The Patternmakers Art: Innovation within a Timeless Tradition
With a few exceptions, architectural metal castings are made using hollow cavity sand and clay molds. The molds are made by compacting the sand around patterns which look like the casting being made. The molds are negatives and the patterns are positives.
All foundrymen know the importance of a pattern, because the casting can never be better than the pattern being used to produce it. It is the beginning of the process. Patterns are the transformation of ideas into reality. Consequently, the pattern shop is usually a popular place, staffed with highly skilled craftsmen who have intricate knowledge of materials and the machinery necessary for precision patternmaking.
In the U.S. the Civil War created a period of substantial growth in the iron foundry industry. At the conclusion of the war, many of these foundries moved into architectural castings offering a wide variety of products for the home, garden and commercial buildings. This period, known as the golden era for Victorian cast iron architecture, and the resilience of cast iron itself, is the reason so many examples remain to this day. Many foundries used catalogs, or pattern books, to make sales.
Patterns would often begin as sketches or renderings. These drawings would be carved in wood by hand to create master patterns. The master patterns were usually used only once to cast working patterns in iron. The working patterns were hand-tooled in the sand molds by skilled molders who, in many cases, poured their own molds with molten iron. In later years, as labor rates increased, metal matchplates were developed to speed the molding process.
In the 80’s the used of plastics for patternmaking became more predominant. The urethane based plastics were a lighter and less expensive option than metal matchplates. Multiple plastic impressions could be mounted to an inexpensive wood board for foundry use. The use of urethane plastics continues to this day.
The Mechanics of Patternmaking
All patterns used for sand castings have a parting line. The parting line is the point at which the two halves of the sand mold are separated, allowing the removal of the pattern without tearing the mold walls. The minimum angle for a parting line is three degrees. Since all molten metals shrink as they solidify, patterns must be larger than the casting being produced. For cast iron, the shrinkage rule is one-eigth inch per linear foot. Consequently, patternmakers must use special shrinkage rulers so that the castings will be dimensionally correct. In order for molten metal to flow evenly into the hollow cavity in the sand mold created by the pattern, a gating system must be designed and attached to the pattern board. The gating system is critical to casting quality. The most typical gating system consists of a long runner with in-gates feeding the casting. The form used to create the sand mold is called a flask. One of the patternmaker’s functions is to co-ordinate the flask sizes with the foundry. This is important because the flask size relative to the pattern size determines the yield of the mold, which is a very large factor in the cost of the casting (ref. Fig. 10).
Recent technological advances have moved the time honored craft of patternmaking into the digital age. The use of three dimensional computer generated models to create patterns is now possible. One way to generate the model is through traditional AutoCad programming. AutoCad models are converted to “G” code using a program called MasterCam. This code is then loaded into a three dimensional router to be cut in solid material (ref. Fig 11). The preferred material is urethane based Renshape tooling board.
Another way to generate the model is through the use of laser scanners. The procedure involves the attachment of 2mm positioning targets to the object being scanned. Much like GPS, the scanner uses triangulation to keep its position in space during the scan process. The scanner renders real time models viewable on the computer screen enabling the operator to monitor the scan quality (ref. fig.12). Once the scan is complete the acquisition software converts the surfaces into an STL file. This file is then imported into RapidForm, a powerful reverse engineering program, to turn the scan into a functional 3D model. The model can be adjusted for shrinkage after it is exported to MasterCam just prior to cutting in a 3D router (ref. Fig. 13).
There are numerous advantages to digital patternmaking and laser scanning. The scanner is accurate to within .002 inches. The process is non-destructive and eliminates the need for physical samples. This is especially desirable for sensitive historic properties. Custom patterns can be stored on disc which alleviates the need to store the pattern after the casting run is complete. The 3D model manipulation solves the shrinkage problem with the push of a button and patterns can be enlarged or reduced with ease.
In summary, the move to digital patternmaking has disproved the adage “You can’t teach an old dog new tricks”.
The author wishes to thank Blake and son Dakota Owen, patternmakers and experts in scanning and router technology for their assistance in the preparation of this paper. Also, W. Wayne Fuller for his assistance with the images.
Striegel: Scotty Howell is the Vice-President and General Manager of Robinson Iron and guides the day to day operation of the firm and the assessment of conditions to the removal, engineering, and installation of restored materials. Scotty launched Robinson Iron in 1975 as a spinoff of the Robinson Foundry Inc. A graduate in engineering from Georgia Institute of Technology, Scotty holds a Masters in Business Administration from the University of Chicago and is part owner of Robinson Iron. He learned the foundryman’s craft from the bottom up so that he could be familiar with the requirements of each demanding job. Over the past thirty-three years, his resume of distinguished projects has included such iconic landmarks as the Raffles Hotel in Singapore, the White House in Washington D.C., Vulcan, and Birmingham, Alabama and others too numerous to mention. Scotty.
Howell: Good morning. It’s a pleasure to be here with you today. So first things first. I would like to introduce (walked away from mike, unintelligible). How do I advance it? Okay. Need you back.
So I’d like to talk to you about pattern making. For most all foundrymen, the pattern is the beginning of the process and everyone pretty much knows that the casting that you make can never be better than the pattern you use to make it with. Something is always lost in the process. So the pattern making part is crucial to pattern makers. This goes way back to the late nineteenth century and you can see from this woodcut from Badger’s Foundry, the molding going on in the lower portion of the foundry operation. So the process has not really changed much in terms of the sand molding process. This is an early image taken from the Robinson Foundry floor in 1975, and the gentleman on the far left in the pork pie hat, his name is Mr. Aubrey Brown, was my pattern maker. Mr. Aubrey carved most of his patterns with a pocket knife. He was an amazing man but he was seventy years old at this point and time and there was no one behind him to take up the craft. So I knew I was going to be in trouble pretty quickly unless I did something to preserve the patternmaking craft. So what I did was I managed to apprentice some people to Mr. Aubrey and those people learned the patternmaking craft starting off with just wood and so forth.
Now how do you begin the patternmaking process? Well in many cases you’ll start with a drawing or sketch. That drawing or sketch then would be carved into wood, like these very old wood patterns in our storage. I see that we don’t really do a good job of our housekeeping but these are original wood patterns from the nineteenth century. How do you do that? Well, originally we just carved them by hand. We used hand chisels and white pine, sometimes mahogany. We made our patterns by hand carving. This is an image of some nineteenth century wood patterns also carved by hand on the wall in my office.
Later, foundries began to make patterns out of metal because the wood didn’t last very long, it got burned up in the process. So they began making metal patterns like these. These are loose metal patterns, again, quite old. Many of these date to the 1860′s and 1870′s. Later, they developed metal patterns that would be mounted to boards like this. These were called match plates.
It was about in the late eighties, early nineties that we began to introduce plastics into the world of pattern making and we used hard plastics, urethane based, to create our patterns. We found this to be very efficient to make urethane based patterns like this collar here without loss of detail, and also the urethane patterns were less expensive than the metal patterns. It’s used by the creation of a mold and then you pour the positive, which a pattern is always a positive, the mold is the negative, and we used, again we’d mount plastics to wood boards like these.
Just for a very brief moment, I’d like to talk about the parting line which is a very crucial part of a pattern. There are a couple of things that are very important about patternmaking. One is the parting line is the point at which the pattern is separated, the halves of the molds are separated. It has to be clean. You can see how the parting line is established here on this particular type of pattern. Also, the shrinkage factor is very important because iron shrinks. All metal shrinks as they are poured into a mold. This is a shot of a typical, what is called green sand mold, and I don’t know why they call it green sand because it’s definitely black and it gets its black color because it’s recycled in the foundry over and over and the sand is burned but that’s the two halves of the mold. The upper half is called a cope and the lower half is called a drag. Now of course, molten iron would be poured into the void created by the pattern. The pattern is compressed in sand, the two halves of the mold is separated, the pattern is removed leaving a hollow cavity in which the molten metal is poured. You can see the parting line is again a very important part of the casting work and it creates what we call plus metal which has to be ground away.
As I mentioned, the shrinkage is important and the foundrymen patternmakers tend to use a lot of instruments such as oversized rulers so that they make their patterns bigger than they need to be. Iron shrinks 1/8 of an inch per linear foot. Other metals shrink at different rates so the pattern has to be bigger. And then of course, there are lots of quality considerations. The development of the pattern is very important because again, the casting that you make can never be better than the pattern used to create it with. Also, if the pattern and the gating is not setup properly, you can have casting flaws. There are lots of ways to go wrong in a foundry, this happens to be a crack in the casting, created by stress in the mold. So what we really want is a nice clean detailed casting and again, the pattern is crucial to that.
We have a lot of coordination issues. The sizes of the pattern with the creation of the flask in this case, is a wood box, which you would use in airset type sand. This is an ancient flask and yes, that is kudzu. We have that in Alabama. Again, an old style steel flask with trunnions, it’s just a very large one with a large cope and large drag. The gating is very important. This is the way the metal flows into the casting. In this case, you have a very large (unintelligible) even that little finial there. The riser acts as a way for the metal to shrink as it flows into the casting so that there would be no voids created inside the casting. Again, a gating system here shown so that you have the down screw and then the feeder is going into the hollow cavity created by the pattern, small rosettes. This is a multiple type pattern where six rosettes would be made per mold. Again, a down screw, feeder, the little gates or runners feeding to the larger rosettes.
So this is kind of a speedy way of getting me into the topic at hand and that is the use of laser scanning for pattern making and this, we think, is our next step. We do have a scanner. The scanner that we use is small enough to go in an overhead bin on an airplane, so it travels very well. We can scan objects, we’re scanning the same leaf today just as you’ve seen it and create a model and the great thing about the model making is that with the press of a button, we can account for shrinkage. So, that’s no longer a big concern to us. We can also take the scanner, we can go to sensitive properties, let’s say it’s the US Treasury Building in Washington, where they don’t want messy molds being prepared on site and they don’t want to give up their iron work. To be replicated, we can scan it on site, bring the model back to Alabama and then we can create the patterns. Once we have the model scanned, we can manipulate it using a program called Creaform and once we have the model exactly like we want it, the right size, we can then convert it to Mastercam and then use the Mastercam models to actually cut them using a three dimensional router.
We have a Haus 3-D router, five foot by ten foot table, that we cut in this case, into a material that we call [ ? ] which is tooling board. Once we cut this individual shape like this in the router, we can then take that, prepare a mold and then develop the pattern. This is the router cutting a pair of patterns. This is a fluted shaft . I think you’ll see in a moment something happening with this particular pattern. This was cut in the [ ? ]. Since this is a deep fluted shaft, we would create the side flutes using a side core, what we call a wing core on the outside of the pattern. That’s our core box that creates the deep fluting on the outside of this particular lamppost. We’re getting pretty close to taking this to the foundry to be poured. There we have a complete casting. This happens to be in aluminum. It’s for a courthouse in Houston, Texas called Harris County. There we have the completed lamppost and it would have a round globe on top of it in cast aluminum, all made with routed patterns.
How much does all that cost? Well, since we own our own equipment and the materials themselves are not that expensive, it’s not as expensive as say sending patternmakers to a jobsite and pulling molds and that sort of thing. So actually, we see it as a cost savings for us and also it is a much more cost efficient process, meaning I have the scanning and 3-D routing turning out patterns faster than a crew of men can get set up and taken to a foundry, so in terms of the way it’s affected us, it has been a very positive thing.
So that’s my talk for today. I’ll be around all morning and so will Jane and if you would like to see more of the equipment, we’d be glad to show you.
Scotty Howell, Vice President and General Manager of Robinson Iron, guides the day to day operation of the firm from the assessment of existing conditions to the removal, engineering and installation of restored materials. Scotty launched Robinson Iron in 1975 as a spin-off of Robinson Foundry Inc. A graduate in Engineering from Georgia Institute of Technology, Scotty holds a Masters in Business Administration from The University of Chicago and is part owner of Robinson Iron. He learned the foundryman’s craft from the bottom up so that he would be familiar with the requirements of each demanding job. Over the past 33 years his resume of distinguished projects has included such iconic landmarks as The Raffles Hotel in Singapore, The White House in Washington, DC, Vulcan in Birmingham, Alabama and others too numerous to mention.
He has been happily married to Jane Benton Howell for all of this time and they along with their son, John, and daughter, Hadley, contribute to church and community affairs through their service as officers and members of various boards. The Alabama chapter of the AIA honored Scotty with their Distinguished Service Award in 1988. In 1989 he was named Georgia Institute of Technology Young Alumni of the Year and in 2006 he received the Boy Scouts of America Distinguished Eagle Service Award. These are but a few of the many honors he has received over the course of his carreer..
He is a published author of articles in the field of historic restoration of cast metals and is a dynamic public speaker who is much in demand.
It is through his special energy, vision and drive that Robinson Iron has garnered
the respect and appreciation accorded the firm.