Progressive Draw Dies
Drawing metal into various shaped cups requires that the metal blank or “cookie” be allowed to flow freely into a die cavity. The following strips show various ways of separating the cookie.
Always machine an air release hole in the draw punch to prevent the part from hydraulically locking onto it. Draw punches can be made from properly heat treated D2 tool steel.
When drawing precision round cups it is a good practice to “float” the punches. This allows the punch to self align with the draw die and eliminate all inaccuracies of the press or die set. This will always achieve an even draw.
The draw die inserts should be draw polished in the same direction as the material flow. Coatings, coupled with draw polishing, should eliminate any chance of galling. If you do not draw polish the draw rings, you may defeat the purpose of the coatings. This is especially true with carbide. Galling may still occur, especially in sizing stations. The AFM (abrasive flow machining) process can reduce draw polishing times substantially. The abrasive flow maching can also be used on molds and form dies as well.
The volume of metal in the first draw station should be equal to the volume of metal of the final part geometry minus any flange for carrying the part.
Drain holes and air release holes should be provided for in every draw station.
In all iron stations, use a tool coating for added lubricity and try to taper the part slightly for ejection.
A good drawing lubricant coupled with a lubricant applicator or stock oiler will benefit productivity and quality.
When running thinner materials it is a good practice to utilize push pull servo feeds. This can increase your production rate dramatically by keeping the material taut.
Unless the bottom of the cup has a flatness specification, stock straighteners may not be needed, but they can improve die performance in keeping the blank coupon flat.
Incorporating die electronics and sensors will improve efficiency.
Proper set up and timing of die cushions is important, especially when interchanging dies in the same press. It is a good practice to use metal spinning as a method to produce working prototypes for evaluation before investing in complex tooling.