This technical information has been contributed by
Summit Steel and Manufacturing

Sourcing Spotlight: FAQs for Selecting a Precision Machine Shop

Answers to frequently asked questions can help OEMs find a reliable CNC machine shop for high tolerance work that is also competitively priced

In industries like aerospace, defense, and medical, finding a CNC parts manufacturer capable of delivering the necessary quality, fast and reliable delivery, and competitive price can seem an elusive combination. It is akin to discovering the rare five-tool player in baseball that can hit for average, with power, field, throw, and base-run that doesn’t have a record-breaking contract.

Fortunately, such precision shops do exist. The difficulty, however, is in finding a shop that will actually deliver all that it promises in reality, and not just on paper.

Muddying the water, CNC shops – whether reliable and skilled, or not – all claim to deliver all of the above and more. Many have the same type and quality of machining equipment, but that alone doesn’t make for consistent parts, every time. Quality standards such as ISO and AS certifications go a long way, but these are also not enough to separate the best from the rest.

And then there is price. While not as important as quality for industries such as aerospace, defense, and medical equipment manufacturing, if the choice is between parts manufacturers that on paper appear similar, it will become a determining factor. But if the price is too good to be true, it probably is, and the result can be sub-par quality, missed deadlines, higher total overall costs, failures in the field, damage to reputation, and even litigation.

So how can OEMs choose the right shop for parts? While there is no magic bullet, the answers to a few FAQs will go a long way toward selecting a CNC machine shop that can deliver the requisite combination of consistent high quality, speedy delivery, and competitive cost.

For high tolerance work, how do machine shops ensure perfect quality every time?
”Perfect” quality means that when an OEM places an order for parts, they will be within the tolerances specified. These tolerances can be 0.0001 inch and tighter, and they apply to straightness, hole size, outer diameter (OD), inner diameter (ID), and other size/shape specifications.

“Industries such as medical, aerospace, or defense require ‘perfect’ quality because you can’t have one part compromising the quality of the assembled system,” says Gary Romig, founder and CEO of Summit Steel and Manufacturing (summitsteelinc.com), a metal component fabrication provider in Reading, Pennsylvania. “If one part goes into an assembly of 100 parts, and 99 are perfect, the single imperfect part can prevent product assembly or render the product defective and undeliverable.”

Quality is usually measured in rejection rate, measured in parts per million. Today, the acceptable rate of rejection continues to drop lower, heading toward the ideal of zero.

To produce a perfect part each time, CNC machine shops look for ways to continually improve and maintain tooling, check fixtures, and upgrade to the latest, state-of-the-art equipment.

Each new generation of CNC equipment, after all, is typically more precise, programmable, and faster than the one that preceded it. Summit Steel, for example, budgets a sizeable portion of its revenue to routinely upgrade to the newest CNC mills, lathes, and Swiss screw machines.

CNC machine shops also look to acquire the highest quality metals and materials, since these workflow through the shop with fewer problems and produce higher quality components. Purchasing from prime sources with full traceability is also essential to ensure that quality begins with the raw materials.

Another way parts manufacturers maintain consistent quality is by investing in automated inspection and measuring equipment, such as coordinate measuring machines (CMM). Instead of manual measurement, which can be lead to inconsistent results, CMM utilizes computer-controlled touch probes to measure various aspects of a part.

Although perfect quality does come at a price, the higher price up front often saves money on the back end.

“For the OEM, perfect quality means they don’t have to reject the part, send it back, have it reworked, and delay their deliveries. For the OEM’s customer, it gives them assurance the product they purchase will work as promised every time,” says Romig.

How do I select a parts manufacturer with confidence they will deliver the quality they promise?
Auditing a machine shop and visiting the facility can provide an OEM with a great deal of assurance that a high tolerance parts manufacturer will be able to deliver a high-valued part every time.



Check for capacity availability, quality control, and testing systems (including how often the instrumentation is re-calibrated). The OEM should also evaluate the staff they meet in regards to how well they understand their quality needs.

In addition, ask about the equipment. As mentioned, precision machine shops should be continually investing in the latest equipment. At Summit Steel, this is done on a tightly regulated, automatic schedule of replacement.

Finally, cleanliness and shop organization can also be major indicators of the quality of the overall operation.

“When an OEM visits a machine shop, they should find it clean, organized, and precise,” says Romig. “It should be a state-of-the-art facility with the latest equipment. Quality shops welcome audits and visits from potential customers.”

What is the best way to ensure on-time delivery without fail?
Most delivery date delays are caused by shipping to multiple companies and locations for secondary operations, such as part finishing, forming, coating, welding, and assembly. Any errors along the way only increase the delays, particularly if a vendor blames a sub-contractor for the error and will not take responsibility for correcting it.

For this reason, more parts manufacturers are adding ancillary services that go beyond traditional machining techniques.

“The surest way to guarantee on-time delivery is to work with a vendor that offers a full range of primary and secondary machine shop services, that will take full responsibility without dropping the baton,” says Romig, who has expanded Summit Steel’s in-house offerings over time to include laser cutting, centerless grinding, powder coating, welding, and assembly, all at the Reading plant. “When all necessary processes are done under one roof, rather than shipped out to a series of vendors, delivery times can often be cut by a month or more.”

Often, speeding turnaround and reducing costs simply comes down to working with a machine shop that spends the time to ask questions and really understand the customer’s expectations.

“Speeding turnaround is often directly related to asking all the correct questions up front,” says Romig, whose company typically ranges from 95 to 98 percent on-time delivery each month. “There are always different ways to achieve the blueprint tolerances, but sometimes unique tolerances are required for a part, sometimes not. Asking the right questions saves lots of time when it’s time to run the parts.”

Are there other ways to keep costs down, without compromising quality?
When it comes to CNC machine shops, sometimes bigger is better.

Smaller machine shops typically buy in small quantities from metal service centers that include a middleman mark-up. Some larger metal fabricators, on the other hand, have the buying power to purchase large quantities direct from the mill. These savings are then passed along to the OEM.

“Buying direct in bulk from metal manufacturers, not distributors, removes a whole layer of cost mark-up from the buying process,” says Romig. “It can translate into a savings of several percent even before the part is made.”

Del Williams is a technical writer based in Torrance, California.