JPH07136845A - Positioning method for gear-positioning jig in group management system of hobbing machine - Google Patents

Abstract

PURPOSE:To carry out positioning for a gear-positioning jig in a simple manner by correcting the working position of a gear positioning jig by adding the correction quantity set according to the property of each hobbing machine. CONSTITUTION:The working position of a gear-positioning jig 13 is calculated on the basis of the measurement of the radius of a hob cutter 10 and the addition of the radius dimension of a gear G. Then, the working position of the gear positioning jig 13 is corrected by adding the correction quantity which is set according to the property of each hobbing machine 1. Accordingly, the working positioning of the gear-positioning jig 13 of each hobbing machine 1 can be carried out through calculation automatically by only determining the specification of the gear G and measuring the radius of the hob cutter 10. Accordingly, the working positioning of the gear-positioning jig 13 is carried out, and a material for the gear which is set on the gear positioning jig 13 is gear-cutting-worked by the hob cutter 10, and the gear G can be automatically manufactured through the preparation in one time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of positioning a gear positioning jig in a hobbing machine group management system.

[0002]

2. Description of the Related Art Generally, a hob cutter of a hobbing machine, which is a gear cutting tool, is worn by machining a gear. When the hob cutter is worn by a predetermined amount, for example, the hob cutter is removed from the hobbing machine and polished, and the polished hob cutter is reattached to the hobbing machine for use.
After repeating this operation a predetermined number of times and using one hob cutter, the hob cutter is replaced with a new one.

When replacing the hob cutter with another hob cutter or when mounting the hob cutter after re-grinding, it is necessary to mount a new hob cutter on the hobbing machine and cut the gear. The hob cutter of such a hobbing machine is shown in FIGS.
3 is shown.

As shown in the figure, the hob cutter 101 is
A plurality of protrusions composed of cutting teeth 102 are arranged on a body portion 103, and each cutting tooth 102 is arranged in a spiral shape along an axial direction passing through an axis 104, and includes a vertex 105 of the cutting tooth 102. The virtual cylindrical surface 106 (shown by the alternate long and short dash line) forms a virtual cylindrical body.

The curve 107 formed at the tip of each cutting tooth 102 is inclined with respect to the virtual cylindrical surface 106 including the apex 105 of the cutting tooth 102, and gradually extends from the apex 105 to the virtual cylindrical surface 106. It is adapted to be displaced inward (in the direction of the axis 104).

Then, as shown in FIG. 10, every time the hob cutter 101 is ground by the scraping amount (a), the cutting teeth 1
Since the portion of the vertex 105 of 02 is scraped off, the radius of the virtual cylindrical surface 106 of the vertex 105, which is scraped off and the position of which is newly displaced, changes.

The hob cutter 10 that changes with each re-polishing
Hob cutter 1 is used to measure the virtual radius of 1 and position the positioning jig for the gear of the hobbing machine on which the gear is set.
Since the work of measuring the virtual radius of 01 requires skill and is troublesome, and so on, the gears are actually matched with each other while being set in the gear positioning jig.

Further, since the imaginary radius dimension of the hob cutter described above changes with each grinding, and even if the gear positioning jig for positioning the gear material is the same, each hobbing machine has its own cutting dimension. It was difficult to keep track of the machining positioning coordinates of the above gear positioning jig from the measurement results of the hob cutter and the dimensions on the drawing, and the machining positioning of the positioning device with the gear material set was determined by what is called physical matching. . The positioning of the positioning jig for the gear of the hobbing machine is more accurate and easier than the numerical matching because of the necessity of re-grinding the hob cutter and the unique characteristics of the hobbing machine. .

Next, a gear machining procedure by actual matching will be described with reference to FIGS. 14 and 15. First, the outer shell of a cylindrical forged material is first ground by a lathe to produce an intermediate material W1 for a hobbing machine shown in FIG. 14, and then this intermediate material W1
Positioning jig 202 for gears of hobbing machine 201
Is moved in the direction of the hob cutter 101 and hits, and hits the surface of the intermediate material W1 with a depth of about 0.1 mm. This confirms the contact state between the intermediate material W1 and the hob cutter 101.

Subsequently, the hob cutter 101 is set to the intermediate material W.
The gear positioning jig 202 is shifted in the direction of the hob cutter 101 by the primary driving dimension in a state of being shifted downward from 1. In this state, the primary processing of the intermediate material W1 is performed.

The matte tooth thickness of about 5 to 7 cutting teeth 102 of the primary processed product formed by completing the primary processing of the intermediate material W1 is measured. The measurement dimension of this Matagi tooth thickness is
Compared with the corresponding drawing dimensions, the difference is plotted on the horizontal axis, the secondary dimension is plotted on the vertical axis, and the secondary dimension is read from the map showing their correspondence. Secondary processing is performed only for the secondary dimension. 2 for primary processed products
Secondary processing is performed to produce a secondary processed product, and the matte tooth thickness of about 5 to 7 cutting teeth of the secondary processed product is measured again. If the measured dimension of the matagi tooth thickness is compared with the corresponding drawing dimension, and coincides, the secondary processed product becomes a gear that satisfies the drawing dimension. If they do not match, the secondary machining is repeated and gear cutting is performed on the gear that satisfies the drawing dimensions.

The positioning dimension of the gear positioning jig 202 of the hobbing machine 201 thus dimensioned during setup is mechanically stored, and the stored dimension is reset during mass production to perform gear cutting. To be done.

A plurality of hobbing machines 201 are installed to manufacture gears of various specifications from a forged material. The hobbing machine 201 gears the intermediate material W1 by gear cutting to produce gears. The positioning of the gear positioning jig 202 in which the intermediate material W1 for gears is set is performed for each hobbing machine 201.

[0014]

However, in the conventional gear cutting method, since the positioning of the gear positioning jig 202 is unknown, at least 1 is obtained by gear cutting the primary processed product.
After performing the secondary machining more than once, the gear positioning jig 20
Since the positioning dimension of 2 is determined, the measurement and processing steps need to be performed twice or more, and the work of setting up the positioning dimension of the gear positioning jig 202 is troublesome.

Further, there is a problem that skill is required for the positioning work of the gear positioning jig 202 and that the setup work time is long. The present invention has been made to solve the above-mentioned problems, and an object thereof is to provide a gear in a group management system for a hobbing machine that can easily perform positioning of a gear positioning jig with one setup. It is to provide a positioning method for a positioning jig.

[0016]

The invention according to claim 1 is
Arrange a plurality of hobbing machines that have a positioning jig for gears that can reciprocate and stop on a saddle, and a hob cutter that cuts the gear material positioned on the positioning jig for gears to make gears. In the hobbing machine, positioning the gear positioning jig on the saddle with respect to the hob cutter respectively.In the hob machine group management system positioning method of the gear positioning jig, the radius of the hob cutter was measured and this was measured. By adding the radius dimension of the hob cutter and the radius dimension on the drawing of the gear, the center-to-center distance from the center position of the hob cutter to the center position of the gear is calculated, and from the predetermined reference point on the saddle to the center position of the hob cutter. The position of the gear to be machined is calculated by subtracting the center-to-center distance from the reference dimension, and the gear position where this gear is set is calculated. The position of the determined jig and correcting by adding the correction amount set in accordance with the specific properties of each hobbing.

According to a second aspect of the present invention, in the first aspect of the invention, the reference line at which the hob cutter and the gear contact each other is:
It is characterized in that it is a tangent line where an imaginary cylindrical surface including the apexes of the cutting teeth of the hob cutter and a root circle including the root of the gear contact.

According to a third aspect of the present invention, the machining dimensions of the gear made by gear cutting the gear material set in the gear positioning jig obtained by calculation on a specific hobbing machine, and the gear It is characterized in that a correction amount peculiar to the hobbing machine is obtained based on the difference from the drawing size.

[0019]

In the present invention, a hob having a gear positioning jig that can be reciprocated and stopped on a saddle, and a hob cutter that gear-cuts the gear material positioned by the gear positioning jig to make a gear. In the positioning method of the positioning jig for gears in the group management system of the hobbing machine, which arranges multiple boards and positions the positioning jigs for gears on the saddle with respect to the hob cutters in each hobbing machine, the radius of the hob cutter Is calculated, and the measured radial dimension of the hob cutter and the radial dimension of the gear on the drawing are added to calculate the center-to-center distance from the center position of the hob cutter to the center position of the gear. The position of the gear is calculated by subtracting the distance between the centers from the reference dimension from the point to the center position of the hob cutter. It is corrected by adding the correction amount set in accordance with the properties of each hobbing.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. 1 and 2 show a hobbing machine group management system according to an embodiment of the present invention, and FIGS. 3 and 4 show hobbing machines used in the hobbing machine group management system.

As shown in FIGS. 1 and 2, from a cylindrical forging material W, a gear G (FIG. 3) is passed through a series of processes including a cutting process A, a gear cutting process B, a deburring process C, and a shaving process D. Is generated). The specification of the gear G is multi-branched, and FIG. 2 shows three gear lines L1, L2, L3 for making gears G1, G2, G3 having different specifications.

In the cutting step A, the outer skin of the cylindrical forging material W is cut by a lathe. The cut intermediate material is hobbing machine 1, 2, 3, 4, 5 in the gear cutting process B.
Gear cutting is performed by the gears G1 and G2 of various specifications.
G3 is built. These gears G1, G2, G3 are finished by a deburring process C and a shaving process D.
The hobbing machines 1, 2, 3, 4, 5 in the gear cutting process B are
Centralized control is performed by the overall control device 6.

According to FIGS. 3 and 4, hobbing machines 1, 2, 3, 4,
The hobbing machine 1, which is one of the five, will be described as an example.
In the drawing, a column 8 is installed on the bed 7, and a hob shift device 10 is provided on a hob saddle 9 which is vertically movable on the column 8. The hob shift device 10 has a hob cutter 101 (having the same structure as the conventional example and the same reference numeral) supported by a horizontal hob shaft 11.

A saddle 12 is provided on the bed 7, and a table 14 of a gear positioning jig 13 is mounted on the saddle 12.
Is mounted so that it can reciprocate in the left-right direction in the figure. A support arbor 15 is fixedly attached to the upper surface of the table 14. On the upper end of the support arbor 15 is set an intermediate material (a material of the forging material W that is cut off by cutting) as a work.
6 is positioned and held by a front end portion 17A of a support arm 17 which is movably supported in the vertical direction. In order to display the amount of movement of the gear positioning jig 13,
A display plate 16A is attached to the support 16. The display board 16A of each hobbing machine 1, 2, 3, 4, 5 is connected to the personal computer 18 of the general control device 6.

FIG. 5 is a block diagram of the personal computer 18 in the general control device 6 shown in FIGS. 1 and 2. The personal computer 18 moves the gear positioning jig 13 of each hobbing machine 1, 2, 3, 4, 5. It is designed to calculate the quantity.

In FIG. 5, the personal computer 1
Reference numeral 8 includes a storage unit 19 and a CPU 20. The storage means 19 stores work data 21 storing specifications of the gear G and data corresponding to the specifications, and cutter data of the hob cutter 101 storing specifications corresponding to the gear G and data corresponding to the specifications. 22, hobbing machines 1, 2, 3,
The correction amount data 23 in which the correction amounts unique to 4 and 5 are stored are stored.

The work data 21 is necessary for calculating the bottom circle of the gear G, and the work No. , Tooth right angle module, tooth right angle pressure angle, number of teeth, twist angle, outer diameter, cutter used. Numerical values representing the specifications of the gear G are entered corresponding to each item.

The cutter data 22 is necessary to calculate the root circle of the gear G together with the specifications of the work data 21. , The tooth right angle module, the tooth right angle pressure angle, the attendant dam on the drawing, the tooth thickness, the lead angle, and the number of threads, corresponding to each item, the numerical values representing the specifications of the hob cutter 101 are entered.

These data are entered in the database in advance by the SUB1 in FIG. 8 and run in the storage means 19 of the personal computer 18 by running the main program shown in FIG. 7 on the personal computer 18.

The CPU 20 stores a positioning calculation means 24 for the gear positioning jig 13 and a root radius calculation means 25 for the gear G. Gear root radius calculation means 2
To the input side of 5, the gear selection means 26 for specifying the gear G is connected, and the work data 21 of the storage means 19,
The cutter data 22 is input.

At the input side of the position calculating means 24, the NO. The hobbing machine selecting means 27 for specifying the above, the hob cutter radius measurement value inputting means 28, and the root radius calculation means 25 are connected, and the correction amount data 23 is input.
In the correction amount data 23, a correction amount specific to each hobbing machine 1, 2, 3, 4, 5 for correcting the positioning of the gear positioning jig 13 is written.

A movement amount display means 29 is connected to the output side of the position calculating means 24. Moving amount display means 29
From the display board 16A, a signal is sent from the display board 16A to the operator while watching the contents output on the display board 16A.
It is designed to operate 2, 3, 4, and 5.

In this embodiment, however, the intermediate material is set on the support arbor 15 of the gear positioning jig 13 and positioned and held by the tip 17A of the support arm 17.

In the hobbing machine 1, the intermediate material set on the gear positioning jig 13 is located away from the hob cutter 101, and the gear positioning jig 13 is brought closer to the hob cutter 101 and stopped. At this time, the hob cutter 101 is in a lowered state, and the outer peripheral portion on the side of the intermediate material is located directly below the intermediate material. At the same time as rotating the intermediate material in the horizontal plane, hob cutter 1
01 is raised while rotating around the hob shaft 11,
After the hob cutter 101 reaches the uppermost point, the hob cutter 101 is lowered to subject the intermediate material to gear cutting.

The specifications of the gear G made of the intermediate material are determined not only by the specifications of the hob cutter 101 but also by the position of the gear positioning jig 13. For example, if the position of the gear positioning jig 13 is close to the hob cutter 101,
The depth of the gear G increases.

The machining positioning of the gear positioning jig 13 will be described with reference to FIGS. 3 and 6 to 9. FIG. 7 shows a main program of the personal computer 18 in the general control device 6 shown in FIGS. By this main program, the machining position of the gear positioning jig 13 of the hobbing machines 1, 2, 3, 4, 5 for continuously machining the gear G is generally controlled in the hobbing machine group management system. Gears with the same specifications have gear lines L1, L2
L3 continuously flows as gears G1, G2, G3,
Gears G with different specifications may flow.

After the initial setting is made in step S1 of this main program, the process proceeds to step S2 and step S2.
Then, the menu is displayed on the screen. Next, in step S3, the personal computer 18 inquires whether or not to execute. If YES is selected, the process proceeds to SUB1 and the procedure shown in FIG. 8 is executed. If NO is selected, the process ends (step S4).

Then, in step S5, the work data 21 is input into the database of the personal computer 18 in advance, and in step S6, the cutter data 22 is input into the database of the personal computer 18 in advance.

In SUB5 in step S7, the correction amount data is input to the database of the personal computer 18 in advance. Next, the SUB1 of step S3 will be described in detail with reference to FIG.

In step S21 of the SUB1, 5 in FIG.
Select one of the two hobbing machines 1, 2, 3, 4, 5. Here, the case where the hobbing machine 1 is selected will be described. In step S22, the correction amount data 23 specific to the hobbing machine 1 set in the hobbing machine 1 is used by the hobbing machine selecting means 27 to determine the position calculating means 2 of the personal computer 18.
Read in 4. The hobbing machine used last time is hobbing machine 1
In the case of, the work data 21, the cutter data 22, and the correction amount data 23 of the gear G that has undergone the gear cutting process last time are read.

In step S23, the work data 21, cutter data 22, and cutter radius of step S22 are displayed on the screen. In step S24, it is inquired whether or not the radius of the gear G to be machined and the radius of the hob cutter 101 are changed. That is, it is inquired whether or not the specifications of the work data 21 and the cutter data 22 are the same as the previous specifications. If the answer is NO, the previous work data 21 and cutter radius can be used, so the flow proceeds to step S29.

If the answer is YES, the process proceeds to step S25, in which the gear No. of the gear G to be machined this time by the gear selecting means 26 is NO. Is entered. Further, in step S26, the work No. The cutter data 22 of the hob cutter 101 corresponding to the specifications of the gear G is read.

In step S27, it is determined whether the cutter radius has changed. When the cutter radius is changed, the process proceeds to step S28, the newly measured cutter radius is input by the hob cutter radius measurement value input means 28, and the process proceeds to step S29. Step S2
If there is no change in the cutter radius in 7 (NO), the process proceeds to step S29. When the cutter radius is changed (YES), the cutter diameter is input (step S28), and the process proceeds to step S29.

In step S29, the positioning calculation means 24 calculates the horizontal distance Y from the reference point 12A of the saddle 12 to the gear positioning jig 13 on which the gear material is set, and the horizontal distance Y from the reference point 12A of the saddle 12 to the hob cutter 10.
It is given by subtracting the center-to-center distance X between the hob cutter 101 and the gear G from the reference dimension L up to the center position of 1 (hob shaft 11).

Here, the center distance X is the measured radial dimension R _H of the hob cutter 101 and the gear G to be machined.
Is calculated by adding the radial dimension R _G on the drawing.

As shown in FIGS. 3 and 6, the reference line P at which the hob cutter 101 and the gear G contact each other is the virtual cylindrical surface 1 including the apex 105 of the cutting tooth 102 of the hob cutter 101.
06 is a tangent line at which the tooth bottom circle C including the tooth bottom G _B of the gear G is tangent.

The position of the gear positioning jig 13 in which the gear G is set is corrected in the correction amount data 23 according to the unique property of each hobbing machine 1, 2, 3, 4, 5. It is corrected by adding the quantities.

Here, the correction amount is obtained as follows.
That is, the gear G is manufactured by subjecting the gear material set in the gear positioning jig 13 obtained by the calculation in the specific hobbing machine 1, 2, 3, 4, 5 to a gear cutting operation. The specifications of the machining dimensions are measured by the Matagi tooth thickness measuring method. Based on the difference between the machining dimension specifications of the gear G and the drawing dimension of the gear G, the hobbing machines 1, 2, 3, 4, 5
A unique correction amount of is determined.

For example, in one of the hobbing machines 1, 2, 3, 4, and 5, the gear material is set on the gear positioning jig 13 which has been machined to the dimension position in the drawing, and gear cutting is performed. When the gear G is cut deeper than the drawing dimension (matagi tooth thickness), the gear positioning jig 13 on the dimension position of the drawing in this hobbing machine has a unique characteristic of approaching the direction of the hob cutter 101. Therefore, in this hobbing machine, the gear positioning jig 13 must be positioned for machining in a direction away from the hob cutter 101 by a predetermined correction amount from the drawing dimension position (regular dimension). The gear positioning jig 1 is placed at the position corrected in this way.
By positioning 3 for the machining position, the gear material is cut to the drawing dimension of the set specifications.

In step S30, the movement amount display means 29 displays the movement amount of the gear positioning jig 13 on the display plate 16A based on the above calculation. When the gear positioning jig 13 is at the position moving from the reference point 7A, the position of the gear positioning jig 13 is read by the magnet scale, and the machining position of the gear positioning jig is set from that position. The amount of movement to the proper position is displayed on the display plate 16A.

In step S31, each data is displayed on the CRT of the personal computer 18. This confirms whether or not there is an abnormality. Step S3
In 2, the data change menu is displayed, and if NO is selected, the process returns to step S31. If YES is selected, "execution" is performed (step S33). As a result, the hobbing machine 1 is driven and the gear material is gear-cut by the gear positioning jig 13 which is calculated and positioned as described above.

Regarding the above correction amount, step S
No. 7 shown in FIG. 9 (FIG. 9), and in step S41, the correction amount peculiar to the hobbing machine used among the hobbing machines 1, 2, 3, 4, 5 is displayed. In step S42, it is inquired whether or not the hobbing machine has been changed.

If the hobbing machine has not been changed (NO), the process proceeds to step SS44 and becomes "execution". That is,
The same hobbing machine correction amount is selected. If the hobbing machine is changed (YES), the process proceeds to S43, the correction amount peculiar to the changed hobbing machine is input, and "execution" is performed (step S44).

According to the above configuration, the hob cutter 1
Measurement of radius R _H of 01 and radius dimension R of gear G on the drawing
The machining position of the gear positioning jig 13 is calculated based on the addition of _G , and the machining position of the gear positioning jig 13 is determined according to the characteristics of each hobbing machine 1, 2, 3, 4, 5. Since the correction is made by adding the correction amount set by the above, the specification of the gear G is determined and the radius R of the hob cutter 101 is set.
_{As long as H} is measured, the machining position of the gear positioning jig 13 of each hobbing machine 1, 2, 3, 4, 5 can be automatically calculated by a computer or the like.

In this way, the machining positioning of the gear positioning jig 13 is performed, the gear material set in the gear positioning jig 13 is gear-cut by the hob cutter 101, and the setup is performed once. The gear G can be automatically created.

In short, it is possible to easily calculate the machining position of the gear positioning jig 13 of each hobbing machine 1, 2, 3, 4, 5 without any skilled work, and to shorten the setup time. There is an effect that can be.

In this embodiment, as shown in FIGS. 3 and 6, the reference line P at which the hob cutter 101 and the gear G contact each other is the virtual cylindrical surface 106 including the apex 105 of the cutting tooth 102 of the hob cutter 101. And the bottom of the gear G G _B
Is a tangent line that is in contact with the root circle C including
The radius of the hob cutter 101 is 1/2 of the outer diameter (the virtual cylindrical surface including the apex of the cutting tooth) and the radius of the gear G is the root radius, but the reference line is not limited to the above tangent line. , The radius of the hob cutter 101 in this case is not 1/2 of the outer diameter, and the radius of the gear G is not the root radius.

[0058]

As described above, according to the present invention,
The positioning jig for the gear that sets the gear is calculated based on the measurement of the radius of the hob cutter and the addition of the radial dimension on the drawing of the gear, and the machining position of this positioning jig for the gear is calculated according to the characteristics of each hobbing machine. Since it is corrected by adding the correction amount set according to, determine the specifications of the gear,
If only the radius of the hob cutter is measured, a computer or the like can automatically calculate the machining position of the positioning jig for the gear of each hobbing machine.

Therefore, it is possible to easily calculate the machining position of the gear positioning jig of each hobbing machine without any skilled work and to shorten the setup time.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a hobbing machine group management system according to an embodiment of the present invention.

FIG. 2 is a production flow chart of a gear to be processed in the embodiment of the present invention.

FIG. 3 is a side view of a positioning device for the hobbing machine.

FIG. 4 is a side view of a hobbing machine.

FIG. 5 is a block diagram showing a method of calculating the position of the table of the hobbing machine.

FIG. 6 is an explanatory plan view showing the reference lines of the hob cutter and the gear when calculating the position of the table of the hobbing machine.

FIG. 7 is a flowchart of a main program of the same calculation method.

FIG. 8 is a flowchart of a first subroutine of the main program.

FIG. 9 is a flowchart of a fifth subroutine of the main program.

FIG. 10 is a front view of a hob cutter.

FIG. 11 is a side view of the hob cutter.

FIG. 12 is a perspective view of the hob cutter.

FIG. 13 is an enlarged cross-sectional view of cutting teeth of the hob cutter.

FIG. 14 is an explanatory diagram of a conventional gear machining method.

FIG. 15 is an explanatory diagram of a conventional gear machining method.

[Explanation of symbols]

1 Hobbing machine 2 Hobbing machine 3 Hobbing machine 4 Hobbing machine 5 Hobbing machine 6 General control device 12 Saddle 12A Reference point 13 Gear positioning jig 101 Hob cutter L Reference dimension X Center distance X G gear _RH Hob cutter radius R _G Gear drawing Top radius dimension

Claims (3)

[Claims] 1. A plurality of hobbing machines each having a gear positioning jig that can be reciprocated and stopped on a saddle, and a hob cutter that gear-cuts a gear material positioned by the gear positioning jig to make a gear. The hob cutter radius is measured by the positioning method of the gear positioning jig in the group management system of the hobbing machine in which the positioning jigs for the gears are positioned on the saddle with respect to the hob cutters on each hobbing machine. By adding the measured radial dimension of the hob cutter and the radial dimension of the gear on the drawing, the center-to-center distance from the center position of the hob cutter to the center position of the gear is calculated, and the hob cutter from the predetermined reference point on the saddle is calculated. The position of the gear to be machined is calculated by subtracting the center-to-center distance from the reference dimension to the center position of the gear. The position of the gear positioning jig is corrected by adding a correction amount that is set according to the unique property of each hobbing machine. Positioning method of. 2. A reference line of contact between the hob cutter and the gear is an imaginary cylindrical surface including apexes of cutting teeth of the hob cutter,
The positioning method of a positioning jig for gears in a group management system for a hobbing machine according to claim 1, wherein the tangent line is in contact with a root circle including a root of the gear. 3. A difference between a machining dimension of a gear made by cutting a gear material set in a gear positioning jig obtained by calculation on a specific hobbing machine and a drawing dimension of the gear 2. A positioning method of a gear positioning jig in a hob group management system according to claim 1, wherein a correction amount peculiar to the hob group is obtained as a basis.