Shrinker / Stretchers are a very handy tool to have in any metal-working shop. As the name implies, they shrink or stretch metal with the aim to create curve in a workpiece that usually has some sort of flange incorporated.
They achieve shrinking or stretching by gripping the piece of metal between 2 jaws and then applying pressure to the jaws to cause them to either move apart (stretch) or move closer together (shrink).
There are number of ways to effect the stretch / shrink, with the entry level units usually incorporating a hand lever that the user actuates to apply pressure to the jaws. The next "step" up is usually to foot operated pedal type unit which lets you exert a little more pressure and keeps your hands free to control the work-piece. The most powerful of units can be hydraulic or air/hydraulic allowing for much more pressure to be exerted but are usually not found in sheet metal working environments.
Some products offer a shrinker and stretcher in 2 separate bodies whilst others have a single body with interchangeable jaws.
Shrink vs Stretch
Stretcher
The stretcher is used when you want create a curve curves away from the outside lip of a flange creating an outer curve. This happens when the stretcher jaws are used to stretch the metal at the furthest edge away from the root of the flange resulting in the outer metal becoming longer that the inner metal (metal closest to the root of the flange) which results in the outer metal curving around the inner metal as it finds somewhere to go. As the metal is being stretched, there is no excess metal and aside from some possible marks from the jaws where they grip the metal, the curve is a very smooth finish that requires minimal finishing.
Shrinker
A shrinker works in the opposite way to a stretcher - instead of stretching the metal at the furthest edge from the root of the flange, it shrinks the metal at the furthest edge. creating an inner curve. As the metal is shrunk and bunches up, the outer edge of the flange becomes shorter than the inner or root of the flange causing the metal to gather around the shorter path and create the inner curve. As the metal is being shrunk and has no-where to go, a shrink can often leave the work-piece with ripples in the shrunken area that may require further post-working to smooth out.
Uni-body vs multi-body
Shrinker / Stretchers come in 2 main body types - the Uni-body which has a single body and interchangeable jaws - or multi-body with 2 sperate bodies where one body has shrinker jaws and the other body has stretcher jaws. There are pro's and cons to each but you will usually find the higher end models are the uni-body type due to the cost of multiple bodies.
Multi-body shrinker / stretchers
Multi-body units are usually the smaller capacity shrinker / stretchers where the material required for the bodies is smaller and hence the cost of the units can be made lower. A big benefit of the multi-body type units is that the user does not need to change jaws to move from shrinking to stretching - they simply use one of the other so there is no time spent changing jaws and you can just get on with the job.
Multi-body pros
- No changing jaws - quick and easy to use and just get straight into the job without making changes to the tool.
- Usually easily identifiable between shrink and stretch - multi-body units are usually 2 different colors so you dont need to go reading the jaws to figure out if you are shrinking or stretching.
Multi-body cons
- Take up more space - due to having 2 units, these products usually take up a little more floor space compared to uni-body items.
- Usually lower capacity / throat depth - this is usually a factor of the cost to produce / ship the 2 bodies so you usually only see multi-body products in smaller throat depths and lower capacity.
- Lack of choice for jaws - most (but not all) multi-body items only give you the choice of the jaws that ship with the product.
Uni-body shrinker / stretchers
Uni-body units are more often the higher capacity (thickness of metal) and deeper throat depth products - This is due to there only being a single body so more time, effort and material can be involved in the construction of the body, allowing for larger size, deeper throat and higher stiffness without the added cost of having to also supply a second body. With uni-body products, you are more often than not getting a larger, stiffer body that allows you to work larger jobs and the product also comes with or has the option for a set of shrinking and stretching jaws. Some products allow you to choose just the stretch or just the shrink if that is all you require.
Uni-body pro's
- Higher capacity for both metal thickness and throat depth due to larger stiffer bodies.
- Choice of jaws - users can often choose form just shrink, just stretch, both, square jaws, rounded edge jaws, smooth jaws etc giving the user maximum flexibility.
Uni-body cons
- Need to change jaws to swap from Shrinking to Stretching.
Hand vs Foot Operated
Most shrinker stretchers give you the choice of how you manipulate the jaws - either by a hand operate lever or by foot operated pedal. Some products give you the choice of hand or foot and purchase time, some give you the option to upgrade from hand to foot and others give you the choice of slightly different products that have either hand or foot operation.
If you are looking for a shrinker / stretcher that you will not be using very often and don't mind a little bit of manual labor, then the hand operated versions are the way to go - the hand levers are sized such that you don't need to exert too much effort to get the desired outcome but do require a little bit of elbow-grease if you are using them often.
Foot operated units are the way to go for anyone using these tools a lot - the foot operated pedal not only requires less effort which makes single-time use easier, but also results in less fatigue for those that are using these machines all day long.
The foot operated unit have the added benefit that the foot pedal incorporates a stand for the shrinker / stretcher to sit on, putting the tool and hence the work-piece at a comfortable height. Hand operated units will usually require a separate stand or to be mounted to a workbench to use.
