Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
39Accuracy Improvement Technology ofMAF-C Floor-type Horizontal Boring MillsSATOSHI FURUTATE*1

KENJI KURA*1

HIROYUKI TAUCHI*2

TOMOHARU HASEGAWA*2

MANABU KAMIJO*2

SHINYA FUJIHARA*3

Horizontal boring mills, which have a protruding boring spindle and ram, have an inherent
problem: deflection due to their own weight and changes in the barycentric position causing
deformation of the machine body, and as a result the straightness and perpendicularity of each axis
vary. Although existing machines have accuracy compensation mechanisms, deformation increases
further in processing with an attachment. In such cases, deformation is resolved by operating
methods when accurate machining is required: for example, table feeding is used instead of ram
feeding. However, some machines have no table feeding function. Thus we have developed new
movement accuracy compensation technology in order to increase the machining accuracy ofhorizontal boring mills to a level nearly equal to that of double column boring mills.

|1. Introduction
Infrastructure industries are growing worldwide, in particular in China and India. Along withthis trend, demand for construction machines, power systems, industrial machines and pressing
machines is increasing. A horizontal boring machine is required for machining medium and large
parts, and the enhancement of cutting ability and accuracy is called for in order to improve
productivity.
MHI has been responding to such requests by offering the MAF-RSC series. Reflecting thecurrent increase in demand, however, the development of a machine with higher productivity thatcan lead the industry further is required. So we have enhanced cutting ability and pursued therealization of a machine that deforms little, even when an attachment is mounted, through theinstallation of new accuracy compensation mechanisms. Accordingly, we have created the MAF-C
floor-type horizontal boring mill, which attains the highest performance level in the world.
This document describes the details of the technology that resulted in the accuracy
improvement described above.

|2. Machine configuration and problems of horizontal boringmachines
2.1

Machine configuration

A horizontal boring machine consists basically of a bed, column, saddle, ram and spindle asshown in Figure 1. The X axis is the longitudinal travel of the machine on the bed. The Y axis isthe vertical travel of the saddle moving along the column. The Z axis and the W axis are the inward
and outward travel of the ram and the boring spindle, respectively. The saddle is connected by awire to the balance weight placed in the column through the pulley on the top of the column tocancel its own weight. Each axis is accurately driven by a ball screw or rack and pinion.
The workpiece is set on the floor plate installed on the front side of the machine and cut byeach axis operation. When a rotary table is mounted, the workpiece can be turned and then its backside, etc., can be cut.
*1

Manager, Engineering Department, Machine Tool

*3

MHI Machine Tool Engineering Co, Ltd.

*2

Engineering Department, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)40

Figure 1 Configuration ofhorizontal boring machine

2.2

Figure 2

Problems of horizontal boring machine

Problems of horizontal boring machines

Because horizontal boring machines have the spindle positioned horizontally, the jutting out
movement of the ram causes accuracy degradation due to the following factors as shown in Figure 2.
(1) Bending of the ram due to its own weight(2) Deformation of the saddle sliding surface, bending of the column, and sinkage of the bed
caused by changes in the barycentric position due to the movement of the ram(3) Leaning caused by changes in the barycentric position when an attachment is mounted
Horizontal feeding without compensation will experience degradation of finished surface
accuracy and perpendicularity between the Z axis and the Y axis as described above. Accordingly,
the accuracy compensation function is essential.

|3. Conventional accuracy compensation method
Our existing MAF-RSC machine uses a full balancing function as a countermeasure to theproblem. This function consists of the three compensation mechanisms shown in Figure 3: themoment balance weight for canceling changes in the barycentric position due to the movement of
the ram, the ram barycenter suspending mechanism for maintaining a certain bending of the ram
regardless of the protrusion of the ram, and the ram suspending position moving device forcanceling changes of moment when an attachment is mounted. These conventional compensation
methods have the advantage that they can fully cancel even changes of moment on the base and thebed, and consequently accuracy stabilizes regardless of the positions of the Y axis and the Z axis.
However, the ram cannot be made smaller and fine compensation specific to each attachment isdifficult.

Figure 4

Figure 3 Current compensation
mechanisms

Ram tension bar

Figure 5 Compensation for saddle suspending force

On the other hand, typical floor-type horizontal boring machines use the ram tension bar
compensation shown in Figure 4 and the saddle suspending force compensation shown in Figure 5
as compensation mechanisms. Ram tension bar compensation is a mechanism that cancels theMitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)41

bending of the ram due to its own weight, using moment generated by the tension bar located at theupper part of the ram. Saddle suspending force compensation maintains the saddle horizontally,changing saddle suspending force in response to changes in the barycentric position of the entire
ram and saddle due to the movement of the ram.
When these mechanisms are used, however, the bending of the column changes in principle
depending on the position of the Z axis, and accordingly the straightness of the Y axis fluctuates in
response to the position of the Z axis. In addition, accuracy degradation caused by sinkage of thebase and the bed due to barycentric changes in the entire machine during the movement of the ram
cannot be compensated. So these mechanisms have the problem where the straightness of the Yaxis and perpendicularity between the Y axis and the Z axis vary and the swing accuracy of thespindle degrades depending on the position of the Z axis.

|4. New compensation system and its effect
The MAF-C aims to make possible a compact ram that cannot be realized by the fullbalancing function and attain the same or higher accuracy. Previously, degradation of accuracy due
to bending of the column and sinkage of the base and the bed could not be compensated for by ram
tension bar compensation and/or saddle suspending force compensation. The MAF-C takes
advantage of our FEM analysis technology to eliminate such a weak point using the following
methods.

4.1

Compensation for bending of the column (column tension bar compensation)

As shown in Figure 6, changes in the barycentric position due to the leftward and rightward
movement of the ram is balanced by changing the suspending force of the right and left sides of thesaddle. Because the balancing wire connecting the saddle and the balance weight runs through thepulley on the top of the column, however, the force on the top of the column varies when thesuspending force of the column is changed. This bends the column, causing accuracy degradation.
To compensate for such bending of the column, we have developed and employed for MAF-C acolumn bending prevention mechanism, installing a tension bar in the column to cancel themoment generated by changes in the saddle suspending force.
This mechanism reduces the bending of the column depending on the position of the Z axis,
and accordingly the straightness of the Y axis and swing accuracy of the spindle are stabilized
regardless of the positions of the Z axis and the Y axis.

Figure 64.2

Force impressed on top of column

Compensation for sinkage of the bed and the base (spatial accuracy
compensation)

When the ram moves leftward and rightward, the barycentric position of the machine bodychanges, causing the bed and the base to sink. As a result, the Y axis leans from the reference
horizontal line depending of the position of the Z axis. In spite of the employment of a design witha higher rigidity for the minimization of such sinkage, complete elimination is difficult. Because

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
42

this accuracy degradation can be compensated for easily by measuring the error vector on the Y-Z
plane, however, we use spatial accuracy compensation, which performs measurement andcompensation with NC, to ensure higher accuracy.
Spatial accuracy compensation enables accurate cutting, even with an attachment mounted,
because the force on each tension bar and spatial compensation amount are controlled dependingon the machine coordinates and the tension amount and spatial compensation amount are set foreach attachment mounted.

4.3

Effect of new compensation system

Figure 7 shows the measurement of displacement in the direction of the Y axis using thecompensation method described above. The displacement shown is 3 μm/1000 mm for theprotruding ram without an attachment and 6 μm/1000 mm for that with an attachment. So thevariation is very small (3 μm/1000 mm).

Figure 7

Measurement of displacement in direction of Y axis

Figure 8

Right angle head finish machining

Figure 8 shows an actual workpiece finish-machined with a right angle head. The machine
cut the workpiece in the direction of the X axis, moving the Z axis at a pitch of 210 mm from theposition where the Z axis juts out 1000 mm (the distance from the gauge line is 1550 mm). The

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)43

unevenness of pitch height was 2 μm at the most, so the effect of the new system wasdemonstrated.

|5. Conclusion
The MAF-C has accuracy compensation mechanisms that represent a different approach tothe status quo. We have realized such a machine due to our extensive analysis technologies. TheMAF-C is a world-leading machine that has improved accuracy, cutting ability and rigidity,enabling even deeper cuts. We will continue to offer horizontal boring machines that provide forcustomer productivity improvement using the expertise obtained in the development of thismachine. Table 1 compares the specifications of the MAF-C with those of competing machines asa reference. We have set specifications that can respond to any customer needs: the stroke of the X
axis is 5000 to 21000 mm, and that of the Y axis is 3000 to 5000 mm.
To make it possible to cut deeply in a small bore, the overall stroke of the Z axis and the Waxis is extended to 2250 mm, and the ram size is reduced from 600-mm-square to 420-mm-square.
Although the ram is more compact, its rigidity is increased from previous models using a ductile
cast iron material and four-plane static pressure support. As a result, this machine can take fulladvantage of the spindle output of 55/75 kW. In addition, the thermal displacement in the Z axis
direction caused by the rotation of the spindle can be compensated for using ATDS (Advanced
Thermal Displacement Suppression System) thermal deformation compensation technology.
When compared to competitors, this machine has attained the best specifications in its class.
In addition, we have obtained an advantage over competitors due to the compensation systemdescribed above.
Table 1Specifications comparison table

min

φ150
*φ180
420×42055/75*80/100
2500

MAF-RSC(existingmachine)
φ150
*φ180
600×600
30/37*37/452000mm

5000-21000

3000-20000

6000-18000

4000-mmmm

3000-5000
1250

2500-4500
1100

3000-5000
1100

2500-3500
1200

mm

1000

900

900

1000

mm/minmm/min

20000
20000
15000
*20000

15000
15000

10000
10000

20000
20000

10000

6000

20000

MAF-C
Boring spindle diameter

mmRam sizemmSpindle output

kW

Spindle speed
X axis travel(column longitudinal)
Y axis travel (saddle vertical)
Z axis travel (ram in/out)
W axis travel(boring spindle in/out)
Rapid traverse-1

XY

Z, W mm/min

Japanesecompetitor A

Foreigncompetitor B

φ160

φ180

420×420400×440

45/55

60/84

2000

2500

* Optional