Shrink Fitting Tool and Die Inserts
A shrink fit holds an insert in its retainer by interference rather than by fasteners, and it does a second job at the same time. The interference puts the insert into radial compression, and that compression works against the tensile stresses which are the usual cause of chipping and cracking in a die insert. This page sets out the interference a die application is built on, the hardness and dimensions each part of the pair is held to, and how the assembly is actually made with heat, with cold or with both. It is written for the tool room building or rebuilding a die and for the buyer who has to supply the steel for both halves of the joint.
What the fit does for an insert
The benefit of shrink fitting tools, dies, guideposts and pins into retainers goes past holding the part in place. A correctly made shrink fit sets up radial compressive stresses in the component that receives the fit, and those stresses counteract the tensile stresses that lead to chipping and cracking. The part receiving the fit is therefore more serviceable than the same part held some other way. The fit also locates the insert and stiffens the assembly, which matters on a die that sees an uneven load across the face.
Mechanically the joint is a pair of stresses. The insert is compressed by the retainer and the retainer is stretched by the insert, and the radial pressure between the two depends on the amount of interference and on the modulus of elasticity of each material. That is why the two parts have to be designed as a pair, and why the choice of material for the retainer is a strength decision rather than a convenience.
Interference, the number the fit is built on
For a shrink fit to work, the outside diameter of the insert is made larger than the inside diameter of the retainer. The difference is expressed per unit of diameter, and there is a working band with a floor and a ceiling on it.
| Interference, in./in. (mm/mm) | What it produces in the assembly | How it behaves in service |
|---|---|---|
| Under 0.001 | Little radial compressive stress on the insert | Too loose to help. The fit does not put the insert into compression, so it does nothing about the tensile stresses that crack an insert in a die application. |
| 0.003 to 0.004 | High joint strength, with radial pressure set up on the mating surfaces | The working band. The retainer carries tensile stress and the insert carries compressive stress, and the compression is what suppresses chipping and cracking. |
| 0.006 or more | Radial pressure beyond what the pair was sized for | Excessive interference. Working stresses plus the shrinkage stresses can pass the strength of the insert or the retainer and produce a failure by stress overload. |
The interference range recommended for shrink fitting tools and dies into retainers, from the shrink fitting section of Tool and Die Making Troubleshooter.
The floor exists because a fit with almost no interference does not develop the radial compressive stress that justifies the whole exercise. The ceiling exists because the same stress that suppresses cracking in service can, at a high enough level, crack the retainer or the insert during assembly or on the first load. Working stresses and shrinkage stresses add together, and the sum has to stay inside the strength of the weaker of the two parts.
The design points that decide whether the fit holds
A shrink fit is designed before it is made, and the rules below are the ones the fit depends on. Several of them are the same rules that apply to any tool steel part, which is why a stressed retainer is drawn with chamfers, fillets and no taper at all.
| Design point | The rule | Why it is in the list |
|---|---|---|
| Retainer material | An alloy steel capable of hardening to 32 to 43 HRC | The retainer is the part in tension, so it has to be strong enough to hold the fit without yielding. |
| Heavy duty retainers | Shock resisting tool steel heat treated to 48 to 52 HRC | Where the die takes impact as well as the fit load, a tougher retainer material is specified in place of the general alloy steel. |
| Stress balance | Working stresses plus shrinkage stresses must stay inside the strength of both parts | The fit adds stress to a tool that will already see service stress. The two add together. |
| Retainer outside diameter | At least twice and preferably three times the inside diameter | The wall around the insert has to carry the hoop stress from the interference without distorting. |
| Taper | A tapered design is not used on either the insert or the retainer | A tapered seat cannot hold even contact around the circumference, and the pressure concentrates where the fit is tightest. |
| Corners and edges | Chamfer corners and edges and provide generous fillets | The same stress raisers that crack a die in heat treatment are present in the retainer. |
| Mating surfaces | Ground to a smooth finish of approximately 5 to 25 microinches (0.13 to 0.64 micrometres) | Contact has to be continuous around the joint for the radial pressure to develop as designed. |
| Machining tolerance | Grind to within 0.0005 in. (0.013 mm) of the required dimension | The interference is the difference between two diameters, so both of them are held to a fraction of the interference itself. |
| Assembly clearance | Provide the clearance needed in the expanded or the contracted state | The parts have to go together while one is hot or one is cold, and a fit that only assembles with force is a fit that has already been damaged. |
Design and preparation points for shrink fitting a tool or die component into its retainer, from the shrink fitting section of Tool and Die Making Troubleshooter.
Assembly, by heat, by cold, or both
Most shrink fitting is done by heating the retainer until it has expanded enough to take the insert. The limit on that method is the heat treatment of the retainer itself, because a part heated above the temperature it was last tempered at comes out softer than it was made. A retainer that has to stay at 48 to 52 HRC cannot be expanded with a torch and no thermometer.
The second method is to shrink the insert. Deep freezing for shrink fitting was once done with dry ice at approximately -120 F (-84 C), and liquid nitrogen at approximately -300 F (-184 C) is used more often now. The lower temperature of liquid nitrogen has made it possible to make many fits without heating the retainer at all, which removes the hardness risk from the assembly step. It brings a risk of its own, because the insert has to be cooled slowly. Dropped straight into liquid nitrogen, an insert can be shocked and cracked by the temperature change, and the crack may not be visible until the fit is under load. Contraction varies with the subzero temperature reached, so the colder the insert is taken, the more it shrinks and the easier the assembly.
Whichever route is used, the finish of the job matters as much as the numbers. Clean and dry both parts, assemble them with a minimum of delay, and let the assembly come back to room temperature slowly rather than quenching or warming it quickly. The shrink fit is not stress relieved afterwards, because the compressive stress in the insert is the point of the operation. The subzero side of the process is described further on the cryogenic treatment page, and the treatment that sets the retainer hardness on the hot work tool steel heat treatment page.
Where a shrink fit goes wrong
Four failures account for most of the trouble. A part heated above its tempering temperature loses hardness, and the retainer then gives way before the load it was designed for. Too little interference leaves the insert without the compressive stress that keeps the edges from chipping, so the die behaves as though the fit were never made. Too much interference cracks the retainer or the insert on assembly, or leaves both parts full of standing stress that adds to the service load. A tapered seat, or a pair of mating surfaces that are not ground true, concentrates the pressure on part of the circumference and produces the same result as excessive interference on that side.
The steel side of the problem is smaller than the assembly side, and the failures above are why a die that cracks in service is not always a heat treatment question. The diagnosis route for a die that has already failed is set out on the tool and die failure analysis page and the cracking resistance page, and the design faults that put stress into a tool before it ever runs are on the tool and die design faults page.
Where to go next
The material for a retainer is chosen on the same tables as any other tooling part, on the tool steel properties chart and the composition chart. For an insert that is loaded in impact, the shock resisting grades are described on the S7 page and the chipping resistance page, and the quench each grade needs before it is fitted is on the quenching chart. For blocks and rounds cut to size for a retainer and its insert together, the supply page sets out how we work.
Before you cut a retainer to these numbers
This page is a reference summary of published practice and it is not an Aobo Steel specification. The interference values and the design points are reproduced from Tool and Die Making Troubleshooter, and the fit actually required depends on the section, the materials and the load in your die. Working stresses and shrinkage stresses have to be checked against the strength of both parts by the person designing the assembly, and the final dimensions are confirmed on the drawing for the job.
Source, Tool and Die Making Troubleshooter, R. M. Leed, Society of Manufacturing Engineers, 2003.