Figure 1: Focusing Tube
Figure 1 shows the focusing cylinder, an aluminum part with a straight arc-shaped wall-penetrating groove and a two-stage spiral-through-wall groove. This component is machined from a car-shaped cylindrical blank. The wall thickness of the part is 4mm, and a Ø4 milling cutter can be used to directly cut the grooves. However, this requires the use of a 4th axis on a machining center or a C-axis function on a turning center. In this case, a turning center was chosen for the operation.
When using the simple axial clamping jig shown in Figure 2, it was found that due to the nature of the through-wall groove and its long shape, applying only axial clamping pressure caused compression deformation during processing. As a result, the width of the groove in the middle could only reach 3.4mm, while the required 4mm width could not be maintained near the ends. Although the deformation was relatively small, it still failed to meet the size requirements. Additionally, the height of the cylinder column changed slightly, and the degree of deformation was proportional to the axial clamping force. If the clamping force was too low, it could lead to relative rotation of the workpiece during milling. Therefore, this type of clamping structure is only suitable for shallow circumferential grooves and not for through-wall grooves.
Figure 2: Axial Clamping Type Fixturing Structure
Figure 3: Elastic Inner Expansion Clamping Structure
To improve the clamping system, an elastic inner expansion method was adopted, which shifts the clamping force from the axial to the radial direction, thus preventing axial deformation. This method is ideal for meeting the positioning requirements of the part. However, standardized fixture structures are often complex. Based on this principle, some modifications were made to the original design, resulting in the simplified structure shown in Figure 3.
An elastic wedge sleeve with a tapered surface was added to the spindle. This improved the clamping reliability while reducing manufacturing costs. To ensure the fixture’s stability, the elastic wedge sleeve and mandrel must be ground according to matching specifications, and the taper should not be too steep. The elastic wedges should be quenched to achieve sufficient hardness. The free state of the elastic wedge sleeve should be 0.2mm smaller than the inner hole of the focusing cylinder. The axial movement of the front end is designed based on the calculated radial expansion and a taper of +1 to +2 mm. For easier wire cutting, the opening groove of the elastic wedge sleeve should have an even number of slots, such as 4 or 6. Since the spiral groove is a through-groove, a 0.2mm deep cutting space can be pre-machined on the outer surface of the wedge sleeve. A spring is used to assist in pulling out the elastic wedge sleeve, and only the lock nut needs to be loosened when unloading the workpiece. Once the workpiece and the wedge sleeve are loose, the part can be easily removed, making the loading and unloading process quick and efficient.
When machining on the turning center, the spindle must be converted to the feed C-axis, and the power head should be mounted at position 11 of the tool turret to enable the rotational motion of the milling tool. The workpiece coordinate origin is set at the center of the right end face. The direction of each coordinate axis is illustrated in Figure 3. The machining program is structured as follows:
01235 G98 (Feed per minute mode)
M24 (Switch to milling mode)
G28 U0 W0 T1100 (Tool 11)
G28 H0 (C-axis zero return)
G0 G54 Z-5.5 (Move to the starting point of the arc groove)
M13 S3000 (Start the power head)
G0 X62.5 (Lower the tool to the outer surface of the workpiece)
M98 P1236 L9 (Call subroutine 9 times)
G0 X100 (Radial retraction)
M15 (Turn off the power head)
G28 U0 G28 W0 (Return to X and Z zero)
G28 H0 (C-axis zero return)
M30
01236 G1 U-13. F100 M08 (Cut in)
H180. F300 (Milling the straight slot)
G0 U13. (Retract)
W-7. (Move to the next slot)
G1 U-13. F100 (Cut in 0.5mm)
W-12. H-180. F300 (Milling spiral groove)
G0 U13. (Retract)
Z-12.5 (Move to the start of another slot)
G1 U-13. F100 (Cut in 0.5mm)
W-12 H-180. F300 (Milling another spiral groove)
G0 U12. M09 (Tool change, 0.5mm less)
Z-5.5 (Move to the start point of the straight groove)
G28 H0 (Return to the start of the straight groove for the next layer)
M99 (Return to main program)
This program is written in the format required by the FANUC-Oi-TB numerical control system. Due to the small tool size and limited power of the power head, the depth is cut in 0.5mm layers, with subroutines used to simplify the repetitive process. After each layer finishes milling three slots, the tool returns to the first slot before proceeding. Using a subroutine call significantly reduces program size and simplifies the code. However, the initial depth and tooling parameters are critical. This program uses incremental programming, where after each layer, the tool moves up by 0.5mm. The height of the first tool must be adjusted accordingly, and the rear tool position is externally determined on the workpiece surface.
In conclusion, the clamping fixture for a through-wall groove differs from that for a shallow groove. Using an elastic inner expansion clamping method provides the best solution to prevent axial deformation. Fixture design does not need to strictly follow standard structures; instead, it should be simplified based on practical conditions to meet usage requirements and improve efficiency. Machining circumferential grooves on cylindrical parts typically requires a turning center or machining center with a rotary axis. However, the milling function on a turning center is usually low-powered, so the programming approach differs from that of a machining center. By mastering the programming rules and key points, a correct and efficient program can be developed.
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