Q. Have you ever heard of the Fairmount method? Is there a wrong way and a right way to bump metal? Do you bump the dent out in the exact reverse of how the dent occurred?
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A. Many years ago, a highly talented metalman named Frank Sargent wrote the first comprehensive books on straightening damaged metal. When I took a collision repair class back in the 1960s, the textbook was written by Sargent. The only work of his that is still in print is a slim booklet titled The Key to Metal Bumping. It’s published by Martin Tools, who took over the Fairmount tool line many years ago, and it is available through the Eastwood Company, among other sources.
That little book is a treasure of information, and there is very little between the covers that I disagree with. In fact, about the only thing I don’t agree with is quenching metal with water when shrinking it with heat – I don’t recommend that!
I’d say you should definitely buy this book if you don’t have it already, I consider it the bible for metalworkers!
Q. I have purchased several of your DVDs. They are very informative, especially for a neophyte metal former. In the “Hammerforming” program, you demonstrate using a rivet gun to form metal into a female mold. You mentioned using “Bondo” to help make the mold. There are so many types of plastic body fillers on the market that it’s confusing. Can you be more specific about the product you recommend for this application?
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A. Great question! As you probably know, Bondo is a brand name for one variety of plastic auto body filler, but that name has become generic (like Kleenex) and is often used to describe all varieties of filler.
Most plastic fillers use a binder of polyester resin with various fillers. Talc is far and away the most common filler, and it’s been used since plastic fillers were first developed, but several significant improvements have been made over the years. “Regular” plastic filler would be a good choice for many light-duty hammerforms.
Most modern fillers are now designated “light.” Light fillers have microballons added (microscopic hollow glass spheres), which increases volume with very little increase in mass. This makes the plastic easier to spread, sand, and featheredge. While this style of plastic filler would work for a hammerform designed for occasional use, it would be the least desirable type, because it will be less durable than the other formulations.
Some plastic fillers have short strands of fiberglass fibers added to them. These are sometimes called “kitty hair” or “tiger hair.” They are much stronger than “light” or “regular” filler, and for this reason they would be much better for a hammerform that would get moderate use. These aren’t used as often for regular bodywork, since they are considerably more difficult to spread, sand, and featheredge.
There are some fillers made with powdered metal used as a filler, rather than talc. These are called “All Metal,” or something similar. These are a great deal stronger than the others, and would be good for a heavy-duty hammerform. Again, these are not commonly used for everyday bodywork, because of the greater difficulty of spreading, sanding, and featheredging this product. They are best used where extra durability is needed.
In the industrial world, “filled” epoxy resins are often used as fillers. Epoxy is heavier, stronger, more expensive, and more toxic than polyester, so I don’t recommend working with these resins unless the regular body fillers just won’t do the job. These “tooling” resins start at about $100 per gallon, and go up from there, but some of them are strong enough to drill and tap! JB Weld is one example you may be familiar with.
Just for reference, I have used “light” or “regular” plastic filler for all of my hammerforms that incorporated filler, and they have worked fine for me, for making just a few parts. If you are doing a large number of parts, or if the forming puts unusual pressure on the form (which could happen if you’re using tougher or thicker metal) you might consider using something more heavy duty.
You can email your questions to Professor Hammer at email@example.com, or mail to Professor Hammer, c/o CLASSIC TRUCKS Magazine, 1733 Alton Pkwy., Irvine, CA 92606. You’ll receive a personal reply! We’ll print your name and city unless you request otherwise. Ron Covell has made many DVDs on metalworking processes, and he offers an ongoing series of workshops across the nation covering all aspects of metalworking. Check them out online at www.covell.biz, or call for a current schedule of workshops and their free catalog of DVDs. Phone 800-747-4631, or 831-768-0705. You can send a request by mail to: Covell Creative Metalworking, 106 Airport Blvd. #105, Freedom, CA 95019.