Regarding the improvement of workshop production efficiency, it actually consists of two parts:
production preparation
Production time
Production preparation accounts for the most production time, especially the processing and production of small batches and multiple varieties (such as the preparation and turnover of materials, tools, fixtures, etc.). This is mainly a matter of management level, and it tests the workshop management ability!
Production time is divided into two situations:
Downtime waiting time
cutting time
The downtime waiting time, such as loading and unloading workpieces, changing clamping tools, etc., is also time-consuming. The cutting time, that is, the program running time, only takes up a small part of the production time, as shown in the figure below:
Double-click the image to enlarge it
Production management is the core of efficiency improvement. This is a matter at the management level. As ordinary employees, how to use the cutting tools well and how to set the cutting parameters reasonably is what we care about!
In today's article, I will introduce you to several important processing parameters in milling from the perspective of cutting parameters:
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The first formula is: metal removal rate formula (Q = F x ap x ae)
The metal removal rate is proportional to F, ap, and ae. That is, increasing one of these three parameters can increase the metal removal rate.
This is why increasing the speed in the program does not directly improve the processing efficiency.
(This refers to the fact that processing efficiency cannot be directly improved)
Improve processing efficiency by increasing cutting parameters. As mentioned before, cutting time only takes up a small part of the entire production efficiency. Therefore, I will focus on this. Simply and crudely increasing cutting parameters may increase the cost of tooling in the workshop and affect the quality of parts. wait.
For example, the feed F in the program is very easy to adjust. If you increase the feed F, the metal removal rate will increase. What impact will such a small change have on the tool and parts?
Specifically, look at the second formula: feed formula (F= n xZn x fz)
Assuming that the other two parameters remain unchanged:
1. As n becomes larger, that is to say, you increase the speed S in the program. This effect is obvious. If n becomes larger, the linear speed Vc needs to become larger (see the third formula for the relationship between Vc and n: n=Vc/3.14*Dc).
Line speed will increase, and line speed has the most direct relationship with tool life.
Tool community: A lot of work has been done on the effects of depth of cut ap, feed F and linear speed Vc on tool life.
As shown in the figure above: the horizontal axis represents the wear amount of the tool, and the vertical axis T represents the tool life.
in:
1. The depth of cut Ap increases by 50% and the blade wear increases by 20%;
2. The tool feed F increases by 20% and the blade wear increases by 20%;
3. When the cutting speed increases by 20%, the blade wear increases by 50%;
That is to say, as the cutting speed increases, the tool life will be shortened sharply. Therefore, when the tool life is too short or the tool wears very fast during the cutting process, the cutting speed can be reduced. This will be reflected in the program and the rotation speed S in the program can be reduced;
2. As z becomes larger, that is, the number of teeth increases. In this way, milling parts with narrow spaces may cause chip removal problems. At the same time, as many blades engage with the workpiece at the same time, the cutting force will become larger, which means that during the cutting process The tendency of vibration will increase.
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If there is vibration during processing, it can be solved by reducing the number of tool teeth. Of course, vibration is related to many factors, such as: the number of teeth of the tool, the leading angle of the tool, the overhang depth of the tool, part clamping, programming, machine tools, etc. Due to space reasons, I will use a cycle diagram to explain the cause-and-effect relationships and corresponding solutions later.
3. As fz becomes larger, that is to say, the feed amount per tooth is larger. When the feed amount per tooth is larger, the most direct impact is that the cutting force becomes larger.
As the cutting force increases, the strength requirements for the cutting edge of the tool also become higher. For example, the cutting edge is shown in the figure below:
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Then, during the cutting process, if the blade is prone to jumping
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There are many forms of blade wear, and the jumping blade is just one of them. (8 common forms of wear, the principles are analyzed and the corresponding solutions are given, which will be shared later)
If the blade is prone to jumping, choose a softer blade (one with a higher grade, see my previous article on tool material classification for details). A soft blade will be impact-resistant and naturally less likely to break.
I have been sharing programming tips, and here I will give you a solution from a programming perspective.
Emphasis:
Milling is a cyclic process in which the cutting edge of the tool enters the workpiece - cuts - exits the workpiece (except for axial feed, such as drilling and plunge milling).
This cycle process tool path often has two forms:
Down milling
Up milling
Many masters who have been in contact with machining centers may know: Climb milling, up milling;
But what is the relationship between these two tool paths and the cutting edge of the tool?
In fact, down and up milling is only a superficial phenomenon. Behind this is the amount of compressive stress and tensile stress that the tool can withstand.
Come on, look at the following two pictures to explain the force principle of the cutting edge of the tool:
This picture is of down milling: when the tool cuts into the workpiece, the cutting thickness is the largest, and when it exits the workpiece, the cutting thickness is the smallest.
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Then, using climb milling, the moment the tool cuts into the workpiece, the thickness of the iron chips is the largest, and the impact force on the cutting edge of the tool is large (that is, a large pressure is given to the cutting edge); when the tool exits the workpiece, the chip thickness is the smallest. According to the force The action force and reaction force of the tool cutting edge are smaller.
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The picture below shows reverse milling: when the tool cuts into the workpiece, the cutting thickness is the smallest, and when it exits the workpiece, the cutting thickness is the largest.
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Then, using up milling, the moment the tool cuts into the workpiece, the cutting thickness is the smallest, and the impact on the tool is small; (that is, a small pressure is given to the cutting edge of the tool); the moment it exits the workpiece, the thickness of the iron chips is the largest, then The maximum pressure endured by the tool is suddenly released. According to the action and reaction force of the force, the cutting edge of the tool is subject to the greatest tensile stress.
As shown below:
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Okay, I understand the force principle of the cutting edge of the tool during the milling process. Please provide additional explanation. How to judge the down milling and the up milling during programming?
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I once said that everything is divided into two states, such as up and down, left and right, east and west, male and female... These two states have given rise to a rich and colorful world. No matter how complex the parts are, they have two forms according to the characteristics of the workpiece, either outer (shape) or inner (shape), thus forming parts of various shapes.
So for milling "shape"
Clockwise cutting is called down milling, and counterclockwise cutting is called reverse milling. (As shown below:)
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So for milling "inner shape"
Clockwise tool movement is reverse milling, and counterclockwise tool movement is down milling.
As shown below:
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Okay, look at the picture above carefully, it is very useful. Remember, you will make a judgment.
Okay, let's first analyze the theories involved in down milling and up-cut milling. What use do these theories have in our actual programming?
For example: (as shown below), it is required to mill the plane
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Before writing this program, we first select the tool. There are usually two options:
1. The tool diameter is smaller than the plane size of the part
2. The tool diameter is larger than the plane size of the part
In the above two cases, I believe everyone will choose a tool diameter slightly larger than the plane size of the part, so that the processing efficiency is high.
Then, the diameter of the tool is larger than the plane size of the part, and there are three ways to move the tool. Zou Jun, I will draw three tool path diagrams for you.
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1. (As shown on the left) When the tool center and the part center coincide, the cutting thickness is always the same when cutting into the workpiece and exiting the workpiece.
2. (As shown in the middle picture) The center of the tool is to the left of the center of the part. The cutting thickness is the thickest when cutting into the workpiece, and the cutting thickness is the thinnest when cutting out the workpiece.
3. (As shown in the middle picture) The center of the tool is to the right of the center of the part. The cutting thickness is the thinnest when cutting into the workpiece, and the cutting thickness is the thickest when cutting out the workpiece.
Okay, let's repeat the important things again (you'd better read it three times at the same time), through the above three knife paths:
The first situation: the tool center and the part center coincide, or it can be understood that when milling the workpiece, full cutting is used, and the cutting thickness of the tool when cutting into and exiting the workpiece is the same.
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The second situation: the center of the tool is to the left of the center of the part, or it can be understood as milling the outer contour of the workpiece (clockwise movement), as shown in the figure, that is, using climb milling, the cutting thickness is thickest when the tool cuts into the workpiece, and the cutting thickness is the thickest. The cutting thickness of the workpiece is the thinnest.
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The third situation: the center of the tool is to the right of the center of the part, or it can be understood as milling the outer contour of the workpiece (counterclockwise tool movement), as shown in the figure below, that is, reverse milling is used. When cutting into the workpiece, the cutting thickness is the thinnest, and the cutting thickness is the thinnest. The cutting thickness of the workpiece is the thickest.
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After analyzing the example, (except for axial feed and plunge milling), whether it is plane machining, contour, or cavity machining, the tool position relative to the part during programming is nothing more than the above three. (Again, although plane milling is used as an example, you can also think of milling contours, pockets, etc.)
So, the first situation is equivalent to full-cut cutting. For example, a groove is milled in the middle of a plate. For example, if a solid workpiece is milled into a cavity, the first cut is full-cut cutting. This situation does not distinguish between down milling and milling. . (Of course, except for some programming strategies for high-speed milling, I will talk about the programming strategies for high-speed milling later).
In the other two cases, the tool position and feed direction determine the down and up milling.
So based on the above explanation, how to apply clockwise and reverse milling during programming? I will focus on giving you a brief analysis from the perspective of tools.
There are many types of cutting tools, and they are also made of different materials, such as high-speed steel, cemented carbide, ceramics, CBN, diamond, etc. Generally speaking, from the perspective of cutting tool materials, there are at least two important indicators: hardness and toughness.
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The horizontal axis represents toughness, (as shown in the figure above). The tool material corresponding to the right side has better toughness, that is, tools made of high-speed steel have good toughness, and tools made of diamond have poor toughness.
The vertical axis represents hardness (as shown in the figure above). The higher the tool material goes up, the higher the hardness. That is, the tool material made of diamond has high hardness, and the tool material made of high-speed steel has low hardness.
Tools with good toughness are resistant to impact, but not wear-resistant; tools with high hardness are wear-resistant, but not resistant to impact.
Combining the programming strategy of down and up milling with the two characteristics of tool toughness and hardness, it is divided into four types:
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1. Tools with high hardness are programmed using climb milling.
2. Tools with high hardness are programmed using reverse milling.
3. Tools with good toughness are programmed using climb milling.
4. Tools with good toughness are programmed using reverse milling.
Which one do you choose when programming?
For example, you are currently using a tool with relatively high hardness (such as cubic boron nitride CBN tool)
The recommended method is to use the first method: use tools with high hardness to program and use climb milling.
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Climb milling, cutting into the workpiece, although the chips cut are the thickest and the tool bears the greatest compressive stress, due to the support of the cutter body (positioning surface), the chips are the thinnest when cutting out the workpiece, and the tool bears the least tensile stress, so it is not easy to jump edge, tool life will be significantly improved.
On the contrary, if a tool with high hardness is programmed using up milling, the chips will be thickest when cutting out the workpiece, and the maximum compressive stress experienced by the tool will be suddenly released (according to the action and reaction force of the force), and the cutting edge of the tool will be subject to the largest tensile stress. The cutting edge is easily carried away by iron chips, causing large pieces of the cutting edge of the tool to fall off.
Okay, let me briefly analyze it from the perspective of tool material. Of course, the clockwise and reverse milling strategy can also be considered from other perspectives during programming, such as processing conditions, roughing and finishing, etc.
For example, taking roughing and finishing machining as an example, let me briefly analyze Zou Jun:
Back to the beginning of the article, the first formula mentioned: metal removal rate (Q = F x ap x ae)
Yes, rough machining is to increase the metal removal rate, so try to have as large a depth and width of cut as possible.
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The large depth of cut and width of cut during the milling process mean that the cutting edge of the tool has more contact with the workpiece. If down milling is used, the tool will cut into the workpiece and cut thickly, which will cause a greater impact (on the power of the machine tool, the parts There are also requirements for clamping rigidity, etc.) It is easy to cause vibration during the cutting process and even the jumping edge of the tool. On the contrary, up milling is cutting thin in and thick out, which can effectively solve the problem of large depth of cut in rough machining, which easily causes vibration.
Okay, the down and up milling strategy in CNC programming can also be analyzed from multiple dimensions such as machine tools, fixtures, workpiece materials, etc., which will be explained later.
In short, [CNC programming] From the analysis of drawings → determination of process route → product clamping → tool selection → programming → CNC processing, the final link must be reflected in the CNC program! service.
