The Art of Milling
New milling tools, combined with proper techniques, can improve efficiency and productivity while reducing costs
April 1, 2010
Removing material quickly is the key to most milling operations.
And most of the material removed is done so during the roughing process, which is the most important part of any milling job.
“The roughing stage is where you can really save time during milling,” explained David Vetrecin, rotating tools product manager for Iscar Canada. “It is during this time that most of the material is removed and finish requirements don’t yet come into play. Finishing operations, depending on the required finish, take time to complete, and you simply can’t rush them.”
Decisions on tooling selection and the movement of these tools will always depend on the application.
“Decisions on what tooling to use should come even before the initial programming is performed,” said Vetrecin. “Time should also be spent figuring out the fixturing for the part and learning what restrictions this may place on the milling tool. For example, you may have to use a tool with a longer reach, depending on how the part is fixtured.”
Fixturing, in fact, plays a large role in any milling operation. If a part can be fixtured in such a way that enables multiple operations in one setup, cycle time is reduced, as is labor and the chance for error. On the other hand, repeatability and accuracy are reduced each time a part is fixtured.
With today’s economic climate and small margins, shops need to be more productive in order to land jobs. Improved milling operations can lead to reduced cycle times, improved quality, and lower costs per part.
According to Vetrecin, the three main areas that should be examined before milling begins are:
- Setups and fixturing. Cycle time and quality both depend on proper fixturing.
- Rigidity. The more rigid the setup, the heavier depths of cut can be made, feed rate can be increased, and tool life can be extended as a result of less vibration.
- Process. An examination of programming techniques, setups, and machine specifications can determine proper tool choices.
“If the customer is committed to improving their cutting process on a certain job, it is something that we can certainly help with. We are seeing more and more requests for process improvement rather than simply requests for inserts,” said Vetrecin. “There are a lot of milling tools on the market, but it is your application that should decide which to use and no other factor.”
Specialty Tools
Special tools are popular in higher-volume jobs, especially in the automotive sector. Saving three or four seconds per part on 1 million parts can mean a lot to a manufacturer. However, in certain job shop environments that typically have low-volume, high-variety parts, special tools may not provide much benefit.
“In a high-volume application, special tools make perfect sense,” said Vetrecin.
While special tools often are used in a drilling operation to drill and chamfer in one operation, they sometimes can benefit a milling operation as well. In slotting, for example, you can easily add a chamfer at the top of the slot with a special tool and take only one pass rather than two passes with two different tools. Tool change time is reduced, as is cycle time.
Standard inserts also have a benefit in milling, however.
“A lot of times we simply have to get our customers to think of different solutions than they may be used to,” said Vetrecin. “For example, I have seen customers who have been going through inserts at a phenomenal rate, and my suggestion was to machine dry. It was just something that they hadn’t considered, and now they are seeing six times the tool life.”
Today’s coatings are designed to run hot, not cycle between hot and cold. All of the heat is designed to escape with the chip while the coating remains hot during cutting. Adding coolant to the equation cools the work environment, counteracting the benefits of new coating technology, which results in thermal cracking of the insert.
However, if your material demands it, for example with titanium and INCONEL® alloys, you still have to run wet. This is because the material being cut is abrasive and the cutting speeds are so low. The coatings don’t have time to get hot in these situations.
“Even stainless steel we try to mill dry now,” said Vetrecin.
Milling Techniques
How you mill is as important as the material that you mill and the tooling you mill with.
“How you enter and exit a part is very important in milling,” explained Kevin Lorch, project manager for Sandvik Coromant. “It’s simply the golden rule of milling. When you are generating a chip, you want it to go thick to thin. If you generate a thick chip on entry and a thin chip on exit, then your cutter is in the best position to protect the longevity of the insert.”
Sandvik recently has been touring the country to educate manufacturers on milling techniques in its Modern Art of Milling show.
“Modern Art of Milling is not a training course or a seminar about our tools; it is about how tooling can be used correctly to improve productivity and efficiency,” said Lorch.
In today’s machining world, being more efficient and productive than the competition will help land new work and keep existing work. While tooling is only a small percentage of production costs, how that tooling is used can have a dramatic effect on the amount of time and money that can be saved. This can be accomplished by the correct tooling choices but also, and in some cases more dramatically, through milling techniques.
“You have to get everything out of your tooling that you can,” said Lorch. “That’s what the Modern Art of Milling is all about. It’s not about speeds and feeds, it’s about how to use tooling in a more efficient and productive way.”
From milling basics such as positioning the cutter at 70 percent engagement to create a thick chip on entry and a thin chip at the exit, to avoiding entering the cut along the centerline of a part, many milling techniques can directly affect tool life.
“When you have 70 percent engagement, you create chips in this thick-to-thin manner, which means removing the maximum amount of metal while at the same time allowing the cutter to ease out of the cut. This will increase tool life and, therefore, productivity,” said Lorch.
Entering the cut correctly, moving the tool while in the cut correctly, and using the correct lead angles are all important aspects of milling.
For example, by employing a roll-on technique rather than simply entering straight into the part, you reduce vibration, which increases tool life.
“Typically, when people program, they feed straight into the part, which causes vibration. The operator will then usually turn down the feed to counter the vibration and turn it back up when the tool is fully into the cut,” explained Lorch. “Adding the roll-on technique, which eases the cutter into the cut, can make a dramatic difference in the tool life and the amount of time it takes to complete a part.”
Change Existing Programs
By spending the time to change programs, manufacturers can extend tool life. It’s no longer really about specific tools; it’s about how the appropriate techniques can be applied.
“As an industry we need to work together to ensure that every job possible stays in this country, and by applying the proper milling techniques to your work, you can enable that,” said Lorch.
These tooling techniques can be added individually or as part of a larger productivity improvement program. By putting a productivity improvement program in place, manufacturers can examine their process as a whole and try to improve each aspect.
“During a productivity improvement session, we try to touch on every part of the process, including the machine tool, tooling, labor, and material,” said Lorch. “While we as a tooling supplier can’t affect what a shop pays for its material, we can help them reduce their scrap rate and the number of nonconforming parts.”
For more information, visit www.iscar.ca and www.coromant.sandvik.com.
blog comments powered by Disqus
