Let me state the conclusion at the beginning: CNC CAM belongs to the soul of CNC machining, but 90% of the scrap products are due to the "taken for granted" situation of the tool path.
At eleven o'clock in the night, the fluorescent lights in the workshop were still on, and the piece of aviation aluminum was rotating happily under the spindle. Suddenly there was a muffled sound and the knife broke. The operator Lao Zhang stared at the screen, which was full of red alarms. This is not a problem with the equipment itself, but a chamfering parameter in the CAM program that is not locked.
Try to speculate that for the same unprocessed material and the same machine used for processing, if another person is replaced to modify the contents of two lines of code, the proportion of qualified products in the total products will rise sharply from 60% to 95%. What exactly is the problem? It's not that the software is defective, it's that you haven't understood the hidden rules of computer-aided manufacturing.
01From drawings to iron filings, what exactly is CAM doing?
CAD can depict the scene of fairies scattering flowers, and CAM can cut out real objects. To put it simply: CAM "translates" the design drawings into G code that the machine tool can execute. However, if the translation is wrong, it will either cause overcutting at the least, or lead to a plane crash at the worst.
For example, there is a factory, let's call it YPMFG, which received an order for a medical part. The surface complexity of that part is as complex as a walnut shell. Then, the programmer Xiao Zhao used the default parameters to generate the tool path, and then performed post-processing. After running, it was checked through simulation. It seemed that there was no problem. Then the machine was put into operation. During the first step of the operation, which was halfway through the finishing process, the ball cutter penetrated directly into the side. As a result, the part was overcut by 0.3mm, causing the part to be scrapped.
Why? This is because the tool path optimization was not carried out in detail. In areas where the curved surface has a steep slope, the point where the tool contacts suddenly changes, and the ordinary constant step distance simply cannot withstand such a situation. Then a strategy was changed, which is to set the residual height according to the area, using an equidistant and parallel method for steep surfaces, and using a spiral form for flat surfaces. After one cut, the surface roughness reaches Ra0.8.
The core point is that CAM is not completed by just clicking "Generate". Each material, each tool, and each geometric feature needs to be treated individually.
02Common car rollover scenes, you must have encountered them before
With the help of YPMFG’s five years of providing services to thousands of factories, we have sorted out three main points of high-frequency difficulties:
1. Post-processing mismatch
For the same CAM file, if a machine tool with a different type of operating system is replaced, the tool path mirror image will show the opposite phenomenon. Please don't ask why. If you ask, the answer is that the post-processing was not filtered correctly.
2. Hard landing with advance and retreat knife
Is it a spiral knife? Or diagonally? Or should it be done vertically? Eight out of ten people will choose the default, but the result is that the tool smashes into the workpiece with a "bang", and the cutting edge cracks as if it has been gnawed by a dog.
3. Giving away the remaining blanks
After roughing is completed, should the second rougher directly load the STL model? There are situations where someone gives a square to save trouble. During the second rough cutting, the knife was left empty for 30 minutes, and the remaining boss was left uncut.
Didn’t the software alert the police for these problems? Report it, but most people will click "ignore".
03How to break it? Quickly clear mines with Q/A
Q1: The tool path simulation is fine, but the cut is over-cut on the machine?
First, check the kinematic model of the machine tool. Second, check the post-processing. Third, the simulation uses an ideal machine tool. Fourth, the backlash actually exists. Fifth, the tool swing length error actually exists.
Q2: Rough cutting efficiency is low. Are you afraid of overcutting when changing to a larger knife?
A: Carry out rough machining according to different areas. Use a Φ20 flying cutter in the open area, use a Φ10 milling cutter for narrow slots, and use residual material recognition to realize tool connection in the middle part.
Q3: Thin-walled parts always vibrate, how to program the tool path?
A: Using the method of down milling and adding the cycloidal angle, the cutting depth of each layer should be less than or equal to 0.5 mm, and the direction of the tool feed is to support from the outside to the inside.
Q4: Does the five-axis linkage tool path overshoot as soon as it runs?
A: The stroke range of the rotary axis is limited, and the 3+2 positioning processing method is given priority. If it is not necessary, linkage will not be performed.
Q5: Is the same program good today or useless tomorrow?
A: Check the rigidity of the workpiece clamping. CAM is only responsible for the tool path. If the blank is overhanging, even if the gods come, there will be no way to save it.
04A practical idea to double the yield rate
YPMFG received a batch of stainless steel valve bodies, and the customer requested that the inner cavity roughness be Ra1.6 and the tolerance be ±0.01mm. The first two suppliers encountered obstacles in finishing vibration marks.
How do they do it? Think of CAM as a "processing preview" rather than a programming tool.
The first layer: For machining simulation, it is not just about observing the cutting situation, but also analyzing the tool load curve. Upon noticing that the load on a certain section of the tool path suddenly increased by 30%, he immediately split it into two tools.
Second level: Relying on the cutting database to reversely calibrate the feed speed. The hardness of the material is HRC35, and the recommended vc is 80m/min. However, in practice, the spindle power fluctuates, and the vibration marks disappear after being reduced by 15%.
Located on the third layer, interference checks must be performed before post-processing output. The gap between the tool holder and the workpiece is only 0.2mm. In this case, it is necessary to replace the extension rod or change the inclination angle of the tool axis.
The results showed that the first piece was judged to be qualified. During the continuous processing, a total of 500 pieces were processed, of which only 2 pieces were scrapped. The scrapping of these 2 pieces was due to the operator's error in tool setting. This operation reduced the cost by 40% and the delivery date was advanced by one week.
Did you see that? CAM masters are not just able to click buttons, they are able to incorporate the characteristics of machine tools, cutting tools, and materials into lines of code.
05From now on, follow these four steps
Don't think about reaching the sky in one step. According to this rhythm:
① Forced simulation verification
Each version of the tool path must be fully run in the "solid cutting" mode. You can't just check the overcut situation represented by the red line, but also pay attention to the residual situation represented by the blue line.
②Create a personal checklist
Examples: post-processing version, the amount the tool extends, the starting point of the coordinate system used by the workpiece, the definition of the unprocessed material, the height of the tool when entering and exiting the raw material.
③Reverse learning using cases
I found three representative waste products, namely overcutting, collision, and knife vibration. I traced the source of CAM and asked myself, "Where are the parameters given less?"
④ Actively ask “What would happen if…”
Change stride length? Change the spiral? Delete an air cut? Run a comparison simulation for every change.
At the end, it is sublimated: CNC CAM is not about drawing lines in the computer, it is about making decisions for the machine tool. Every time the tool cuts in, there is resistance from the material behind it, there is the gasp from the tool, and there is the groan from the workpiece. And there is a tendency that the more you know about them, the docile they become.
Repeat the truth that is easily overlooked: if the program has errors, it can be corrected, but if the workpiece is scrapped, it can only be started again. Before you click "Generate" next time, spend three minutes thinking about it – do you really dare to let the machine tool run this tool path?
Suggestions for action: Before leaving get off work today, pick out a program you have written recently, open it into the simulation state, and only look at one parameter, which is the density of the tool path at the minimum radius of curvature. Is this enough? Do I need to add a root clearing operation? Save after modifications are completed.
You will notice that the scrap rate decreases from the moment you start to doubt the "default settings."
