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Selection of Machine Tools
The CNC engraving machine has only seen significant development domestically in the past one or two years. Related processing factories and users constantly keep a keen eye on manufacturers' trends, which is the real reason why producers of engraving and milling machine hosts dare not relax at all.
As a user, of course, you need to choose the right equipment. If the selection is inappropriate, not only will it fail to make money, but it will also lead to the bitter situation of working for the machine. So, what kind of machine tool is a good machine tool?
We believe the definition of a good machine tool is this: a machine tool that can recoup its investment in a short period is a good machine tool. The design service life of CNC machine tools is generally 7 years, mainly based on the service life of the CNC system. The ratio of spending to earning will directly affect your business, so carefully analyzing functions for selection is a necessary condition for effective investment.
The term engraving and milling machine (CNC engraving and milling machine) has existed abroad for a long time. Strictly speaking, engraving is part of milling. Whether to purchase an engraving machine or a CNC milling machining center is a question often asked. Additionally, there is the currently popular high-speed cutting machine tool (HSC MACHINE).
Let's first clarify the differences among the three machine types:
1. -- CNC milling and machining centers are processing equipment used to complete workpieces with larger milling volumes;
2. -- CNC engraving and milling machines are used for finishing or deburring parts with more complex shapes. Their milling volume is small but precise and fast, and they can replace some EDM processing;
3. -- High-speed cutting machine tools are used to complete medium milling volumes and minimize the polishing amount after milling.
A deep analysis of the structure of the above equipment can help us make the right choice.
I. -- From the mechanical perspective
The machine tool's mechanics are divided into two parts: moving parts and non-moving parts. The worktable, slide plate, cross slide, etc., are moving parts; the bed, column, etc., are non-moving parts.
1. -- CNC milling machining center:
Non-moving parts require very good rigidity; moving parts also require very good rigidity.
Advantages: capable of heavy cutting; Disadvantages: because the moving parts are also large, it sacrifices machine flexibility and is powerless for small parts and rapid feed.
2. -- CNC engraving and milling machine
Non-moving parts require good rigidity; moving parts must prioritize flexibility, be as light as possible while maintaining certain rigidity.
Advantages: capable of relatively small processing, high machining accuracy, and can replace some EDM processing. For soft metals, high-speed machining is possible; Disadvantages: due to poor rigidity, heavy cutting is impossible.
3. -- High-speed cutting machine tool
Non-moving parts require very good rigidity; moving parts require relatively good rigidity and be as lightweight as possible.
Advantages: capable of medium to small volume cutting (e.g., generally a φ10 flat-bottom cutter, for 45# steel (300), a cutting depth of 0.75 is optimal);
Disadvantages: when used correctly, it can achieve high efficiency and low cost, minimizing polishing; when used incorrectly, it will quickly cause a mountain of tool scrap.
How to achieve the above contradictory requirements of being light and rigid mechanically depends on the mechanical structure's craftsmanship.
1. -- The bed adopts a mesh structure with high and low ribs, some directly use a honeycomb connected internal hexagonal mesh structure;
2. -- Extra-wide columns and beams. It is known that the gantry structure, due to its excellent symmetry and rigidity, has always been the preferred structure for high-speed cutting equipment manufacturers;
3. -- For moving parts, a significant difference from CNC milling is the widened distance between many guide rails to overcome adverse torque issues;
4. -- Material-wise, generally Meehanite cast iron, also known as inoculated cast iron, is used. During casting, a certain proportion of silicon (Si) is added to change the internal structure of the iron, making it more impact-resistant and significantly improving rigidity;
5. -- The rigidity of the machine tool mainly overcomes the strong impact of moving parts on non-moving parts during high-speed movement, so guide rails and ball screws require being thicker, and connection parts' rigidity is strengthened.
II. -- From the CNC perspective
1. -- CNC milling machining centers require moderate CNC system speed, with spindle speeds around 0-8000 RPM.
2. -- Engraving and milling machines require high-speed CNC systems, with spindle speeds around 3000-30000 RPM.
3. -- High-speed cutting machine tools require high-speed CNC systems and excellent servo motor characteristics, with spindle speeds around 1500-30000 RPM.
III. -- From the programming software perspective
From the software perspective, CNC milling machining centers, high-speed cutting machine tools, and engraving and milling machines can all use standard CAD/CAM software such as MasterCam, Cimatron, PRO/E, UG, etc.
Milling machines usually consider Cimatron's toolpaths better. The new version of the software fully considers the uniformity of the tool's cutting volume at every moment, especially the speed and smoothness when the tool enters and exits the workpiece, as well as the following error algorithm at turning points, making the results closer to the design graphics. The CAD part heavily uses intuitive 3D solid modeling like Solidworks, then transfers via IGS to CAM software for processing.
But don't worry, the development speed of CAD/CAM far exceeds that of CNC machine tools.
Due to the special nature of engraving tools and the need for very fine angle control, TYPE3 or Jingdiao software is preferred.
IV. -- Automatic tool changer and fully automatic tool setting requirements
Machining center is a respectable term because it is equipped with an automatic tool changer. However, I think the requirements for the tool changer must be combined with actual conditions.
A machining center is a CNC device designed to complete multiple processes automatically, mainly for fixed large-batch production operations. For example, if we are machining a box with many holes and threads and need to produce similar batches daily (minimum batch size over 200), then a machining center is definitely the choice. For the mold industry and small-batch production units, do not rush to use machining centers because many manufacturers buy machining centers but use them as CNC mills. Using a tool magazine is simple in terms of CNC system cost, but the spindle, tool magazine, air compressor, and various tool holders increase costs. The price difference for one machine can exceed 100,000 RMB, and programmers must be clear-headed. Otherwise, disasters and many problems will occur. Regarding efficiency, for production volumes of the same type of workpiece under 100-200 pieces, try not to use machining centers as efficiency is too low.
What is a good way to improve efficiency without using a tool magazine and without causing manual tool change errors? Only a fully automatic tool setting system can do this: install the tool upwards, press one button, and the machine tool automatically sets the tool and starts machining, with errors within 0.001-0.0003 μm. Compared to automatic tool changing time, it is not slower at all. If it is a machining center without an automatic tool setting device, the efficiency in practice is much lower than a machine tool without a tool magazine but with automatic tool setting. Also, please note that the best brand of automatic tool setters (such as Marposs) costs about 10,000 RMB and is unlikely to break. Analyzing this, for domestic mold processing and small-batch industrial parts processing with abundant labor, if the value of capital utilization is fully considered, machining centers should not be used. Additionally, domestic manufacturers' tool magazines still have many problems, and imported ones are quite expensive (a BT40 10-tool magazine costs no less than 100,000 RMB).
5. Views on High-Speed Cutting Equipment
Smart processors will spend a lot of time considering machine tool setup time and losses caused by human factors, and we never need to remind them about this. What we should do is provide them with higher machining efficiency from the machine tool's processing time and reduce their grinding time as much as possible, even to zero grinding. The dazzling debut of high-speed cutting equipment is an event that excites processors. Don't consider its expensive price (around 1.5 million RMB) for now. From a usage perspective, I believe the old saying applies: it is a delicious piece of meat for some people and deadly poison for others. The Japanese and Europeans tirelessly promote the benefits of high-speed cutting to encourage adoption of their equipment, though some practices are impractical. My 8 years of practical experience in the CNC machine tool industry tells me their cost should be around 600,000-700,000 RMB. Such a high profit margin reveals their true nature. The gross profit margin for domestic CNC machine tools should be between 30-40%, which is reasonable and honest profit. If you want to maintain their high profits, I think you cannot earn more money in the same time. Also consider after-sales service, etc. I don't believe their service can solve all problems within 1-2 days because our current technology cannot achieve "solid fax".
From the perspective of revitalizing national industry, domestic machine tool manufacturers should be supported. The essence of high-speed cutting is to complete the tool's travel length in a short time. Everyone knows
Distance = Speed × Time
The tool's description and fixed cutting method determine that longer length means no profit, and increasing speed means shortening time.
High-speed cutting is currently limited by the machine tool's spindle and the tools used, so it cannot always be better. The common practice abroad is not to use high-speed cutting on the first machine but to consider it for the second or third machine.
Successful practices include:
One profiling mill completes most of the cutting (heavy cutting), leaving 2-3mm machining allowance, and the remaining cutting is quickly finished on a high-speed machine. This approach has many advantages:
1) Many heavy cutting machines, low processing cost
2) Large processing volume, easy to achieve low precision requirements
3) Cheap tools
4) After switching to high-speed cutting, since the workpiece is close to forming, the first cut's cutting volume is very uniform
5) Fast and accurate forming
6) Less grinding, precise dimensions
7) Although tools are expensive, the time is short, cutting volume is small, and the performance-price ratio is good
8) Many fine details, such as small angle draft, are finished in one detailed machining without needing EDM assistance
9) Excellent surface finish (mirror effect)
Thus, equipment coordination is the key to success. Because the remaining allowance is 2-3mm, two calibration G54 stops on the XY table solve the workpiece clamping problem, and for workpieces with small cutting volume, machining is directly completed on the high-speed machine.
6. Another issue to discuss is the spindle problem
For CNC milling machines and machining centers, because low speed and high torque are required, a spindle gearbox is needed to reduce speed and increase torque. Low speed and poor precision are inevitable, so small radius tools are unlikely to be used. For engraving and milling machines, the spindle must operate at 10,000-20,000 RPM, with a runout accuracy of about 2 μm; otherwise, tool breakage will be severe. Therefore, an electric spindle, where the motor and spindle are integrated, must be used. For high-speed cutting equipment, a built-in electric spindle is required, and even at low speeds, a certain torque is needed with oil-water cooling to maintain spindle operating temperature.
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