade is severely worn;
3. The cut in volume is too large.
|1. Reduce the feed rate;|
2. Carry out grinding in time;
3. Reduce the cut in amount.
|Poor chip removal||1. Cutting fluid pressure is low;|
2. Small space for empty chips;
3. The cut in volume is too large.
|1. Increase the cutting fluid volume and pressure;|
2. Use milling cutter with few edges;
3. Reduce the cut in amount.