Examples of manned single station cells include:

·     A CNC centre producing identical parts, with an operator required to load and unload parts as the processing programme is completed by the machine.

·     A CNC centre producing non-identical parts, with an operator required to load new part programmes as necessary.

·     A cluster of two CNC turning centres, each producing the same part, but operating independently. A single worker attends to both machines.

·     An assembly workstation where an operator performs mechanical assembly of a simple product (or sub-assembly) from components located in tote bins at the station ENDLIST.

Examples of single-station automated cells include:

·     A CNC machining centre complete with parts carousel and automatic pallet changer producing identical parts.

·     A CNC machining centre complete with parts carousel and automatic pallet changer producing non-identical parts. In this case the appropriate part programme is downloaded automatically as necessary.

·     An automated insertion machine assembling electronic components onto printed circuit boards in a batch operation.

·     A robotic assembly cell with one robot assembling a simple product or sub­assembly.

Numerous examples of both manned and automated single-station cells can be given, right across the manufacturing environmental spectrum.

Analysis of Single-Station Systems

In our analysis here we can determine:

·     The number of single stations required to satisfy specified production requirements.

·     The number of machines to assign to a worker in a machine cluster.

Number of Workstations Required

We must determine how many workstations are required, given a certain production rate, or a given quantity of work units. This is generally done by determining the total workload that must be accomplished over a certain period, and dividing that by the hours available on one workstation during the same period. Workload is determined thus:

where WL is the workload scheduled for a given period; Q is the quantity to be produced during the same period; and T_c is the cycle time required per piece. If the workload includes multiple part or product styles that can all be produced on the same workstation, then:

where Qj is the quantity of part or product style j produced during the period; and T_cj is the cycle time of part or product style j.

We must now divide the result by the number of hours available on one workstation, thus:

where n is the number of workstations; and AT is the available time on one station in the period under consideration. These equations do not take into account a number of potential complicating factors, which makes it more difficult to assess the number of workstations required; these include:

·        Set-up time in batch production.

·        Availability of machines.

·        Utilization of machines.

·        Defect rates from various machines.

Example:

800 parts are to be produced. Cycle time is 11.5min. Determine number of machines given 40hrs availability.

WL = 800(11.5) = 9,200min = 153.33hrs AT = 40hrs

n = 153.33/40 = 3.83 or 4 machines

Availability time may be measured as follows, with the available time becoming the actual shift time in the period multiplied by availability and utilization:

AT = H_shAU

where AT is available time; H_sh is the shift hours during the period; A is availability; and U is utilization. The defect rate—that is, the fraction of parts produced that are defective—must be assessed so that it can be factored-in to the starting batch size, so that the output can compensate for defective parts produced.

Example:

800 shafts are in 20 different types. Average batch size is 40. Set-up time between batches is 3.5hr.

WL = 20(3.5) + 20(40)(11.5/60)

= 70 + 153.33 = 223.33hrs n = 223.33/40 = 5.58 or 6 machines

The relationship between starting quantity and actual quantity produced is:

Q = Q_o (1 - q)

where Q is the quantity of good units made in the process; Q_o is the original or starting quantity; and q is the fraction defect rate. This formula can be rearranged to give us the amount of starting units we require, thus:

Taking these factors into consideration, we can now amend our original formula, thus:

Example:

Using previous data and Availability is 100% during set-up and 92% during running. Utilisation is 100%. Fraction defect rate is 5%. Determine number of machines.

For set-up:

WL = 20(3.5) = 70.0hrs AT = 40 (1.0)(1.0) = 40 n_su = 70/40 = 1.75 machines.

For production runs:

WL = ((20)(40)(11.5/60)) / (1-0.05) = 161.4hrs AT = 40(0.92) = 36.8hrs per machine n_pr = 161.4 / 36.8 = 4.39 machines Total Machines = 1.75 + 4.39 = 6.14 or 7 machines.

The number of workstations that are required is determined by the total workload that must be accomplished over a certain period, divided that by the hours available on one workstation during the same period; together with a consideration of any mitigating factors.

Analysis of Machine Clusters

Sometimes opportunities exist to allow a worker to oversee more than one machine at a time, owing to the semi-automatic machine cycle of individual machines. Worker attention remains important; however not as regularly as every work cycle; instead, the worker will be required on the basis of a manning level of less than one for each workstation. This type of organisation is referred to as a machine cell, or machine cluster, which is defined as a collection of two or more machines producing parts or products with identical cycle times and serviced by one worker. A machine cluster must satisfy several conditions in order to exist:

·        The semi-automatic machine cycle must be long relative to the service portion of the cycle that requires the worker’s attention.

·        The semi-automatic machine cycle time must be the same for all machines.

·        The machines that the worker would service must be located in close enough proximity to allow time to walk between them.

·        The work rules of the plant must permit a worker to service more than one machine.

If we consider a situation where we have a collection of single workstations, all producing the same parts and operating under the conditions outlined above, then we can say:

Each machine operates for a certain portion of the total cycle under its own control T_m (machine time), before requiring servicing from an operator T_s (service time)

The total cycle time (T_c) of the machine is therefore machine time plus service time; or, in equation form:

T_c = T_m + T_s

If we add another machine to the operator’s purview, then the operator will lose some time walking to this machine, called the repositioning time (T_r)

The total time that an operator needs to service one machine must be adjusted from simply T_s, to: T_s + T_r

We must also factor-in the time to service n machines, which is: n(T_s + T_r)

Thus, the original equation takes the form:

Further, we can determine the number of machines that should be assigned to one worker by solving:

Note that the result for n is unlikely to be an integer, so worker time in the cycle cannot be perfectly balanced with the cycle time of the machines. In reality, either the machines or the workers will experience some idle time. There arise two potential cases from this consideration:

Case 1: the number of machines will be an integer greater than the value of n (call this n₁); or

Case 2: the number of machines will be an integer less than the value of n (call this n₂).

Example:

Machine shop contains number of machines with cycle time for one particular part of 2.75min. One worker can load and unload machines in 25sec. It takes 20sec to walk between machines. How many machines can one worker manage if no machine idle time is allowed.

T_m = 2.75min; T_s = 25sec = 0.4167min; T_r = 20sec=0.33min

The most favourable case is dependent on other factors, such as the labour cost rate (C_L) and the machine cost rate (C_m). For case 1 (where n₁ < n) the worker will have idle time, and the cycle time of the machine cluster will be:

If we assume that one work unit is produced by each machine during a cycle, we have:

For case 2 (where n₂ > n) the machines will have idle time, and the cycle time of the machine cluster will be the time it takes for the worker to service the n₂ machines, which is:

The corresponding cost per piece is given by:

The selection of either n₁ or n₂ is based upon whichever results in the lowest cost per work unit. In the absence of precise cost data, the layout should be arranged so that any idle time is taken by the workers, and not the machines. This is because the total hourly cost rate f operating n machines is larger than the labour rate of one worker. The corresponding number of machines to assign the worker is given by:

A machine cluster is defined as a collection of two or more machines producing parts or products with identical cycle times and serviced by one worker.