Gantry Type Machine Center

if the components we make is small, we can use normal size machine tool.  If we need to manufacture quiet large parts, such as jet wing, that a large size machine tool is needed. This large size machine tool is so called gantry type machine center, especially to make molds of car, fuselage, turbine engine components, wind generator parts, large parts of power plant. We can say that if we want to do well at heavy industry, this kind of large machine tool is essential.

The appearance of this  machine tool is as Fig 1 shown:

 

Figure 1. Gantry Type Machine Center schematic sketch 

 

A common adult aside this type machine tool, proportion is as Fig 2 shown, this makes readers to know how big is this kind of machine tool.

 

Figure 2. Proportion of gantry type machine center to an adult

 

The workpiece is set on the base to do machining job, as Fig 3 shown.

 

Fig 3. Schematic sketch of machining

 

Because of nowadays the workpieces of aerospace and automobile are mostly with complex streamline shape, this type of machine tool must be capable to cut complex streamline shape, therefore, this kind of machine tool is mostly 5-axis machine tool. Besides XYZ linear moving axis, 2 rotary axes are also equipped on the spindle, as Fig 4 shown.

 

Fig4. Rotating direction of spline.

 

 

This time we introduce a company in Taiwan that manufacturing this type of machine tool, it’s one of the few in Asia that firstly using Linear Motor at  large size machine tool.

 

We need to describe the difference between linear motor and traditional motro. A nornal traditional motor, as same as everyone had learned in junior high physics & chemistry class. There is a fixed SN maget, centre shaft is an electromagnet, as Fig 5 shown.

 

Fig 5. Schematic sketch of traditional motor

 

When the power is on, the magnetic pole of electromagnet and fixed magnet are the same, the repulsive force is generated, then the shaft will rotate, as Fig 6 shown.

 

Fig 6.Rotating of shaft

After rotating half round, we found out that magnet and electromagnet are in different polarity and attract each other, if motor were in this situation, it had not keeping working, as Fig 7 shown. Therefore, at this moment, the current through electromagnet will change its direction and make polarity of electromagnet changing, to keep repulsion so that motor can continue rotating. This is the principle of motor working. For certain, there are plenty kind of motors and their unique working way, we just save words for this.

 

Fig 7. Motor Rotating principle

 

Furthermore, how normal motor transform rotary motion to linear motion? Readers could refer figures below. Fig 8 is saddle, sliding guideways, carriage, ball screw, coupling and motor of machine tool.

 

Fig 8. Carriage assembly of machine tool

 

The way of operation is as descripted below, motor and ball screw linked firmly by coupling to make them rotate synchronously, as Fig 9 shown.

 

Fig 9. Coupling links motor and ball screw

 

the block upon ball screw is assembled  with carriage, carriage is put on bearing surfaces of two parallel sliding guideway, as Fig 10 shown.

Fig 10. Block, carriage and sliding guideway

 

When motor rotates, ball screww will also rotate, thread of ball screw will push the block so the block will  move along the axial direction of ball screw. Because block is assembled with carriage, carriage will move together as Fig 11 shown.

Fig 11. Motor motion transform 

 

But the principle of linear is different from normal motor, it is like maglev train, fully arrange magnets along its railway, as Fig 12 shown.

Fig 12. Arrange of magnets

 

Carriage is equipped with electromagnet as Fig 13 shown.

Fig 13. Carriage 

 

Fig 14 is assembly of magnetic railyway and carriage

Fig 14. Assembly of railway and carriage

When power if on , electromagnet will generate magnetic force. Because of same polarities, there is repulsive force between carriage and railway so that the carriage will float as Fig 15 shown.

Fig 15. Float situation

 

At this moment, it is not stable between carriage and railway, it needs tiny force to trigger carriage, the carriage will start to move as Fig 16 shown.

 

Fig 16. Carriage moving

While carriage moves, we just need to change the current direction of electromagnet, polarity will also change to avoid carriage and railway suck together and keeps carriage moving as Fig 17 shown.

 

Fig 17. Changing polarity of electromagnet

 

If we need to brake, then we make electromagnet and railway different polarities and suck together as Fig 18 shown. 

Fig 18. Electromagnet brake

The above is the simple introduction of linear motor, the practical operation is much more complex.

 

Compare with linear motor and normal motor assembled with ball screw, linear motor gets advantages as blow:

 

First, motion transform mechanism is not needed, therefore the mechanical structure of machine tool is much simpler. Simpler structure means it reduces tolerances of assembling and moving.

 

Second, the motion of linear motor is non - contact so that it doesn’t produce friction wear and heat. For precision machine, heat will cause thermal expansion, that will make machine loss precision. 

 

Third, the velocity of linear motor  is at least 2 times faster.

 

But linear motor also has its drawbacks,

First, it is expensive.

 

Second, it is highly environmental requirement during assembling or it will lose its precision.

 

Third, you need a group of well-skilled technicals to assembly it.

 

For this kind of large size machine tool, temperature changing rapidly affact. If there is 1 ‎°C changes, for 1m long iron bar , the deviation of length will be 0.01mm. 

Readers don’t underestimate this 0.01mm deviation, for many aerospace parts, their tolerance of size  doesn’t exceed 0.05mm. For this kind of large machine tool, 1°C  deviation will probably cause more than 0.05mm deviation. Thus, this kind of  large machine tool must be assembled, installed and utilized in well-controlled environment.

 

Because this kind machine tool is firmly large, it can not be packed in ocean shipment container, so when it is tested done in machine tool factory, it is disassembled and then packed into containers. Til the machine arrived at customer’s factory then re-assembled and test it. So it also needs a group of technicals at customer’s factory to do installation jobs.

Fig 19 is parctical disassembling of machine tool.

Fig 19. Disassembling of machine tool

 

To manufacture such high precision and large size machine tool, engineers must be good at mechanical, material, electrical and control specialities, it also needs a group of well-skilled technicals to do machining and assembling missions.

 

This company is proud of having a group of excellent technicals, the manager of this company also emphasized such machine tool can’t produce without them.

 

Hope everyone figure out that this large size machine tool can’t be transport overall unit, it shall be disassembled and then reassembled by technicals after arrived destination.

If there is some defects occurs during assembling, the machine tool will be wasted. So the technicals of this company are fairly experienced as well as well-educated. They are all able to speak English, or they can’t communicate with foreign customer. it is regret that the education reform of our country has almost eliminated institutes of technology, because it needs good technicals to make expensive machine tool.

 

The amazement of this company is that their machine tool has reached the upstream supply chain, and produce critical components of aerospace. All the more astonishing is, this company is not rely on cost reduction, but on its abundant application techniques, skilled technicals and highly flexible customized services. It’s really precious in Taiwan’s industrial field, where is full of vicious circle of price reduction competition. We should be glad that we have such good company, and should encourage these engineers.

 

There is one more thing worthy for us to be glad, the linear motor is also made in Taiwan