Approaches To ACME Thread Machining Utilizing Internal Groove Tools

Views: 268     Author: ANEBON     Publish Time: 2024-12-03      Origin: Site

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Approaches To ACME Thread Machining Utilizing Internal Groove Tools

Content Menu

1. Preface

2. Existing problems

3. Thread turning method

4. Specific process methods

5. Processing effect display and precautions

6. Summary


Approaches to ACME Thread Machining Utilizing Internal Groove Tools

The "tool tip trajectory profiling method" is infrequently used in daily production due to its low processing efficiency. However, it can serve as an emergency solution to address specific processing requirements under certain conditions, compensating for the limitations of traditional thread processing methods.


This text provides a detailed discussion on the applicability and necessity of the "tool tip trajectory profiling method" when manufacturing ACME threads. It covers topics such as the modification of the inner groove tool, the manual compensation of the tooltip arc radius, and the compilation of macro programs.

1. Preface

New product trials are often urgent tasks that involve small quantities, short production cycles, and high-quality requirements. Ensuring timely delivery and seizing market opportunities are the top priorities. Over many years of trial production practice, operators have frequently encountered challenging processing issues with specially shaped threaded parts.


To process these parts efficiently using the "forming tool method," special thread tools must be customized. However, these tools can be expensive and have long delivery times, making it difficult to meet customer requirements. As a result, when addressing urgent processing tasks, we typically utilize the "tool tip trajectory profiling method."


This processing method, which is based on the principle of "interpolation," has lower single-piece processing efficiency and is not suitable for mass production. However, its flexibility is advantageous and can enhance the overall completion efficiency of trial production tasks. Practical experience has demonstrated that this method is very effective during the trial production of new products.


The following section provides a demonstration of how this method is applied, specifically in addressing the ACME thread processing challenges encountered during the trial production of new products.

2. Existing problems

The component shown in Figure 1 is an adjustment nut, which is a part of a railway locomotive. The thread parameters highlighted in "I zoom" in Figure 1 reveal that the thread profile angle is 29°, indicating that it is a short-tooth trapezoidal thread. The thread profile has a half-angle deviation of only ±0.25°, which sets high precision requirements for processing. Additionally, the lead of the three-line thread is substantial. The most challenging aspect of manufacturing this part is the transition processing of the 0.4 mm radius arc at the thread's minor diameter.

internal groove toolsthreading techniques

For mass production, a specialized thread blade needs to be customized to process the thread, which is typically achieved through a forming process. However, because of the short production timeframe and the limited quantity of this product, the customization cycle for the special thread tool is often insufficient to meet user requirements. To address this issue, a solution has been developed using CNC interpolation principles combined with a specialized macro program. This allows the processing technology to replace the special thread tool with a standard inner groove tool, ensuring the timely completion of the product's trial production.


The general processing method for machining special threads with small-angle tools, known as the "tool tip trajectory profiling method," has proven crucial in the trial production of various new products and in capturing market demand swiftly.



3. Thread turning method

3.1 Forming tool method

When the angle of the tooltip matches the angle of the thread profile, the general thread processing method is typically utilized. This method is particularly suitable for producing small lead threads. While there are various feed methods available—such as straight feed, oblique feed, and staggered feed—they all fundamentally rely on the contour accuracy of the tool to ensure the actual processing accuracy of the thread.


Thread-cutting tools can be categorized into two types: full-profile thread cutters and pan-profile thread cutters. Full-profile thread cutters, illustrated in Figure 2, are designed for processing threads of specific specifications. They are specialized tools that offer excellent processing quality and high efficiency but lack versatility, making them ideal for large-scale production. On the other hand, pan-profile thread cutters, shown in Figure 3, have a broader processing range but provide lower processing quality compared to full-profile cutters. They are well-suited for single-piece and small-batch thread processing.

ACME thread machiningCNC threading tools


3.2 Tool tip trajectory profiling method


The method for achieving shape accuracy by utilizing the motion trajectory of the tooltip is referred to as the "tool tip trajectory profiling method." In this approach, the tool is allowed to move in a controlled manner relative to the die cast workpiece, and the tooltip is interpolated point by point to generate the desired contour geometry.


The shape accuracy achieved through this method depends on the precision of the forming motion, while the actual processing accuracy is influenced by the geometric accuracy of the machine tool and the CNC interpolation method used. This technique is particularly well suited for small-batch processing of specialized threads, making it ideal for the trial production of new products.

solid carbide threading



4. Specific process methods

4.1 Tool manufacturing

(1) Tool style

To meet the technical parameter requirements for the ACME thread trial and considering the geometric structure of the thread, the prototype machined parts material, and the tool overhang ratio, the focus was on enhancing the rigidity of the process system. As a result, we selected a φ 16mm inner groove tool along with Iscar's high-quality anti-vibration inner groove tool holder, as illustrated in Figure 5. The blade used was a premium carbide blade, designated IC908, featuring a cutting edge width of 2mm and a tip arc radius of 0.1mm.

ACME thread profiles

(2) Thread lead angle Figure 6 shows the principle of calculating the thread lead angle. The lead S of the three-line thread is 19.05 mm, and the thread diameter d2 is 23.35 mm. Before modifying the tool, the thread lead angle θ (°) is first calculated. The calculation formula is


θ = arctanS/(πd2) (1)


Substituting the relevant parameters into formula (1), we can get θ = arctan19.05/(23.35π) = 14.56°.

grooving tool applications

From formula (1), it can be seen that the lead angle θ is usually calculated using the thread median diameter, but in fact, the lead angles at different locations on the entire thread surface are different. Taking the minor diameter as an example, the maximum lead angle at the minor diameter is about 16°.


In order to avoid tool interference, the left rear angle of the blade is sharpened using a diamond grinding wheel. Considering that the tool's rear angle will decrease when the tool is cutting longitudinally, the left rear angle of the blade needs to be sharpened to 25°~30°[2]. The effective depth of the thread should be greater than 2.15mm, and the grinding length should be greater than 2.5mm. The preferred method is to sharpen the corresponding part of the toolbar to prevent interference. The modified tool is shown in Figure 7.

precision machining

4.2 Program design and calculation


(1) To achieve flexibility and efficiency in manual macro programming, it is essential to configure the appropriate macro variables and apply well-reasoned algorithms. This approach ensures a program that is concise and efficient. We adopt a double-layer nested loop structure: the outer layer employs the axial head division method to control thread division, while the inner layer manages thread processing.


During CNC rapid prototyping, we first perform the tooth top chamfering, followed by the trapezoidal thread processing. The radial feed serves as the independent variable, from which we calculate the axial point position and begin processing at the center. Then, we work on both sides in layers, progressing from shallow to deep. It is crucial to control the independent variable and step size appropriately to ensure processing accuracy.


As straight thread processing is a form of point processing, the thread-cutting instruction cannot accommodate tool tip arc compensation. To maintain contour accuracy with an arc radius of R ≤ 0.4 mm, it becomes necessary to manually calculate the tooltip arc compensation amount. This is based on the CNC machining tool tip arc compensation principle. We will use the tooltip arc center trajectory to compile a program that compensates for tooltip arc deviation.


The specific operational method is as follows:


1. Use electronic drawing software to create the theoretical geometric contour of the thread.

2. Apply the equidistant line principle, which states that the center trajectory of the tooltip arc is the theoretical contour line, to draw the center trajectory of the tooltip arc (see Figure 8).

3. Perform the necessary numerical calculations to obtain the program control parameters.

internal threading solutions


The parameter calculation process is as follows.

OT=0.4sin14.5°=0.1(mm)

OK=(23.35-21.25)/2-0.4=0.65(mm)

KT=OK+OT=0.65+0.1=0.75(mm)

MA=KTtan14.5°

=0.75×tan14.5°=0.194(mm)

AK=0.4cos14.5°=0.387(mm)

OO'=M N+2×(MA+A K) = 6.35/2 + 2 × (0.194 + 0.387) = 4.34 (mm)

OC = 0.5sin14.5° = 0.125 (mm)

CF = 0.5-OC = 0.5-0.125 = 0.375 (mm)

E'F = (23.35 × 1.08-21.25)/2 = 2.15 (mm)

CF and E'F are the ranges of the independent variables, and OO' is the thread width calculation parameter.


(2) CNC program: The details are as follows.


T0404

S300M3

G0X18Z12.7

M8

#3=6.35

WHILE[#3LT19.1]DO3

#1=0

WHILE[#1LT0.38]DO1

#2=SQRT[0.25-[0.5-#1]*[0.5-#1]]

G0Z#3

G92X[21.05+2*#1]Z-30F19.05

G0Z[#3+1.27-#2]

G92X[21.05+2*#1]Z-30F19.05

G0Z[#3+#2-1.27]

G92X[21.05+2*#1]Z-30F19.05

#1=#1+0.01

END1

#1=0.375

WHILE[#1LT2.16]DO2

#2=TAN[14.5]*[1.872+#1-0.375]

G0Z#3

G92X[21.05+2*#1]Z-30F19.05

G0Z[#3+1.27-#2]

G92X[21.05+2*#1]Z-30F19.05

G0Z[#3+#2-1.27]

G92X[21.05+2*#1]Z-30F19.05

#1=#1+0.02

END2

#3=#3+6.35

END3

G0X200Z200

M30


In the program, the #2 intermediate variable is obtained by applying the geometric relationship formula of arc and oblique lines, respectively. The "1.27" in the expression of head positioning "G0Z[#3+1.27-#2]" is calculated by (OO'-1.8)/2=(4.34-1.8)/2=1.27 (mm). The "1.872" in the oblique line geometric relationship formula "#2=TAN[14.5]*[1.872+#1-0.375]" is calculated by 0.5cos14.5°/tan14.5°=1.872 (mm).


5. Processing effect display and precautions

Figure 9 shows a cross-section of the actual part. The figure demonstrates that the part's appearance quality is satisfactory. After inspection with the projector, all thread parameters meet the product design requirements.

pitch-specific threading tools

The specific precautions for processing are as follows.


1) When straight thread processing cannot perform tool tip arc compensation, manual calculations become necessary. Therefore, the efficiency and convenience of the drawing software should be fully utilized during the process.


2) The programmed tool position point is at the tooltip arc center. When setting the tool, it is important to ensure consistency within the toolset by measuring the tooltip arc center.


3) When sharpening the tool, care should be taken to avoid damaging the tooltip arc contour.


6. Summary

The "tooltip path profiling method" is a processing technique designed for manufacturing special threads. However, it has a notable disadvantage: it employs point-by-point interpolation during the machining process under typical working conditions. When compared to the "forming tool method," this technique results in longer processing times per piece and lower processing efficiency, making it less suitable for mass production.


Despite these drawbacks, this method offers several advantages, which are outlined below:


1. Cost Advantage: This processing method utilizes universal standard tools to manufacture special threads, eliminating the need for customized tools. For small batches or even single-piece production of special threads, this approach provides significant cost-effectiveness.


2. Quality Advantage: The point-by-point interpolation processing principle employed in this method reduces the accuracy requirements for thread tools. It eliminates the tooth profile angle errors that can be problematic in the "forming tool method" and avoids the complexities of repeated adjustments. Consequently, the theoretical processing accuracy is superior to that of the "forming tool method," making it more suitable for high-precision thread production.


3. Processing Technology: The method exhibits favorable cutting characteristics with lower cutting resistance. It is particularly effective for thread processing under conditions of poor rigidity in the machining system, especially when dealing with large lead and weak rigidity.


4. Timeliness Advantage: This technique can significantly reduce the overall production cycle time, leading to enhanced overall efficiency.


Every processing method has its own set of advantages, disadvantages, and appropriate applications. During production, it is essential to select the most suitable processing method based on a comprehensive assessment of product quality, cost, delivery requirements, and demand.




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