The Machining Buyers Guide


Choosing and working with a Machining supplier is not straightforward, so we have published this guide to help. We buy and sell machining services so have learned a few things that you mind find useful.

Free Download

The Machining Buyer's Guide e-book

All of our top tips for buyers of machining services in a downloadable pdf e-book or free printed booklet.


Get the Guide
GWM Machine Shop From the Balcony

Grainger & Worrall buys a lot of subcontract machining from other companies but we also have our own machine shop (GW Machining) which sells its services to the world. 

We know some of the difficulties and pitfalls from many years of experience. Here we have attempted to boil it down to the main things you need to consider, together with some useful tips, to reduce lead time and cost.

Revise the basic terms and concepts, read our suggested supplier questions and see what we look for on a supplier visit.

The Machining basics

What is a micron?

We all know it's a unit of measurement that represents quite a small distance. One thousandth of a millimetre in fact.

However, it’s surprising to many people just how small it is in real life. For example, a micron is one seventieth (1/70th) the thickness of a piece of paper or about the size of a single bacteria cell.

A relatively small part can grow or shrink by several microns just with tiny temperature changes, which is why it’s important to get a grip on what one is and how hard it is to achieve micron-level accuracy in machining. If not, extra cost can be incurred chasing that micron down when it may not be needed.

GW Machining Micron A1 Poster 600x826



To really understand just how small a micron is, we think you need to see it.

GWM has created a wall poster to show the relative sizes of common objects against a 1mm circle. It always gets great comments from visitors so we are offering you the chance to own one.

Click HERE to get your free download or mailed* version.

*qualifying companies

Why do we need to have tolerances on drawings anyway?

In the current age of 3D models, not every engineer has quite got their head round tolerances. In models on a CAD screen, all dimensions are always exact. It is a perfect world where it all fits, everything is makeable, and mistakes are (almost) free to correct.

You may need a dimension on your part to be 100mm, but what exactly do you mean? There is no such thing as exactly 100.000mm all the time – in the real world anyway.

The problem arises because of course nothing is perfect in real life. Everything would be so easy if it were. In real life, temperatures can cause expansion, internal material stresses can relax, cutters can wear and even measurement equipment has a limit to its accuracy.

You have to say how close to 100mm you can accept. The tighter that tolerance, the more cost you are committing to. If you had enough time and a big budget, then you could get close. In most cases though, you do not need it everywhere. To get the most cost-effective part, it’s important to agree what is important and what isn’t.


How much does a micron cost?

At one end of the spectrum, it can cost a scrapped part or even a warranty claim on an expensive finished product. In the most famous example, a two-micron error on the Hubble telescope mirror resulted in a blurry image and it cost $1.5bn for a Shuttle trip to repair it.

At our more down-to-earth level, microns can still cost big sums. The most likely cause of this is the over-tolerancing of dimensions. There is a whole field of expertise called GD&T (Geometric Dimensioning and Tolerancing). Someone on your team will have expertise on this if it is not you.

There is a range of different manufacturing processes out there, each suited to different tasks. Some are quick and dirty (low process capability) others are more accurate but either slower or more expensive. If there was such a thing as a highly accurate, fast, cheap machine then that would be the only one everyone used.

However, be aware that the tolerances on your drawing tell your supplier which process they need to use to meet your requirements and how they need to measure it. Will a steel rule do the job? Or £1m of CMM (Coordinate Measuring Machine) be required? It also has implications on the temperature control of the machine, the coolant and even the entire internal volume of a huge factory. All of this has a cost.

While this gives some idea of what factors affect the cost, there are further complications. Not all machining facilities have all processes. You might end up in a situation where the suppliers that can meet some requirements and not others, and you end up with subcontract loops. It is not unknown for parts to travel twice round a continent visiting different specialist machinists to get a finished part.


Which process will I need?

An engineering student will be taught which processes can machine to what accuracy, to give some indication of what is possible. It is a gross over-simplification, but it’s a start.

Some holes could be milled or drilled, more tightly-toleranced holes will need boring. Going further up the accuracy scale you may need grinding or even honing. The accuracy of all these processes is further affected by:

  • Temperature
  • Bespoke tooling
  • Machine accuracy
  • Stability of the part
  • Cleanliness

What can the supplier do to help you up front?

Ideally during the design phase and no later than the beginning of the quotation stage, it is important to talk with experts in you supplier base.. They will usually be quite happy to share experience based on what they have seen before, what works well and where the pitfalls can be.

They will not be able to tell you exactly what level of tolerance your part requires. This is down to you or your designers and will depend on the application, tolerance stack, materials, failure modes and effects and so on. However, they can tell you if it is out of the ordinary – in either direction. Building a trusting relationship with experts in your supply chain could potentially save you significant cost and lead time later down the line.

For information on the right questions to ask your supplier, read chapter two below.


How do I know if my part is in tolerance?

It may be written on the inspection report but be cautious and ask yourself these questions:

  • Have you understood what level of inspection you have asked for?
  • Does your purchase order ask for 100% inspection?
  • Of all dimensions?

Bear in mind that a high-performance engine block may have 7,000 dimensions on it.

A Formula 1 customer may ask for a detail report on every one of them, on every part made, using more than one measurement technique and potentially also with CT scans of the internals included.

You may ask for a ISIR (Initial Sample Inspection Report) and then agree a sampling rate for other parts. Every 10th? 100th? Critical dimensions only? What does that mean if a process leads to out of tolerance measurements of parts or dimensions that are not measured?

Also, when is ‘out of spec’ the same as scrap? You will probably have a concession process to deal with these situations. It has been known however for a customer to refuse to accept a part that is one micron below bottom limit. Ok, if you are machining the mirror for NASA’s next telescope then maybe.

You will also encounter very different attitudes in different companies for how they treat out of spec situations. Any professional machinist should be extremely strict. They should be 100% honest and record dimensions consistently according to their process all the time.

It is important your supplier is not only compliant to all standards as well as passing all internal and external audits, but you need to know that you can trust them. Unless you are going to match their measurement capability and measure it all again at Goods In. That is a big cost implication.


What is the difference between machining a casting or machining from solid?

It is a common mistake to assume all lumps of metal are the same – bar, billet, forgings, castings. All of these have different manufacturing processes, and they leave their ‘fingerprint’. They all have different internal stresses ‘baked in’ to the material which you cannot see or measure.

When you machine any piece of material, you are removing something that contributed to its shape – not just the stock you have removed but also how that material held the rest of it in position.

Castings are particularly fickle. The way the molten metal solidifies in the mold is part of the pattern makers art. The arrangement of the gating system, risers, chills – all have an impact on the final part. A machinist with experience of milling castings will have learned how to predict not only how a part moves when stock is removed, but how to fixture it and minimise distortion.


What will be the cycle time for my part? Speeds and Feeds

How much material is going to be removed in each pass of the cutting edge, and how fast will it happen? Your machinist will be able to show you how these are calculated for different cutters and materials.

However, due to the infinite number of variables that exist, e.g. material, shape, size – it is not until the cutting process begins that the optimal conditions can be established. Under proper management from your machinist, a more accurate process can be established by fine-tuning the values based on sight, sound, temperature, and tolerance holding, whilst running the machine. This is a process that can improves with the number of parts machined.


What will be the cycle time for my part? CNC Programming

Potentially a more significant factor in determining the cycle time for machining your part will be the skill of the machinist’s engineers. To start with they must select the right machine, part fixturing, machining orientation, machine type, tooling, coolant, and a myriad of other conditions. They will also have to programme the CNC machine.

It is easier and less risky to develop a programme with long tool travel distances (and times), slow approach speeds (where the tool is moving but not cutting) and logical but not optimised tool changes.


What is the real difference between Turning and Milling?

To the casual observer, this is a trivial question. When Turning, the part moves and the tool stays still. When Milling the part stays still and the tool moves.

The real difference though lies in the complexity of the machine and therefore what you can do with it. The machine that does the Turning is a lathe, and this is a reasonably simple machine. It has to have a chuck to hold the workpiece, and be able to spin it on its axis, but apart from that the cutting tool only moves in two axes.

The result is a cheaper machine. Turning also has the advantage that it can use cheaper feed stock – bar is easier to make than billet. The shapes you can create though are limited.

Milling on the other hand has up to 5 axes of movement to manage – 3 linear ones in the X, Y & Z planes but also two rotational movements. With Milling you can achieve more complex shapes than with Turning.

With more axes you can also drastically cut down on the total cycle time required by eliminating the need to remove and reset the part in a different orientation. For simpler parts or smaller batches though, you can be more effective using a simpler 3 or 4 axis machine.


How many axes do you need?

It is important to understand the differences between 3+2 and true 5-axis machining.

It is often the case that a buyer will approach a supplier thinking they absolutely need 5-axis machining, which can be misleading. The reality is that many parts simply don’t require it or are more efficiently machined with 3+2 movement.

This still gives you the benefit of fewer set ups and fixtures due to the flexibility of the machine. Running a simultaneous 5-axis machine can cost twice or three times as much, but for some parts it is the only solution.

True simultaneous 5-axis machining

Free Download

The Machining Buyer's Guide

Get your download or mailed* copy now

*eligible companies

Get the Guide

Good Questions to Ask Your Machining Supplier


You may be amazed at how few customers ask good questions before deciding where to place their work. If you care about your product, then here are some questions you should think about asking before you decide who gets your order.


Can I visit?

This may seem obvious, but you are going to get a much better feel for how a machine shop is run if you can go and see it in person. See the next chapter on things to look for on a visit. Use it to verify some of the answers you get from these pre-visit questions too.

How can I reduce the Lead Time?

Everyone wants to get the best price to machine their part, but often the decision rests on who can provide the quickest lead time.


There are many factors that influence the lead time. Asking how to reduce it isn’t an easy question. You will be told it depends on many things including the available capacity at the time the 2D drawings are received (with all details and tolerances, the batch size and the complexity of the part. It’s still worth asking though as it gives an insight in to how well your supplier thinks.


One thing that will definitely affect the lead time is the number of operations, the total cycle time and set up times – for both machining and inspection, as well as any post-machining operations. Lack of temperature control can add 24 hours between machining and inspection, while a part ‘soaks’ to reach dimensional stability. A tight cleanliness specification can require a long process time.


This is where good design and appropriate specifications can help. Your supplier will have seen the good, the bad and the ugly – so tap into this expertise. Small changes to design can sometimes make a big difference.



How would you improve my design to reduce cost?


You may think this is a waste of a question as your design is fixed. There are two reasons why this may not necessarily be true.


Firstly, the answer you get will give you confidence that your machinist knows what they are talking about. They may suggest things you hadn’t thoughts of, and in the spirit of VA/VE (Value Analysis/Value Engineering) they may even highlight a seemingly simple feature or specification that costs a lot more than you realised.


The second is that you may be able to accommodate some changes after all. They might not all change the functionality of your part. Sometimes something as fundamental as the Datum Structure or Pick Up Strategy can have a very large impact. Tolerancing of some features can mean the difference between dedicated tooling or not, or specialist inspection equipment.


How are you going to machine the features on my part?

If it’s a simple part with wide open tolerances then don’t worry about this; just leave it to the experts. If you have a critical dimension or an unusual feature then challenge the machinist to talk you through the options. Are they going to interpolate a circular hole and expect it to be perfectly circular? Or purchase dedicated tooling?

How are you going to measure my part?

There is no point focussing only on the machining without an equal amount of attention paid to the inspection. CMMs need programming too, just like CNCs.


It’s amazing how many machine shops invest all their capital in the machines and then leave the inspection room as a poor relation. You are not going to want to inspect the parts when they arrive with you, so you need to make sure the supplier can do the job properly.


Some specifications require very specific measurement equipment so make sure they can cope. Some additional questions you should ask include:

  • How do they measure cleanliness?
  • Do they measure surface finish in the same units as you?
  • What is the repeatability of their measurement machines?
  • How is traceability managed? To serial number level?
  • When do they pressure test? There is an optimum point and it isn’t at the end when you may cause damage blocking up leak paths.

How do you propose routing and fixturing this part?

This is an opportunity to see whether your machinist is thinking about this creatively. For example, how have they divided up the features into operations on the routing? What machine is it going on – is it worth fewer set ups on a 5 (or 3+2) axis machine? Or more set ups (and fixtures) on a 3-axis? Can one fixture be designed to hold the part in different ways for two operations? Routing, machine choice and fixturing is all about trade-offs.

Have they got a solid understanding of how the part will move when machined and therefore how it needs to be clamped? What order do the clamps need to be tightened? This is critical with castings as discussed in an earlier chapter, but even with machine from solid you might be removing 90% of the material from the billet, so this is not without its implications.


Is your facility temperature controlled?

This is not about making it comfortable for the employees on hot and cold days. Metal moves a lot as its temperature changes. The coefficient of thermal expansion of Aluminium is 24 x 10-6/oC (twice that of iron). This means a 500mm component will grow by 12 microns per oC. Given a machine shop temperature could vary by 10oC easily (for example by a door left open while taking a delivery), this is a change of more than 0.1mm.


Coolant chillers will help, but ultimately to achieve the best precision you are going to need a fully temperature-controlled shop. This isn’t cheap, either to install or run, so it’s often a corner that a machinist will cut when kitting out their machine hall. A good standard is +/- 1oC.


Don’t forget about the inspection room either, as this is even more critical. Usually, inspection rooms are smaller spaces so are more commonly temperature controlled. However, a part coming in from an uncontrolled hall will need to soak for a period of time relative to the part’s mass adding lead time and cost. For large parts this has been known to be up to a week.


It’s also important to remember that parts with varying cross-sectional areas will expand and contract in bizarre, unpredictable ways. It’s nearly impossible to achieve a tight flatness tolerance in these circumstances – and that may be your datum face.



 What CNC programming software do you use?


Some parts only need the most basic CAD/CAM CNC Programming software. But a complex part may have all 5 axes moving at once, and this is going to need some pretty high-level processing power.


Even typical designs requiring more common 3+2 axis machines will still benefit from having the fire power available in more advanced systems. More advanced software will also have powerful simulation features for generated tool path inspection and error checking.


Not only that, but if your supplier uses top-end systems then it shows they are more than capable to handle whatever you throw at them. Ask your supplier’s engineers what software they use and look it up. Does it sound like they are using the best? Make sure they know that their software can handle the size and complexity models you will be giving them; several have been known to crash when loading file sizes in the 100’s of MB.



Have you got your own Tool Room?

 Lots of machine shops will have a corner they call their Tool Room. This is important if you want to be able to produce prototyping fixtures quickly rather than having to subcontract it out. A dedicated Tool Room shows that your supplier cares about controlling the whole process to the same high standard.

How do you protect the confidentiality of my product?

 Your terms and conditions will almost certainly have a confidentiality clause. After all, you didn’t invest all that time in design for someone else to ‘take inspiration’ from your work without your permission.


And yet, it is surprising just how few machining companies take this seriously. You can check this for yourself on a tour (see next chapter), but before you travel, ask some basic questions. These can include:

  • What are your protocols for guarding my product confidentiality?
  • How do you prevent visitors seeing my parts?
  • Do you have secure storage?
  • What happens to my data?
  • What physical access controls do you have?

What Quality Accreditation do you have?

This is almost certainly a given and you will only be considering accredited companies. Which standard though, and is it appropriate for your industry? As a minimum it is likely to be ISO9000. If you are in Aerospace you will already be familiar with ISO9100. The Automotive Industry relies on IATF16949. These are all specifically written for ‘series’ manufacturing. If you are a prototype or small-scale producer you will have to decide if this is appropriate for your job.

None of these standards are easy to get and maintain, but what really matters is whether they are a badge on the letterhead, or ingrained in the way of working. Ask your supplier what benefit they get from their quality standard
and see if you are convinced.

What to look for on a Machining Supplier Visit

Our previous chapter covered great questions to ask your supplier before a visit. But no matter how many questions you’ve asked as part of your supplier selection process, you can’t beat an in-person visit to really tell whether you have found your trusted machining partner. Here we run through what we would look for if visiting a new supplier for the first time.


Tidiness, cleanliness & organisation

This is important not only as it’s the first thing you see, but also because it’s a reflection of everything the supplier stands for. Open your eyes and assess how the place looks. Cover everything, from the reception area to the meeting room where you are hosted – does this feel like a well-looked after place? Do they care about the small things?

When you get to visit the shop floor, a seasoned visitor will know instinctively what to look for. Does it look safe? Are the PPE requirements clear and well-enforced? Machine shops normally insist on safety footwear and eye protection as a minimum. Are there clear pedestrian walkways and effective forklift truck separation? Attention to health and safety details is also a sign of a good general approach to quality.

Does the place generally look and feel clean? Remember cleanliness is critical in the accurate measurement of parts. Does the temperature feel right? A cold draft or leaves blowing in the back door are not going to inspire confidence.

Look at the space surrounding the machines. Are there clearly marked out areas for WIP? Are all parts identified with the appropriate paperwork? Do they track by batch or to part level by serial number?


Inspection Room

Before you even go to the machine hall, ask to see the inspection room. This is arguably even more important, as it’s where your part will be checked before it is passed for sending to you. This may surprise your guide who may be keen to show off the investment made in machines. But it’s essential – you need to check that they have invested as much in their inspection equipment.

Think about whether the inspection machines are relevant to your part. Have you already talked through how they will measure the required dimensions? Also, does it look like they have enough machines to match the capacity of the CNCs? If not, maybe they aren’t used to much of their work requiring tight tolerances.

Another important issue is whether the inspection room is temperature controlled. If so, how accurately? And if the inspection area is, but the main hall isn’t, make sure to ask where they soak the machined parts prior to measurement, as well as for how long.

You should also ask how they clean the parts prior to measurement. Is it a quick blow down with an air line? Or a wash?

Finally, enquire about their internal inspection capability. Do they use X-Ray, or CT scanning? What about pressure testing? What do you think about the lighting levels? Task Lighting for inspection should be 1000 Lux (compared to 200-300 in a warehouse). Some F1 customers even insist on 2000 Lux which is equivalent to a bright sunny day. It’s all dependent on the requirements of your part, but understanding their capabilities and testing processes is important.


Other customers’ parts

You can tell a lot about the general capability of a machine shop by the type of work for other customers you see while on a visit. Remember to look in the inspection area as well as the WIP in the machine hall.

Look for the general difficulty of work of the other parts. Are they mainly simple 3-axis jobs with limited fixturing? How many dedicated fixtures can you see? Do they have a fixture store? How big is it?

It may be that you have specific confidentiality requirements, but even if not, then it would be good to see that adequate provision is made to prevent you from seeing critical parts from their other customers. You may have asked about their protocols before you came. Do they live up to them?

Can you see other customer names on paperwork? If you can see parts that are almost certainly sensitive, such as military components or high performance automotive parts, ask what measures they take to prevent unauthorised access. Were you allowed to bring your camera phone on to the shop floor?


It might seem unlikely, but going behind the machines is one of the richest places for information on competitors’ parts- and accordingly, the general level of complex work undertaken by the supplier.

On your tour, ask to go round the back of a machine and take a good look inside the tool carousel. Most companies will leave the carousel full of tools to save unnecessary changes, which can be very useful to you.If behind the machine is full of drills and end mills then they probably do mainly simple work. However, if it has some fancy bespoke form tooling in there, then you know they do some clever stuff from time to time.

It’s also useful for consider their machine fleet generally, including their machine buying strategy. Do they have a mix of all sorts of manufacturers? Or have they settled on one or two? There is real benefit in having ‘sister machines’, both of which are approved. This gives a fail-over option in the event of machine problems.

You might also ask about the relationship they have with the machine manufacturers. The best ones will have very robust Service Level Agreements in place to ensure fast response times to breakdowns.

Post machining operations

Machine shops don’t just remove metal and then measure it. As a minimum they should have some form of cleaning process to remove swarf, coolant and other contaminants.

There will probably be a post-machining operations area and this can be interesting to see what else they can do for you. This might include coatings, painting or pressure testing. In many machine shops there is also an assembly area as well. It is more economical to get parts assembled where they are machined than having them shipped separately.

Make sure you get into the detail of post machining operations. For example, where do they package the parts up ready for dispatch? Do you have demanding packaging requirements? Returnable packaging loops? Air freight packaging? What are they used to doing? These type of questions demonstrate quality and commitment to high standards, so don’t skip them.


Does the general condition of the equipment look like new – even if it is 15 years old? These machines are relied on, sometimes 24 hours a day, to produce consistent, accurate results. They need careful looking after- do they appear to be looked after well?

Any shop can have completely new kit – for a while. This is evidence of a big budget though rather than a robust maintenance programme. Do they keep the machines clean? Inside and out – and on top and underneath? Are there leaks and spills?

Ask if the coolant is maintained properly, especially with regards to the coolant concentration. Bacteria is one of the worst enemies – does the machine hall smell? Rancid coolant is hazardous to health and can negatively affect tool life.

How do they manage their planned preventative maintenance (ppm) programme? Do they have fixed shutdowns? If they do, when are they and are they likely to impact your project? If they don’t, how do they make sure they complete their ppm tasks?

If you are particularly inquisitive, ask to see their machine service records. Check what recommendations were made at the last machine service and then ask whether they have been completed.

Quality controls

No self-respecting machine shop would wait until a part reaches Inspection before it is checked. There will be in-process checks to ensure critical dimensions are within tolerance and to monitor tool wear. A comprehensive approach will also check non-critical functions to double check the process is still capable. Often these can be done in cycle by the operator without extending floor to floor times. Check that the level of task lighting is bright enough and shadow-free to be able to visual inspect the part.

Ask the supplier where the Measurement Plan (Control Plan) is, the master document for what needs recording on all parts. Is there 100% monitoring and recording to a serial number level? Where is this recorded? Is it paper-based or digital? Electronic Control Plans are no easy to implement. They also have the added benefit of aiding in real time WIP monitoring and automatic time stamping.


Free Download

The Machining Buyer's Guide

Get your pdf or mailed* version of the Guide today

*qualifying companies


Get the Guide