Hobbing processing principle and common defect handling methods

Gears are common parts in the design of mechanical products. In the gear tooth profile processing, hobbing is the most widely used. In addition to machining straight and helical gears, it can also process worm gears and spline shafts. Hobbing is suitable for single-piece small batch production and mass production. This article will introduce you to the machining principle of hobbing and the treatment of common defects.

   Hobbing processing principle and common defect handling methods
   The hobbing gear can be seen as a gear and rack drive without meshing clearance. When the hobbing rotates one revolution, it corresponds to the rack moving one tooth in the normal direction. Hobbing is the most widely used method of cutting teeth. It can process involute gears, arc gears, cycloidal gears, sprockets, ratchets, worm gears and enveloping worms. The precision can generally reach DIN4~7. At present, the advanced technology of hobbing has multi-head hob hobbing, hard tooth hobbing technology, large gear hobbing technology, high-speed hobbing technology and so on.

01, the number of teeth is incorrect

main reason

1) The split gear exchange gear is not adjusted correctly

2) The hob selection error

3) The workpiece blank size is correct

4) When the helical gear is rolled, the additional movement direction is wrong.

Solution

1) Re-adjust the split gear and check if the intermediate wheel is installed correctly.

2) Reasonable use of hob

3) Replace the workpiece blank

4) Increase or decrease the intermediate wheel in the differential exchange gear

02, the tooth shape is not normal - the tooth surface is out of the edge

main reason

Hob tooth shape error is too large or the tooth is incompetent and weak, the instantaneous speed is relatively large, and there are four kinds of workpiece defect conditions.

1) After the knife is sharpened, the teeth are equally pided

2) Hob axial rocking forest

3) The hob has a large radial runout

4) The hob is blunt

Solution

The main method: focus on the quality of the hob sharpening, the accuracy of the hob installation and the geometric accuracy of the machine spindle:

1) Control the quality of the hob sharpening

2) Ensure the installation accuracy of the hob, and do not knock when installing the hob; the end face of the washer is flat; the end face of the nut should be vertical; the inside of the taper hole should be clean; after the bracket is installed, no gap can be left.

3) Review the rotation accuracy of the machine tool spindle and repair and adjust the hob spindle bearing, especially the thrust washer

4) Replace the new knife

03, the tooth profile is not normal - tooth profile asymmetry

main reason

1) The hob is not properly installed

2) After the knife is sharpened, the radial error of the front blade surface is large

3) After the hob sharpening, the spiral angle or lead error is large

4) The error of the hob mounting angle is too large

Solution

1) Align the knife with the "sickle flower" method or the knife rule

2) Control the quality of the hob sharpening

3) Re-adjust the installation solution of the hob

04, the tooth shape is not normal - the tooth angle is wrong

main reason

1) The tooth profile angle error of the hob itself is too large

2) The radial error of the front blade surface is large after the hob sharpening

3) The error of the hob mounting angle is large

Solution

1) Reasonable selection of the accuracy of the hob

2) Control the sharpening quality of the hob

3) Re-adjust the mounting angle of the hob

05, tooth shape is not normal - tooth shape periodic error

main reason

1) After the hob is installed, the radial runout or axial yaw is large

2) Machine table is not evenly rotated

3) Install eccentric or tooth surface bumps on the cross-wheel or split-tooth exchange gears

4) The knife holder slide is loose

5) Unreasonable workpiece clamping results in pendulum

Solution

1) Control the mounting accuracy of the hob

2) Check the axial movement of the machine table's indexing worm and adjust the repair

3) Check the installation and operation of the cross-wheel and split-tooth exchange gears

4) Adjust the iron of the knife holder slide

5) Proper selection of the correct solution for workpiece clamping

06, the ring gear radial runout tolerance

main reason

The center of the hole in the workpiece does not coincide with the center of rotation of the machine table

(1) Regarding machine tools and fixtures:

Worktable radial runout

Mandrel wear or radial runout

The upper and lower thimbles are biased or loose

The fixture positioning end face is not perpendicular to the table rotation center line

Work clamping components such as washers and caps are not accurate enough

(2) Relevant work:

Workpiece positioning hole diameter is out of tolerance

When installing the outer circumference of the workpiece, the coaxiality between the outer circle and the inner hole is too bad.

Poor workpiece clamping rigidity

Solution

Focus on controlling the rotation accuracy of the machine table and the correct installation of the workpiece

(1) Regarding machine tools and fixtures:

Check and repair the table swivel rail

Reasonable use and maintenance of the workpiece spindle

Repair the accuracy of the rear column and the upper thimble

The positioning end of the clamp is not perpendicular to the center line of the table

Improve workpiece clamping component accuracy, such as gaskets and caps

(2) Regarding the workpiece:

Control the dimensional accuracy of the workpiece positioning hole

Control the coaxiality error between the outer circle and the inner hole of the workpiece

The main reason why the clamping force should be applied to the parts with sufficient rigidity

07, tooth direction error is out of tolerance

main reason

The vertical feed direction of the hob is too skewed toward the axis of the inner hole of the tooth blank. When machining helical gears, there is also an inaccurate additional motion.

(1) Regarding machine tools and fixtures:

Column triangular guides and table axis are not equal

Workbench end face is big

Top and bottom top different axes

The meshing clearance of the indexing worm gear pair is large

There is periodic error in the transmission of the indexing worm gear pair

Vertical feed screw pitch error

Large tooth and differential exchange gear error

(2) Relevant work:

The ends of the blank are not parallel

The workpiece positioning hole is not perpendicular to the end surface

Solution

Focus on controlling the machine geometry and the correct installation of the workpiece.

(1) Regarding machine tools and fixtures:

Repair column accuracy and control machine thermal deformation

Repair the rotation accuracy of the workbench

Repair the accuracy of the rear column or the upper and lower thimbles

Reasonable adjustment of the meshing clearance of the indexing worm gear pair

Repair the accuracy of the parts of the indexing worm gear pair

If the vertical feed screw is not accurate due to wear and tear, it should be replaced in time.

The calculation error of the differential exchange gear should be controlled

(2) related to the workpiece

Control the parallelism error of both ends of the tooth blank

Control the perpendicularity of the positioning hole and the end face of the blank

08, the cumulative error of the pitch is out of tolerance

main reason

The maximum error of uneven rotation in each revolution of the hobbing machine table is too large:

Indexing worm gear pair transmission accuracy

The radial runout of the table and the end face jump

The tooth exchange wheel is too loose or has a bump

Solution

Focus on the accuracy of the split tooth kinematic chain, especially the indexing worm gear pair and the hob:

Repair indexing worm gear pair transmission accuracy

Repair the rotation accuracy of the workbench

Check the split gear exchange gear and mesh tension and operating conditions

09, tooth surface defect - tear

main reason

Uneven material of the tooth blank

Improper heat treatment of the tooth blank

Unreasonable cutting amount is used to produce built-up edge

Cutting fluid is not efficient

Hob is blunt, not sharp

Solution

Control the quality of the tooth blank material

Correct selection of heat treatment methods, especially hardness after quenching and tempering, it is recommended to use normalizing treatment

Correct use of cutting amount to avoid the formation of built-up edge

Correct use of cutting fluid, paying particular attention to its lubrication performance

Replace the new knife

10, tooth surface defects - caries

main reason

Due to sudden changes in the mutual position of the hob and the blank:

The triangular guide rail of the column is too loose, causing the feed of the hob to suddenly change. The triangular guide rail of the column is too tight, causing creeping phenomenon.

The tool holder helical gear has a large meshing clearance

Oil pressure instability

Solution

Adjusting the column triangular guide: the requirement is tight and appropriate

If the tool holder wears out due to long use, it should be replaced.

Reasonable maintenance of the machine, especially cleaning, keeps the oil path clear: the oil pressure remains stable

11, tooth surface defects - vibration lines

main reason

Due to vibration:

Large gap in a certain drive link inside the machine tool

The clamping rigidity of the workpiece and the hob is not enough

Cutting amount is too large

After the rear bracket is installed, the clearance is large

Solution

Find and eliminate vibration sources:

Timely overhaul of machine tools that have been worn for a long time

Improve the clamping rigidity of the hob, for example, reduce the distance between the supports: the large diameter of the hob should be selected as much as possible. Improve the clamping rigidity of the workpiece: for example, try to increase the supporting end surface, and the supporting end surface (including the workpiece) is only concave: shorten the distance between the upper and lower thimbles

Correct selection of cutting amount

Correctly install the rear bracket

12, tooth surface defects - fish scales

main reason

The tooth blank is not properly heat treated, and it is more common when processing steel after quenching and tempering.

Solution

Control the hardness of the quenching and tempering treatment as appropriate

It is recommended to use normalizing treatment as pre-heat treatment of the tooth blank.