How to machine thin-walled parts?

As we all know, CNC machining of parts “difficult” that thin-walled parts due to poor rigidity, in the processing of parts easily because of the increase in the shape of the parts error is not easy to ensure the quality of parts processing. Turning or milling thin-walled parts, due to the poor rigidity of the parts, the deformation of thin-walled parts is usually manifested in the volume of expansion or contraction, accompanied by bending, twisting, elliptical, warping, and other deformations in the process of processing.

thin wall part

The factors affecting the machining accuracy of thin-walled parts

Factors affecting the machining accuracy of thin-walled parts are mainly two aspects:

1. Due to the thin wall of the workpiece, the workpiece, tool, and fixture cutting system are composed of poor rigidity, and the clamping workpiece, under the action of the clamping force, is very easy to produce deformation. Cutting vibration is easy to produce during the cutting process, in the cutting force, especially the radial cutting force is the role of the backward force, which easy to produces vibration and deformation, affecting the dimensional accuracy of the workpiece, shape, positional accuracy, and surface roughness.

2. Due to the thin wall of the workpiece, the cutting heat will cause thermal deformation of the workpiece, making it difficult to control the size of the workpiece. Especially for thin-walled metal parts with large coefficients of linear expansion, if rough turning, semi-finish turning, and finish turning are carried out continuously in a single setup, the thermal deformation of the parts caused by the cutting heat will have a great impact on the dimensional accuracy, and sometimes even make the parts jammed on the fixture.

In order to prevent and reduce the deformation of thin-walled parts, you can take the following technological measures: take the workpiece phased processing can reduce the impact of thermal deformation of the workpiece on the machining accuracy, take to increase the clamping contact surface and the use of axial clamping fixtures, in order to reduce the deformation of the workpiece clamping, and should also be a reasonable choice of cutting tool geometric parameters and reasonable choice of cutting amount.

Process analysis

The main principle of processing is to use the sharpness of the tool, speed, etc., as well as the machined parts extrusion occurs to remove the blank allowance and the formation of the cutting force in the process is very large, coupled with the specificity of the structure of the thin-walled parts, so in the machining of each stage of the processing process parameters must be set reasonably, or else it will be deformed because of thin-walled parts of thin-walled and deformation of the forces occurring.

1. Roughing

Due to the thin-walled parts, the force can not be too large, and for other reasons, roughing should be processed first cavity, and then processing the outer contour. First machining cavity, mainly because the outer contour is not machined and can withstand a larger cutting force. According to the nature of the material and parts for thin-walled processing, the use of software, the main process parameters are set as follows :

Roughing cavity of the tool diameter of 10mm alloy end mill, process type for the cavity milling, toolpath settings under the cutting mode to follow the part, the step distance is constant, the maximum distance of 1mm, the public depth of cut for each cutter is constant, the maximum distance of 10mm.

Click [Cutting Parameters], connect the open toolpath set to change the cutting direction, margin under the bottom surface of the part margin of 0.1mm, the side of the part margin of 0.5mm, the side of the margin set to a greater extent is mainly to increase the wall thickness of the processed parts, so as to increase the machining process of the parts to withstand a greater cutting force, to avoid excessive deformation due to the machining process of the parts deformed by the force and the impact of the subsequent machining.

Click [Non-cutting movement], the feed type under the closed area is spiral, the slope angle is 0.5°, and the height is 0.5mm.

Click [Feed Rate and Speed], the spindle speed is 4200r/min and the cutting feed rate is 1800mm/min.

When machining the outer contour, the tool is unchanged, where the unchanged parameter settings are the same.

Changes in parameter settings, click [cutting parameters], the strategy under the extension on the edge for 5mm, check to add finishing toolpath, the number of toolpaths is set to 4, the finishing step is 0.2mm, the purpose of adding finishing toolpaths is mainly to reduce the machining step settings near the contour, thereby reducing the cutting force during processing, to prevent the workpiece from being overstressed and produce a large deformation.

Click [non-cutting movement], the type of feed under the closed area is plunge cutting, the type of feed under the open area is arc, the radius is 5mm, the arc angle is 10°, the arc angle is set to be smaller mainly because of the reason to reduce the cutting force when feeding near the part, to prevent the cutting force from being too large and lead to excessive deformation of the part, which will lead to the machining of the part is unqualified.

2. Semi-finishing

The role of semi-finishing machining is mainly to reduce the roughing process of the surface of the parts generated by the error and improve the quality of parts processing. This stage of the parameter settings is mainly introduced to the processing of the contour.

Processing the inner contour, the software’s main process parameters are set as follows, the machining tool diameter of 8mm alloy end mill, the process type of cavity milling, toolpath settings under the cutting mode for the contour, step for a constant, the maximum distance of 0.2mm, additional toolpath s number of 3, the public depth of cut for each cutter for a constant, the maximum distance of 2mm.

Click [Cutting Parameters], and the part side margin under the margin is 0.05mm.

Click [Non-cutting Movement], the feed type under Closed Area is Slant Feed along Shape with a slope angle of 0.5° and a height of 0.5mm, and the feed type under Open Area is Circular Arc with a radius of 5mm and a arc angle of 10°, and the starting point of the default area under Starting Point / Drill Point is Corner.

Click [Feed Rate and Speed], the spindle speed is 5800r/min, and the cutting feed rate is 800mm/min.

When machining the outer contour, the tool is unchanged, and its unchanged parameter settings are the same, as the changed parameter settings, you need to click on [non-cutting movement], the type of feed under the closed area is an interpolation, and the rest of the parameters are unchanged.

3. Finishing

As the role of finishing is mainly to improve the quality of parts processing and processing of qualified parts. Parameter settings at this stage mainly introduce the processing of the contour.

Processing the inner contour, the software’s main process parameters are set as follows, the processing tool diameter of 8mm alloy end mill, process type of cavity milling, toolpath settings under the cutting mode for the contour, step for a constant, the maximum distance of 0.02mm, the public depth of cut for each cutter for a constant, the maximum distance of 1mm.

Click [Feed Rate and Speed], the spindle speed is 6800r/min, the cutting feed rate is 500 mm/min.

Click [Non-cutting movement], the feed type under the closed area is slant feed along the shape with a slope angle of 0.5° and a height of 0.5 mm, the feed type under the open area is arc, with a radius of 5 mm and an arc angle of 10°, and the default area starting point under the starting point/drill point is the corner.

When machining the outer contour, the tool is unchanged, its unchanged parameter settings are the same, and the changed parameter settings are the type of feed under the closed area is an interpolation, and the rest of the parameters are unchanged.

The structure and use of special fixtures

Parts in the clamping force, cutting force, and cutting heat is easy to produce deformation, but machining accuracy is difficult to ensure. In order to solve this problem, the design of a thin-walled set of parts processing lathe fixtures can be expanded taper set expansion fixture. When the rough machining is completed, the end of the bore finishing, the fixture is used to the workpiece bore as a positioning reference surface to carry out semi-finishing and finishing of the outer circle of the workpiece.

The fixture consists of a taper mandrel, an expandable taper sleeve, a spring, a lock nut, and a washer. Taper mandrel (see Figure 4) for the main body of the fixture, the material is 65Mn. Installation, of the left end of the taper mandrel with Grass’s No. 6 taper shank installed in the lathe spindle cone, and tighten the bolt rod. Then install the spring, expandable taper sleeve, washer, and lock nut in turn.

In this case, the taper center and expandable taper sleeve are fitted with 1: 30 internal and external tapers. The two end faces of the washer should be parallel and should normally be ground. Spherical washers may be used if necessary. Tighten the lock nut before machining, thus pushing the expandable taper sleeve to move left on the taper heart axis, so that the expandable taper sleeve rises evenly along the circumference, realizing the reliable clamping between the workpiece and the expandable taper sleeve without any gap, and ensuring the machining accuracy of the parts.

After the parts are processed, just loosen the lock nut, and the expandable taper sleeve under the action of spring force to the right and produce radial contraction, will be processed workpiece removed.

Expandable taper sleeve

In the whole set of fixtures, the expandable taper sleeve plays an important role, expandable taper sleeve rise and contraction control of the workpiece clamping and relaxation. As shown in Figure 4, there are 6 symmetrically distributed grooves on the circumference of the expandable taper sleeve, and the inner hole of the expandable taper sleeve adopts a taper of 1:30 to match with the taper mandrel, which can effectively ensure that the elastic sleeve can be quickly opened and contracted, and make the circumferential force of the workpiece uniform.

So that the workpiece and expandable taper sleeve can be clamped reliably without clearance, to ensure the dimensional accuracy of the parts. To improve the service life of the expandable taper sleeve, the parts are made of 65Mn material with economic fire treatment. The hardness can reach HRC55~58, and then grinding with the taper mandrel, and then wire cutting openings.

Conclusion

CNC milling can process a variety of parts, and processing methods are also many, but due to the different structures of the parts, the processing process and parameter settings are also different. For conventional parts, the process analysis and parameter settings are more or less the same, but for the special structure of the parts, the process analysis and parameter settings are not the same.

In fact, in the machining process, there are many factors affecting the processing, such as the type of blank material, tool performance, the accuracy of the machine tool, the precision of the fixture, the choice of cutting fluid or cutting oil, as well as the structure of the parts process analysis and parameter settings, the technical level of the operator, etc., these factors are interlocked, once a certain part of the problem, the machining of the parts will be affected!

2 Comments

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  2. Every time I read a new post, I feel like I’ve learned something valuable or gained a new perspective. Thank you for consistently putting out such great content!

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