When manufacturing machined components, the key challenge is to ensure the consistency and repeatability of the dimensions and tolerances. With metals, this challenge is not so acute. There is a wealth of industry experience around the machining, punching and shaping of metals and therefore enough data to support how the material will behave when subjected to external forces. Metals also adhere to much closer tolerances. Precision metal part manufacturers will frequently cite tolerances of as low as one micron (1µm), which are unheard of in the polymer space. In addition to this, polymers are not as well traversed as metals and manufacturers who develop specific methods to ensure close tolerances will usually keep these as proprietary. Furthermore, each polymer behaves in its own manner and as such throws up its own limitations to what can be allowed by way of dimensional accuracy. A few such examples are given below:
- PEEK – tends to absorb stress during machining and can deform post machining (sometimes after days of being kept on the shelf), if not treated properly
- PTFE – does not absorb heat from the tool during machining, but can experience thermal expansion and its softness, can make it difficult to achieve close tolerances
- Nylons – can be affected by the heat from the tool, causing melting at the part’s surface. At the same time, coolant is not an option as the material tends to absorb moisture and swell post-machining. Methods need to be developed to balance the machining RPM with the feed rate to ensure tolerance and part finish are both achieved
The above examples are only a few to illustrate how each polymer behaves. There are literally thousands of polymers grades and each would come with its own set of properties to influence how it must be handled.
One feature of most polymers is their softness when compared with metals. This is especially true when considering a thin cross section, where the part will deform even when held by a human hand.
For such thin sections, traditional inspection tools – such as callipers, micrometres and probes – do not work. The mere act of holding the part and applying the pressure of the tool will throw out the dimension, rendering the process useless. To remedy this, non-contact methods, such as profile inspection and vision inspection were developed.
Vision inspection is not only useful for non-contact inspection, but can also be used to inspect radii, chamfers, angles and other unique profiles that gauges and tools cannot be easily developed for. In addition to inspection, the process also allows for the dimensions to be captured on the part and a visual to be created to be send to the client or end-user as proof of the dimensional accuracy.
Inspection of complex profiles
More recently, we encountered a set of parts which can only be inspected by sectioning the final component and using the vision inspection system to capture the dimensions. Obviously, once the part is sectioned (cut through), the part itself is destroyed.
Given the size of the parts, this kind of destructive testing can be cost prohibitive. Furthermore, as the parts were made from an especially expensive PTFE-based compound, the challenge to inspect and approve the FAI parts before beginning manufacture was proving to be an expensive one. To be absolutely sure that the dimensions did not deviate during bulk manufacture, it would be optimal to perform this destructive test at the beginning, the middle and the end of the machining cycle. As such, a batch of say 50 such parts would require the destruction of 3 parts to ensure the dimensions were within tolerance. This represents a wastage of 6% - which is high and very costly.
To remedy this, we made three suggestions:
- We machine the parts needed for destructive testing from a polymer that is inexpensive. When making both PTFE and PEEK components, we usually machine a few parts in POM (Delrin) first. POM is dimensionally very stable and mirrors the behaviour of PTFE and PEEK to a high extent. Once we are happy with the dimension, we shift over to the more expensive polymer
- The second option was to machine the FAI part in pure virgin PTFE, which would most closely match the behaviour under machining of the PTFE compounded material. This would work out more expensive than option 1, but still cheaper than destroying the compounded parts
- The final option was to ask the client to specify the percentage of parts they needed tested and work this back into the commercials. In such a scenario, we would need to define a minimum batch quantity – say 40 parts – and come to an agreement that 1 out of the 40 would be checked, leading to a 2.5% increase in the costing
Eventually, because of the sensitivity of the part and the stringent need for quality, it was decided that option 3 would be used.
The client was not comfortable with option 1. This is understandable, since as we have described above, no two polymers behave exactly alike. While we may be comfortable using POM as an approximation for PTFE and PEEK for internal dimensional verification, the same cannot always be considered accurate for bulk manufacture.
In order to better map the behaviour of POM with PTFE and PEEK, we will be undertaking a correlation study to be done over 3 months across 200-300 parts. Once the data is collected, it would put us in a better position to recommend option 1 to clients in the event that cost restrictions limit the level of destructive testing allowed.