Method for dynamically compensating for tool abrasion in a numerically-controlled machine tool

A method of the invention detects a machining load acting on a tool during a machining process, estimates abrasion amount of the tool based on the machining load and a tool abrasion characteristic registered in advance in a memory, and changes automatically tool position compensation amount based on the tool abrasion. The estimating computation of tool abrasion amount is performed by, for example, obtaining in advance a tool abrasion coefficient based on tool abrasion amount per unit time when a main spindle load current value is taken as a reference machining current value, and integrating for a machining time the product of a ratio of a main spindle load current value during an actual machining process to the reference current value and said tool abrasion coefficient.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention is described in detail in the 
following and in the accompanied drawings. FIG. 1 shows an example of a 
numerically-controlled (NC) apparatus implementing a method for 
dynamically compensating for tool abrasion according to the invention. 
This numerically-controlled (NC) apparatus, which is a computer numerical 
control (CNC) system, comprises a CPU 1, a ROM 3 for storing a system 
program and the like, a RAM 5 as a working memory for temporarily storing 
various data, a RAM 7 for storing a machining program, tool data and the 
like, a servo controller 9, a main spindle controller 11, an input part 13 
of a tape reader, a ten-key pad and the like, and a display 15. As 
illustrated in FIG. 1, the parts are all connected with one another by a 
bus 17. 
The servo controller 9 is connected with a servo amplifier 19, and the 
servo amplifier 19 is connected to a servomotor 21. The main spindle 
controller 11 is connected to a main spindle motor 23 for turning a tool 
(not illustrated in the drawing). 
FIG. 2 shows a concrete composition of a numerically-controlled (NC) 
apparatus implementing a method for dynamically compensating for tool 
abrasion according to the invention. This method is performed by the CPU 1 
which executes a system program. 
This numerically-controlled (NC) apparatus comprises a machining program 
reading buffer 31 for reading a machining program. A 1-block analyzer 33 
analyzes the machining program block by block. A target position computing 
part 35 computes a target position of a tool length compensation axis 
based on machining program analysis. A tool length compensation amount 
storing area 37 stores a tool length compensation amount. A tool length 
compensation computing part 39 performs compensation of the target 
position of the tool length compensation axis computed by the target 
position computing part 35 based on a tool length compensation amount 
registered in the tool length compensation amount storing area 37. An 
abrasion compensation amount computing part 41 estimates tool abrasion 
compensation amount (tool abrasion) per unit time based on a reference 
current-value, a main spindle load current value and a tool length 
compensation amount. A tool abrasion compensation amount storing area 43 
stores a cumulative value of tool abrasion compensation amount computed by 
the abrasion compensation amount computing part 41. A tool abrasion 
compensation amount variation integrating part 45 computes variation of a 
tool abrasion compensation by subtracting the last cumulative value of 
tool abrasion compensation amount stored in the tool abrasion compensation 
amount storing area 43 from a new tool abrasion compensation computed by 
the abrasion compensation computing part 41. A moving distance computing 
part 47 computes a moving distance to a target position of the tool length 
compensation axis after compensation of a tool length. A tool abrasion 
compensation computing part 49 subtracts a tool abrasion compensation 
amount when executing a block from the moving distance to a target 
position of the tool length compensation axis. A moving distance per unit 
time computing part 51 computes a moving distance of each axis at 
intervals of a unit time (a sampling time) specified corresponding to 
kinds of machines. Finally, a tool compensation performing part 53 
subtracts in real time a variation of a tool abrasion compensation amount 
computed by the tool abrasion compensation amount variation integrating 
part 45 from a moving distance computed by the moving distance computing 
part 51. 
A cumulative value of tool abrasion compensation quantities is computed by 
the following expression in a range of proper use of a tool. 
EQU Tool abrasion compensation amount=k(main spindle load current 
value/reference machining current value)dt (1), 
where k is the abrasion coefficient of a tool, and the abrasion coefficient 
k is obtained by performing a machining process under a proper condition 
and measuring (actually measuring) a machining abrasion amount per a 
certain unit time as taking a main spindle load current value as a 
reference current value. The abrasion coefficient k is stored in the RAM 7 
of the numerically-controlled (NC) apparatus. 
Tool abrasion compensation amount (a cumulative value of tool abrasion 
compensation quantities) obtained by the expression (1) is a value 
obtained by integrating for a machining time the product of a ratio of a 
main spindle load current value during an actual machining process to a 
reference machining current value and the abrasion coefficient k of a 
tool, and is a value obtained by estimating tool abrasion. 
The above-mentioned computation of tool abrasion compensation amount is 
performed in real time during a machining process, and the compensation is 
performed in real time as subtracting a tool abrasion compensation amount 
from a spindle moving distance. 
As an example, tool length compensation is described in the following. When 
performing the tool length compensation, as shown in FIG. 3, the data 
provided are tool length compensation amount storing area for storing 
compensation amount of tool length for each tool to be used, a tool 
abrasion coefficient storing area for storing the abrasion coefficient of 
each tool, a reference-machining current storing area for storing a main 
spindle load current value at a time of measuring the abrasion 
coefficient, and a tool abrasion amount storing area for storing tool 
abrasion (tool abrasion compensation amount) for each tool. 
As to the tool, first, tool length compensation amount A of the tool is 
read from the tool length compensation amount storing area and the tool 
abrasion amount storing area. Then, when the 1 block which has been 
analyzed is actually executed, tool abrasion amount B of the tool is read 
and the tool length compensation is performed by the following expression: 
EQU Total tool length compensation=(tool length compensation amount A)-(tool 
abrasion amount B) (2). 
After a machining process has been started using the tool, an actual tool 
length compensation amount is adaptively changed by updating tool abrasion 
amount B in the expression (2) with the latest tool abrasion amount B 
after computing the tool abrasion amount B (tool abrasion compensation 
amount) in real time by the expression (1). The tool length compensation 
amount computed by the expression (2) is reflected in the block next to 
the current block being currently executed. 
A particular embodiment of the invention has been described above in 
detail, but the invention is not limited to this and it will be apparent 
to a person related to this industrial field that other various 
embodiments can be implemented within the scope of the present invention.