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

Austenitic Grain Size is a material characteristic that is usually reported on test reports and certification documents for the steel materials that we machine in our shops.
Coarse Austenitic Grain Size is a result of NOT ADDING grain refining elements to a heat of steel. Because these Grain refining elements have not been added, the steel has a “Coarse Austenitic Grain Size.”

Friday, May 16, 2008 (3).max
This is Coarse Grain Austenite. You like it for machining.

Typically this practice is applied to free machining grades such as 11XX and 12XX steels. These steels are sold primarily for their ability to be machined at high production rates.
What does Coarse Austenitic Grain Size imply for the parts that you make?

  1. Better Machinability– Coarse Grained Steels are more machinable and provide longer tool life than Fine Grained Steels. (The elements added to make the Austenitic Grain size fine create small, finely dispersed  hard abrasive particles in the steel)
  2. Better Plastic Forming–  than Fine Grained Steels
  3. More Distortion in Heat Treat- than Fine Grained Steels
  4. Lower Ductility at the same hardness- than Fine Grained Steels
  5. Deeper Hardenability– than Fine Grained Steels 

Coarse Austenitic Grain Size will show up on the test report as an ASTM value of 1-5. Values of 5 and higher are called Fine Grained Steels, and are the result of additions of Aluminum, Vanadium, or Niobium in North American  commercial practice for most Carbon and Alloy steels.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods For Determining Average Grain Size.
A nice discussion can also be found HERE.
While  we think that chemistry may be the controlling factor for machining performance of the steel in our machines, the contribution of austenitic grain size is also important. As long as you are ordering your free machining steels (11XX and 12XX series) to Coarse Grain Practice, Austenitic Grain Size should not be an issue in your shop.

In North America, the AISI/SAE steel grade nomenclature system is widely used.
In this system, 4  numeric digits (XXXX)  describe the base grade. The first two digits tell you whether the steel is a carbon or alloy grade.
If the first digit is any number other than a “1”, that steel is an alloy steel. We’ll discuss alloy steels in a later post.
If the first digit is a ” 1 “, the steel is a carbon grade. 10XX is the template for the plain carbon steels. We’ll explain those last two digits at the end of our post. (Exception: if the second digit is a “3”- then its one of the  alloy manganese 13XX grades- grades we don’t encounter very often these days.)
If the second digit is a “1”, the steel is a resulfurized  carbon steel.  11XX. Guess how many “extra” elements were added to the grade? If you guessed 1- thats right. Sulfur is  the one element added  to promote machinability in the 11XX grades of steel.
If the second digit is a “2”, the steel is called a rephosphorized and resulfurized steel. Both sulfur and phosphorus,-2 elements- are added to make these free machining steels.  1215 and 12 L14 are the grades we mostly see today. (As many of you know, that “L” as an infix tells us that there is a lead addition in the 12L14 steel.)
If the second digit is a “5” the grade is a high manganese carbon steel.  Grades 1524, and 1541 come to mind as the principal 15XX grades seen by our industry.
A “B” infix tells us that the steel has been treated with boron. This makes it especially adept at being heat treated. 15B21 is used  to make fasteners  that are heat treated.
So, now that you know what the first 2 digits mean in a US grade designation for steel, what about the last two?

synthetic diamonds
Diamonds are just a special form of 'carbon'. Same as in steel.

The last 2 digits in the grade are the mean or average carbon content of the steel. In weight percent.  
So grade 1018, is a plain carbon steel, 0.18% average  carbon content.
1144 is a resulfurized 0.44% average carbon content steel for higher strength and machining.
And 1215, well- 1215 is a resulfurized, rephosphorized 0.09 max weight % carbon steel for machining. 0.09% max!
 Don’t you just love exceptions?