The CNC process can be simply divided into roughing and finishing, and CNC roughing is to make the material into a rough shape, and finishing is to cut the material into fine shape. Rough machining first removes excess parts, followed by precision machining in the second step of finishing.
Rough-processed products refer to products made by simple processing or the primary processing of raw materials. They are generally prepared for semi-finishing and finishing, which is convenient for the subsequent processing process to be faster and more convenient. Rough-processed products have low processing accuracy and surface Poor quality and other characteristics.
Rough machining does not have high requirements on the surface quality after machining. Generally, it is to prepare for semi-finishing and finishing. Because the rough machining allowance is large, the processing speed is high, and the heat generated by the machining is also large, so the processing Tool requirements are relatively high. Generally, alloy materials with high hardness are used as tool materials. At the same time, heat treatment measures must be taken during roughing, and if necessary, manual cooling of the tool, such as oil bath cooling and air cooling to extend the tool life.
Optimizing rough machining is three to four times faster than traditional machining methods, and makes the service life of cutting milling cutters in titanium alloy longer. The design of parts with straight prismatic walls requires a longer axial cutting depth and can engage all grooves on the milling cutter, which is ideal for optimizing roughing. In these cases, this strategy optimizes the often-challenging corner features and achieves high metal removal rates in superalloys and various stainless steels.
However, to avoid errors and imperfect results, for applications that are not in the optimal parameter range, the workshop should skip optimizing roughing. For example, in a complex three-dimensional mold cavity, optimized roughing may produce a stepped surface, requiring a lot of semi-finishing. In this case, high-feed roughing will produce better results.
Optimized roughing provides efficient results on applicable parts and features, including grooves with longer axial cutting depths, challenging corners and straight walls. This strategy can significantly improve part cycle time, surface finish, milling cutter life, and machine tool utilization. Taking the time to understand the workshop that optimizes rough machining can increase productivity, efficiency, and profitability, and these parts are the best choice for this strategy. To achieve the best results, the workshop should use the expertise of milling cutter suppliers to adjust their methods for individual work.
Milling of curved surfaces belongs to the category of contour/contour CNC milling. It involves machiningirregularly shaped contours or continuous curves with different angles (inclined, concave or convex). This is the key process to complete most of the customized parts with unique shapes and requires CNC machinists to have advanced expertise in basic machiningprocedures and principles.
This article will give a detailed introduction to the surface CNC milling technology. What aspects should be paid attention to when milling curved surfaces? What should the complete curved surface machiningflow look like?
The processing of curved surface parts also follows the processing rules of CNC machining centers. Before reaching the final processed parts, they are first subdivided into different categories: rough machining/semi-rough machining, semi-finishing, finishing and super finishing. Because free-form curves and surfaces cannot be clearly expressed by geometric drawing and mechanical drawing, it has become the primary problem to be solved in the roughing stage of the machining center.
In order to improve the efficiency of milling when the rough parts of the curved surface parts are preliminary processed, according to the margin given by the surface to be processed, an end mill can be used to mill them layer by layer according to the contour surface. After rough milling, the shape of the curved surface will appear hierarchical distribution, and the height of the steps depends on the rough milling accuracy. During rough machining, due to the large machining allowance and cutting amount, a large amount of cutting heat is generated during the cutting process, which is easy to make the tool wear quickly. At this time, the temperature of the cutting area should be lowered, and the cutting fluid should be applied at this stage. Lubrication and cooling effect.
Semi-roughing is indispensable. It is an important stage from roughing to finishing. Its purpose is to mill out the extra part left in the roughing step. Semi-finish milling should be carried out with a ball-end milling cutter, and its line and step distance should be larger than that of fine milling. It should be noted that semi-finishing milling should leave a machining allowance of about 0.5 mm for the subsequent finishing milling process. After this process, the shape of the processed surface is close to the theoretical curved surface.
Finishing is the process of finally processing the theoretical surface. Ball-end milling cutters are the first choice for finishing tools, and line cutting is usually used. Attention should be paid to the selection of the turning point and the determination of the feed speed during programming. For parts with better openness, the turning point should be selected outside the curve table, that is to say, the curved surface should be appropriately extended outwards during programming.
We use various round-end milling cutters to machine smooth curved surfaces on the manufactured parts. These include ball head indexable end mills, round inserts and ball head solid carbide. We prefer to use circular contour tools in contour processing because it will not leave any obvious marks on the tool path.
These types of end mills usually leave a very good surface finish on the machined parts. Due to their structure, they may have lower stability. Therefore, it can be used for finishing instead of roughing.
Before choosing to manufacture custom parts through contour milling, it is important that you first be aware of the various factors that may affect the entire milling process. In addition, in order to choose the ideal processing method, we must also determine some specific matters. This is a further understanding of the main preparatory work we carried out.
The feed and speed determine the speed at which we subtract material from the part. This is why it has a major impact on obtaining an excellent surface finish. In order to achieve this goal, we need to follow some calculation methods. The feed rate may also depend on the tool used, the depth of cut, the material cut, the required accuracy, and the contour of the machined part.
In todays market, there are many tools available. We prioritize the rigidity of the tool for roughing operations to suit the feed requirements and the aggressive cutting depth for roughing. At the same time, the end mill must be very sharp in finishing, so as not to leave any obvious traces of the tool path. Another difference is the tool diameter. Generally, the diameter of the roughing tool is larger than the diameter of the finishing tool.
Vibration is one of the main causes of poor surface finish, chattering, and damage to cutting tools. During milling operations, vibration can come from a variety of sources, including clamping stability, tool stiffness, material hardness, and errors in the machine spindle. Here are some tips to reduce this processing barrier:
Machining or metal cutting is one of the secondary manufacturing processes by which excess material is gradually removed from a preformed blank to obtain desired shape, size and finish. There exist larger number of processes to fulfil the basic requirement of machining. Such processes can be broadly classified as conventional machining processes (turning, threading, facing, drilling, boring, shaping, planing, milling, grooving, reaming, etc.), abrasive cutting processes (grinding, lapping, honing, polishing, superfinishing, etc.), micro-precision machining processes (micro-milling, micro-drilling, diamond turning, etc.), and non-traditional machining processes (ultrasonic machining, electro-discharge machining, electro-chemical machining, laser beam machining, ion beam machining, hybrid machining, etc.).
All of these are subtractive manufacturing processes, which indicates layer by layer material is removed from a solid workpiece to obtain desired three dimensional features; however, they follow varying principles of material removal and thereby possess varying capability in terms of machinable materials, stock removal rate, surface quality, production rate and cost, etc. Most NTM processes and micro-precision machining processes are not suitable for removing bulk volume of material; instead, they can generate fine features with high accuracy. Conventional machining processes are suitable for high stock removal as well as imparting reasonably good surface quality. However, achieving both in a single pass is not possible. Thus machining is usually carried out in two steps with varying process parameters (cutting velocity, feed rate and depth of cut).
In first step, bulk amount of material is quickly removed from workpiece as per required feature. Higher feed rate and depth of cut are employed for this step so that high stock removal rate is obtained. This step is called rough cut or roughing pass. It cannot provide good surface finish and close tolerance. After rough cut, a finish cut or finishing pass is carried out to improve surface finish, dimensional accuracy and tolerance level. Here very low feed rate and depth of cut are employed. So stock removal rate reduces in finish pass but surface quality improves. Various differences between roughing and finishing in conventional machining processes are given below in table format.
Objectives of rough cut and finish cut: Rough cut is carried out to quickly impart a basic shape according to desired feature. Here surface roughness is not important factor; instead, removing maximum unwanted material is ultimate objective. Contrary to this, finish pass is carried out to improve surface finish, dimensional accuracy and tolerance of the desired feature. Stock removal rate has no importance in case of finish pass.
Process parameters and MRR: Cutting velocity (Vc), feed rate (s or f) and depth of cut (t or a) are three process parameters for every conventional machining process. These parameters greatly influence overall machining action and capability. Higher velocity, feed and depth of cut can increase material removal rate (MRR) but with the sacrifice of surface finish. MRR is proportional to velocity, feed and depth of cut and thus can be mathematically expressed by the multiplication of velocity, feed and depth of cut with a positive constant for unit conversion. During machining, velocity is normally maintained unchanged as it is selected on the basis of work and tool material, machine tool capability, vibration level and other important factors. To fulfil the basic objective, higher feed and depth of cut are employed in rough pass and as a consequence MRR increases. On the other hand, low feed and depth of cut are employed in finish pass and thus MRR reduces.
Surface finish and dimensional accuracy: Presence of scallop marks or feed marks on the finished surface is inherent to every conventional machining process due to feed velocity. Such saw-tooth alike scallop marks cause primary surface roughness. Apart from cutting tool geometry, surface roughness directly relies on feed rate. Higher feed rate can lead to poor surface finish. Higher depth of cut also tends to degrade surface finish and machining accuracy. In rough cut, higher feed and depth of cut are utilized and thus poor surface finish is obtained. It also fails to provide high dimensional accuracy and close tolerance. On the other hand, finish pass can improve finish, accuracy and tolerance as very low feed and depth of cut are employed.
Usage of old cutter: An old cutter may have less sharp edges (i.e., higher edge radius and nose radius) as it has already worn out during machining. Edge and nose sharpness limit the achievable surface finish in the process. A sharp edge cannot take high chip load but is mandatory to obtain better finish and accuracy. So an old cutter can be utilized in rough pass without noticeable problem as surface quality does not matter. However, a sharp tool should be used in finish pass so that better finish, accuracy and tolerance is achieved. Here feed and depth of cut remain low, so chip load possesses no detectable problem in tool breakage or edge chipping.
Scientific comparison among roughing and finishing in machining processes is presented in this article. The author also suggests you to go through the following references for better understanding of the topic.
In this paper, experimental investigations are carried out by end milling process on hardened tool steel, Impax Hi Hard (Hardness 55 HRC) a newly developed tool steel material used by tool and die making industries. Experiments are performed with an aim to study performance investigations of machining parameters such as cutting speed, feed, depth of cut and width of cut with consideration of multiple responses viz. volume of material removed, tool wear, tool life and surface finish to evaluate the performance of PVD coated carbide inserts and ball end mill cutters. It has been observed through scanning electron microscope, X-ray diffraction technique (EDX) that chipping and adhesion are active tool wear mechanisms and saw-toothed chips are formed while machining of Impax Hi Hard steel. It is also noticed out that tool life is not enhanced while machining with minimum quantity lubricant than dry machining. From the investigations, it is observed that hard machining can be considered as an alternative to grinding and EDM, traditional methods of machining difficult-to-machine materials i.e. hardened steel with hardness greater than 50 HRC with a scope of improved productivity, increased flexibility, decreased capital expenses and reduced environmental waste.
Senthil Kumar A, Raja Durai A, Sornakumar T (2006) The effect of tool wear on tool life of alumina-based ceramic cutting tools while machining hardened martensitic ceramic cutting tools while machining hardened martensitic stainless steel. J Mater Process Tech 173:151156
El-Wardany TI, Kishawy HA, Elbestawi MA (2000) Surface integrity of die materials in high speed machining, Part 2: micro hardness variations and residual stresses. Trans ASME J Manuf Sci Eng 122:632641
Klocke F, Schulz A, Gerschwiler K et al (1996) Saubere Fertigungstechnologien-Ein Wettbewerbsvorteil von morgen? In: Wettbewerbsfaktor Produktiontechnik-Aachener Perspektiven, Aachener Werkzeugmaschinen-Kolloguium (AWK), VDI, Dusseldorf
King RI, Vaughn RLA (1984) Synoptic review of high speed machining from Salomon to the present. In: Komadurai R, Subramanian K, Von Turkovich BF (eds) High speed machining (presented at the winter annual meeting of the American Society of Mechanical Engineers, New Orleans, LA). ASME, New York
Kishawy HA, Elbestawi MA (1997) Effects of process parameters on chip formation when machining hardened steel. In: Proceedings of the International Mechanical Engineering Congress and Exposition, Dallas, Texas, vol 6-2. ASME-MED, pp 1320
Kishawy HA, Elbestawi MA (1998) Effects of edge preparation and cutting speed on surface integrity of die materials in hard machining. In: Proceedings of the International Mechanical Engineering Congress and Exposition, vol 8. MED, pp 269276
The principal author would like to express his gratitude to Dr. Gopal P. Sinha, Director, CMERI, Durgapur for his kind permission to publish this work. He is grateful to DAAD for sponsoring fellowship programme and to Fraunhofer Institute for Production Technology, Aachen, Germany for providing necessary facilities for experimental work.
Gopalsamy, B.M., Mondal, B., Ghosh, S. et al. Investigations on hard machining of Impax Hi Hard tool steel. Int J Mater Form 2, 145165 (2009). https://doi.org/10.1007/s12289-009-0400-5
Optimized roughing, also called high-efficiency milling, is an effective way to improve material removal rates and tool life for titanium and hard-to-machine alloys, but knowing when to use it is as important as knowing how. #highspeedmachining
As new machining techniques become more widespread, it can be tempting for shops to try them out and put their promises to the test. It is important to not only learn how to use a new machining strategy, but how to make it the most effective for your application needs. Before testing out these new strategies, it is vital to learn when to use it and how to achieve the best results.
Optimized roughing has achieved widespread use in much of the metalworking world thanks to its ability to improve tool life and feed rate. Known by many different names, including high-efficiency milling (HEM), it is an aggressive machining strategy that makes use of the full flute length of an end mill and light radial step-overto dramatically improve tool life and boost material-removal rates as high as 60%.
Typically, the depth of cut in an optimized roughing toolpath will be at least twice the tool diameter, making use of smaller radial stepovers to avoid generating too much heat with that depth. Despite the smaller radial stepover, the large depth of cut enables to cutter to remove material at a much faster rate with excellent chip control. The chip control is vital, as a constant chip load must always be maintained.
High-performance optimized roughing has also been seeing more use lately. Similar to HEM strategies, high-performance optimized roughing uses a larger radial stepover to increase the material-removal rate even further. Both approaches require robust tooling and advanced CNC controls capable of handling the intense forces and complex movements that are involved.
The core benefit of HEM is simple: remove more material faster. However, the precise chip control of tools designed for it enables the user to control heat generation, making it ideal for roughing titanium and high-temperature nickel alloys. As an added benefit, this machining strategy provides more reliable tool life, thanks to both heat control and consistent engagement with the material.
The first and arguably most important consideration is the part itself. Optimized roughing is extremely effective at achieving high feed rates, but only on prismatic parts with large, simple edges. The process is designed for making large, aggressive cuts quickly and efficiently, but a part with multiple complex features may not be the best choice, according to SecoTools Product Manager Jay Ball. When I look at a part, I look for straight walls and cavities that allow the use of the full flute length of the end mill, Ball says. This is a cutting strategy that is focused on high metal-removal rates, and parts that force you to reduce the depth of cut will necessarily force you to lower your metal-removal rates.
Control heat generation in your optimized roughing operations with proper chip evacuation. This is especially important to do when working with difficult materials such as titanium and high-temperature nickel alloysto preventhardening that can eat up your cutting tool.
Optimized roughing is an aggressive machining strategy that involves a high feed rate with precise chip control. This means a shopneeds to consider whether its machine tool can handle the intense forces and processing power necessary for optimized roughing. A spindle speed of at least 10,000 rpm is necessary, and more is recommended. Additionally, the machine must be exceptionally rigid, with a heavy-duty spindle. According to Ball, For optimized roughing, the machine tool has to maintain rigidity in all aspects from the spindle to the ball screws. This is a demanding process, and not just any machine can handle it.
In addition to a machine tool with high rigidity and a fast spindle, the CNC controllermust have sufficient look-ahead capabilities to accurately maintain high feed rates.Most newer machine tools have these capabilities, which evaluate thousands of lines of code ahead of the spindles current position. This enables the machine to make complex adjustments in tight corners to maintain a consistent angle of engagement. If the tool does not keep the angle of engagement consistent, you run the risk of scorching high-temperature alloys, Ball says. This can cause small areas of workpiece hardening, which is detrimental to both the part and to your tools. Programming these kinds of tool paths by hand is nearly impossible, which makes the advanced CAM software absolutely necessary.
If you have the necessary machinery and processing power in your CNC, and your parts fit the bill, then it is time to think about how to set up a machine for optimized roughing. Once you confirm that your machine can handle the process, the next step is ensuring that your tooling and fixturing are also up to the task, Ball says. At Seco, we recommend that the toolholder and workholding fixture are both extremely rigid, and we advise using end mills specifically designed to handle this process.
Starting with the cutting tool, Ball recommends six- to nine-flute carbide end mills such as Secos Niagara CutterMulti Flute family of products.The higher flute counts enable machines to achieve much higher feed rates when using the advanced toolpaths associated with optimized roughing, and the flutes themselves are designed to provide consistent chip evacuation, even when dry-machining difficult materials. According to Ball, The most unique feature of the Niagara Multi Flute family of products is the advanced helical chip splitter design. This innovative engineering enables unsurpassed chip control in applications requiring depths of cut up to three times the diameter of the tool. Our MultiFlute mills are designed with HEMin mind, he says. From coating to flute, from geometry to edge prep, these are designed to make optimized roughing as efficient as possible.
Of course, yourtoolholder has to be as reliable as the cutting tool and machine. Runout should be less than 0.0004" for this process, according to Ball. The thing to remember about Multi Flute cutters is that there are a lot of points of contact between the tool and the material, he says. That means there are a lot of points where the material pulls down on the cutter, so toolholders have to have anti-pullout capabilities. The slightest offset can create nasty runout, so maintaining a firm grip on the tool is paramount. Ball recommends shrink-fit holders, high-performance collet chucks or milling chucks with anti-pullout technology.
Finally, the workholding has to be robust and rigid. Every machinist knows that you have to keep your part still, says Ball, but this is an aggressive process that involves some very high forces, so it bears emphasizing. Heavy-duty workholding is needed, as any shifting in the part could be disastrous.
This study is focused on developing and fabricating ball micro end mills with spiral blades by using low speed wire electrical discharge machining (LS-WEDM). Firstly, the new type ball micro end mill is designed and the mathematical model of wire movement trajectory is established for optimizing machining parameters and realizing visual prediction. Then, the diameter, spiral width, and the length consistent accuracy of ball micro end mills fabricated by LS-WEDM is investigated in detail, and more importantly, the three-spiral blades ball micro end mill with the diameter of about 278 m and the length of 1860 m is firstly and efficiently manufactured by LS-WEDM. Besides, the micro machining performance and tool wear of LS-WEDM fabricated ball micro end mill are evaluated by conducting micro milling experiments on nickel-based single-crystal superalloy DD5 material, and experimental results revealed that the main tool wear of the new type ball micro end mill is friction wear and adhesion wear rather than diffusion wear. Furthermore, the surface roughness of machined surface can be reduced to 183 nm, which indicates that the ball micro end mills with spiral blades fabricated by LS-WEDM are significantly potential to achieve high-quality machining for complex micro structures on hard material.
Thepsonthi T, Turul (2013) Experimental and finite element simulation based investigations on micro-milling Ti-6Al-4V titanium alloy effects of CBN coating on tool wear. J Mater Process Technol 213(4):532542
Hanif M, Ahmad W, Hussain S, Jahanzaib M, Shah AH (2019) Investigating the effects of electric discharge machining parameters on material removal rate and surface roughness on AISI D2 steel using RSM-GRA integrated approach. Int J Adv Manuf Technol 101(5-8):12551265
Oliaei SNB, Karpat Y (2016) Investigating the influence of built-up edge on forces and surface roughness in micro scale orthogonal machining of titanium alloy Ti6Al4V. J Mater Process Technol 235:2840
Nakamoto K, Katahira K, Ohmori H, Yamazaki K, Aoyama T (2012) A study on the quality of micro-machined surfaces on tungsten carbide generated by PCD micro end-milling. CIRP Ann Manuf Technol 61(1):567570
Cheng X, Wang ZG, Nakamoto K, Yamazaki K (2009) Design and development of a micro polycrystalline diamond ball end mill for micro/nano freeform machining of hard and brittle materials. J Micromech Microeng 19(11):115022
Katahira K, Nakamoto K, Fonda P, Ohmori H, Yamazaki K (2011) A novel technique for reconditioning polycrystalline diamond tool surfaces applied for silicon micromachining. CIRP Ann Manuf Technol 60(1):591594
Gong, S., Meng, F., Sun, Y. et al. Experimental study on fabricating ball micro end mill with spiral blades by low speed wire electrical discharge machining. Int J Adv Manuf Technol 108, 25412558 (2020). https://doi.org/10.1007/s00170-020-05446-z
Many solid carbide ballnose end mills feature coatings such as aluminum titanium nitride (AlTiN) that combat the high temperatures associated with hard-milling applications. Images courtesy of Seco Tools.
Successful hard-milling operations require the perfect balance of all the factors present in a system, including cutting tools, CAM software and machine tools.
Solid carbide end mills can help bring high productivity and reliability to mold shops that machine a large portion of their products from hardened steels.
To maximize productivity and reliability, mold shops often machine a large portion of their products from hardened steels. Historically, hardened steels have been rough milled at low feeds and speeds, with large depths of cut and stepovers. The process is agonizingly slow and can produce deep stair-steps on the part, which necessitate multiple semi-finishing and finishing operations. Alternatively, shops will rough mill a soft block, have it heat-treated, and then bring it back to the milling machines for a number of setups for semi-finishing and finishing. Another approach to hardened steel machining has been EDM, a process that is also very time-consuming.
These lengthy processes are increasingly being replaced by high-speed hard milling, which involves taking light depths of cut and using high feed rates. This process enables shops to drill holes and water lines in a block, perform heat treatment, and then apply the high-speed strategies to rough and finish in one setup. Metal removal rates are high, and semi-finish and finish operations are minimized, because the hard-milling process results in near-net-shape parts. Surface finishes in the range of 10-12 rms are possible. The result is increased productivity, and decreases in the costs of setup and repetitive part handling.
The measured hardness of typical hardened steels is in the range of 48-65 HRC. However, when it comes to real-world machinability, the Rockwell number does not represent the whole story. For example, D2 tool steel hardens to about 60-62 HRC, but it machines more like 62-65 HRC due to a chromium content of 11-13 percent that increases toughness. For D2 and similar multi-constituent alloys, it is necessary to apply machining parameters from the tooling supplier that are intended for harder materials.
A key to tool life and part quality in milling, and especially in high-speed milling of hardened steels, is maintaining constant chip load on the milling tools cutting edges. Chip load equals the feed rate divided by the spindle speed multiplied by the number of cutting flutes, and chip load that varies widely or is too low or too high will cause tools to wear out too fast, chip or break.
Maintaining constant chip load is a particular problem when machining the 3D contours that are characteristic of moldmaking. Programming a straight high-speed, high-feed tool path generally is routine, but in milling complex shapes, the load on the tool changes and the machine may not be able to maintain the desired chip load. For example, when a cutter arrives at a 90-degree corner, the tools engagement angle doubles and cutting forces increase. If the feed rate is not reduced, the tool will wear rapidly or break. Machinists can manually reduce feed via feed-override controls, or the CAM program and machine tool control can combine to back the feed rate down in order to machine varying mold contours.
A machinist can confirm if the specified feed rate is being achieved by loading the CAM program and tools into the machine and setting the tools Z height about 1 inch above the part. A dry run will reveal the actual feed rates. Basic physics dictates that maintaining the desired feed rate and chip load 100 percent of the time is not possible. A good rule of thumb is: If the programmed feed rate is not maintained for 80 percent of the cycle time, then spindle speed must be reduced accordingly to maintain consistent chip load.
For example, in an application where a feed rate of 100 inches per minute at 30,000 rpm will produce the desired chip load, the correct feed may be achieved for some portion of the operation but may fall to 40 inches per minute for other parts of the process, producing an average feed of 50 inches per minute. In this case, cutting the speed in half will most likely produce the desired chip load. Lowering the spindle speed will nominally increase cycle time, but tool life will increase in turn. Milling calculators are available to provide the parameters needed to achieve constant chip load.
Another critical but often overlooked factor in milling operations is tool runout. In general, runout greater than about 0.0004 inch (one-seventh the diameter of a human hair) can cut tool life in half. Minimizing runout grows in importance when employing very small tools. For some small tools, 0.0004-inch runout will double the chip load on a single tooth, causing accelerated wear on the tools cutting edge. Using expensive machine tools and expensive cutting tools but employing bargain toolholders is a recipe for problems. High-precision holders, including shrink-fit and hydraulic, among others, will essentially eliminate runout as a negative factor.
Many shops make the mistake of leaving excess part stock for finish milling. For cutters of about 1/8-inch diameter and larger, leaving about 1 percent of the cutter diameter for finishing is recommended. For example, when applying a -inch-diameter tool, stock for finishing should be about 0.005-inch, or for a 1/8-inch-diameter cutter, finishing stock should be 0.002-0.003 inch.
For smaller tools, determining a sufficient amount of stock for finishing may be a case of feel, or trial and error. One percent of a 0.020-inch-diameter ballnose cutters diameter is 0.0002 inch, but the amount of stock may be insufficient and the tool may rub the workpiece material instead of shearing it, thus hastening tool failure. For the 0.020-inch-diameter cutter, finishing stock of 0.001 inch or 0.0008 inch would probably be more appropriate.
With small tools in particular, excessively large steps between finishing tools will cause problems. Starting with a 2-mm tool (1-mm radius) to create a 0.2-mm radius in a corner, some machinists will next apply a tool with a 0.4-mm radius. Under those circumstances, the chance that the tool will break is high. A better progression might be to next use a 0.8-mm-radius tool, then a 0.6-mm, and finally tools with 0.4-mm and 0.2-mm radii. This conservative method will consume a few more tools, but tool life will be greater for the tools that are applied, and the risk of breakage is small.
Programming software is critical in maintaining chip load. Top-of-the-line CAM systems employ a larger number of individual points to define a tool path than less-capable programming systems. The CAM program also manages tool entry and exit to moderate forces on the cutting edge. Although more capable CAM software is usually also more expensive, the benefits generally outweigh the initial higher cost.
Machine controller capabilities play a role in efficient milling as well. To efficiently carry out high-speed milling strategies, a machine must have the computing power to look ahead and smoothly handle the rapid changes in machining parameters dictated by the CAM program. Older controllers and servos cant necessarily process as many blocks per second as are needed to follow the complex machine movement commands of high-speed milling.
Close consideration of chip load, runout and other issues such as machine rigidity can produce surprising results regarding tool life in high-speed milling. Properly applied tools can last hours when milling hardened steels. Of course, the definition of tool life is a factor as well; the demands of the customer receiving the mold regarding surface finish may limit the amount of time that a tool can be run before being changed.
Extreme heat negatively impacts tool life, so the light depths of cut used in high-speed milling can boost tool life by maximizing the amount of time the cutting edges have to cool while outside the cut. To avoid thermal shock, air blast or oil/air mist generally replaces coolant when materials harder than 48 HRC are being milled. Although in some cases liquid coolant flow can clear chips and prevent recutting, an air blast is a better choice because it does not subject the tool to rapid, large changes in temperature.
The industry-wide trend toward tighter tolerances includes moldmaking products, and those demands are reflected in the tools used to machine the molds and their components. A few years ago, a typicalradial tolerance for a ball mill was 10 microns; now it is closer to 5 microns. Aballnose end mill that is not true to formwill produce parts that do not match. Avoiding that kind of error is critical in moldmaking where, for example, liquid silicone rubber can form flash in mold mismatch gaps as small as 2 microns.
Because milling hard materials generates a significant amount of heat, many of the carbide end mills used in hard milling feature thermal-barrier coatings such as aluminum titanium nitride (AlTiN). These tools generally have hard micrograin carbide substrates (8 percent cobalt content) for heat resistance and strength, and negative cutting edge rake geometries to resist chipping. Cubic boron nitride (CBN) tools can be used in finishing operations, and inserted end mills are effective in roughing.
Very small milling tools can create features that formerly were only achievable with EDM. Tools as small as 0.1 mm (0.0039 inch) in diameter are available, and even such tiny tools can be effectively applied at high speeds with short flute lengths.
Precision tools, sophisticated CAM software, high-capability machine tools, premium toolholders and details such as coolant alternatives should be applied together to maximize the productivity and quality of hardened steel milling. Tooling, machine tool and workpiece material suppliers typically are more than willing to provide their expertise to help shops achieve a true process balance and meet their productivity goals.
The use of high speed milling (HSM) for the production of moulds and dies is becoming more widespread. Critical aspects of the technology include cutting tools, machinability data, cutter path generation and technology. Much published information exists on cutting tools and related data (cutting speeds, feed rates, depths of cut, etc.). However, relatively little information has been published on the optimisation of cutter paths for this application. Most of the research work is mainly focused on cutter path generation with the main aim on reducing production time. Work with regards to cutter path evaluation and optimisation on tool wear, tool life, surface integrity and relevant workpiece machinability characteristics are scant. Therefore, a detailed knowledge on the evaluation of cutter path when high speed rough and finish milling is essential in order to improve productivity and surface quality. The paper details techniques used to reduce machining times and improve workpiece surface roughness/accuracy when HSM hardened mould and die materials. Optimisation routines are considered for the roughing and finishing of cavities. The effects of machining parameters notably feed rate adaptation techniques and cutting tools are presented.