What’s it gonna be? Feed or Speed?

For a given production rate of metal removal, better tool life is obtained by using heavy feed and low speed.
 Sorry, Flash.
Less horsepower per cubic inch of metal removal is required for heavier feeds (see the diagonal lines on the chart below.)

This also means  fewer revolutions of the work (or tool) to get the job done.
This reduces wear on the tool.
Slower speeds results in less friction, less heat.
Surface finish declines as feed rate  increases, but  it is usually acceptable until a critical rate is reached (see  the numbers along the curves above- they are the values for surface finish in RMS).
In steels, grades that are rephosphorized and renitrogenized can take heavier feeds than steels that are not. (That’s why I’m showing C1213 at 0.07-0.012 phosphorous compared to C1215 at 0.04-0.09 Phos.)
Here is another graph to illustrate the effect of feed rate and surface finish.

As feed rate increases bottom (horizontal) axis so does surface roughness (vertical) axis measured in RMS.
The contract shop industry remains seduced by the siren song of speed to reduce cycle time.
Perhaps the proper use of the feed approach can make you some new friends among your customers…

These data are based on HSS tools. Obviously using carbide one needs to have sufficient speed to take advantage of the carbide.
Bottom Line: Increased feed rather than speed can result in longer tool life and less problems than increasing speed and  dealing with the heat that results.
What is your approach? Speed for cycle time? Or feed for  minimizing HP for removal and longer tool life and fewer problems?
Feed or speed? What’s it gonna be?
Photo credits:
The Flash: http://www.ramasscreen.com/wp-content/uploads/2009/07/Flash-Adam-Strange-Aquaman.JPG
The Incredible Hulk: http://keneller.typepad.com/photos/uncategorized/2008/06/14/lou_ferrigno_as_incredible_hulk.jpg
Playstations’ genius image of  Finger of the Hulk beckoning link: http://www.sparehed.com/wp-content/uploads/2007/03/ad-hulk-playstation-2-2006.jpg

Surface finish issues are especially critical in aerospace and medical applications. Chips recontacting the work and high or unstable Built Up Edge  (BUE) are the usual suspects of poor surface finish on machined parts, regardless of material.   There can be other factors, such as a poorly maintained machine or exhausted metalworking fluids, but these are seldom the case when “the last job on this machine ran just fine.”

Surface finish is critical on precision machined parts.
Surface finish is critical on precision machined parts.

Here are our 5 tips  that you can address on the machine to make poor surface finish go away:
1) Increase the speed SFM (especially on Carbide!). This will help reduce BUE.
2) Reduce the feed per revolution (IPR- inch per revolution). This will help reduce the flank wear.
3) Increase the top rake angle.
4) Add a chip breaker / chip curler.
5) Increase tool nose radius.
We have seen increasing speed to be especially helpful on aerospace and medical machining jobs on stainless steel. Increasing speed is also important when using carbide- carbide likes speed.
If you can see that the chip is recontacting the workpiece,  then address your chip control issues first. Chip control  is the first place to start. Adding  chip control geometry on the tool is  probably the easiest change on non CNC machines.  Modifying the cam to break the chip should also be considered.   On  CNC’s, adding chip breaks into the program is also an easy adjustment. These are especially effective if the workpiece is a gummy material.
Built Up Edge (BUE) is impacted by three primary factors: material chemistry (which you can’t change- you already have the material);  surface footage (slower speed means hot chip is in contact with tool longer, creating higher BUE); and tool geometry (the point is to slice or cut, not rub off the material).
Of course, you should make sure that your setup is rigid, your tooling properly seated, your coolant lines  are delivering plenty of coolant  to the tool/work interface, etc., etc.. But these 5 tips are ‘Tools You Can Use’ to improve the surface finish on your problem jobs, including stainless and other aerospace and medical materials.