Chucks and CNC lathes

[itsme]

Hi there,

I have been wondering how chucks on industrial type CNC lathes work.

I was looking at an Emco catalogue and from what I can see the chucks look very similar (externally) to a standard 3 jaw chuck on the average hobby machine. These chucks must, however, be automatic (ie. open and close by themselves). What I would like to know is if they are operated using compressed air and some kind of solenoid valve or if they use something else. I'm not sure how the mechanism would work if it did use compressed air, as extremely high pressures would be needed (I think) to grip the parts being machined and a fail-safe system would be a problem if pressure was suddenly lost. Also, how is compressed air directed to a moving chuck? Does anyone have any technical detail on how these chucks work? I am not at all familiar with this type of machine.

I was also wondering how the tool changers work. Do they use compressed air to index the tool changer? How is the tool changer locked in position for maching?

Finally, does anyone know how the 'powered' tools on the lathe's tool changer are powered? Do they use an electric motor with an elaborate gearing system?

Sorry for all the questions, but I'm keen to find out how all this stuff works...

Thanks
Warren

[miljnor]

There are such things as air operated chucks although all of the air chucks I have seen have been collet style (which don't tend to loosen with higher RPM).

All 3 jaw automatics Ive seen have been Hydralic and they typicaly use 100psi to 300psi, some higher. The thing with air is air compresses so instead of "hydraulicing" the air is "squishy" and will tend to let the jaws back off a little and BAM! goes the part.

[aus-newb]

Hi Itsme

Most commercial cnc lathes use hydraulic chucks (not cheap) they the older type chuckers (no hollow spindle) use a cylinder with a draw bar which move the jaws in & out on a taper. Newer hollow spindle type units are a little more complex than that. Air chucks are also used, they use compressed air to actuate a piston inside the chuck to open and close the jaws. They may also use special wachdog systems to monitor air pressures for safety.

Tool changers can be Hydraulic, air, or electric driven. I have a lathe that has 1 hydraulic and 1 electric operated, it is not uncommon for a turret to use both ie hydraulics to lift and lock the turret, and electric motor to index. There is a good example on the zone of such a home built system which uses this principal. Newer turrets use high speed servomotors to index, and one of mine does 0.2 second index tool to tool and 0.5 s index to tools on opposite side of a 12 tool turret (this is quite scary to watch in rapid). The live tooling on this unit is driven by a servo motor through a drive shaft and a coupling.

The new Morries actually have the servo drive motor inside the turret to drive the tools.

Craig

[Geof]

On the hydraulic chucks I am familiar with there is a large diameter low pressure hydraulic cylinder at the opposite end of the spindle with a hollow piston rod otherwise know as a draw tube passing through the spindle to operate the chuck. The linear movement of the draw tube moves the chuck jaws by means of a sliding taper system where about 15mm of draw tube movement translates into about 1mm of chuck movement so the result is a multiplication of the clamping force. The hydraulic cylinder is supplied through a rotating gland on the end of the spindle so the pressure can be applied even while the chuck is spinning. The chuck has jaw carriers that have jaws bolted to them and these are serrated on a about a 1mm pitch so the jaws can be adjusted to accommodate different diameters. This is needed because of the limited jaw travel as mentioned above. The grip of the chuck as adjustable by varying the hydraulic pressure. Hydraulic chucks can grip internally or externally and on some machines can be opened while the spindle is running to advance bar stock.

Air chucks are quite different; the only style I am familiar with just uses the compressed air to open the chuck while a powerful spring hold the chuck closed when the air is off. The draw tube, taper mechanism and serrated jaws are the same as the hydraulic. Air chucks are more limited in that often they can only grip externally and their grip is not as powerful as a hydraulic chuck. Also they have to be brought to a full stop before opening because the air cylinder is stationary and pushes against a thrust ring on the draw bar.

[itsme]

Hi there,

Thanks for the information, everyone - it is very interesting. There are just a few things I'm not clear on.

It sounds like a lot of the spindles are not hollow. Does this mean that bar stock can't be fed in from the back and the entire part is always on one side of the chuck? Also, if the spindle is hollow, is the mechanism to operate the chuck jaws very similar to the solid spindle version, but with a few differences so that stock can pass through the middle of it?

I also didn't realise that the chucks only had limited travel and the jaws needed to be moved for different diameters. I suppose I am used to messing around at home where every part is different, but when hundreds/thousands of the same part needs to be made, then I guess it's no problem.

Thanks again
Warren

[Geof]

The lathe spindle does have a hole through, that is how the hydraulic cylinder operates the chuck from the far end of the spindle. That is why the piston rod is called a draw tube, it has a hole through so it is possible to feed bar stock through the entire assembly. Naturally the hydraulic cylinder also has a hole; it is an annular cylinder. Because the draw tube is inside the spindle hole the maximum size of bar that can be passed through is limited; a machine with a 2.6" diameter spindle bore will probably only take something like 2" through the hole in the draw tube.

I have seen what are called 'front closing chucks' where all the mechanism is at the chuck end and there is no draw tube. This means the full spindle bore is available but they are not very common.