Turning on an HMC

Reducing the labor involved in a setup has a direct effect on the cost of the part, and you also keep your spindle utilization high.

While technologies exist that allow HMCs to perform turning functions, including U-axis heads and index chucks, other technology eliminates the need for such add-on hardware.

Combined with software and control technology, a single machine tool can be used to turn parts of multiple diameters as well as perform the obvious milling functions. Fewer tools and setups reduce machining costs and part handling, and specialized fixturing is not required.

CIM—Canadian Industrial Machinery asked Okuma America Corp. Machining Center Product Specialist Jim Endsley to explain how this operation is accomplished. Here is what he had to say.

CIM: How is a turning operation performed on an HMC?

Endsley: By loading a boring bar in the machine’s spindle, you can perform lathe functions, including turning the OD and ID, threading, grooving, and contour generation.

A common example of a turning functionality reproduced on a horizontal machine is threading. When you thread on a lathe with a single-point tool, what you are doing is synchronizing the spindle RPM with the X and the Z axes to control depth and the pitch of the thread. What happens with our system is that the synchronization of the spindle RPM and the X, Y, and Z axes keeps the tool perpendicular to the surface of the part being cut.

CIM: What type of part is commonly created?

Endsley: A good example is a ball valve. Without some type of technology that can create complex forms, you need to load this part on a lathe and perform the ID bore, then change setups to perform work on the back of the part or on the OD. Then you must move the part to a machining center to mill the faces and create bolt circles. With technology like Okuma’s Turn-Cut system, you can put the part on an HMC and do all of this work in one setup.

CIM: How did this technology evolve?

Endsley: We have been offering this technology for about 10 years now. Our expertise in building multitasking equipment, along with the abilities of our PC-based control, is really the background of the package. We also build our own machine encoders, spindles, and software, and that gives us the ability to be able to control the machine’s function even in the most complex parts.

Many builders are now offering this type of technology, and shop owners always are finding new and unique ways to implement it.

CIM: How important is the machine’s control for this type of work?

Endsley: The CNC is important in every aspect of machining, but it becomes even more important as the part, and therefore the machine’s movements, gets more complex.

However, a control’s processing speed needs to be judged accurately. Alone, processing speed has nothing to do with how fast you can physically do things on the machine. It’s more about how fast the control can talk to the encoder and get an answer back. The faster this talking happens, the more points can be processed on a complicated surface at a cutting speed of 500 IPM and higher.

CIM: What are some of the benefits of this function?

Endsley: The business side of machining is becoming more and more important. Shop owners understand the importance of reducing overhead and labor content in parts. Reducing the labor involved in a setup has a direct effect on the cost of the part, and you also keep your spindle utilization high.

The average spindle efficiency of a vertical machining center today is 25 percent. On a horizontal machine it’s 80 percent. The more you keep the machine in operation, the more money you will make.

Part quality also is improved when you don’t handle a part as much. Also, the chance that an offset is entered incorrectly is now a thing of the past, and that means that scrap production also is reduced.

Essentially, if you have pallets of half-finished work lying around the shop, you need to figure out some way of combining operations.

CIM: What are the requirements of running this type of technology?

Endsley: Over the course of the last 10 years we’ve been able to witness a lot of these machines being installed and run. We’ve also been able to get a pretty good handle on the requirements.

First of all, you need a machine that doesn’t have boxways. The stick-slip on these boxway machines creates marks on the part every time you cross over into a different quadrant during machining. All of our horizontal machines use roller ways, which eliminate stick-slip while enabling high feed rates.

Another requirement is what we call “abso-scales,” a system of guaranteeing accurate positioning.

Finally, you should have a ball screw cooling system. The machine makes so many repetitious movements during the cut that if you don’t have a cooling system, the ball screws will wear prematurely.

CIM: Are there limitations in the types of features that can be created?

Endsley: Yes. There are limitations in terms of the relationship between RPM and bore diameter. As diameters go down, RPMs need to go up to maintain the surface footage, so I recommend that you bore diameters 2 in. and larger.

When you get down to 2-in.-diameter bores, you can start running out of RPM. The reason is that during a Turn-Cut operation, you are making small circles and letting the f dimension (distance between the center of the bar and the tip of the insert) on the boring bar cut the needed diameter. This can be accomplished easily above 2-in. diameters, but not below that size.

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