There are two main methods of forming a wire coil through medical engineering;
- By wrapping it over a mandrel and then removing the mandrel
- By deflecting the wire against one or more coiling points.
The deflection coiling method is the one we use at Shannon MicroCoil. There are few limitations to what can be achieved using this method but one key parameter to bear in mind is the coil index. Coil Index is defined as follows:
Index = D/d Where D is the coil outside diameter and d is the wire diameter.
Some people prefer to use the Mean Coil Index which utilises the wire centreline and is given as follows:
Mean Index = D/d -1
Lower Limit For coil forming, the lower Index limit is 4 (Mean index 3). Although with certain materials it may be possible to go slightly below, in general it’s not possible. So, for example, if you wish to make your coil at an O.D of 0.016” then the maximum wire size you can use is 0.004”. If you require tight tolerance on the OD or ID then we would suggest a 0.0035” wire to allow adequate headroom.
Upper Limit Coil Indices greater than 15 can have their own particular problems in that the stability now becomes the issue. While a 0.010” wire can be taken to an index of 30, a 0.002” will be completely unstable at this index and cannot even be handled without damage. In general, flat wire profiles are more stable at these indices.
Droop is usually specified for guidewire coils and endoscopic coils and the deflection coiling method can control droop over a wide range of values by setting the initial tension of the coil. Initial tension may be thought of as a virtual negative pitch. However, too much initial tension for stiff droops can cause climb out of the coils and while this might not be visible at the production stage, any exposure of the coil to low temperatures or high temperatures can trigger climb out. This may present as intermittent roughness or sometimes a sinusoidal wave along the coil. If really high droops are required then switching to flat wire profiles is the best option.
Tolerances A common problem in specification of wire coils and forms is specifying too tight a tolerance or using tolerances more suited to machining operations. Wire is “live” and its spring quality from foot to foot is variable. All forms of wire forming and coiling that use wire in the spring condition compensate for “springback”. While very tight tolerances in the micron range can be achieved it comes at very high cost since the wire specification tightens and the production speed slows and tooling costs go up.