In complex CNC machining, a standard tolerance of +/- 0.0050” is sufficient for most parts. When tighter tolerances are necessary, Reata Engineering can reliably hold tight tolerances of +/- 0.0005” before costs significantly increase.
However, we often see customers specifying tolerances of +/- 0.0002” and tighter when those tolerances aren’t necessary for the part’s function. This is when customers start to see high costs due to tooling needs, longer labor hours, and increased material waste.
We get it—you want your parts to look good and function perfectly. We feel the same way! But over-tolerancing isn’t the best path to an optimal part, and making this mistake can significantly increase your costs.
Here’s our advice from over 30 years in the precision machining industry.
How to Use Tolerances in Precision Machining
As a rule, Reata engineers advise customers to specify the loosest tolerance that maintains part functionality. This recommendation aligns with our Design for Manufacturability (DFM) principles, which help customers produce the highest quality parts as quickly and cost-effectively as possible.
Let’s use steel dowel pins as an example. There are three primary types of fits for dowel pins:
- Slip fit: parts move past each other and almost touch, but not quite. We recommend medium-tight tolerances with at least 0.0005” between surfaces to avoid friction.
- Interference/Press fit: parts are pushed together and held by friction to create a joint. The dowel is often slightly larger than the hole. We recommend tight tolerances whose sizes interfere between 0.0002” and 0.0008” for a secure fit. These numbers can change proportional to the size of the assembly.
There are some common assumptions about tolerances that are just plain wrong. For example, many customers believe threads require tight tolerances. But the opposite is true. Threads are air-fit and able to move, so tight tolerances are unnecessary. In fact, most commercial off-the-shelf screws are actually undersized!
In our experience, extremely tight tolerances are necessary for metal-to-metal interference fits, bearing shafts, and other critical fits. These applications require high precision for parts to function correctly and are worth the extra expense of tight tolerances.
The Impact of Over-Tolerancing in Complex CNC Machining
When customers over-tolerance parts, the main consequence is increased costs and lead time. Parts with tolerances of +/-0.0002” cost significantly more to produce as parts with tolerances of +/- 0.0005”. Just because Reata can do it doesn’t mean it’s in your best interest!
The tighter the tolerance, the more difficult it is to maintain part consistency on production runs. Tight tolerancing increases scrap rates as well as material, labor, and tooling costs. Ultimately, you’re paying for parts you’ll never even see!
Sometimes, tight tolerancing can even negatively impact part functionality. For example, over-tolerancing surface finishes can inhibit paint adhesion because there isn’t enough texture for the paint to adhere to.
When in Doubt, Communicate with Your Manufacturing Partner
Over-tolerancing is common when intent isn’t properly conveyed in the quoting process.
Often, customers specify tight tolerances without explanation, which puts shops like ours in a difficult position. If a shop suggests that a customer loosen their tolerances, the customer might think it’s because the shop can’t achieve tight tolerances and take their parts elsewhere. The shop then loses the job. To avoid this outcome, most shops avoid starting this conversation and build the parts to print, costing the customer extra money for tight tolerances they don’t need.
That’s why communication is so important. At Reata, we encourage you to start a conversation with us when you request a quote—especially if tight tolerances are involved. Ask us for our DFM advice, and we’ll happily provide you with it.
Our goal is always to save our customers money on high-quality parts. You can trust our decades of engineering and precision machining experience, especially paired with our customer-first mindset.
And if your part does require tight tolerances, we’ve got you covered.
Request a quote to work with us.