Achievable Machining Tolerances

[Othon Sdcd]

Thereforemanufacturers often go through the various types of manufacturing processes and compare them while keeping the machining tolerances as a major factor. To understand the machining tolerances of different processes, it is vital to know the concept of machining tolerances, how to measure them, and the different types of tolerances that exist.

Machining tolerance is the value limit till which variation in a dimension can be allowed in relation to its ideal blueprint values. Machining tolerances depict the accuracy of any manufacturing process.

Since there is no such thing as a perfect process, the value of machining tolerances can never be zero in practice. However, modern manufacturing techniques such as CNC machining have brought this value quite down and to the minimum.

The basic size of a workpiece is the size mentioned in the blueprints. Manufacturers and designers know that the manufacturing processes will have a certain level of tolerances. Therefore, designers choose the basic size keeping in mind the deviation that will occur during the manufacturing process.

The actual size is the dimensions of the final product after the machining process is finished. While the basic sizes are theoretical values, the actual size is the practical realization of the finished part. While it is almost impossible to make the actual size exactly the same as the basic size, manufacturers aim to bring these two values as close as possible.

Limits are the maximum and minimum allowed dimensions of the part. The maximum allowed dimension is called the upper limit and the minimum allowed dimension is called the lower limit. If the actual size of the part falls outside of these two limits, the part is considered unusable and rejected.

Deviations are the variances of the maximum allowed size from the basic size. Since there are two types of maximum allowed size- upper and lower limits, there are two types of resultant deviations: upper deviation and lower deviation. Calculation of these deviations is easy:

In physics, a datum is an imaginary line or plane chosen arbitrarily as a reference point for measurement tools. The concept of Datum is also used in many types of geometric dimensioning and tolerancing areas, which will be discussed in the sections to come.

Maximum Material Condition (MMC) occurs when a feature or segment of the workpiece contains the maximum amount of material in all places. Examples of MMC can be the smallest size hole or the largest pin in a workpiece. The occurrence of MMC provides bonus tolerances to work with.

Similarly, the Least Material Condition (LMC) occurs when a feature or segment of the workpiece contains the least amount of material in all places. Examples of NMC can be the largest size hole or the smallest pin in a workpiece.

The use of MMC and NMC dictates the clearance fit for an assembly. MMC is the worst condition scenario in which the part would still fit. Any increase in size beyond the MMC would not allow the assembly of the product.

The shift from MMC to LMC allows for a greater allowed tolerance in the workpiece area, which is called the bonus tolerance. The calculation of bonus tolerance depends on how much lower material the actual part has compared to the maximum material. Therefore,

In high-precision processes such as CNC machining, tolerances occur in very small amounts. The actual value of tolerances in CNC machining is so low that it requires decimal places to measure it. A higher number of decimal places correlate to tighter tolerances and higher accuracy.

Sometimes, instead of mentioning upper and lower limits, the limits are described in the form of variation, such as 10 0.2 mm. In this case, the upper and lower limits can be calculated by adding and subtracting the variation respectively.

Tolerances in CNC machining are expressed in different ways, due to the different geometries of parts and the different types of machining processes. Let us go through these different tolerances one by one:

Unilateral tolerances in CNC machining hint that the allowable variance can only occur in one direction. The basic size of the component is the same as the upper limit or the lower limit, and the tolerance can only be either positive or negative but not both.

For instance, if a pipe has a diameter of 10 mm with a unilateral tolerance of +1 mm, both the basic size and lower limits of the process would be 10 mm. The upper limit in this case would be 11 mm. All the acceptable parts should fall within this range, and any part smaller than the basic value of 10 mm will be rejected.

Similarly, if a pipe has a diameter of 10 mm with a unilateral tolerance of -1 mm, both the basic size and upper limit for the process would be 10 mm. The lower limit in this case would be 9 mm. The manufactured parts should fall between this range and all the parts even slightly larger than the basic value of 10 mm will be rejected.

Contrary to unilateral tolerance, bilateral tolerances allow variation in both directions. The basic size of the component lies between upper and lower limits and the value of tolerance can be both positive and negative.

To take an example, if there is a pipe with a diameter of 10 mm and a bilateral tolerance of 1 mm, the basic size will be 10 mm, the upper limit will be 11 mm, and the lower limit will be 9 mm. All parts between 9 mm and 11 mm will be acceptable. Therefore, the actual part can be smaller or bigger than the basic intended part.

Limit tolerances are easy to use and eliminate the need for any calculations. If limit tolerance is depicted in a graph, the upper limit is stated over the particular dimension and the lower limit is stated under the upper limit and over the particular dimension.

A major thing to remember is that while limit tolerances use different expressions than bilateral tolerances, the part outcome is going to be the same. The difference only comes in the ease with which the blueprint reader comprehends the design criteria.

Profile tolerance is very different from the other types of tolerances mentioned above. While the other tolerances so far were variations in dimensional accuracy, profile tolerances relate to the curvature of the cross-section of the part. Its symbol is a semi-circle lying on its cross-section diameter.

For understanding the concept of profile tolerances in cnc machining, it is important to know what is profile line. Profile line is the line running along the cross-sectional area of a workpiece. Profile tolerance range implies that the curve of this line should be within the acceptable variance. This value is measured in dimensional units (mm or inches).

Orientation tolerance is the variation of a form of the workpiece in relation to a reference form. The reference form or plane used to check the relative variances is called the datum. Measuring orientation tolerance is done with regards to the perpendicularity of the workpiece or its angularity. Even when measuring a shift in angularity, orientation tolerance is also measured in mm or inches, instead of degrees.

The location tolerance range is similar to orientation tolerance. Location tolerance in CNC machining refers to the shift in the location of particular features of the workpiece. For measurement of the shift, a reference line called the datum is used. The intended position of the feature is called its true position.

Form tolerances pertain to the physical features of a workpiece, such as its flatness, roundness, or straightness. These tolerances are also measured in mm or inches, with measurement tools such as height gauges, calipers, micrometers, etc.

Runout tolerance refers to the fluctuation of a particular feature of the workpiece with reference to a datum when the part is rotated 360 degrees around a central axis. Runout tolerance can be important and measurable for any or all features of the workpiece. The symbol for this tolerance is a square box containing an arrow pointing to the top right corner.

Geometric Dimensioning and Tolerancing is a system to detail and communicate the standard machining tolerances. Since there are many types of tolerances in many different types and shapes of parts, a standardized system helps various parties involved in manufacturing to communicate with each other. GD&T is the most widely adopted system of standard tolerances used across the globe.

CNC machining is a wide field with many different processes under its umbrella. The CNC machining tolerances for each of these processes are different due to variations in the types of cutting tools used. Here are the standard CNC machining tolerances for common processes:

Since tolerance directly reflects the accuracy of a part, the first look can make it appear that it is always better to have tight tolerances. However, for CNC machined parts, tight tolerances can increase the cost of production and lead to a time-consuming process. Therefore, use of tight tolerances should be incorporated when it is required.

Tight tolerances are generally needed in cases when a part is going to be used in secondary assembly processes. Loose tolerances in these cases can lead to a failure in acceptable assembly. Therefore, there is a high focus on the tolerance band.

Another use case of tight tolerance machining is when designing prototypes of innovative parts. Designers expect the prototype to function exactly like the finished product. Therefore, they use as tight requirements as possible.

For better optimization of resources, manufacturers do not aim for the absolute least tolerances. Instead, they use the least tolerances that will fit into their budget. A good way to incorporate the budget factor is by plotting the increase in cost with the reduction in the tolerance band, and finding out the acceptable range where these two values meet for the particular project.

Most projects made with CNC machines or any other manufacturing process have a quality control phase to check out if the final product is in the acceptable range. In case it fails to meet the acceptable range, the product is rejected.If using very tight tolerances, the time in the inspection stage is considerably increased. Additionally, complex equipment can be needed to measure the tighter tolerance level.

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