Locating Material Properties

When I need to locate material properties I look on the web. The most comprehensive single place I look is MatWeb. MatWeb includes metals and plastics. A useful feature of MatWeb is the ability to locate materials by their UNS numbers which makes ordering the correct grade easier.

For plastic properties only, a comprehensive site is IDES Prospector.

A useful site when I need to make quick rough comparisons between materials is McMaster Carr. McMaster Carr is a large vendor (I highly recommend them). To use their web site to obtain material properties in the find producst box type in your material: steel, aluminum, plastic, copper, etc.... A list of choices for products which include the material in their name will pop up. Near the top of the list will be an item title "About...." Click on the About link and you will get summary info on the material.

For example, if I am interested in stainless steels I type in stainless steel in the find products box. I list of links pops up. The list includes items such as stainless steel flat washers, stainless steel chain, etc.... The second item on the list is "About Stainless Steel." Clicking on "About Stainless Steel" brings up a page comparing the properties of different grades of stainless steels. It includes comparisons of formability, weldability, corrosion resistance, machinability. It also has graphs of yield and hardness for the different grades of steel.



Example of graph of yield strengths from McMaster Carr.

McMaster Carr does not have the extensive databases that the other two sites have but is is useful for quick comparisons.

Seen in the workplace

A poster on an office door:


Cowboy wisdom

Cowboys know that when you discover you are riding a dead horse the best strategy is to dismount and change horses. In business, however, it seems that we try other strategies with dead horses:

1. Buy a stronger whip.
2. Change riders.
3. Say things like, "This is the way we always have ridden this horse."
4. Appointing a committee to study the horse.
5. Arraging to visit other sites to see how they ride dead horses.
6. Increasing the standards to ride dead horses.
7. Appointing a tiger team to ride the dead horse.
8. Creating a training session to improve our riding ability.
9. Change the requirements to declare "the horse is not dead."
10. Hire a contractor to ride the dead horse.
12. Harness several dead horses together for increased speed.
13. Declare "No horse is too dead to beat."
14. Provide additional funding to improve the horse's performance.
15. Purchase a product to make dead horses run faster.
16. Declare the dead horse "better, faster, and cheaper."
17. Form a quality circle to find uses for dead horses.
19. Revisist the performance requirements for horses.
21. Promote the dead horse to a supervisory position.

Unfortunately the option of budgeting for a new horse and feed to keep said horse alive is almost never considered.

Tolerancing software

In keeping with my last post on process capability, here is some software that helps automate tolerancing

What I design is produced in low quantities. Consequently tolerancing does not get close attention. The items that are produced from my designs typically have 100% inspection. When I design I base my tolerances on published handbook values for the production process being used. My sources include Machinery's Handbook, Tool and Manufacturing Engineer's Handbook or Manufacturing Engineering and Technology.

In those references one can find charts and graphs like the one below.


Source: Manufaturing Engineering and Technology, Serope Kalpakjian, 1989.

The problem with these charts and graphs is that they don't give statistical properties. There is no information on the standard deviation of the process.
For me, in designing low production run items, not having the statistical properties doesn't matter. Generally, using the tolerance bands indicated in the charts and graphs produces parts that fit together.

Occasionally, a part might not fit. The cost of an occasional goof, however, is not worth the cost of the analysis it would take to remove that risk. In large production runs it becomes important to know the capability of the process.

The software, Tolerance Capability Expert, I linked to at the top of this article addresses the problem of not knowing the statistical distribution. It queries the user for the process, the size of the part, and the tolerance. It returns a Cp, the capability index for that specific feature.

I am not going to offer a formal review of the package. I tried a demo version of it a few years back but have never had access to the full package. The demo impressed me. For someone designing for large production runs I think it could be truly useful. For me, I didn't think I could persuade my boss or my customers to spring for it. The occasional extra bit of hand fit-up is seen as cheaper than the cost of the software.

Finally, a good reference on capability indices is Measuring Process Capability: Techniques and Calculations for Quality and Manufacturing Engineers by Davis Bothe.

Process Capability

I had some parts made recently that were formed on a press brake. There was one critical dimension, the width of an opening. The specification limits on the width of an opening was 3.13" to 3.19". The limits were decided as most of these things are by a long meeting.

The fabricator made an initial three items to tune the press brake settings. The opening width on the first three items was 3.1870, 3.1880, and 3.1745.

The opening width for the run on the successive items, produced after the press brake program was finalized, are as follows:
3.1765
3.1885
3.1760
3.1785
3.1775
3.1695
3.1880

The mean on those items is 3.1792
and the standard deviation is .0068

We can now calculate the Cpk for the process. Cpk is given by (See the discussion here)



Calculating

Cpk= min(.52, 2.4)

Therefore, Cpk=.52. Another way of looking at this is a Cpk=0.5 gives 6.68% nonconforming fraction. This can be seen in the graph below.




Looking at the mean as opposed to the specification limits we see that the process is not centered within the specification limits. Given the process standard deviation we calculate what the process ought to be capable of doing.



We calculate Cp=1.47. This is pretty good.

Hmmm. A Cpk=.52 and Cp=1.47, it seems that all we need to do is center the process within the specification limits. A few words are in order now.

This was a small run. The fabricator had deliberately decentered the process with my knowledge. There were some other design features that the fabricator was worried about destroying if the opening was too narrow. So we opted to go wide. This resulted in having to inspect every item to ensure that the specification was met.

For a small run such as this it was not a hardship. For a really large run the optimal solution would have been to do some minor redesign to allow the process to run centered. This is a much more robust solution than inspecting each item for acceptability.

Finally, this data was not from a true capability study. I analyzed the data to satisfy my curiosity about the process capability. For a true study I would need to verify that the process was stable and check that the data is normally distributed.

Who knew ?

When working in a domain outside one's experience one should proceed carefully. There could be unexpected surprises

I have recently finished a project designing an airlock for a high temperature furnace (1600 C, the melting point of steel is ~1500 C). The airlock is a commercially available vacuum gate valve. To protect it from the furnace heat flux there is a set of thermal barrier shutters which rest on a cold plate.

The thermal barrier shutters consist of tantalum and graphite sheets supported by tantalum studs. The multiple layers of tantalum and graphite sheets act as radiant barriers. The studs are supported by thermal isolators. The thermal isolators are fastened into copper shutter plates. Tantalum has a high melting point (3017 C) but is fantastically expensive. One of the goals of my design was to minimize its use.

I looked at a number of materials for the thermal isolators: titanium, molybdenum, and tungsten. But not nickel or ceramics. I didn't want to have to consider dealing with brittleness and fabrication difficulties of ceramics. I didn't consider nickel simply through oversight.

I presented my design to my customer. He was satisfied with the design. Then he added as an aside, "By the way you don't have any nickel touching any tantalum surfaces, do you?"

It turns out that nickel and tantalum form a low temperature (for the purposes of this project) melting point eutectic with a melting point of 1000 C. So wherever tantalum and nickel touch at elevated temperatures there is effectively corrosion.

I had no idea. Worse I did not even imagine the problem to investigate this failure mode.

Postscript: I changed the design from that described here. The thermal flux through the studs was as great as that through the multiple sheets of the radiant thermal barrier. The heat flux through the studs raised the temperature of the copper uncomfortably close to its melting point.