A heavy duty hammermill having a fragmentizing chamber and a hammer assembly mounted for rotary motion in the chamber and cooperating with a cutting comb and grate bars for fragmentizing auto parts fed into the chamber, the hammermill assembly being formed from a number of spiders mounted on a drive shaft and an end disc mounted on the shaft at each end of the spider assemblies, the spiders being spaced apart equal distances by spacer members provided between each pair of spiders and the end discs, each spider assembly including a spider having six arms with freely rotating hammers mounted on hammer pins in the spaces between the arms, the spider arms having a hub portion spaced outwardly from the spacer members and protective caps mounted on the impact areas of the spider arms and end discs.