Selection of materials for precision machined parts should be held to a higher standard than just  “cheapest price per pound.”

Here are 4 questions to help decide if a grade of steel (or other material) is appropriate:

  1. Is it economical in assuring a satisfactory end product?
  2. Does it provide sufficient safety factor for the properties called upon in the design?
  3. Does it provide the most economical means of production?
  4. Is it the lowest cost raw material?

The order in which these questions are asked is critical. Answering number 4  first puts the entire company in jeopardy for product liability lawsuits if the first three questions are ignored in the buyer’s holy quest for cheapest raw material price.

In steel, with many possible carbon and alloy grades, qualities, and finishes (cold drawn, turned and polished, ground and polished, or combinations of these) the end use is of particular importance in arriving at the grade, type, and quality of the steel.

Human safety critical components (airbags, anti lock brakes, climbing equipment) require different thinking than parts for less critical applications where failure is merely inconvenient, not life threatening.

Components for expensive machines and production equipment also fall into this category, where the failure of a part purchased under the assumptions of “false economy” result in extensive downtime of a very high value production asset.

Once the suitability for the end use and safety factor as designed has been determined, then the suitability of the material for the production method becomes the next selection criteria. In high volumes of relatively simple parts,  for example, very low carbon, plain carbon steel is the appropriate choice using cold heading. If the volumes are not there, attempting to use this same steel on screw machines would result in inferior finishes and far more expensive parts than if a free machining grade of steel were chosen. Selecting for Manufacturability can help lower the total cost over the entire supply chain as well as for the final consumer.

The final criteria then becomes transactional cost. But even this is more than just dollars and cents- it is both dollars and sense! Is the supplier a legitimate source? Do they have statistically controlled systems? Do they have a mature quality system that has demonstrated it’s strength over time? Do they limit their number of suppliers so that you will not be subjected to the full range of variability of inputs possible in an increasingly global,  interconnected world?

If buying for mere cheapness was the point, we could replace all purchasing agents with third graders. By third grade, most kids know which number is larger, and which is smaller.

It might take some effort to get them to choose the lower number, though...

The professional value that purchasing adds is by establishing and  following a process that assures an optimum outcome for the entire value chain, not just one part of it.

Photo credit CNN

Machinability is one of those words that everyone uses but everyone also seems to have a different meaning.

Nudge, Nudge. Know what I mean?

Here is a look at just a few of the aspects of what that person you are talking with might have in their mind when you say “machinability.”
1) Surface feet per minute. High surface feet per minute equates to fast cycle times. Fast cycle times mean lots of finished parts per hour. Thus surface feet per minute equals machinability. (But too high surface feet per minute can mean premature tool failure and higher costs and downtime).
2) Tool life. Rapid tool wear is a sign of poor machinability. Long tool life equals better machinability. (Too long tool life can mean overpaying for tools or too slow cycle times).
3) Ability to hold surface finish and close tolerance. If you are constantly fighting the setup to keep the finish acceptable or to hold the specified tolerance, you are not experiencing “good machinability.”
4) Uptime. If the doors are open and your operators head is in the machine and his backside is pointing out, you aren’t making parts. Downtime equals not so machinable.
So what are the units of machinability? Is machinability measured in surface feet per minute? Tool Life? Surface finish or tolerance? Machine uptime?
In order to measure anything, you first have to have units with which to measure.
May I humbly suggest that the proper units of machinability are parts produced by the end of the shift, conforming to print, and requiring the least amount of operator intervention to produce at the quoted cost?
Only when we agree on this definition can we get a meaningful discussion between Purchasing, “I want the cheapest material.” Operations, “If you gave me better tools or material I could get this job running.” Engineering, “Why can’t you guys hit the cycle time, we figured that job ourself?” And Management, “Why can’t you guys hit plan? I buy you everything you want…”
The value that shop management adds is to facilitate the organization’s arrival, together, to the optimum state for the shop to produce given the resources available. To do that everybody needs to be on the same page.
When we’re talking about machinability, that page ought to read “parts produced by the end of the shift, to print, and requiring the least amount of operator intervention to produce.”
So how do you define machinability?
Have you seen the tragic results of a department maximum that cost the rest of the organization dearly?
(Not at your current employer, of course!
” Nudge. Nudge. Know what I mean?”