Cold Forming Die Casing Material Selection
Cold Forming Die Casing Material Selection
(OP)
We are presently taking 5/8 dia. wire and forming it into a parallelogram shape. The die section is split into two pieces and has a six thousandths press fit into a casing that is 5.810 dia. and made from 4340 unheattreated material. We experience some loosening of the die after running approximately one hundred thousand parts with it getting progressivly worse until a new casing has to be used. I was wondering if the 4340 material is the best choice for this and if I might not be better off using the shrink fit method instead of press fitting. The die section must be split due to some finishing, etc. that are easier to accomplish with it being split.
Thanks
Thanks





RE: Cold Forming Die Casing Material Selection
Heat treat: 1450-1625F, oil quench, temper at 325-350F.
RE: Cold Forming Die Casing Material Selection
RE: Cold Forming Die Casing Material Selection
RE: Cold Forming Die Casing Material Selection
What is the outer diameter of the die insert before press fitting?
RE: Cold Forming Die Casing Material Selection
The die section is .900 thk.; 2.316 at the top with a one and a half degree taper to the bottom. The bottom diameter is 2.363. The die is made in two "L" shaped parts following the shape of the paralleogram. The casing has a diameter of 2.310 with the matching one and one half degree taper. It has always seemed to me that it was a lot of interferrence but I was told by the people that initially built the die that that was the right amount due to the tremendous pressures involved in cold forming.
RE: Cold Forming Die Casing Material Selection
Cases and stress rings for cold forming tools typically use a diametrical interference of 0.3 to 0.6% of the interface diameter. In your case, 0.3% would be an interference of 0.0069 inches and 0.6% would be 0.0139 inches; this means that the current 0.006 inch interference is on the low side of the recommended range.
I am trying to calculate the stresses due to the press fit, but I am not sure that I have the geometry exactly right. Is the 0.900" thickness in the same direction as the diameter, or are you describing how "tall" or "deep" the die segments are? What is the length of the 2 parallelogram sides? Once we calculate the stresses due to the press fit, we can add in the working stresses, and see what the total stress will be on the 4340 case. Are the die sections made from a High Speed Steel such as M2?
RE: Cold Forming Die Casing Material Selection
The long leg of the parallelogram is 1.211 and the short leg is .444 with a .375 fillet between them. The other two corners of the parallelogram are sharp, with no radius. The angle that forms the parallelogram is 2 degrees. The finished part thickness is .300.
There is no clearance between the form punch and the die section. Do you think that with the press fits involved, that there might be some reduction of the interior die section dimensions to the point where instead of a line to line fit, there is some interferrence?
RE: Cold Forming Die Casing Material Selection
1. The interference fit is too small.
2. The strength of the die case is too low.
The minimum size of inserts and stress rings/case should roughly follow this developed formula:
B (inside diameter of ring/case) = 1.5*A (inside diameter of insert or segment)
C (outside diameter of ring/case = 4.5*A
The ideal ratio is B = 2*A and C = 6*A. Your application is currently B = 1.91*A and C = 4.8*A, so this acceptable. Using the ideal ratios would be an improvement, though.
I would increase the amount of press fit to around 0.008" so that the die segments would be under more compression. And I would quench and temper the die case to be 45-50 HRC. The thickness of the die case is too large to satisfactorily quench 4340 to that type of hardness, so I recommend changing to H13 tool steel. When the press fit is too small, the interface between the segments and the case deforms due to the high forming pressure. I am guessing that the pressure is fairly large in order to fill out the corners of the parallelogram, so either making the assembly stiffer (die segment to case ratio) or stronger (press fit + heat treating) would improve the life of the case. Many die cases are only replaced when the inner diameter has become exceesively worn due to many insertions and removals of the die segments.
RE: Cold Forming Die Casing Material Selection
RE: Cold Forming Die Casing Material Selection
RE: Cold Forming Die Casing Material Selection
RE: Cold Forming Die Casing Material Selection
RE: Cold Forming Die Casing Material Selection
For extreme applications in which the expense can be justified, I have ocassionaly seen maraging steels (C250, C300) used for die cases. These materials have an exceptional combination of yield strength (required to retain the insert in compression) and "toughness".