To best answer your question, I had reviewed several reference books and the best description regarding "machinability" is defined in the ASM Metals Handbook, Desk Edition.
"Machinability" is loosely defined as the ease with which materials can be machined. In most cases tool life test results are used for comparison. Tool wear rates can be used.
AISI Type 1212 steel is normally selected as a reference standard and assigned a value of 100. Other materials are compared based on the ratio of tool life obtained with the material and the tool life of the reference material in a standardized test.
Other criteria for rating machinability can be machining forces, power consumption, cutting speed, chip form and achievable surface finish.
My take on this is that as long as you use a consistent test method for ranking material, this can be used to define "machinability".
The evaluation of Machineability compromises innumerable variables but basically there is a relationship to economics which maybe expressed as:
"Cubic inches of metal cut or net pieces per hour at maximum productive efficiency combined with minimum rejections of the end product for surface finish and/or tolerance"
As stated by metengr there are a tremendous number of variables to consider.
The basic evaluation is against cold drawn bars of B1112 turning at a surface speed of 180 ft/min with feeds from .0003" to .0040" with HS tools.
unclesyd,
Did you mean to say comprises insteaed of compromises?
heavymember,
How much do you weigh? Just joking. There is a thread on the Industrial -- Manufacturing/Machines & Machining engineering forum about machinability. My take on it is that currently the art outweighs the science when it comes to choosing cutting tools. As mentioned above, there are many variables and one thing to be carefull about is carbide insert manufacturers claims. There is not anything basically wrong with the ratings as a starting point for broad brush comparisons.
As a metallurgist/toolmaker I use the published (Machining Data Handbook) data as a starting point and derate it but because of all the variables -- rigidity of setup being a prime one -- each part of a paricular material becomes a test especially if you are talking high production.
You mention a point that I was made aware of many years ago.
When it comes to machining "You can make a little part on a big machine, but not a big part on a little machine".
Like you, we had to develop/adopt tables for almost every production machining job we undertook. It was a never ending quest to stay on the curve of cost vs production.
There is no miracle tool form or cutting fluid, there is one. That is until the next iteration comes around.
Several years, a steel supplier told me of a machinability test developed by Volvo, I believe. Basically, you take the steel sample in questiion and run it on an instrumented milling machine until you generate a certain weight of chips.Then the power usage required for the milling is factored into an equation to give a machinability index. I could find only one place in the U.S. to perform this test and it was too expensive to be practical. I thought it was a good concept.
Thank you for excellent information on this subject. My Ryerson stocklist shows machinability ratings. Does every manufacturer come up with a list of their products' machinability? If this is the case, are they comparable from one manufacturer to another?
They are generally comparable but you have to look at what the parameters are on each list.
Look at the materials feeds and speed tables from your supplier of a particular product. If the tables have a reference check it out.
A lot of times the number will vary based on how the material is produced. Say for bars, hot rolled, cold rolled, high temperature drawing, etc.
The tables should help you get in the ball park and with all machining operations to get the maximum productivity and efficiency the operation will require tweaking
