The machining accuracy of high-precision hole parts is usually very high, and the hole diameter, depth, shape, location, and other parameters need to be strictly controlled.
The machining accuracy level directly affects the parts’ performance and life.
CNC milling machines are important equipment in modern manufacturing industries. Their processing accuracy and efficiency directly affect the quality of products and market competitiveness.
Therefore, the application range of CNC machine tools is expanding constantly. The CNC milling processing of high-precision hole parts has important practical significance.
High-precision Hole Parts Processing Methods
High-precision hole parts generally use reaming and boring processing methods, which differ in processing range, accuracy, economy and applicable scenes.
Reaming is more suitable for processing smaller holes and meeting high-pressure applications’ precision and surface quality requirements, and boring is more suitable for these purposes.
Boring is more suitable for processing larger holes or expanding the hole diameter to improve accuracy and reduce surface roughness.
In practice, choose the appropriate processing method according to the specific processing needs and workpiece characteristics.
Tool selection
In the CNC milling processing of high-precision hole parts, the right tool should be chosen based on the material’s nature, hole diameter and depth, and other factors.
Center drills, twist drills, reamers, and boring tools are commonly used for hole-making. These tools are generally clamped using a self-tightening drill chuck, as shown in Figure 1.
When clamping, avoid using wrenches; hand-tightening is sufficient. During the drilling process, the tool’s cutting force acts in the opposite direction of the spindle’s rotation.
This opposite force causes the chuck to tighten further, ensuring the tool does not loosen or fall out.
Using a wrench to tighten the self-tightening drill chuck can damage its accuracy.
Boring tools have special shanks for clamping. Generally, boring tools come in sets, and depending on the hole size, different boring shanks and blades are selected.
For holes requiring higher accuracy, the process generally begins with a center drill to establish the hole’s center.
Next, an appropriate drill bit is used to drill the hole to the desired size.
Finally, depending on the hole’s requirements, a reamer is used for reaming, or a boring tool is used for boring.
Reaming Process Control
Reaming involves removing a trace of metal layer from the workpiece’s hole wall to improve its dimensional accuracy and hole surface quality. It is one of the hole finishing methods widely used in production.
Reaming is a more economical and practical machining method for smaller holes than internal grinding and fine boring. The size of the forming reamer limits the scope of reaming.
•Reaming process
> Reamer preparation
A suitable reamer is selected according to the machining requirements. Commonly used reamers include the straight shank reamer shown in Fig. 2 and the tapered shank reamer shown in Fig. 3.
> Accuracy classes
Reamers are available in three accuracy classes, H7, H8, and H9, so the appropriate reamer can be selected according to the process’s accuracy requirement.
> Preparation before reaming
(1) Before reaming, it is usually necessary to center the hole with a center drill and then drill it with a twist drill to ensure that the hole’s initial size and shape meet the reaming requirements.
(2) Install the selected drill and reamer on the spindle of the CNC milling machine, rotate it, and carry out the centering operation to prevent the drill and reamer from swinging.
> Reaming operation process
(1) When drilling holes, The drill bit should be 0.3mm smaller than the reamer diameter.
As shown in Figure 4, the drill bit must be sharpened and qualified before use. The cross-cutting edge should be positioned at the center of the drill bit.
The main cutting edges must be equal in length on both sides. The turning points of both outer edges should be at the same height, and the outer edge turning point should be higher than the tail rotating point.
If the drill bit is not sharpened correctly, the holes may become too large, affecting the reaming process.
This is especially important for new drills, which should be carefully inspected by rotating them left and right. Rotate the drill from side to side and observe.
After the drill has been qualified, you can first drill to a depth of about 4 mm and then use a reamer to test the hole size before continuing to drill to the specified depth.
(2) When setting the cutting volume, choose the appropriate volume for processing the material.
For example, if the material is 45# steel and a pin hole of Φ10 mm is to be machined, a drill of Φ9.7 mm should be used to drill the hole.
For example, when machining a Φ10 mm pin hole in 45# steel, a Φ9.7 mm drill bit should be used for drilling.
First, check whether the drill bit is qualified. Sometimes, a Φ9.7 mm drill bit may produce a hole with a diameter larger than 10 mm.
To ensure accuracy, verify the drill bit’s quality. Start by drilling to a depth of about 4 mm. Then, test with a Φ10 mm reamer.
If the reamer does not fit into the hole, continue drilling until the correct depth is reached. This method provides the most stable and reliable results.
After drilling, use a Φ10H7 reamer to ream the holes. When reaming, the rotating speed of the reamer should not be too fast.
The general rotating speed is about 120 r/min, and the feeding speed is 80 mm/min.
For holes requiring high accuracy, it may be necessary to rough ream the hole first and then finish ream it.
In general, the larger the diameter of the tool, the slower the speed, the smaller the diameter of the tool the faster the speed.
(3) Selection of cutting fluid: A suitable cutting fluid must be selected to reduce the cutting temperature and extend the tool life.
For example, when reaming tough materials, emulsion or extreme pressure emulsion can be used; reaming cast iron and other brittle materials, kerosene or kerosene and mineral oil mixture can be used.
•Reaming notes
(1) reamer exit:
reamer exit workpiece can not stop, wait for the reamer to completely exit the workpiece and then stop, to prevent the reamer from reversing caused by the expansion of the hole diameter.
(2) Axis coaxial:
After drilling and expanding, the axis of the reamer should be coaxial with the hole’s axis. To ensure the hole’s accuracy, it is better to drill, expand, and ream continuously.
(3) Preventing knocking:
The cutting edge should be prevented from knocking during use to avoid damaging the accuracy of reamed holes.
•quality inspection
(1) Hole diameter detection:
You can use the standard pin detection by using appropriate measuring tools to check whether the hole diameter is in line with the requirements, such as pin holes.
(2) Surface quality:
Observe the surface quality of the holes to make sure that they are smooth, free of burrs and defects.
(3) Follow-up treatment:
This is determined according to the hole’s needs for follow-up treatment, such as cleaning and rust prevention.
Boring Process Control
Boring is forging, casting, or drilling holes for further processing. It can expand the hole diameter, improve accuracy, reduce surface roughness, and better correct the original hole axis skew.
Boring can be divided into rough boring, semi-fine boring, and fine boring. The dimensional accuracy of a fine boring hole can reach IT8IT7, and the surface roughness Ra value 1.6~0.8μm.
•Boring process
> Workpiece clamping
Fix the workpiece to be processed on the table of the CNC milling machine. Make sure the workpiece is stable and will not move or shake.
> Boring tool selection
Select the appropriate boring tool or other cutting tools according to the workpiece’s material, size, and machining requirements. Other cutting tools include a center drill, drill, milling cutter, etc. The selection of tools directly affects the quality and efficiency of machining.
Boring tools are widely used for hole machining. Various types of boring tools are available, including single-flute, double-flute, and micro-adjustment boring tools. You can choose the appropriate type according to the machining requirements.
Generally, CNC milling machines use fine adjustment boring tools, as shown in Figure 5.
Micro-adjustable boring tools have a fine adjustment device to fine-tune the cutting diameter, which is suitable for machining tasks that require high precision.
> Preparation before boring
When boring, the amount left for the boring tool should not be too large, generally the diameter of the hole can be left about 1mm.
Before boring, the hole should be drilled with a drill and then milled with a milling cutter.
This is to prepare the basic shape of the hole on the workpiece to facilitate the subsequent boring operation. The following are the general steps and points of the boring process.
(1) Determine the processing requirements:
According to the workpiece material and processing requirements, determine the hole size, precision, surface roughness, and other requirements to select the appropriate tool and cutting parameters.
(2) Workpiece preparation:
clamped workpiece for tool setting operation, general programming zero point set in the center of the hole, that is, X, Y-axis tooling to take the center of the circle, so as to facilitate the boring programming.
Check whether the surface of the workpiece is flat to ensure the accuracy of the machining datum, the surface of the workpiece is the zero point of the Z-axis.
(3) Tool installation:
Select the appropriate milling cutter, ensure that the cutter bar extends the length of moderate, leakage should be longer than the depth of the hole, do not leave too long, which affects the rigidity of the tool machining.
> Programming processing
(1) Processing parameter setting:
Prepare processing programs using digital control programming software or manual programming, according to the shape of the workpiece and processing requirements.
If it involves more than one hole or deeper holes, you can use fixed-cycle commands to simplify the programming, such as drilling cycles G81, G73, and boring cycles G85, G76.
After programming, verify and simulate the program to ensure accuracy and feasibility. When programming, set the appropriate machining parameters according to the material of the workpiece and machining requirements.
These parameters include spindle speed, feed rate, depth of cut, and machining radius. Ensure that these parameters are set correctly to ensure machining accuracy and quality.
(2) Monitoring and adjusting:
During the boring process of the CNC milling machine, we should strictly follow the operation rules and safety regulations to ensure the operators’ personal safety and the equipment’s safe operation.
During the machining process, pay close attention to the operation status and machining quality of the CNC milling machine.
If necessary, make appropriate adjustments according to the processing conditions, such as changing the spindle speed, feed rate, etc., to obtain the best processing results.
> Boring operation process
(1) Firstly, rough milling the round hole, remove most of the residue, and form the basic shape of the hole.
Based on rough milling, finish milling is carried out further to improve the accuracy and surface quality of the hole.
For example, to bore a round hole of R30, as shown in Fig. 6, a Φ18 drill is used to drill the hole in the center of the circle, and then a Φ16 alloy milling cutter is used to mill the hole to R29.5.
To summarize, the milling process before boring is a critical preparatory stage, which directly affects the quality and efficiency of the subsequent boring process.
Therefore, in the actual operation, we should strictly follow the technical requirements to ensure that each step of the process achieves the predetermined quality objectives.
(2) Boring tool boring is divided into rough boring and fine boring.
First, touch the tip of the boring tool to the inner wall of the circle. Then, lift the boring tool.
Next, fine-tune the boring tool dial and adjust the cutting amount to 0.3 mm.
Fine-tune the boring tool on the dial to adjust the size, which refers to the diameter.
After cutting off 0.3 mm, use the internal diameter gauge as shown in Fig. 7 to measure.
Finish the remaining size twice, using the internal diameter gauge to measure each time to ensure the accuracy of the boring size.
> Usage of internal diameter gauge
(1) The specifications of the inside diameter gauge are 18~35mm, 35~50 mm, 50~160mm, etc.
According to the size of the hole, choose the appropriate specification of the inside diameter gauge.
The inside diameter gauge comes as a complete set with screws for measuring different diameters.
Select the corresponding screws based on the hole size and tighten them onto the gauge rod.
At the same time, install the dial gauge on the gauge rod.
(2) According to the measured size tolerance, first choose a micrometer, such as an R30 round hole with a diameter of 60mm.
You can choose a 50-75 mm micrometer, adjust it to 60mm, and lock it.
(3) One hand holds the inside diameter of the meter, and one hand holds the micrometer.
The meter will be placed in the probe for micrometer calibration. Make sure the meter is as vertical as possible in the micrometer rod.
(4) Adjust the meter so that the pressure gauge is 0.2 ~ 0.3 mm and set the needle to zero.
(5) After boring, insert the internal diameter gauge into the hole to be measured. Slightly swing the gauge up and down, left and right.
Observe the changes in the needle to determine the maximum diameter value of the hole.
When the table needle points to zero, just the micrometer calibration size, that is, the diameter of 60mm, if not to zero, according to the actual situation, continue to bore until the size requirements.
•Boring notes
(1) in the adjustment of the boring tool cutting amount, such as using the trigger hand to screw over the need to reach the scale, to be more back some, re-screwed to the need for the scale position, the reason is to adjust the clearance eliminated.
(2) Control the cutting speed and feed volume to avoid damage to the cutter or deformation of the workpiece caused by excessive speed.
(3) Maintain adequate supply of cutting fluid to reduce cutting temperature and improve tool life and machining quality. Regularly check the tool wear during the boring process and replace the badly worn tools and inserts in time.
(4) Check whether the internal diameter gauge is intact when measuring the hole’s diameter after machining.
Touch the head of the gauge with your finger to see whether the needle can return to the initial position. If it can’t, then the gauge needs to be replaced.
(5) do not immediately lift the boring tool out of the hole after completing the final boring. This helps ensure the quality of the bore surface.
First, keep the boring tool inside the hole and stop the spindle rotation.
Next, adjust the dial to move the boring tool away from the hole wall. Then, lift the spindle to avoid scratching the bore.
This method applies to manual boring tool removal. Alternatively, the boring cycle instruction G76 can be programmed to achieve the same result.
(6) According to the needs of the hole for subsequent processing, such as chamfering, deburring and so on.
High-precision hole parts processing future development trend
(1) Science and technology are continuously progressing, and new and advanced processing technologies will continue to emerge.
These include laser processing technology, electron beam processing technology, and microfabrication technology.
Such technologies are widely used in high-precision hole parts processing.
(2) Artificial intelligence technology is continuously developing, and its application in high-precision hole part machining will continue to expand.
Intelligent technology will play an increasingly important role in this field.
Technologygy will be integrated with more advanced technologies, such as the Internet of Things and blockchain.
This integration will drive continuous innovation and upgrades in high-precision hole part machining technology.
For example, multiple sensors can be arranged on the machining equipment to monitor various parameters, such as cutting force, vibration, and temperature, in real time.
This data allows machining parameters to be dynamically adjusted using adaptive control algorithms.
This approach helps cope with uncertainties in the process and improves machining precision and stability.
(3) Big data analysis technology can be used to process and analyze the data collected by sensors in real time.
This analysis helps identify potential problems and anomalies. Warning signals can be issued in advance to avoid errors and losses in the machining process.
Conclusion
CNC milling of high-precision hole parts requires comprehensive consideration of tool selection, machining programming and techniques, and process control.
By reasonably selecting and configuring these factors, the quality and accuracy of machining can be ensured to meet the requirements.
In the future, with the continuous development of manufacturing technology, the method of CNC milling for processing high-precision hole parts will be further improved and optimized.