Enhanced Cavity Machining Precision via Angle Head and Dual-Pilot Reamer Application
To address the challenges of precisely processing ear holes in cavities, this paper outlines the complete workflow for the components involved. It introduces a specialized drill, reamer, and machine reamer that are designed to work with an “angle head” on a horizontal CNC machining center, enabling accurate processing of ear hole positions. Additionally, a custom hand reamer is created to ensure the coaxiality and size of the ear holes are maintained. This approach offers valuable insights and guidance for the future processing of similar structural products.
01 Preface
Currently, as the aviation industry continues to demand improved aircraft maneuverability, stability, and fuel efficiency, the design of landing gear has evolved significantly. While a variety of lightweight specialized materials, such as ultra-high-strength steel, aluminum alloy, and titanium alloy, are increasingly utilized, the overall structure of landing gear has also been innovatively optimized. Some components that were previously fastened together using matching relationships have now been streamlined into an integrated structure. This change reduces the number of individual parts in the landing gear and effectively meets the technical requirement for lightweight designs.
However, after reducing weight, ensuring the functionality of the landing gear becomes more challenging. As part of the main load-bearing structure, this can complicate production and processing due to the introduction of additional features, such as ears. This article focuses on a machining method for creating high-precision tab holes designed within a mold cavity.
02 Analysis of Part Structure and Machining Difficulties
2.1 Part Structure
A primary load-bearing component is constructed from 300M ultra-high-strength steel and has an overall “Y” shape. It features two sets of tabs located in the tail lightening cavity, each with two φ22H7 holes. The spacing requirements for the holes, measured from the center main datum, are (68 ± 0.03) mm. Additionally, the perpendicularity requirement is 0.05 mm, and the parallelism requirement with respect to the B-C datum is also 0.05 mm. The surface roughness requirement for the holes is Ra ≤ 0.8 μm. The specifications for the location and dimensions of the holes to be machined are illustrated in Figure 1.
2.2 Machining Difficulty Analysis
The lug is situated within the reduced cavity at the rear of the component. The primary areas that need machining are the φ22H7 holes and the end faces of the four lugs. Machining the end faces of the lugs is relatively straightforward and can be done using a solid carbide milling cutter, with a recommended cutter diameter of 16-25 mm, on a horizontal four-axis machining center.
However, machining the four φ22H7 holes while ensuring the required geometric tolerances is exceptionally challenging due to positional and orientation constraints. The specific reasons for these challenges are as follows.
1) Conventional machining experience indicates that the spindle of the machine must be parallel to the hole axis during the machining process, which means that the spindle needs to extend into the cavity. However, due to interference from the cavity’s contour, existing machining equipment—whether three-axis or four-axis horizontal machining centers—cannot position the spindle perpendicular to the lug end faces when drilling, boring, and reaming.
2) The part is constructed from 300M ultra-high-strength steel, achieving a hardness of 55HRC after heat treatment. The distance between the outer end faces of the two sets of lugs measures 231 mm. During machining, the four φ22H7 holes must maintain their diameter, parallelism, perpendicularity, and position, while also ensuring that both sides of the same set of holes on the two sets of lugs are coaxial. To meet these coaxiality requirements, the holes in each set of lugs must be machined simultaneously from a single direction. Additionally, the aspect ratio of the reamer used must be greater than 10. It is important to note that when the tool rotates at high speed, centrifugal force can affect machining stability, ultimately impacting the quality of the machining process.
03 Processing technology
3.1 Processing scheme design
Before quenching, the parts are processed using a horizontal machining center to complete the rough machining of the outer contours of two sets of lugs. A machining allowance of 1mm is reserved for the mating surface of each lug. The holes are not machined prior to quenching. After quenching, the holes are machined using both rough and fine machining techniques.
Using an “angle head” along with a “special tool” on the horizontal CNC boring machining center, the four φ22H7 holes are machined down to φ21.9H7 through drilling, reaming, and additional reaming. To ensure the positional accuracy of the holes, a “special hand reamer” is employed. The four φ21.9H7 holes serve as guides to guarantee the coaxiality requirements for each hole size within the same group of holes.
3.2 Implementation of the scheme
(1) The method of rough machining four φ21.9H7 holes is as follows.
1) Product clamping and alignment requirements.
The part was securely clamped onto the worktable of a four-axis machining center. The alignment requirements specified that the radial runout of the B and C datums in the Z and Y directions should be ≤0.03 mm. Additionally, the radial runout of the A datum along its axis was also required to be ≤0.03 mm. The A datum was centered to establish the X origin, while the B and C datums were centered to determine the Y and Z origins, respectively.
2) An angle head was used to change the machining axis.
Due to the structural limitations of the tail relief cavity and the four φ22H7 holes in the lug, the spindle head could not align parallel to the axis of the machined holes during machining. As a solution, a specialized “angle head” was employed to alter the machining direction. The T90cn-2.5 angle head, illustrated in Figure 2, ensures that its axis is parallel to the axis of the machined hole. The size of the angle head was chosen based on the actual internal dimensions of the two lugs, the effective overhang required by the machining tool, and the torque output of the angle head.
3) Design of Specialized Tools.
Our company utilizes a proven 300M heat-treated hole processing technology. For the rough machining of the four φ21.9H7 holes, we employed a “drill-reamer-reamer” method. Considering the characteristics of the material being machined (with a hardness greater than 55 HRC) and the operating environment of the tools, we developed three custom tools (as illustrated in Figure 3) specifically for drilling, reaming, and reaming tasks.
4) Processing process.
To install the special tool, attach it to the angle head using the reducer sleeve, and then connect the angle head to the horizontal machining center. Utilize the alignment table to ensure that the radial runout of the alignment reference surface on the angle head is less than 0.005 mm. Set the tool at reference point B and confirm the Z-axis origin for the drill. The tool setting methods for the special drill, special reamer, and special tap are the same. The origin calculation formula is:
Z0 = Z actual – D1/2 – D2/2 (1)
Where Z0 represents the program coordinate origin during machining, Zactual is the actual coordinate value after tool setting, D1 is the measured diameter of the process mandrel at datum B (in mm), and D2 is the theoretical diameter of the tool (in mm).
After the tool is set, the CNC machining program is executed to sequentially complete the drilling and reaming of four holes. The hole roughing process is illustrated in Figure 4. First, orient the angle head toward one of the ear pieces (Figure 4 shows it directed toward the right ear piece). Next, call the drilling program to create two φ21mm holes on the right ear piece. After drilling, replace the drill bit with a reamer (there’s no need to reset the tool) and immediately call the reaming program to expand the two holes to φ21.7mm.
Then, replace the reamer with another reamer and execute the reaming program to perform the rough five axis machining of two φ21.9mm holes on the right ear piece. Once this is complete, change the direction of the angle head to point toward the left ear piece and repeat the same operations to finish the rough machining of the two φ21.9mm holes on the left ear piece.
Once the process is completed, we can ensure that the spatial dimensions of the four φ21.9mm holes, the verticality of datum A, and the parallelism of datums B and C meet the specified requirements. As this is a rough machining of the bottom hole, the surface roughness requirements are relatively low, with a standard of Ra ≤ 1.6μm being satisfactory.
(2) Method for Finishing Four φ22H7 Holes
After rough machining the φ21.9mm holes, the key challenge is to accurately remove the final 0.1mm allowance from the hole diameter while ensuring the coaxiality of the group of holes. Reaming is an effective method to enhance the accuracy of small to medium-sized holes. Additionally, the design of a special hand reamer is essential for this process.
1) Design of special hand reamer.
When designing a special hand reamer for reaming holes after quenching 300M steel, several factors must be considered. These include the structure of the processing custom CNC parts, the feasibility of benchwork reaming, the distribution of reaming allowances, and the surface roughness requirements of the inner hole post-reaming.
If the aperture has a remaining margin of 0.1 mm, reaming must be completed in two passes. This necessitates the creation of two distinct reamers: one for roughing and another for fine reaming. The roughing reamer is responsible for removing the margin and making initial corrections to the coaxiality of the aperture. In contrast, the fine reamer ensures that the surface roughness and aperture size meet the specified requirements.
To maintain coaxiality for the same set of holes on both tabs, the reamer design must also consider forward and backward guidance. Additionally, simultaneous machining of the same holes on both tabs must occur in a single reaming pass. A schematic of the dedicated hand-held fine reamer is illustrated in Figure 5.
Both rough and fine reamers are composed of five main parts: a front guide sleeve, a sleeve reamer, a cylindrical pin, a rear guide sleeve, and a reamer. Using the fine reamer as an example, the functions of each part are as follows:
Front Guide Sleeve: This part has an outer diameter that fits into the front hole, guiding the sleeve reamer during the reaming process. It ensures the accuracy of the front ear when reamed.
Sleeve Reamer: The cutting edge of the sleeve reamer is designed with a tapered inner hole at a ratio of 1:30. It is mounted on the conical surface of the reamer and secured in place by a cylindrical pin. Its purpose is to simultaneously ream the left ear hole.
Cylindrical Pin: This pin is essential for the stop sleeve reamer; it prevents the sleeve reamer from rotating and moving axially during processing.
Rear Guide Sleeve: Similar to the front guide sleeve, this part also has an outer diameter that fits into the rear hole, guiding the reamer and ensuring the accuracy of the rear ear during reaming.
Reamer: The overall length of the reamer is designed to be 325 mm. Its rear end features a 12 mm x 12 mm cube for securing a wrench or connecting to a pneumatic adapter sleeve. The cutting area of the reamer is sized appropriately to meet the required dimensions of the finished part after reaming. The central outer diameter is φ16H7, which serves as the positioning support for the rear guide sleeve and connects the front sleeve reamer to the sleeve reamer. The sleeve reamer mounting area is designed with a 1:30 taper to ensure a precise fit upon installation. The front end of the reamer is φ16H7, providing positioning support for the front guide sleeve.
The design of the roughing reamer mirrors that of the finishing reamer, with the only difference being the dimensions of the front and rear guide sleeves and the cutting area of the reamer.
2) How to Use the Special Hand Reamer
First, install the rear guide sleeve into the right lug hole. Next, insert the reamer body from the right end. After passing through the rear guide sleeve, attach the sleeve reamer to the tapered surface of the reamer body and secure it with a cylindrical pin. At the same time, install the front guide sleeve in the left lug hole. Continue moving the reamer to the left until you reach the position shown in Figure 5. You can ream two lug holes manually using a wrench or by connecting a pneumatic adapter to a compressed air system. While manual reaming may lead to variability in the quality of the holes, using a pneumatic adapter, when conditions allow, is the preferred method for better consistency.
The processing sequence begins with the rough reamer. After rough reaming, replace the front and rear guide sleeves, the sleeve reamer, and the reamer body. Then, finish reaming the two lug holes using the same method. This process ensures that the original positional accuracy of each lug hole is maintained while achieving the required hole diameter.
This method was employed for processing landing gear parts. The measured dimensions of the four φ22H7 holes were all within specifications. The verticality of Datum A, the parallelism of Datums B and C, and the spatial position dimensions of each datum met the technical requirements of the parts. After delivery, there were no issues during assembly, resulting in a qualification rate of 100%.
04 Conclusion
This article outlines a method for machining lug holes in mold cavities. By using an angle head to change the spindle direction, specialized tools such as a drill, reamer, and machine reamer are designed to position the lug holes accurately on a horizontal CNC machining center. Additionally, a specialized hand reamer is created to ensure both the necessary coaxiality and the required diameter of the lug holes. This method offers guidance and insights for the machining of similar structural products in the future.
If you want to know more or inquiry, please feel free to contact info@anebon.com
Anebon’s primary objective is to provide our customers with a serious and responsible business relationship, offering personalized attention to each of them. We specialize in new fashion design, OEM services, precision hardware fabrication, online CNC milling service, precision casting, and sheet metal fabrication services