Originators and Designers of Specialty Reamers and Cutting Tools Since 1918

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Tolerance Explained

Basic Types of Tolerances

Limit Dimensions

Upper and Lower dimensions are placed on top of each other. The dimensions show the largest and smallest values allowed. This means that any dimension in between these values will be accepted.

Unilateral Tolerances

The tolerance variation only occurs in one direction. For example, in this case it's going up by +0.005.

Bilateral Tolerances

Bilateral Equal occurs when variation from the target dimension is shown going both positive and negative directions equally.

Bilateral Unequal is the same as Bilateral Equal, but both directions are going unequally.


Tolerance Problems in The Manufacturing Industry

In the manufacturing world, the important concept of tolerance is used all the time because it defines the accuracy necessary to create a functioning product. Imagine that you work for a company that builds engines and you need to ream a precise hole in the cylinder head where the valves reciprocate. You look around and eventually find a company that manufacture reamers for this specific job. You take a chance and order 100 reamers per your company needs, and they arrive in few weeks. Wonderful right? Maybe not! As your staff inspects the parts, they begin to realize that the size of the holes are not consistent. Half of them are fitting and the other half don’t. You realize that the reamers you purchased are not accurate enough for the valve to function properly. What happened? Why don’t the valves fit the holes?  In order to explain this, let us go back to the beginning and explain what “tolerance” means in manufacturing.

According to Webster, tolerance is defined as: “the range of variation permitted in maintaining a specified dimension in machining a piece”. When a product is being made, it’s never “perfectly” accurate and having a tolerance tells us how close it needs to be. These dimensional tolerances have a high and low limit, denoted as, + and –, respectively. For example, if the diameter of an object is 45 mm and the tolerance is written as +.003 and -.003. This means that the diameter is acceptable between 44.997 and 45.003. When a reamer, in this example, is made, the customer needs to specify the tolerances beforehand. Usually machinists have blueprints that have all the dimensions and tolerances needed for a job. Let’s say you specified the reamer diameter to be .6250”, but you did not specify a tolerance. As your staff informed you of some inconsistencies, you take a micrometer or a vernier caliper and measure the diameter. You are in shock! The reamer diameter measured .6255”, larger than what you asked for by .0005”. You were expecting it to fall between + .0003 and - .0000, and this is concerning because if the reamer cuts the hole oversize or undersize, the clearance between the valve guide and the valve increases, causing the valve to seat improperly and the lack of compression can generate an engine misfire. This can also cause an increase in buildup of hydrocarbon deposits in the combustion chamber, triggering an increase in compression and ultimately a fatal detonation.

What happened? You call the reamer manufacturer and they tell you that their shop follows the ANSI Tolerance Standards, not Aerospace Standards. Unless specified, and based on the diameter given, the tolerance applied was +.0001 / +.0005. Some cutting tool companies will apply their own “manufacturer standards” to unspecified dimensions, especially when they are not given a tolerance to hold, giving you a product that may or may not work based on your application. As we see in this scenario and in many others in manufacturing, tolerances are extremely important. Specifying proper tolerances will save time, bypass design issues, reduce unnecessary costs, and in some rare cases even help save lives.


The Gammons Hoaglund Company guarantees tolerance-tight products and will ensure that you save time, money and manufacturing do-overs. We figure out how to effectively balance tolerance requirements while reducing costs. By taking aggressive measures, we make sure to avoid both “over- tolerancing and “under-tolerancing” your products.

Some steps we follow:

  1. Review the details of the product’s application and blueprint.
  2. Determine the required tolerance
  3. Analyze tolerances for other components of the reamers and how they will affect the other tolerances.
  4. Determine any outside inconsistencies and address any machine-related fixes
  5. Consistently train engineers/machinists to reduce the costs of tolerance specifications


Whatever tolerance needs to be applied; we get it done the first time around.


Most cutting tool manufacturing companies that produce reamers don’t meet aerospace standards. At the Gammons Hoaglund Company our standard tolerances are as tight or tighter than the aerospace standards. We vow that our reamers will meet tolerances better than anyone else in the industry. We strive to make it our mission to deliver each reamer with unparalleled precision and outstanding performance. If you want more information on our reamers and their tolerances contact us today and get a free consultation. You can also check out our Tolerance Calculator by clicking here.