A Look at Milling

Milling is evolving, and the same techniques and tools used even five years ago may not be cutting it anymore

August 1, 2010

New milling tools and techniques can aid a shop's bottom line.

Increasing the toughness at the cutting edge of the tool and new ship former geometries can add strength to the tool.

If the recent economic stagnation has taught us anything, it’s that smart companies are examining every part of their business structure and looking for ways to save money. The slow and steady upward trends that we are seeing now will continue only if shops take a long, hard look at their manufacturing processes.

For example, no single part of the milling equation is more important than another, but there are ways to save both time and money in each step along the way.

Getting Rough

“Roughing is where you can save the most amount of time during a milling operation,” said Iscar Canada’s Rotating Tools Product Manager David Vetrecin.

During this stage the tools can be pushed to the maximum to achieve the most amount of metal removal. Removing material as quickly as possible shortens the time it takes to run a job. This material removal is best accomplished with the latest in milling tools, which make deeper depths of cut with higher feed rates compared to those of even a few years ago.

“New tools are designed for a higher metal removal rate compared to older tools,” said Vetrecin. “Higher cutting speeds can also be achieved thanks to new coating technology on the latest incarnation of milling tools. Our SumoTec process, for example, increases the toughness at the cutting edge of the tool, and new chip formers can add strength to the tool because of their geometry.”

Pocket geometry also is important to successful milling because it helps lock the insert into place. Dovetail clamping, for example, allows negative geometry inserts to be used during the roughing stage. Previously most roughing was done using positive inserts, according to Vetrecin.

The only restriction to reaching very high speeds in milling comes from the fixturing.

“If the fixturing is solid enough, you can usually push your machine as hard as the tool can take,” said Vetrecin. “Fixturing and roughing really go hand-in-hand. You have to design the fixtures to be able to take the forces that roughing will place on them.”

Finishing Stage

“The success of the finishing operation starts way back when you first start your metal removal,” said Walter Tools Head of Product Development Thomas Benjamin.

The finishing process in milling is an important stage because the mistakes made here will most often produce scrap. So much time and money have been invested in the part by the time it reaches the finishing stage that scrapping it can have catastrophic effects on profitability.

The correct style of tool for the material being cut will break the chip correctly, making ti much easier to evacuate that chip.

“If you make a mistake in the previous stages, such as roughing and semifinishing, you can often correct it without totally scrapping the part,” said Benjamin. “In the finishing stage, however, you can often find yourself in a situation in which you cannot rework the piece-part to save it.”

However, this problem is potentially worse than just a scrapped part; it could lead to more business-related problems. Not only does scrapping a part at this final stage mean that you will invariably eat the cost that is already in the part, you may also end up spending more by having to work overtime, and you may even miss a delivery.

Success in the finish stage usually occurs when the job is prepared correctly in the roughing and semifinishing stages.

“You need to leave the correct amount of material on the part for finishing pass or passes to remove,” said Benjamin. “Also, the correct tool is needed to create the surface finish that is required.”

While Benjamin calls one-pass finishing the most cost-effective method, it is not always possible. If the correct tool can finish a part in one pass and obtain the needed finish, the process is more economical because it uses less machining time, with less tooling costs, and less labor.

“If you can get the correct finish with your indexable tooling, you may be able to eliminate a postmachining grinding operation, but this is totally dependent on getting the required finish,” explained Benjamin.

Typically, today’s finish cutters have a number of features built-in to make this possible. They include the ability to adjust the inserts in the cutter both radially and axially. Also, wiper stations using wiper inserts can greatly improve the type of finish that is possible.

Tooling suppliers also sell various finish cutters for specific materials, with the design of the cutter being based on the properties of the material being cut.

“New types of titanium are being used in the aerospace industry as well as the medical industry, and a new type of cast iron is now being used in the creation of engine blocks,” said Benjamin. “This leads to more research and development work at tooling suppliers in order to get customers the finish requirements that they need.”

The correct style of tool for the material being cut will break the chip correctly, making it much easier to evacuate that chip.

“Chip evacuation in the finishing process is critical,” said Benjamin. “You don’t want to be recutting chips, and you don’t want the chips rubbing across the surface of the workpiece. To be able to evacuate the chip from the work zone either through a blast of air or coolant is very important.”

Coolant Usage

Proper coolant flow means improved tool life and maximum cutting speeds.

Mechanical cutting generates frictional heat, and the rate at which the tool absorbs heat increases as velocity increases. To optimize performance, tools must be adequately cooled. Proper coolant flow means improved tool life and maximum cutting speeds. If not properly cooled, the insert will heat up rapidly. This can shorten tool life and affect surface finish because of built-up edge (BUE), where the workpiece material bonds to the cutting edge.

One way to battle this problem is by channeling coolant through the insert.

Kennametal recently developed the Beyond BLAST™ system that delivers coolant at the interface of the milling cutter and insert.

“In tests involving titanium turning, [these] inserts at 100 PSI were showing 75 percent improvement in tool life over the same inserts with flood coolant at 100 PSI,” said Kennametal’s Senior Product Manager Osny Fabricio. “In a different test to evaluate the influence of coolant pressure, tool life for Beyond BLAST at 100 PSI was nearly that of identical insert geometries at 1,000 PSI.”

No matter what type of coolant-delivery system is employed, reduction of heat and increased lubricity at the point where the tool meets the workpiece is necessary to maintain the surface finish.

Tooling Storage

Automated tool storage systems can help control inventories, understand tool usage, and ensure that necessary items will always be available.

Before and after the milling operation takes place, tools are not being used. However, this does not mean that process optimization cannot be applied.

“The inventory value that today’s shops have is far greater than 15 years ago,” explained Seco Manager of Business Solutions Magnus Tillman.

By employing an automated tool storage system, companies can control inventories, understand tool usage, and ensure that necessary items will always be available.

“You will now know that you will not run out of an 80 cent screw that holds an insert down in a mission-critical operation,” said Tillman. “I think that [this type of] system can be implemented at any kind of customer that is interested in reducing inventory costs and tracking their usage and has a need to restrict access to parts.”

A tool management system also can help companies reduce their inventories.

“The goal is to turn the inventory 20 to 25 turns per year,” explained Tillman. “Most metal cutting shops turn their inventories five to 10 turns per year. Systems like our SecoPoint will also be able to give access to certain parts to certain operators on the shop floor.”

The reports from tool management systems are very detailed. They can help identify areas where tool usage is high.

“The SecoPoint™ system, for example, will also inform the customer when inventories do not move at all, and then suggest removing inventories and replacing them with new items that will be used,” said Tillman. “As mentioned before, we will be able to free up some inventory-carrying costs that the customer can use in other areas.”

For more information, visit www.iscar.ca, www.walter-tools.com, www.kennametal.com, and www.secotools.com.