HWooldridge,
Within the metalshaping community the hammers that Fabnut asked about are not referred to as planishing hammers. Many people group the two together even though their functions are different. The Pettingell hammers pictured above are power hammers and they are built for stretching and shrinking sheet metal very quickly. A pneumatic planishing hammer while capable of some forming operations, is a tool used primarily to finish a worked sheet metal surface. A metalshaper might use a planishing hammer after major forming operations under a power hammer.
Power hammers also differ from the pullmax, Trumpf, and other nibblers that many metalshapers have modified for shaping operations. These modified nibblers/shears are direct acting, with fixed linkages, whereas the power hammers have a floating head like their larger brothers in the blacksmithing business.
Just thought I would clear up the differences between these machines that are often grouped together.
There are numerous metalshapers who have fabricated their own replicas of vintage power hammers. Their are others who have designed their own with modern adaptations or found object engineering, and there are others who have designed their own machines from the ground up. It is all predicated on the endlessly rising value of the originals thanks to a tremendous upsurge in people who want to build custom motorcycles and hotrods. More power to 'em.
<img src=http://ccookenterprises.com/images/yoder/yoder_lg.jpg>
<img src=http://allshops.org/community/CommunityAlbum/9980101699000.jpg>
-Adair
P.S.
Where did you dig up that photo, John?