Are You Wasting Money With Tolerances That Are Too Tight?

“We Machine to Tight Tolerances” 

One question we’re often asked is “what tolerances can you machine to”? Of course, when we’re talking about precision machining, tolerance requirements are critical to successful machining projects, so it’s a valid question. However, “tight tolerances” is a phrase that is often loosely thrown around in the machining industry. You’ve undoubtedly seen it on other machining websites. “We machine to extremely high tolerances”. “High tolerance machining”. Sound familiar?

Let’s talk about tolerances for a minute.

What Are Tolerances in CNC Machining or Engineering?

Tolerances identify acceptable variations for given dimensions (a machined part, for example). Tolerances are typically provided in engineering drawings or specifications. Engineers rely on tolerances to call out how far a measurement can range from its intended dimensions. If we think about a railroad spike for example, versus, say, aerospace components, or medical device components, they are going to have different levels of acceptable tolerances.

In reality, all modern CNC machining equipment (CNC mills, CNC lathes, wire EDM, waterjet, etc.) are capable of tolerances that are extremely tight. Maximizing the potential of today’s CNC machining technology isn’t as straightforward. Achieving the tightest tolerances in CNC machining involves several variables.

Factors that impact CNC machining tolerances

The quality of tooling plays a part. The frequency of tooling changes will also impact achievable tolerances. Of course the machinist, or operator plays a role as well. How many setups will your part require? With our 5-Axis machines we’re able to complete multiple operations with one setup. Less handling means less opportunity for error – human or otherwise. 

At Blanda, we certainly have the ability to machine parts to extremely high tolerances. In fact, a significant part of our business has been machining components for the military, aerospace, computer and medical industries, which often require the most accurate machined parts possible. 

Why are tolerances so important?

Part design, dimensions, materials, and engineering tolerances all play a part in optimizing parts for both reliability and manufacturability. Without a doubt, determining proper tolerances, and the acceptable amount of variation is so critical to the success of any machining project. 

± .1 vs. ± .01 vs. ± .001 – More is better, right? So Why not default to the tightest tolerances possible?

Sounds like a reasonable question, but let’s explore this.

At Blanda, we’ve been producing quality machining for over four decades now. As a precision machine shop, we are accustomed to producing machined parts with extremely tight tolerances. There are definitely instances where the tightest tolerances possible are called for. 

Examples: aerospace machining, medical machining, machining for the computer industry. That said, not all of our machining work for these industries requires the tightest tolerance machining available. There are other factors to consider that can potentially save you money on your machining project.

Does the environment call for tight tolerances?

What’s the part’s function? Does it interact with any other parts? Is there perhaps just one critical dimension as opposed to the entire part?

There are also times where “loosening” the tolerance a bit makes sense. Have you considered testing parts that are slightly out of spec?   

We often see tolerance requirements that may be a bit too restrictive for the intended use of the machined part. In fact, many times we see tolerances set in the CAD software and all of the dimensions are set to the same high number. When this happens, here’s what we often hear next:

“Why is it so expensive?”

This is where it gets a bit more complicated. 

Let’s go back to our question above, which is “why not default to the highest tolerances possible?”

With an increase in tolerance requirements comes an increased number of in-process inspections, more frequent tool changes, a lower percentage of acceptable parts, higher scrap rates and higher material costs. Any one of these will add to the cost of a project, but collectively, can have a significant impact on your project cost.  

In fact, when we extend your tolerances out by just one decimal, it can double or sometimes triple the cost of your machining project. 

Aside from the additional hands-on and inspection requirements, it may also restrict your production method to a less-economical method than is really necessary for your application.

It’s important to have an understanding of the true impact of your default tolerances, as they greatly influence the price. Our recommendation is for the tolerances to be as generous as possible, without sacrificing quality or reliability.  

Your machining partner should help you make an informed decision regarding tolerances. At Blanda, we help our customers bridge the gap between machining and manufacturing, by leaning on our decades of experience. 

Whether you need assistance with material selection, part design, or have questions regarding proper tolerances for your upcoming machining project, the team at Blanda is happy to assist.