Understanding Sheet Metal Tolerances

Sheet Metal

A Quick Guide to Countersinks, Curls, and Hems

Compared to other machining processes, designing Sheet Metal parts is relatively straight-forward. Sheet metal parts are made from a single metal sheet by punching, cutting, stamping, and/or bending and are known for their end-use durability. But there are still many important design-for-manufacturability issues to consider.

Check out our list of quick tips and tolerance specifications for designing Sheet Metal countersinks, curls, and hems! And for even more information, download our free Sheet Metal Design Guide!



The maximum depth of a countersink is 3.5 times the material thickness of the part. Countersinks must be at least 8 times the material thickness from each other, 4 times the material’s thickness from an edge, and 3 times the material’s thickness from a bend.

  Sheet metal curl


Outside radius of curls must be at least 2 times the material’s thickness. Holes should be placed away from the curl at least a distance of the radius of the curl plus the material’s thickness. Bends should be at least 6 times the material’s thickness plus the radius of the curl.

Tear drop hem


Hems are folds to the edge of a part to create a rounded, safe edge. Hems may be open, flat, or tear-dropped, and tolerances depend on the hem’s radius, material thickness, and features near the hem (Note: flat hems risk fracturing the material at the bend and should be avoided if possible).

For open hems, minimum inside diameter is equal to the material thickness (larger diameters tend to lose circular shape), and the return length is at least 4 times the material’s thickness. Tear-dropped hems must maintain an inside diameter of at least one material’s thickness, an opening of at least ¼ the material’s thickness, and return length is also 4 times the material’s thickness.

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Xometry Partner Network Profile: True Precision Machining, Inc.

Watch our latest video and learn how Xometry’s Partner Network is helping small manufacturing shops like True Precision Machining fill their excess capacity and grow their businesses!

The Xometry Partner Network provides members with free access to jobs through our online platform where you can view job details and only take the jobs that are a fit for your shop and fit within your shop’s capacity. There’s no bidding on jobs. Sign up today!

About Xometry

Xometry offers instant manufacturing pricing for precision CNC machining, 3D printing, sheet metal fabrication, and urethane casting services, plus live engineering support. Upload a 3D CAD file to get an instant quote.



How a Pittsburgh machinist adapted to the changing manufacturing industry


Anyone who’s even remotely familiar with the history of Pittsburgh knows that it’s now the “Steel City” in name only. In the 1920s, Pittsburgh steel companies produced one-third of the world’s steel and the industry generated over 300,000 jobs that served as the backbone of the local economy. But in the early 1980s that dominance came to an abrupt end.

Deregulation and foreign competition sent the price of steel plummeting and it wasn’t long before Pittsburgh steel companies began hemorrhaging jobs — 133,000 between 1979 and 1987. In at least one Pittsburgh-area county, the unemployment rate reached 27 percent, which is higher than the peak national unemployment rate during the Great Depression.

But Pittsburgh eventually endured and adapted to the 21st century economy. Today, the city is the home of thriving companies in the technology, health care, and education industries. And though many of those steel-era manufacturing companies closed down, some managed to evolve with the times and are still operating today.

One such company is Ashby Manufacturing. Now owned and operated by Tim O’Donnell (his brother Kevin is also a part owner), it was founded by his father, Manus, in 1976. “My father came over from England in 1965,” O’Donnell recalled. “He went to school for engineering and wanted to see what America was like, so he and a friend came over here and he just fell in love with it. Came back six month later and started renting an apartment from my grandmother, which is how he met my mother.”

This was when steel was a still-thriving industry, and O’Donnell’s father started as a machinist for a shop called Miller Printing. It wasn’t long though before he started taking jobs on the side and then eventually left to start his own machine shop. It started as a small operation in Pittsburgh and as it grew he moved to a larger facility in the suburbs. “As it got bigger, everybody started pitching in and helping out,” recalled O’Donnell. “My mom would have a phone in the kitchen for business calls. She was basically our secretary.”

Ashby Manufacturing
Ashby Manufacturing

It was fairly early in his life that O’Donnell realized he would end up joining the family business. He began working for it as a teenager and then later went to school for engineering and business. He learned CAD technology, which was relatively new at that time, as well as the intricacies of fulfilling government contracts. And by working in his father’s shop, O’Donnell was exposed to some of the most experienced machinists in the business. “Really our strength is the people who have been working here for decades, which translates to a lot of experience on the floor,” he said. “The second guy we hired is still here, and so is the fourth guy we hired. There’s another gentlemen who has worked here for 22 years. Bouncing around with all those guys allowed me to learn from their wealth of experience, and then I was able to add some expertise of my own on the technology side of things.”

By the time the steel market crashed in the 80s, Ashby Manufacturing was well-positioned to weather the storm. “We bought some high end machines that a lot of our competitors didn’t have, and that allowed us to go after some high end work back then,” said O’Donnell. “That set us apart.” It also primed the company for what came to replace the steel industry; the rise of tech, health care, and service companies shifted much of Ashby Manufacturing’s focus from high-volume orders to low-volume jobs with quick turnarounds. “Small quantity, quick turnaround kind of work is what we found we shine to. We were small enough that we realized we could take on things like that.”

But even though Ashby Manufacturing adapted to the changing economy and secured business relationships that have lasted for decades, it was still subject to the boom and bust cycles that most smaller machine shops face. Such shops are often highly reliant on local industries, so they’ll struggle to fulfill a rush of orders and then wait for new orders to come in — a wait that could last days or even weeks. These manufacturers also experience difficulty securing new customers because they don’t have a marketing budget or sales team in place. “A potential new customer might come along who wants to manufacture a part, and so you spend several hours making sure you have the right materials and determining the cost,” said O’Donnell. “But then when you get back to them you find out you’re not the lowest bidder, or they were only toying with the idea of making a part and not all that serious about it. You do have to be careful how much time you spend quoting things.”

Earlier this year, O’Donnell solved this problem by signing on to Xometry’s Manufacturing Partner Network. “We had a representative come in from a large supply house, and he said, ‘There’s this thing called Xometry, and its guys are looking for small shops and you guys are a real good fit,’” recalled O’Donnell. After spending time on the Xometry website and speaking with someone from its partnership team, he decided to register for the program. “I think we had our first job within three or four days. It’s a pretty good system. Whenever someone posts a job to Xometry, we can see the delivery date, the materials, and the price, and so then all we have to do is decide whether we’re able to take the job or not.” He likes the fact that he doesn’t have to waste time with marketing or quoting prices.

So how does O’Donnell decide whether he’ll take on an order from the Xometry network? “Every day I come in I know I am going to have five or six jobs on Xometry that I can fit into any free space that I might have out on the floor,” he explained. “It is just really nice to be able to have it , because always in the back of my mind, I’m thinking, ‘Oh man, I have no new orders coming in from my current clients, but I could take this job that right here [on Xometry] and keep busy for a while.’ That is a huge for us.”

Not only does the Xometry partnership allow Ashby Manufacturing to pick up work during down time, but it also expands its client base beyond local industries. “Normally, it is really difficult for me to say, ‘Hi, I’m in Pennsylvania, and I would like to do some quick turnaround work for you and you are out in Colorado.’ A lot of buyers are like, ‘I want the guy down the street. But Xometry has taken out that distance factor.”

Since joining the network in mid-June, Ashby Manufacturing has taken on and completed 15 projects worth a combined $30,000. “I never have come in and not seen a job on your board,” he said. “In fact, I’m staring at my computer right now and there are six jobs here to look over.”

“In fact,” he added, laughing, “I might take a job right after we get off the phone.”

About Xometry

Xometry offers instant manufacturing pricing for precision CNC machining, 3D printing, sheet metal fabrication, and urethane casting services, plus live engineering support. Upload a 3D CAD file to get an instant quote.  

Why 2 engineers quit their jobs and launched their own drone imaging company

Overwatch Imaging

When Nick Anderson embarked on his career in unmanned aerial vehicles (the technical term for what’s commonly referred to as drones), the consumer market for this type of technology didn’t yet exist. While enthusiasts today can purchase handheld drones for less than a thousand dollars, when Anderson graduated college — in 2008, with a degree in aerospace engineering– the market mainly consisted of high-end products that were only available to the military and large-scale industries. “When I graduated, the only way to work on UAVs was to join one of the handful of companies that produced them,” he said in an interview.

And so Anderson took a job at Insitu, a drone company that, around the time of his hiring, was acquired by Boeing. He was hired on as a mechanical engineer and was responsible for structural layout, CAD design, and fabricating rapid prototype and composite parts. Two years later he took a job at Cloud Cap Technology, which built drone avionics and stabilized turrets for larger UAVs. By this point the consumer drone market had begun to heat up, and though he enjoyed his work, he started noticing that lower-cost drones were beginning to take on tasks that weren’t well served by the higher-end UAV industry. “I bought a drone off Amazon and began playing around with it,” he recalled. “I saw it as this cool technology and individuals and small companies were making some neat data products out of these low-altitude drones. What struck me was that I had been working on these higher-end drones and they couldn’t produce images as good as the ones taken by a GoPro.”

Anderson and Greg Davis, a colleague who had worked with him at both Insitu and Cloud Cap, noticed that land management personnel, forest firefighters, and especially farmers were using these low-altitude drones rather than their high-altitude counterparts. “With the rate at which the imaging technology was developing, it seemed like there was an opportunity to marry those two together” — high resolution cameras with high-altitude drones — “and come up with some pretty cool ideas that would allow these larger, more expensive platforms to take the same quality of images, but at a much larger scale.”

What did this mean from a practical standpoint? Well, farmers were already using consumer drones to take aerial photos of their crops to assess their health, but, due to these drones’ ability to only fly at low altitudes, they were limited in scope and usefulness. A high-altitude drone, the kind that costs tens of thousands of dollars, can cover a larger geographic area at a faster speed.

Realizing there was a market opportunity, Anderson and Davis quit their jobs at Cloud Cap last year so they could launch Overwatch Imaging, a hardware and software company that builds high-quality imaging technology for drones.

One of Overwatch Imaging’s products, Cropwatch, uses two daytime cameras that are modified to be sensitive to the particular frequencies to tell if a crop is healthy or not.

Overwatch’s first product, launched late last summer, is called Firewatch and is designed for use by forest fire fighting teams. “We took a thermal camera and a daytime camera and coupled them together with a big computer on board to analyze the images that those cameras are taking in real time,” he explained. “It’s designed for forest fire mapping, to create a real time heat map of what is going on that shows up in Google Earth and other data programs.”

Currently there are only a handful of planes used nationwide for forest fire monitoring. “They fly over the Western U.S. and map these big fires at night” Anderson said. “Obviously they can’t go everywhere, so if your fire isn’t big enough or you don’t put in your request soon enough you may not get data for two or three days.” With the Firewatch system, a forest service employee could get the same quality data, and then they wouldn’t have to wait for one of those planes to become available.

Anderson and Davis developed the initial concepts for Firewatch in March. “We came up with three different prototypes and then around May we ordered all of the aluminum parts and expensive cameras,” said Anderson. For the prototyping, Overwatch relied on two in-house 3D printers and Xometry. “The printers we have at the office are great for when we want to get an immediate look at something,” he said. “But it is not the level of quality or refinement that we could get from a part printed by Xometry. So we had some parts that were built in-house, but the vast majority of the prototyping stuff was built through Xometry along with all production parts.”

Once the product was ready for market, it was then a matter of leveraging their personal network and attending trade shows to sell it. “There was the reaction of, ‘Yeah, we were waiting for something like this,’” said Anderson. “But also because you have these thermal cameras and the amount of electrical and software work that had to go into it, it’s quite expensive, so it’s not something a company decides to acquire overnight. So there was some serious work to get that first initial order in.”

But the customer response was strong enough that Anderson felt confident there was sufficient demand. So Overwatch set about developing its next product: Cropwatch. “That one we kicked off in August and we delivered in October,” he said. “For Cropwatch, we were able to use a lot of the same design and intellectual property we had come up with for Firewatch. The main distinction is that the Firewatch includes a thermal camera to detect heat, whereas Cropwatch uses two daytime cameras that are modified to be sensitive to the particular frequencies to tell if a crop is healthy or not.”

Overwatch Imaging
Nick Anderson, Co-founder of Overwatch Imaging.

Because Overwatch doesn’t build the drones itself, just the imaging technology that can be added with the drone, it teams up with drone companies that sell their services to these large agriculture companies and park services. “We are selling to the large agri-businesses of the world,” said Anderson, “those large companies that operate lots and lots of farms. The smaller, traditional farmers can use the consumer drones because they only need to map a thousand acres at most. For our product, we are talking about the ability to map all of North Dakota with a single UAV.”

While Overwatch is focused largely on expanding its penetration into the agriculture and land management services market, Anderson said there are plenty of new areas for which the company can customize its technology.

“Part of the company’s value proposition is the customized aspect,” he said. “We are trying to solve challenging problems that other companies have, so that kind of branches us out in a lot of different directions. We’re developing a core architecture that is mechanical and software based that we can then very quickly leverage into the right solution for different sized-aircraft, be it domestic or international. The plan is to grow, so right now we are hiring and expanding.”

About Xometry

Xometry offers instant manufacturing pricing for precision CNC machining, 3D printing, sheet metal fabrication, and urethane casting services, plus live engineering support. Upload a 3D CAD file to get an instant quote.  

See what’s new with our SOLIDWORKS Add-in!


We’re excited to announce improvements to our SOLIDWORKS Add-In!

Now, quoting with our Add-In is even faster, and the new look and feel gives you easy access to design-for-manufacturability feedback on your parts.


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  • Save time working in one integrated CAD workspace
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Getting Started

  1. Download and install the Add-In from xometry.com/solidworks-add-in.
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