In the world of precision machining, there’s a saying we’ve all heard about tolerances: every decimal place adds a zero onto the end of the price. Manufacturers know that 90% of the time, tolerance is what escalates the cost of parts. But still, customers make requests that require over-toleranced part designs that are challenging to bring to life. It isn’t their fault, though. There is a very clear gap between what can be designed and what can be manufactured. As a precision machining partner, it’s our job to help fill the gap so that you are able to get the parts you will need at a price that makes sense.

Being Incredibly Precise

When you really stop to think about the actual differences between a standard tolerance like +/-0.005” and a precision tolerance like +/- 0.002’ or +/-0.0005” it can really blow your mind how deeply precise these measurements are. Did you know that a single strand of human hair is approximately 0.003” thick? So when you specify a +/- 0.002” tolerance, you are asking for one with less permitted variance than the width of a piece of hair. A +/- 0.0003” tolerance is like taking one single strand of hair and slicing it into 10 pieces. Seems impossible, huh? Just one of those tiny pieces would be the equivalent of the permitted variance. Some applications will require these kinds of precision tolerances. We are experienced in machining tight tolerance parts and have the skills and equipment needed to meet your specifications. Nothing like a tight tolerance to challenge our team to excel. However, tight tolerance parts come with a cost as they add much complexity to the precision machining process. We remind our clients to think really hard if their project needs that level of precision before submitting their designs. We want to make sure you’re satisfied but not hit with an unexpected fee.

What Makes Tight Tolerance Parts So Costly?

The majority of precision machine shops are able to achieve a standard +/- 0.005” tolerance with ease. But when that tolerance is cut in half to +/-0.0025, it becomes more than twice as difficult to machine. High quality aerospace machine shops are still able to get these kinds of jobs done without a problem. Once we have a +/-0.0025” tolerance programmed into our machines, we are able to repeat it– but this process is more time consuming than your standard tolerance. When a part requires a tolerance of +/- 0.0005”, we are not able to achieve those and repeat them with ease.

What Are The Challenges We Typically Face?

Tooling Wear

Considering The Temperature And Climate

Materials will fluctuate in size at different temperature ranges, and some more than others. Alloys such as Monel and Inconel are hardly affected by changes in temperature. In contrast, certain plastics tend to shrink or expand when exposed to even a slight temperature fluctuation. While you might not notice this just by looking at it, the variation can easily compromise precision tolerances. Therefore, we have to consider potential temperature changes as we inspect tight tolerance parts when they come out of a hot machine. If we are machining a project with a +/- 0.0005’ tolerance and it needs to be inspected, we must first allow it to sit in our quality control department for up to 90 minutes just to help regulate the temperature before we can ensure the readings are accurate. Projects with standard tolerances might be iterated several times a day. With precision tolerances, we are only able to fit one or two iterations in a day as they require more time to perform an accurate inspection. A step that might only take a couple minutes can take hours or even days to be done, yet another reason the cost rises.

How It Pans Out

If we make a part that has a tolerance of +/- 0.0005”, then we go to cut an additional part, but the tooling has been worn down to +/- 0.0005”. Now we have to cut that part one time, take it out, allow it to adjust to the temperature, thoroughly inspect it, realize that it is no longer in tolerance, put it back in, and repeat. This process can be very time consuming.

Avoid Making A Big Deal Over A Slight Difference In Unnecessarily Tight Tolerances 

Precision machine shops provide a quote based upon the designs that clients submit. We have worked with many customers who have unknowingly over-toleranced their parts without being aware of the challenges we face trying to make them happen. Many times, when we reach out to our clients and explain their tolerance options, which are either accepting the part with slightly looser tolerances or allowing us to achieve the precision tolerances as desired with a cost increase and longer production time, they make the decision that they would be okay with a looser tolerance. Sometimes designs can be adjusted in the beginning before the process starts to take place. Consider this when drafting your parts and tolerance specifications.

But We Do Enjoy A Good Challenge And Tight Tolerances

There are several shops that are not willing to offer a quote on projects that have tolerances below +/- 0.0005” but we, at Palmer Tool Company, like a good challenge. We have the precision machining capabilities to achieve tight tolerances and top-of-the-line inspection equipment to ensure that all specifications have been met. Be aware that tight tolerance parts will require more planning, tooling selection, and quality control preparation, all of which add to the costs and production time. So, before you submit your designs, reassess your tolerances and decide whether or not they are unnecessarily tight. Even a slight difference can save you quite a bit of time and money– it is worth the double check.