The proper selection of mechanical design tolerances ensures projects stay within budget. And more importantly, designs remain functional.
To get it out of the way, I’m not a mechanical engineer or machinist. But, I’ve done my share of tinkering with devices at home and at work as an electrical engineer. So I know a thing or two about mechanical design tolerances.
At first, though, I was utterly confused with mechanical design tolerances. I remember I’d mess around with a design not knowing what to use as a reasonable tolerance. I’d ask my friends for help, and they’d tell me a number without explaining the why. Or, they’d tell me to write “critical dimension” for the factory to figure things out themselves.
Not only was their advice not helpful, but it was a recipe for disaster. Instead, what I needed, was the following questions explained to me:
- What tolerances are realistically achievable?
- How repeatable can a given tolerance be in a design?
- What’s the cost for varying tolerances?
- How do tolerances vary when using different materials and machines?
I’ll address each of these questions in the 8 tips I discuss. Because I know firsthand how frustrating choosing design tolerances can be. Before we start though, let’s define tolerances.
What is dimensional tolerancing?
Tolerance is the amount of variation in a measurement you can tolerate from the dimension’s nominal value. In other words, the amount a measurement can be off by, and your design still functions.
For example, say you need a piece of metal to be cut 64 inches long. You wouldn’t just tell a factory the following:
“Cut a piece of metal 64-inches long.”
Because you’ll never get a piece of metal cut to exactly 64 inches. Your measurement may end up being 64.1 or maybe even 68 inches. Instead, you say you want a +/- 0.1-inch tolerance for your 64-inch long part.
In return, your machinist will know your part can range anywhere in length from 63.9 to 64.1 inches. This becomes your max and min limit, or the boundaries for quality control. To further illustrate tolerances, take a look at the following example:
Now not surprisingly, the tighter your tolerance the better it theoretically is. Because parts will better fit and you’ll have a more aesthetic-looking design. BUT, there’s a catch.
The tighter your tolerance, the more expensive and time-consuming your factory work becomes. The factory will need to use more precise expensive machinery with high-skilled workers.
One of my favorite examples of poor tolerances is with cheap furniture. Say you find an awesome deal on a dining table. So you race home to assemble together the table you just bought. Soon thereafter, you find out screw holes don’t line up. Your excitement quickly fades and turns into deep frustration. In the end, though, you get what you pay for.
#1 Understanding the costs associated with tolerance selections
The first step is to always determine how precise your tolerances need to be for a functional design. The best designers I know can maintain a product’s function with the loosest tolerances. And this is what makes design work an art.
The following table from the Machinability and Machining of Metals shows the increase in cost with tighter tolerances:
|Surface finish technique||Tolerance||Approximate relative cost|
|Rough machining||+/- 0.030||$101|
|Standard machining||+/- 0.005||$200|
|Fine machining||+/- 0.001||$440|
|Very fine machining||+/- 0.0005||$720|
|Fine grinding||+/- 0.0002||$1,400|
|Very fine grinding||+/- 0.0001||$2,400|
|Lapping, polishing||+/- 0.00005||$4,500|
You can see how the cost rapidly increases as the tolerance becomes tighter. Now, imagine if the factory has to apply tight tolerances to many dimensions of your product. Your costs will jump up fast. Even more, imagine you need to manufacture thousands of copies of your product. Your costs will then skyrocket. This increase in cost for tight tolerances comes from the following sources:
- Expensive tools
- High paid expert machinists
- Advanced processes to support the operations of machines and tools
- Intricate inspections to test and verify the quality of tolerances
To avoid the exponential high costs, make sure you actually need tight tolerances. I always find starting as loose as possible with your tolerances is the best way to go. Only then, tighten your tolerances, as necessary.
Also, learn about parts to make good judgment calls. For example, the tolerance for computer hardware will be tighter than with furniture. And to help you conceptualize necessary tolerances, become familiar with tolerance standards. You’ll find many tolerances already exist in parts of the engineering world. This way you can better pin down an appropriate tolerance and not burn money. But also, just as important, not burn time as illustrated below.
#2 Errors with measuring instruments
No measuring device is perfect.
What’s more, a tool’s placement in dimensioning always has an associated error. To make matters worse, this positioning error grows the greater the measurement distance is.
To illustrate this increase in positioning error, look at the below table. You can see as the distance increases, from A to G, the error increases as well.
|Segment parts of a single product||Target measurement||Actual measurement|
|A to B||10 inches||10.5 inches|
|B to C||10 inches||10.3 inches|
|C to D||10 inches||10.4 inches|
|D to E||10 inches||10.6 inches|
|E to F||10 inches||10.2 inches|
|F to G||10 inches||10.5 inches|
The total target measurement from A to G is 60 inches. While the total actual measurement from A to G is 62.5 inches. Now, imagine if you had to align a hole at part G with another part that measured 60 inches. You’d be off by 2.5 inches!
So, always find factories that’ll meet your expectations with tools and measurements. I’ll discuss more on tolerance accumulation in part #8.
#3 Tight tolerances and common sense
If you make your tolerances too tight, you’ll always run into problems. I’ve seen before where an engineer marked a +0.05/-0.00 tolerance for a 2.00-inch hole. Then, they marked a +0.00/-0.005 tolerance for the mating 2.00-inch shaft.
If the machinist stays near the median of these tolerances, the shaft will freely turn in the hole. But, what if the machinist cuts both the hole and shaft to exactly 2.00-inches? You’ll have 0.00-inch clearance, and your shaft won’t turn.
Then guess what, none of this is the fault of the machinist. The machinist cut within your designed tolerances. So ALWAYS consider the worst-case tolerances in your design. This way you maintain clearances to ensure proper function.
What’s more, you always want to use various existing proven tolerance guides. BUT, be aware of how crazy fine some of the tolerances can be. A lot of the time, the tables are statistical and theoretical based. They don’t consider manufacturing methods.
And this is where you come in. You can refer to a standard table in your drawing but also direct the machinist. Direct them on the best manufacturing method to use that’ll get you what you want.
Important Note: oftentimes with large dimensions, you don’t need pinpoint accuracy. For example, you don’t need to cut a 20-foot long beam to 4-decimal places. In these cases, make it clear to the machinist what you explicitly want. Otherwise, the machinist will waste endless time trying to cut the beam to 20.0000 feet, which is ridiculous.
#4 Real-world factors impacting tolerances
Many real-world factors affect your tolerance choices. Thus, always think over each factor before you start your design work. This will save you A LOT of future headaches.
Before I do any design work involving tolerances, I think over each of the following factors:
- Bearing load
- Length of engagement
For example, let’s talk about material choice. You always need to consider a material’s physical and mechanical properties. Because certain material properties will heavily impact your tolerance levels. Maybe a given material may become too brittle if it’s cut too thin.
As another example, parts in extreme temperatures need heavy consideration. This is because of thermal expansion. Materials, in general, expand when heated and contract when cooled.
So always look at the big picture before diving into choosing your tolerances. And over time, your gained experience will simplify a lot of the tolerance selection work.
Important Note: the machinist who makes your part just needs your design. Don’t expect the machinist to know everything about your material. For example, how your material behaves in heat treatment. This is especially the case when you’re only paying them to cut for you.
Thus, your drawing should list all requirements for the manufacturing of your part. This includes calling out the manufacturing process, to ensure the result is acceptable.
If you need your manufacturer to fill in the holes in your design, your cost can drastically increase. The machinist will need to research on their own, trying to figure out your design.
#5 Verification of CAD settings and standards
Verify the set tolerances in your CAD software are what you want. For example, your CAD software may be set to use 4 decimal places versus 1 for all your dimensions. As a result, you’ll unknowingly use tighter tolerances than necessary. In return, your project costs will shoot up.
What’s more, check your design drafting standards in your title block as shown below. For one, make sure your drawings have your standards listed. These are default tolerances applied to dimensions when explicit tolerances aren’t listed. So, this is a legend your machinist will use to guide them. Secondly, make sure your listed standards match the specs of your project.
Just as important, use ordinate dimension sets in your drawings. This way you’re specifying a zero location in the X and Y-axis. Make sure you do this for each view of a part too.
Important Note: not always do you need to list out your dimensions. If you know a machinist, and the project isn’t overly precision-driven, you can spell things out.
For example, you can point to a hole with a leader reading “press-fit dowel.” The machinist will then know how to best ream it for a good fitting for your dowel.
#6 Drawbacks with limit dimensions
Limit dimensions are a set of min and max values, with any value in between being acceptable.
The common problem I encounter with limit dimensions is with production personnel. They often will machine the parts consistently to the higher boundary. And I get why.
It’s a time-saver, the tool wear and tear goes down, and it’s a hedge in case anything going wrong. Because production folks like to stay on the safe side of tolerances. So if something goes wrong, they’ll have material left to rework. You don’t have this luxury when you cut at the mean of a tolerance.
The point is, be aware this happens. If you’re okay with it, then it’s a moot point. If you’re not okay with it, then use bilateral tolerances. You provide a variation from your target dimension in the positive and negative directions. For example, plus or minus 0.1 from the target dimension. In return, the machinist will more times than not cut at your target dimension.
Important Note: limit dimensions cut out ambiguity. Sometimes machinists become confused with unilateral tolerances. Imagine in your design you list a 6-inch shaft with a +0.05/+0.10 tolerance. In your mind, you expect to have a shaft made that’s 6.05 to 6.1-inches.
But, I’ve received the wrong-sized parts before, because the machinist thought I meant +0.05/-0.10.
#7 Limitations of manufacturing methods
From a design standpoint, you can specify any tolerance your heart desires. But the real question is, what can your workshop actually deliver on? Different manufacturing processes are capable of delivering different tolerances.
This is the classical problem for mechanical design engineers. So always ask a machine shop what tolerance they can hit at a reasonable price.
Because you need to fit your design into the resources you have available to you. For example, let’s say your factory can only hold +/- 0.00X tolerances. But if your design calls for +/- 0.000X, you’ll need to outsource, or you’ll get low yield rates.
Just as important, always figure out your tolerances first. Your tolerance will dictate the manufacturing process you choose. Don’t let the manufacturing process dictate your tolerance for you. Of course, your budget plays a big role in what manufacturing processes you have available to you.
Important Note: a given manufacturing process isn’t always constant. Achievable tolerances will vary between workshops, depending on the experience of the machinist.
At the same time, don’t overly specify manufacturing processes. A machine shop will help you decide the best method to use given their tools and your project.
#8 Tolerance Accumulation (stack up)
The tolerance between two points on a design depends on the controlling dimensions. This is why the max variation between two points is critical to know. Because too much of a variation can throw your entire design off.
As an example, the below design graphic shows the best and worst-case scenarios. As the number of controlling dimensions increase, your tolerance accumulation increases too. Your variance is then equal to the sum of the tolerances from the controlling dimensions.
Important Note: some dimensions don’t list tolerances. But there’s always an implied tolerance.
Relative dimensions with mechanical design tolerances
Each tolerance in the path to the final dimension, adds to the total system tolerance.
Say you have 6 back to back dimensions, each with a +/- 0.01-inch tolerance. The worst-case for the sixth dimension is the summation of 0.01 + 0.01 + 0.01 + 0.01 + 0.01 + 0.01. The accumulated worst-case tolerance is thus 0.6-inches above the target dimension.
This is not the ideal outcome, because your design may not function. If you only want the sixth dimension to be +/- 0.01 inches relative to your first point, you need to specify a new relationship. So you provide a new dimension and tolerance and leave out the middle tolerances.
Important Note: you need a solid understanding of your end design. This means understanding how all parts of your design fit together. At the same time, do tolerance stack-up analysis throughout your design work. This way you won’t end up with a bunch of parts that don’t properly mate.
Selecting mechanical design tolerances is an art and a necessary part of a lot of design work. So familiarize yourself with tolerances, to avoid future headaches and costs.
I recommend using specialized software, to help you analyze mechanical design tolerances too. The software can tell you if there will be any tolerance stack-up issues among other things.
More importantly, use common sense. Figure out how parts will fit together and work. Then, design with as large as a tolerance possible, without compromising on function. This will make you a better designer, and you’ll make a lot of machinist friends.
What’s your biggest challenge when selecting mechanical design tolerances? What tips and tricks do you use when it comes to mechanical design tolerances?
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Koosha started Engineer Calcs in 2020 to help people better understand the engineering and construction industry, and to discuss various science and engineering-related topics to make people think. He has been working in the engineering and tech industry in California for over 15 years now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.
Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).
Koosha is most interested in engineering innovations, the cosmos, our history and future, sports, and fitness.