Design for Manufacturing: Guidance for Durable Tapped Holes
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When you’re designing a part that can’t accommodate nuts and bolts, the best solution is to design tapped holes. Our precision machine shop will pre-drill the part’s holes and then insert a machine screw to “tap” the hole’s internal threads.
A valued customer recently requested a complex vacuum plate comprising nearly 40 tapped holes. We used forming taps and lubricating oil to tap the small holes with M3 x 0.5 and M4 x 0.7 threads. This approach allowed us to manufacture a precise and durable part quickly, efficiently, and without any tool breakage—all while ensuring impactful time and cost savings for our customer.
Keep reading to learn our Design for Manufacturing (DfM) guidelines for parts with tapped holes.
Understanding Tapped Holes
Unlike threading, which forms threads outside of a component using a die tool, tapping tools allow us to form threads inside a pre-drilled hole. It is then possible to screw directly into a tapped hole to secure it to a mating component.
There are five different standards of tapping tools:
ISO: short overall length (OAL), metric shank diameter, used worldwide
ANSI: short OAL, imperial shank diameter, used in America
DIN: long OAL, metric shank diameter, used worldwide
DIN/ANSI: DIN/long OAL, imperial shank diameter
JIS: short OAL, metric shank diameter, used in Asia
Additionally, there are two standard tooth forms, each with two name variations:
National Coarse (NC) and United National Coarse (UNC) are the same
National Fine (NF) and United National Fine (UNF) are the same
The difference between UNC and UNF tooth forms is the pitch—the distance between the crests of each thread. Coarse threads have a larger pitch, resulting in fewer threads per inch, and fine threads have a smaller pitch, delivering more threads per inch.
For example, a UNC tapping tool with a pitch diameter of 0.25” will form 20 threads per inch, whereas a UNF tapping tool with the same 0.25” pitch diameter will form 28 threads per inch.
Durability and Tapped Holes
For high-risk applications, durability is essential. If the part’s threads do not securely attach to the part’s mating component, the application could fail.
Summit CNC implements various strategies to minimize tap breakage, part loss, material waste, and overall cost based on your part's material and the quantity of tapped holes.
Many shops forgo these steps, but in our precision machine shop, we’re confident that our techniques—along with the DfM best practices we recommend—can make a tremendous difference in the longevity of customers’ parts.
3 DfM Recommendations for Precise, Durable Tapped Holes
1. Designate forming taps for your part’s threaded holes
Whenever possible, specify forming taps instead of cutting taps for tapped holes. While not a viable option for the hardest materials, formed threads are known to last longer than cut threads.
Forming taps literally “form” the threads by compressing the part’s material, making the threads stronger, more durable, and smoother. (The forming tapping tools themselves are more durable, too!)
Cutting taps, on the other hand, cut the material away to create the threads, resulting in weaker and less consistent threads.
2. Use our simple table to ensure properly sized threads
Before designing your next part with tapped holes, download our precision machine shop’s easy-to-use Internal Threads Table.
This simple chart provides actionable information for engineers designing tapped holes:
Learn to select the ideal combinations of form/cut threads and drill/hole size to enable us to quickly and accurately tap your part’s holes using SOLIDWORKS Hole Wizard or other CAD hole-generating tools.
Learn how to ensure that your interior threads meet ASME guidelines for inner thread diameter measurements.
By including critical information on your drawings, you’ll provide Summit CNC with the details we need to tap inner threads that adhere to ASME guidelines.
3. Specify the correct tap type
Sometimes, form tapping is not an option due to a part’s required tap size. Other times, a part’s material is too hard for form tapping.
When form tapping isn’t possible, simply request cut threads with inner hole diameters that follow ASME guidelines, and we’ll machine your part’s holes accordingly.
Get the Tapped Holes Table
When you need cut or formed threads, Summit CNC has the expertise to deliver beautifully machined parts with tapped holes that meet ASME guidelines and standards.
Use our Internal Threads Table, include your requirements on your part’s drawing, and get moving on your next project!
For parts with (or without) tapped holes, request a quote.