This month’s column discusses the details of hammer and dolly use.
I have a few questions on hammer and dolly work. When you hammer off-dolly to shrink, how much off the dolly is the hammer struck? Is there any overlap of the dolly and hammer or are the edges of the hammer and dolly separated by a certain distance?
I’m trying to repair some dings on the sides of a 1951 Ford truck bed. With the dolly backing the low spot, I find that hammering around the circumference of the low area is causing the depth of the dent to decrease, but the size of the low area is getting larger. What am I doing wrong? Should the dolly to be held tighter against the metal or more loosely?
Via the Internet
Let’s start with the very basics, for those who may not be familiar with the distinction between hammering “on-dolly” and “off-dolly.”
When you hammer on-dolly, the hammer strikes the sheetmetal right at the point of contact between the sheetmetal and the dolly beneath it. This compresses the metal, making it slightly thinner, and the metal will stretch—normally causing it to bulge more in the direction it is already domed. If you are working on an “outside curve,”, one that is bulging toward you, stretching any portion will cause it to bulge more. If you happen to be working in a “hollow” area or a portion of the panel that is concave on the side facing you, stretching any portion will cause it to “sink,” or move farther away from you.
If the hammer hits any point EXCEPT where the dolly is touching the metal, that’s off-dolly hammering. As long as you hit with enough force, the metal the hammer hits will go down. Sometimes the metal the dolly is touching will go up. There are a few variables here—principally the distance between the points of contact of the hammer and dolly, and how much pressure you exert on the dolly. You should be thinking about the points of contact of these tools, not their edges!
It’s difficult for beginners to know exactly where the hammer and dolly are touching a panel. If the metal is clean, you often can see a small shiny mark where the hammer hits. It’s much more difficult to know exactly where the dolly is touching. I suggest holding the dolly stationary and tapping around with the hammer until you find the spot where it “rings.” That’s where the dolly is touching the metal. If that’s not the spot you want, move the dolly, and “hunt” again with light tapping until you find it. After doing this “hunting and finding” hundreds of times, you’ll get much faster.
The offset between the points of contact of the hammer and dolly can be as little as 1/4-inch, or it could be a large distance. As long as the hammer is swung with enough force to make the metal move, the sheetmetal that’s hit by the hammer will go down. If the dolly is 1 inch away, or less, and if it’s being pushed up against the metal with sufficient force, the area the dolly touches will probably move up too. If the dolly is held “loosely” against the panel there will be much less lifting force generated by the dolly. If the hammer and dolly touch the metal more than an inch apart, it’s unlikely that you’ll get much benefit from the rebound of the dolly, even if the part struck by the hammer moves down.
There’s nothing harder to straighten than a flat panel, and pickup beds on older trucks are definitely flat. If there is a dent in a flat panel, the best way to think about the damage is not as a “low” spot, but an area that has been stretched to create a mound. I’d suggest orienting yourself so the dent is a “peak” pointing toward you, and hold the dolly on the other side.
On flat panels, you can only lower high spots a tiny bit by hammering off-dolly. The more curvature a panel has, the more strength it has, and the more you can lower high spots with off-dolly hammering. For medium to large dents on flat panels, you’ll probably have to use heat to correct the problem. You can use a shrinking disc, or to do some off-dolly hammering after heating a small section with an oxy-acetylene torch.
You can email your questions to Professor Hammer at email@example.com, or mail to: Covell Creative Metalworking, 106 Airport Blvd Ste 105, Freedom CA 95019; you’ll receive a personal reply! Ron Covell has made many DVDs on metalworking, and he offers an ongoing series of workshops across the nation. Check them out online at covell.biz, or call for a current schedule of workshops and a free catalog of DVDs. Phone (800) 747-4631, or (831) 768-0705. You’ll also enjoy Ron’s YouTube channel; www.youtube.com/user/covellron