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Choosing the appropriate fastener when automating

Choosing the appropriate fastener when automating

The fastener is as important when automating as the automation equipment itself. Choosing the right fastener may prevent exorbitant machine and fixture costs, decrease set-up and cycle times, as well as reduce the manufacturing cost of the components. Christie Jones of Spirol explains.

One of the biggest motivators for companies who choose automatic fastening over manual fastening is increased productivity and reduced costs. Unfortunately, many don’t realise the impact the fastener has on achieving these goals. Not all fasteners are easy to orient, feed, or install. Also, the more tooling necessary to orient and deliver the fastener, the more expensive the equipment is going to be.

It is in the design stage of the assembly where the decisions are made that will either make or break the success and ease of automation. A common mistake that is made is when designs allow the cost of the fastener to take precedence over the cost of fastening. Any money saved on the cost of the fastener can be quickly eroded by the money spent on intricate automation equipment, and decreased productivity through increased assembly cycle times, and equipment downtime. 

Companies should focus instead on the lowest installed cost fastener. Typically, these are permanently installed fasteners that are pressed into, rather than threaded into, a hole and do not require any secondary operations for retention. Design engineers and assemblers should become familiar with the features of fasteners that can affect the success of automating. For the sake of this discussion, the features have been broken down into symmetrical and non-symmetrical.

Non-symmetrical fasteners can be a challenge to automate. They require end to end orientation; thus more expensive tooling is necessary than required for symmetrical fasteners. In order to utilise traditional automating methods, headed parts should be able to hang by the head. A good rule of thumb is that there should be a minimum of a 20% differential between the head and body diameter in order to provide enough distinction to allow for orienting and hanging the parts. If the diameter differential can be held between 20%-30%, additional tooling costs can be avoided. Headed parts that do not have a consistent head diameter, or are inconsistent beneath the head tend to get jammed on the feed rail. Flat heads are also better than round heads for automatic installation. This is because it is easier to press a flat insertion quill onto a flat surface versus a round surface while keeping the fastener straight. 

The added costs to feed, orient, and install headed fasteners make it fundamental to ensure that the application truly requires a headed fastener before specifying one.

Fasteners that are symmetrical and have a continuous profile are ideal for automating. They are the easiest to feed because they require minimal orientation. Basically, all you need is a machine that will deliver the parts in a straight line to the feed tube. Once oriented, these parts are typically fed in a tube down to some type of insertion equipment. 

There are some disadvantages associated with some types of symmetrical fasteners. For example, the straight dowel pin is highly dependent on the host material for retention. This means that the cost of the hole preparation can be expensive. To compensate for some of the disadvantages of the straight solid dowel pins, grooved pins and knurled pins were developed. The diameter across the grooves and knurls is designed to be larger than the hole. When a hardened grooved pin is used for strength, the host material deforms but not to the same extent as a straight solid pin.

The knurled pin is designed to cut its way in the host component, however neither the knurled nor the grooved pin requires the tight tolerances that the straight solid pins do. Regardless, insertion forces are usually much higher for all types of solid pins, which can dramatically affect the cost of the automation equipment. In addition, since solid pins require deformation of the host material for retention, there is the possibility for cracked and/or damaged components during the installation process.

To compensate for the disadvantages of the solid pin, the spring pin was developed. When a spring pin is driven into a hole, the spring action of the pin allows it to compress as it assumes the diameter of the hole. Once installed, the radial force exerted by the pin against the hole wall provides self-retention. Since spring pins do not require deformation of the material for retention, there is no host component damage and installation forces are lower. In addition, the spring pin is able to absorb hole tolerances and minor hole mismatch.

By considering the fastener during the design stage, companies can implement automatic fastener installation at the lowest installed cost.

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