Superior surface finishes

Getting the right speed and feed to establish a required surface finish is sometimes a guessing game due to so many factors that are required to be “just right”. There are some general rules of thumb like: kick up the surface footage and turn down the feed. Parameters like surface footage, IPM feed and depth of cut are best set at the programming stage and not left for operators to experiment with. Learning the best guesses is very important for an efficient operation.

Tool pressure plays the most important role in a superior finish. The most important factor in increasing tool pressure is depth of cut. Getting this right should be first. We have all tried to sneak up on a close tolerance and missed it by one thousandth and had to take that little cut that made the dimensional tolerance but it totally blew the required finish. A sanding we will go.

A Cracker Jack machinist will calculate tool push off into the final offset and hit the tolerance and finish on the money every time. This comes with experience and a little knowledge about the machine tool that the part is running on. Depending on the hardness of the material a finish cut should be between .015 - .09 of an inch. A properly set finish cut will glaze the material causing it to shine.

Setting the correct RPM’s for the spindle should not be a difficult undertaking. If you are roughing efficiently with good insert life then the same grade of insert should finish well at about a ten percent increase in surface footage.

Tool nose radius plays a role in the final feed rate needed to return a desired finish. Smaller radius equals slower feed but there are limits to how slow you can feed with a given radius. The cutting insert gets to the point where it is no longer shearing the material and it begins to rub, negatively affecting surface finish. At this point a smaller nose radius should be used and the feed can be reduced again. Normally, a .015 nose radius tool at a .002 feed is the bottom of the barrel and different tactics need to be employed.  

By taking a CNMG that has a leading angle of five degrees and laying it down in the tool holder to cause this angle to be almost zero and then feeding inwards the flattened angle on the now trailing edge will “wipe out” the peak in the previous feed line causing an increase in surface finish. New inserts are now available that take advantage of this concept called wiper inserts and they work very well in most applications. Check with your insert supplier for these cutters that fit in standard holders.

Ceramics could be the answer. To get the most beautiful surface finish in a fraction of the time due to greatly increase surface footage employ ceramics in your turning operations. Seems like nowadays everybody knows about them but they just are not used to their potential. Many types and styles are available now. The newest ceramics are a hybrid called whispers and they can cut with coolant keeping the part cool and reduce the chance of fire.