10 Tips to Improve Surface Finish

Creating the best surface finish means choosing the proper insert, cutting conditions, technique

Scrapping parts during the finishing stage is an outcome that no shop desires. A combination of proper tools and technique can keep jobs on time and on spec. But, what variables need to be considered before entering the finishing stage? To find out, CIM—Canadian Industrial Machinery asked experts John Mitchell of Tungaloy Canada and Tom Hagan of Iscar Tools Canada.

1. Increase the Speed 

Increasing surface feet per minute (SFM) reduces built-up edge (BUE). This will prolong tool life and reduce the chance that catastrophic tool failure will damage a finished part.

2. Reduce the Feed

Reducing the inch per revolution (IPR) will reduce flank wear and also prolong insert life.

“When roughing, it is best to use a tool capable of a high feed to remove material quickly. When finishing, it is generally best to have a light depth of cut and conservative feed rate,” explained John Mitchell, general manager, Tungaloy Canada. “However, if the depth is too light, it may cause the tool to rub and not cut. A 0.020-in. depth of cut is good for finishing in manyapplications.”

3. Increase the Top Rake Angle

Rake angle is a variable in the insert’s design that can be tailored to achieve the best surface finish.

For example, when you’re using a 45-degree lead face mill, the cutting force will act downward at 45 degrees, perhaps causing the part to flex after the cutter has passed over the part. This in turn causes the back half of the cutter to recut the machined part, resulting in a negative impact on surface finish. A 90-degree cutter, however, creates cutting forces parallel to the part and willnot flex it, thereby producing a better finish.

“A lead angle will ease the insert into the material,” said Mitchell. “The lead angle starts to cut at the strongest point on the insert, unlike leading in with the nose radius. It will also help to lead out of the part. However, tool pressure is a consideration. If the part you are milling is thin-walled, flimsy, or poorly fixtured, a lead angle may have detrimental effects on the surfacefinish.”

4. Use a Chip Breaker

Chip breakers can reduce cutting pressures and produce chips that can be evacuated more easily. In materials that produce long, stringy chips, a chip breaker can help produce smaller chips that exit the cutting zone quickly and easily.

“Every material is different and choosing a chip breaker for a material is not as easy as opening a catalog,” said Tom Hagan, milling product manager for Iscar Tools Canada.

It is important to examine the stability of the setup, type of milling application, and coolant usage. These are all key factors when choosing a chip breaker.

“When milling a slot, the cutter should be changed rather than the chip breaker for better chip evacuation. If the setup is unstable, then the grade of insert should be changed to a tougher grade,” said Hagan.

Once again, the chance of a catastrophic tool failure occurring has been reduced, and long chips are no longer able to scratch the surface of the part.

Safety also is improved when this so-called birdnesting is eliminated because you no longer need to clear chips from around the tool and toolholder by hand.

5. Use a Large Nose Radius

There is a direct relationship between the size of the insert’s nose radius and the surface finish produced. While it’s true that a smaller nose radius decreases the pressure on a tool, it also limits the feed rate that can be used.

“An insert is capable of feeding only at one-half of the nose radius. Once this is exceeded, the surface produced is similar to a thread,” said Mitchell. “Therefore, use the largest radius possible to produce the best finish and not create chatter.”

A larger nose radius also enables a heavier cut to be made, which is necessary when you’re cutting hard-to-cut materials.

“An insert with a large nose radius can take more punishment,” said Hagan. “This means that tool life can be extended, particularly if you are working with difficult-to-chip materials.”

However, with a large nose radius, more material must be left on the workpiece for removal during the finishing pass.

“If you have an insert with a 0.032-in. nose radius, you can’t perform a finishing pass where only 0.010 in. of material is left on the part,” said Hagan.

6. Use a Wiper

“Use an insert with a wiper is always my first piece of advice when surface finish requirements are important to a manufacturer,” said Hagan. “By using an insert with a wiper, you can create a smoother surface in the milling pass.”

Employing an insert with a wiper also allows you to increase feed rates.

Also, ensure that the correct tool diameter is being used. The cutter should be engaged between two-thirds and three-fourths of its diameter.

7. Use the Correct Technique

Technique also plays a role in achieving fine surface finishes, and creating a chip that is thick-to-thin should be the goal.

“Arc-in and arc-out milling reduces impact on the insert edge because it creates an easy transition as the cutter engages the workpiece,” said Hagan. “The same applies when going into a corner.”

Choose a cutter that is smaller than the radius so you can program the cutter for a smooth transition from line to line.  This will eliminate sharp moves and a sudden dwell in the tool path.

8. Use Different Tools to Rough and Finish

While some experts say that the same inserts can be used for both roughing and finishing, with the roughing being performed by a slightly used insert and finishing with a new one, Mitchell suggests using one tool for roughing and another for finishing.

“Roughing the part could be done with an insert with a large nose radius, large rake angle, and using a rapid feed rate. Then the finishing tool with the desired lead angle and radius could utilize a wiper flat, which flattens the part, giving a better surface finish,” said Mitchell.

Another factor that needs to be taken into consideration is the insert material. In a light finishing pass, a cermet insert often can produce a better finish.

“When roughing, a coarse-pitch cutter can be used for better chip evacuation, and a fine pitch for finishing if the surface finish is important,” said Hagan. “A light depth of cut for finishing is good, but it must be the same or more than the radius. If not, the insert will push the material, rather than cut, resulting in poor surface quality, burred edges , and shortened insert life.”

9. Clear the Chips

The decision whether or not to use coolant is one that is often hotly debated. It usually comes down to the type of operation being performed (such as deep cavity milling), material type, and type of insert.

“Coolant in milling applications or interrupted cuts in turning applications should be avoided,” advised Mitchell. “It causes thermal cracking, shortens tool life, and will negatively affect surface finish. However, in a sticky material such as aluminum, nickel-based alloys, and low-carbon steel, coolant will prevent the material from sticking to the tool.”

10. Check the Toolholding and Workholding

“One factor that is often overlooked when trying to improve a finish application is the toolholder. If the toolholder is old, and the pocket that holds the insert is worn, the insert may move. Any movement of the insert will create chatter and result in poor surface finish,” said Mitchell.

Chatter caused by improper toolholding and fixturing, or by a machine tool that is not rigid, will create nothing but problems.

Rigid, stable workholding is also key. And, the higher the metal removal rate, the more important stable workholding becomes.

www.iscar.ca

www.tungaloyamerica.com

About the Author
Canadian Metalworking

Joe Thompson

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Joe Thompson has been covering the Canadian manufacturing sector for nearly two decades. He is responsible for the day-to-day editorial direction of the magazine, providing a uniquely Canadian look at the world of metal manufacturing.

An award-winning writer and graduate of the Sheridan College journalism program, he has published articles worldwide in a variety of industries, including manufacturing, pharmaceutical, medical, infrastructure, and entertainment.