Rapid Prototyping - not just for prototypes

We - the group at my work - purchased a rapid protoping machine a few years back. The machine, a Dimension 3D printer, makes parts out of ABS plastic. We do use it for making sample prototypes to show customers. Our main use however is to make functional parts for actual use.

Parts made with it include electronic boxes, mounting brackets, drilling jigs, joystick controller handles. Some sample parts are shown below.




The parts in the photo are clockwise from the top: A sector of a gear for a belt drive mechanism, a locating bushing, an alignment tool, a sensor mounting bracket, and a tubing mounting bracket. All of them except the gear sector designed by me. All of the parts shown went into field use.

A more sophisticated use of the rapid prototyping machine is using it to fabricate molds. Molds are required when an elastomeric part is desired or if the part is to have metal inserts. Below is a picture of a mold and the parts molded in it.



The part molded is an elastomeric mounting bushing for supporting some equipment. The material used in the mold is polyurethane.

A good source for casting material is McMaster Carr. Search for two part casting compounds. Be sure to coat the mold with mold release prior to filling with casting compound.

Arghh.....

I designed an airlock for a high tempereature furnace this past winter. One of the key components is a cooling flange that fits between the furnace and a vacuum gate valve.
The cooling flange is made of copper. It has a cooling channel cut into it with a copper plate welded over the channel. There are additional features for sweep gas connections, mechanical connections for actuating thermal shutters, and bolt holes for mounting the gate valve.

This piece has been one of the most trouble plagued parts I have ever tried to get fabricated.

The first problem was the cooling channel was cut too far. The machinist programming the machine didn’t pay close attention to the drawing. The channel ended up intruding into the area where the holes for the sweep gas were to go. Rather than forcing the fabricator to start over I redesigned the part rerouting the sweep gas porting.

The reasons for redesign instead of starting over were it maintains good will with the fabricator and the time required to get a new billet was not insignificant.

After the redesign the next step was to weld on the cooling channel cover plate. The cover plate warped leading to concerns about leakage. So it was machined off and the surfaces dressed for another attempt at welding.

I asked to be notified when the second attempt at welding was going to be attempted. I wanted to witness the welding. I don’t know what went wrong the first time but I thought that by being present it would discourage any attempts at shortcuts. The full welding procedure is as follows: clamp cover plate on billet over the cooling channel, preheat to 800 F, tack weld, reheat to 800 F, weld while pausing to reheat billet as needed. I watched the welding and it proceeded according to procedure.

A few days later I got another phone call: the weld had porosity in it which was uncovered when machining on the weld to form a sealing surface for an O-ring. In addition a counterbore on a large through hole formed oversize. The oversized counterbore leaves some components exposed to the furnace atmosphere and greatly increases their heat flux.

At this I finally threw up my hands and said we need to start over on this piece.

What I am reading

Creating and Maintaining a World Class Machine Shop: A Guite to General and Titanium Machine Shop Practices, Edward Rossman, 2007.

As the title implies the book is about machining practice. It is written to the shop owner/foreman and assumes a detailed knowledge of machining practice and shop management. As the title implies a large section of the book is about speeds and feeds for titanium machining, specifically milling.

What are some of the practices leading to a world class machine shop: optimization of cutter life versus machining time, a facility allowing for flexible tool layout (multiple utility drops throughout the facility allowing easy equipment repositioning), tracking of scrap rate (less than 1%), tracking of job costs and time, well maintained equipment, (tracking of spindle runout, .001 TIR desired), tracking of pecentage on time delivery.

A check list for reviewing a shop is presented. The check list items are: initial impressions and greeting prior to going out on the shop floor, clarity of safety instructions, general cleanliness, equipment appearance and condition, in-process job-tracking with appropriate paperwork (heat numbers, operation sign-off, inspection results), operator training, fixturing and set-up. This check list is intended to aid a shop owner improve his shop.

The practices and checklist are useful for an engineer in locating or qualifying a shop to perform work. Perhaps the single most important item is the tracking of on time delivery rates. If a shop tracks on time deliver they probably will pay attention and deliver on-time. My experience has been that any shop that has been in business a while delivers parts accurately made to print. On time delivery, however, is another matter.

So what else did I learn that is useful in the book to the practicing engineer, as opposed to the machinist? The most useful item I found was a discussion relating machine accuracy to required design tolerance. A rule of thumb was presented: The resulting error in machined features is N times the machine accuracy or

error=N X machine accuracy

N is a number determined from experience. The value of N given in the book ranges from 4 to 10.

So it the machine accuracy of .0002" and N is conservatively taken to be 10 the resulting error in the machined feature will be .002"

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.