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.
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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.
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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
Vetting Your Supplier an Interview Checklist for the Buyer
Quality. Delivery. Price. Three basic things every buyer wants to be promised. But what assurance does a buyer have that those commitments will be met, time and time again, over the length of the program? When choosing a deep draw stamping and precision metal stamping company, you want to approach it deliberately and with forethought. Here are some tips to help you plan.
Like interviewing a prospective new employee, an interview with a prospective precision metal stamping supplier should include topics that give you a good idea of what their capabilities are, and whether they will be able to produce a quality product, reliably. I’ve discussed below the subject areas you should explore with prospective suppliers. We can assume that they will be open and willing to share this information as well, they should have the answers readily available. If not, these alone are indicators of their overall (in)ability to accommodate your needs.
- Quality Standard Certifications
One assurance is that a potential supplier is certified to a quality standard, whether it’s IATF (International Automotive Task Force), ISO (International Organization for Standardization), or one of the many medical, military, or aerospace quality standards. Whether your industry requires it or not, if they’ve done the work to achieve and maintain registration to a quality standard, it demonstrates an investment in their organization. But a good vetting process shouldn’t stop there.
- Well-Defined, Measurable Objectives
Your prospective supplier should have a well-defined set of objectives that are measured on a regular basis and are thoroughly understood and practiced throughout the organization. A quick interview of the company’s management should yield some honest answers about what their objectives are, how they measure up, and what they are doing to meet and improve their objectives. Performance objectives should include things like on-time delivery, customer complaints, customer reject rates, etc. These should be measurable and reported on a periodic basis. A report on cost of quality should not only include the cost of poor quality but also show an investment in prevention and improvement. Any reports should show a trend toward improvement or plan for corrective action.
Click here to download a checklist of interview questions to ask suppliers.
- Reinvestment Priorities
Part of your conversation should be about reinvestment in their company. Are your suppliers expanding their capacity and capabilities, or at least keeping them up to date? Are they able to add capacity for increased production needs? Are they willing to add capacity if you bring them enough business? If they are willing to add capacity, how quickly will they be able to ramp-up for full production and delivery?
- Value-Added Relationships
Ask about their relationships with their material sources. Do they buy enough from the type of supplier your product will require so that they have buying power and a dependable source? Do they have long-standing relationships with a complementary network, with possible contingencies and options in place should the supply chain get interrupted or compromised for any reason? Ask about their lean manufacturing initiatives. Is there an active program and is everyone trained and committed? You would want your product to be run as efficiently as possible to avoid costly mistakes and delays.
5. Workforce Training
At the core of any operation is the workforce. Find out what percentage is permanent versus contract help. It’s important to know how well-trained and supervised are they are ask. It’s increasingly common for companies to use a higher percentage of contract help, which can lead to a poorly trained workforce with high turnover, unless they are working in a very controlled and supervised environment. It’s too easy for expensive mistakes to be made under those conditions, with serious consequences to both your product and tooling. Costly events such as equipment crashes could result in interruptions to your product supply.
- Visual Check
Last but not least, a tour of the facility will be insightful. Is the space well-organized and clean? Does the equipment look clean and well-maintained? Does there appear to be adequate room for your product? Visualize, if you can, your product being processed through their plant. Does it appear there is a good workflow and an overall good fit for your requirements?
Although these questions may sound like a full-blown audit, some clear and structured questions during an interview with a prospective metal stamping supplier should yield some evidence and general indications fairly easily. ISO or IATF standards require these kinds of records, but any well-run company should be tracking their performance against objectives they set for themselves and have the information readily available for you.
The best vetting process still will not guarantee you won’t have any start-up issues or minor delays now and again, as things can happen beyond anyone’s control. But it will certainly provide some assurance that you’ve placed your product in capable hands that have a system in place to logically, nimbly, and scalably flex to your production needs with consistency while maintaining top quality.
AQP
We discussed advanced quality planning (AQP) in a recent blog. The intention of a AQP is to establish a system, from the very beginning of the manufacturing relationship, that ensures that customer requirements are met (and exceeded) throughout the tenure of the customer/manufacturer partnership for any given tool or metal stamping part. AQP provides a framework of communication among all stakeholders within the manufacturing company, and the process rolls out in five phases. There are some distinct benefits to highlight when following AQP.
Advanced Quality Planning Benefits
The benefits are always two-fold both to the customer and the manufacturer. They include:
- Saving time and money through the efficient use of time and resources
- Clear communication to set expectations and mitigate misunderstandings and risk
- Continuous improvement model that incorporates assessment, feedback, and nimble corrective action when needed
- Increase in overall customer satisfaction by avoiding or reducing disruptions (and their subsequent containment contingencies)
- Superior quality in designs and products, process flow and controls
Larson has has been practicing AQP for many years, which allows us to be a nimble and flexible precision metal stamping supplier—and we can do so in a scalable manner. We apply those characteristics to all projects, passing those advantages on to our customers.
To help you vet your precision metal stamping suppliers, we’ve compiled this interview checklist that will guide you through the information-gathering process. Download this checklist that guides you through the information-gathering process when vetting a precision metal stamping supplier.
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