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How to repair the spindle of CNC cnc machining center machine?

Release time: 2020-01-10 13:47:11

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As an old man doing dry machining, I still know a little bit about the maintenance of CNC machining center machine tools, because during our processing, it seems that the main axis of the vertical machining center has problems. I am very careful about cnc machining center .

The optical axis I am talking about is the "main optical axis", which is the spindle of the machine tool used in cnc machining. As the core component of a CNC machining center machine tool, the quality of the spindle will directly affect the machining accuracy and service life of the entire CNC machine tool.

What is the main axis

The main shaft is an important part of the machine tool. It includes the main shaft, bearings, and transmission parts (mounted on the main shaft). The main shaft requires torque to be transmitted. It directly accepts cutting forces and is also satisfied with general vertical machining center machines, horizontal machining center special machines, and numerical control. Machine tools have different requirements.


Elements of spindle precision
Popular science concepts:
Rotation accuracy
The rotation accuracy of the spindle assembly refers to the radial and axial runout of the workpiece or tool part of the front-end equipment of the spindle under the condition of no load and low speed rotation after the equipment.
Static stiffness
The static stiffness (referred to as stiffness) of the spindle assembly reflects the ability of the assembly to resist the deformation of the static external load. The bending stiffness K of the spindle assembly is defined as: the force required to be measured at the displacement direction when the front end of the spindle produces a unit displacement, as shown in the figure, that is,
K = F / δ



Temperature rise and thermal deformation
When the main shaft assembly works, it will generate heat due to the conflicts and oil stirring in the relative movements, which will cause a temperature rise, which will cause the shape and position of the main shaft assembly to change (thermal deformation).

Abrasion resistance
The wear resistance of the spindle assembly refers to the ability to adhere to the original accuracy for a long time, that is, the accuracy persistence.

Structure of the main shaft
The shape of the front end of the spindle of various machine tools is shown in the figure:


a) the end of the lathe spindle;
b) end of milling machine and machining center;
c) the end of the spindle of the cylindrical grinder;
d) Spindle end of internal grinding machine;
e) Spindle end of drilling and boring machine;
f) Spindle end of combined machine tool

Information and heat treatment
The choice of spindle data is mainly based on wear resistance and heat treatment deformation.

Ordinary machine tool spindle, 45 or 60 high quality medium carbon steel can be used, tempered to 220HBS-250HBS.
Precision machine tool spindles are expected to have less quenching deformation and stress. 40CR or low-carbon alloy steel 20CR, 16MNCR5, 12CRNI2A, etc. can be selected to be HRC> = 60.

Skill requirements of the spindle

The work and measurement benchmarks are directly related to the accuracy check of the machine tools:
Projects related to work and benchmarks
First, ensure the roundness of the two journals; second, ensure that the taper of the two journals is consistent with the tapered bore of the rolling bearing.
  
Items related to machine tool accuracy check
The machine tool uses the axis of the taper hole to represent the centerline of the spindle. After the spindle component equipment, insert a check rod with a length of more than 300mm into the tapered hole.


What can we do overhaul
Mechanical spindle overhaul generally refers to: replacement of bearings, grinding of tapered holes, and balance running. The common spindle of machining center machinery is bt30 bt40 bt50.

We use bt40 spindle to give you an example:

Record before dismantling:
Look at the traces left by the fault to find the cause of the bearing failure. Inspect the taper hole and the front cover for signs of impact. Look for looseness in the rear coupling and connecting parts. If these dangers are not ruled out, no matter how good the spindle is repaired by you, it will not be able to withstand later wear. It is best to make a spindle repair form that records all hazards and spindle type information for later troubleshooting.

Disassemble the spindle:
Symbols should be made before disassembling the spindle, especially for things that are relatively easy to misalign the equipment such as glands and pulleys. If the parts are not symbolized, subsequent misalignment of the equipment will have a greater impact on the dynamic balance in the later period, and the broach mechanism equipment will be misaligned. It will cause the tool to change normally after the spindle is installed on the machine tool. More careful people can take pictures of each part and part before disassembly, which is more convenient and clear.

主轴的尾端 Trailing end of spindle
The part of symbol one is a device to prevent the broach rod from coming out at the back. It plays the role of pre-compressing the broach rod, and together with a dynamic balancing screw hole on the outer circle.

The component of symbol two is a typical belt tensioner equipment, which is tightened by the taper preload inside. Generally, the concentricity of this structure is better. When disassembling it, you can use a pin puller to pull it outward at the end surface.

Symbol three is a device for preloading the inner ring of the main shaft of the main shaft. It also has the purpose of dust prevention. It also has a dynamic balance screw hole on the top. It is preloaded and tightened by transmitting force on the end face of the pulley.
Disassemble the bearing

Disassemble the bearing

Take the bearing and the mandrel out of the spindle housing. In the future, if the interference between the inner ring of the bearing and the outer ring of the mandrel is not large, you can quietly shake the rear end surface of the main shaft on a wooden board or aluminum copper plate. The bearings trembled quietly;

If the bearing is hard to disassemble due to burning, you can use the method on the picture to pull it out, but you must pay attention to pull the inner ring of the bearing as much as possible; if there is no way to pull the inner ring of the bearing, you can quietly pull the outer ring to the bearing. With a certain clearance, you can use the bearing Lamara bearing inner ring (the inner ring must be pulled again), otherwise the bearing is easy to fall apart.

Bearing drawing equipment

Bearing drawing equipment

This set of bearing drawing equipment is suitable for customers' on-site operations, and the advantage is that the transfer is relatively light. After disassembling all the parts, take the shaft core to grind the taper hole in the future, and check its inner and outer diameter runout and straightness.
Spindle equipment
First, clean all the parts. Generally, use alcohol to add 3m 100 clean cloth to clean. After cleaning, wipe with clean paper and then wrap it.



Then measure the bearing spacer ring, bearing height and bearing gland with a high-precision altimeter. The primary intention is to calculate the preload of the bearing outer ring and check the flatness of the bearing spacer and gland. If there is a slight error, you can A piece of fine sandpaper is placed on the marble platform to quietly grind manually. Generally, the flatness of the part is controlled within 3um. For the preload of the bearing outer ring, please refer to the bearing information.

After checking the positive and negative tolerances of the bearing outer ring preload, the sizes of the main shaft housing and the bearing outer ring gland can be adjusted accordingly. Of course, surface grinders can also be used.
Spindle angular contact bearing rigid preload adjustment
Mechanical spindle bearing pairing methods generally have the structure of <<> or << >>. There will be a bearing inner spacer ring and an outer spacer ring in the bearing center. If the height of the outer spacer ring between the bearings is higher than the height of the inner spacer ring, then the bearing's The preload will be large. Similarly, if the inner spacer ring is higher than the outer spacer ring, its preload will be smaller.



Equipment bearings generally use a bearing heater to heat the bearing inner ring to 90 to 120 °. After the bearing equipment is lowered than the bearing temperature, the bearing is preloaded, and the runout of the bearing spacer ring is adjusted. Table to view runout of spindle taper and equipment accuracy.

Use a hot air gun or other heating equipment to heat the spindle housing, and then fit the new shaft with the bearing. Of course, it is also possible to first install the bearing and the spacer into the housing and then integrally cover the mandrel equipment, but I think this is more dangerous and more difficult to do. Than the heat of the case is dissipated, put the other accessories together, and then you can run and balance.