Manufacturing and Production Processes

Once, or every time

This topic gives you four things to learn. The processes that shape, form, join and finish materials. The scales of production. Tolerances and quality control. And making things with the help of a computer. Almost everybody revises the first list hardest, because it is the longest. That is the wrong way round. ⭐⭐ EVERY DECISION IN THIS WHOLE TOPIC IS ONE QUESTION IN DIFFERENT CLOTHES: WHAT DOES THIS COST ONCE, AND WHAT DOES IT COST EVERY TIME? Take one shape and imagine making it by hand from a drawing. There is nothing to set up. You can start this afternoon. And a skilled person has to spend that time again on the second one, and the third, and the four hundredth. Now imagine having a mould made so the shape can be formed. That costs a great deal, and it costs it once. After that each piece takes very little. ⚠️⚠️ SO WHICH IS BETTER? THE QUESTION HAS NO ANSWER UNTIL SOMEBODY SAYS HOW MANY ARE WANTED. ⭐ FOR ONE, THE MOULD IS ABSURD. FOR HALF A MILLION, MAKING THEM BY HAND IS ABSURD. THE OBJECT DID NOT CHANGE. THE NUMBER DID. Which is why the scales of production are not a separate list to memorise. They are the thing that chooses the process. Now the two items students find most arbitrary, because the same question explains both. ⭐⭐ WHY IS A TOLERANCE ALLOWED AT ALL? WHY NOT JUST MAKE EVERY PART THE RIGHT SIZE? Because making every part exactly the same size costs far more than it is worth. So instead you state a range within which a part still works, and anything inside that range is accepted. The tolerance is not sloppiness. It is a decision about how much precision is worth paying for. ⭐ AND WHY CHECK A SAMPLE RATHER THAN EVERY ITEM? Same trade. Inspecting everything costs more than accepting that a small number of faults will get through. How big the sample is depends on what a fault would cost if it reached somebody. ⭐ AND MAKING THINGS WITH A COMPUTER IS THE SAME MOVE AGAIN. Time goes into getting the instructions right once, instead of skill going into every repetition. Carry one question through everything that follows: what does this cost once, and what does it cost every time?

Three scales, three different sums

The same object, three quantities, three completely different right answers. Notice that nothing about the object changes.

Tap the two that are setup costs

Tap the TWO costs that are paid once for a whole production run, rather than again for every item.

• Having a mould made so that a shape can be formed repeatedly
• Writing and testing the program that tells a machine what to cut
• The material that each finished item is made from
• Applying a surface finish to each item after it has been made

Why the mould was worth it

A company makes a plastic part by hand for a customer who wants one. A year later another customer wants fifty thousand of the same part, and the company has a mould made instead. Nothing about the part has changed. What changed?

• The number it can be spread across. A large cost paid once is absurd for one item and sensible for fifty thousand, because it is divided by the quantity
• Moulding produces a better part than making it by hand
• Moulds have become cheaper than they used to be
• The company no longer has anybody skilled enough to make it by hand

Five terms for making things

Five terms, each defined by what it is. The first is the one most often recited and least often understood.

Match each choice to what it moves the cost to

Two that follow from spreading a cost

Select the TWO statements that follow from every decision here being about cost paid once against cost paid every time.

• Naming the best process for a product is impossible until somebody says how many are wanted
• A tolerance is a decision about how much precision is worth paying for, rather than an allowance for poor work
• Mass production is the most efficient method and should be used wherever possible
• The tightest possible tolerance always produces the best product

Answering a which process question

You may already have met the idea that in any mechanism or circuit you never get something for nothing, and that every gain is paid for somewhere. ⚠️ MANUFACTURING IS THE SAME PRINCIPLE APPLIED TO MONEY AND TIME RATHER THAN TO FORCE. A shape that works on any question asking which process suits a product. ⭐⭐ START WITH THE QUANTITY, AND SAY IT OUT LOUD IN THE ANSWER. If the question tells you how many are wanted, that is the most important fact in it, and most answers never mention it. THEN SAY WHERE THE COST SITS FOR EACH OPTION. ⭐ WHAT IS PAID ONCE, AND WHAT IS PAID EVERY TIME. THAT TWO-PART SENTENCE IS THE ANSWER TO MOST OF THIS TOPIC. THEN DIVIDE THE ONCE-COST BY THE QUANTITY, AT LEAST IN WORDS. A large setup spread over a very large number becomes small. Spread over three, it stays large. THEN ADD WHAT ELSE THE CHOICE COMMITS YOU TO. ⭐ A SETUP BUILT AROUND ONE DESIGN MAKES CHANGING THAT DESIGN EXPENSIVE, AND THAT IS A REAL COST EVEN THOUGH IT APPEARS ONLY LATER. AND SAY HOW THE FINISHED WORK WILL BE CHECKED, since at larger quantities checking everything stops being affordable and a sample has to stand in for the rest. ⚠️ THREE HABITS THAT COST MARKS HERE. ⚠️ THE FIRST IS NAMING A PROCESS WITHOUT THE QUANTITY. Saying that a product would be injection moulded says nothing unless you say why that suits the number being made. ⚠️ THE SECOND IS TREATING TIGHTER AS BETTER. ⭐ A TOLERANCE TIGHTER THAN THE JOB REQUIRES COSTS MORE AND REJECTS PARTS THAT WOULD HAVE WORKED. THE RIGHT TOLERANCE IS THE WIDEST ONE THAT STILL LETS THE PARTS DO THEIR JOB. ⚠️ AND THE THIRD IS REACHING FOR A FIGURE. A cost, a batch size, a tolerance in millimetres. If the question supplies numbers, use them. If not, argue from where the cost sits, which is worth more and cannot be wrong. One last thing. This module is about how these decisions are reasoned, not about how to carry any process out. For the practical work itself, follow your centre's own rules and your teacher's instructions.

One product, three quantities

Here is an invented brief, written for practice. A workshop is asked to make a small wooden bracket with two holes in it, used to hold a shelf. The design is fixed and does not change in anything that follows. Only the number wanted changes. Watch the right answer change with it. ⭐ SOMEBODY WANTS ONE. It gets marked out from the drawing, cut, and the holes drilled where the marks are. Somebody skilled spends perhaps an hour. There is nothing to prepare beforehand, and if the customer asks for it slightly different halfway through, that costs almost nothing. ⚠️ EVERY MINUTE OF THAT HOUR IS PAID AGAIN FOR A SECOND BRACKET. THERE IS NOTHING TO SPREAD. ⭐ NOW A SCHOOL WANTS FIVE HUNDRED. Marking out five hundred brackets by hand would be slow, and worse, no two would be quite alike, so the holes would not line up from one to the next. So the workshop spends a morning making something that holds each blank in the same position and guides the drill to the same place every time. That morning is spent ONCE. After it, each bracket takes a fraction of the time, and they come out consistent. ⭐⭐ THE MORNING WAS WASTED ON ONE BRACKET AND OBVIOUSLY WORTH IT ON FIVE HUNDRED. NOBODY HAD TO CALCULATE ANYTHING TO SEE THAT; THEY ONLY HAD TO ASK HOW MANY. ⭐ NOW A RETAILER WANTS HALF A MILLION A YEAR, EVERY YEAR. Now it is worth building a setup that produces the bracket almost without a person touching it, and that setup may cost more than the entire workshop. Divided by half a million a year, it is small. ⚠️⚠️ AND NOTICE WHAT HAS QUIETLY BEEN GIVEN UP. THAT SETUP WAS BUILT AROUND THIS EXACT BRACKET. ASK FOR THE HOLES IN A DIFFERENT PLACE NEXT YEAR AND THE CHEAPNESS DISAPPEARS, BECAUSE THE THING THAT MADE IT CHEAP HAS TO BE REBUILT. Two other things change with the quantity, and both are the same trade. ⭐ AT ONE, EVERY BRACKET IS LOOKED AT, BECAUSE THERE IS ONLY ONE. AT HALF A MILLION, LOOKING AT EVERY BRACKET WOULD COST MORE THAN THE BRACKETS DO, SO A PROPORTION IS EXAMINED AND THE REST ARE TAKEN ON TRUST. ⭐ AND AT ONE, THE HOLES CAN BE PUT EXACTLY WHERE THE SHELF NEEDS THEM. AT HALF A MILLION, THEY WILL VARY SLIGHTLY, SO SOMEBODY HAS TO DECIDE HOW MUCH VARIATION STILL LETS THE SHELF GO UP. DECIDING THAT IS A DESIGN DECISION, NOT A FAILURE. ⚠️ SAME BRACKET. THREE COMPLETELY DIFFERENT RIGHT ANSWERS, AND THE ONLY THING THAT MOVED WAS THE NUMBER.

Order how a batch is planned

Put these six decisions into the order they are sensibly made in when planning to make a quantity of something.

• Establish how many are wanted, and whether more will be wanted later
• Choose a process whose setup cost is justified by that quantity
• Prepare whatever makes each repetition identical, and pay that cost once
• Decide how much variation a part can have and still do its job
• Decide how the finished work will be checked, and how much of it
• Consider what changing the design later would now cost, before committing

Build the process justification

This is the sentence a which-process answer needs. Assemble it.

The manufacturing run

Five questions on where the cost sits. Three lives.

Complete the manufacturing facts

The allowed range of variation in a dimension for a part still to be acceptable is the _____. Making a set number of identical items, then changing the setup to make something else, is _____. A device that holds work in place so the same operation can be repeated identically is a _____. Checking finished work against a standard to decide whether it is acceptable is _____.

What a tolerance is actually for

A student defines a tolerance correctly and then writes that a good manufacturer keeps tolerances as tight as possible. What is wrong with the second half?

• Tightening costs money and rejects parts that would have done the job, so the right tolerance is the widest one that still lets the parts work
• Tolerances should actually be as loose as possible
• Nothing: tighter tolerances always indicate better manufacturing
• They should have stated the tolerance in millimetres

Three choices to justify

Three student answers about choosing a process. The technical knowledge in each is sound, and the reasoning is what needs work.

• Asked which process suits a product, a student names one and describes accurately how it works, without mentioning how many are being made. Why does that score poorly?
• A student argues that a company should always use the tightest tolerance it can achieve, to guarantee quality. How would you correct it?
• A student recommends a very large automated setup for a product a company expects to redesign within a year. What is the weakness?

Explain why quantity decides process

A friend has learned the manufacturing processes thoroughly but keeps losing marks on questions asking which one suits a product. Explain what they are missing, and how to answer such a question properly.

• Explain the one question every decision in this topic comes down to, in terms of cost paid once and cost paid every time
• Give an example of a process with almost no setup cost and a high cost per item, and one the other way round
• Explain why the same object can be made in completely different ways depending only on the quantity wanted
• Explain what a tolerance is really deciding, and why tighter is not automatically better
• Explain why a sample is checked instead of every item once quantities are large
• Say what a large setup quietly commits a manufacturer to, and why that belongs in the decision at the time it is made
• Finish with the shape of a good answer, and avoid any cost, batch size or measurement you were not given