Tolerancestacks

Statisitical Variation Analysis applies statistical controls and methods to relax component tolerances without negatively impacting product quality.

Worst-case tolerance analysis is the traditional type of tolerance stackup calculation. Each tolerance is set to its largest or smallest limit in its total tolerance range. This does not consider the distribution of tolerance range, only that each tolerance stays within its expected range. This method does guarantee that that the parts will fit and function properly, regardless of actual variation. However, because this method often requires very tight individual component tolerances, as the total stack up at maximum conditions is the primary attribute used in the design, it promotes expensive manufacturing and inspection process and high scrap rates.

This method is often requested by customers for critical interfaces in assemblies, but as mentioned, suffers from over-tolerancing parts. The worst-case scenario rarely if ever occurs in actual production, and therefore often incurs unnecessary costs in manufacturing and quality.

As we discussed in our article, What is Tolerance Analysis?, there are really four different methods of doing Stack Analysis -

The coefficient λsi in a linearized model also provides the sensitivity relationship from a tolerance Ti to a measurement Ms.

A drill bit may appear to be a simple hunk of metal; the average person probably doesn't put much thought into its design. However, there are actually several types of distinct drill bits that vary based on the type of material you need to drill into and how big of a hole you want to make. Wood is the most commonly drilled material, but there are also bits made to punch through metal, glass, ceramic tiles and concrete. They all come in standard sizes based on bit length and diameter, usually measured in metric units.

The main issue in Tolerance Analysis is how to calculate the total variation from accumulating tolerances. There are two major categories in this area:

However, if we consider that tolerances are small relative to dimensions, we can approximate a non-linear relationship with a linear model. Besides the calculation advantage, the sensitivity analysis is also an important reason to have the linearized model (although, there is a coming solution in AAO Add-on for non-linear models; see the webinar at the bottom).

When manufactured, parts are never made to perfect specifications. Due to variation caused by material characteristics and manufacturing processes, such as stamping and machining, parts are always made larger or smaller than their nominal design. This variation is captured in design as tolerances, depicting the range of variation acceptable in the design.

Drill bits with a tungsten carbide tip are the strongest; when it comes to solid concrete, the sharper the better. Masonry bits cut holes through concrete in two steps. The tip of the drill bit is larger in diameter than the shaft below so that when the shaft reaches the hole, it fits right in. Drilling at a lower speed is more efficient for drilling through concrete, and it prevents the bit from overheating. Masonry bits with a carbide or diamond tip specifically intended for hammer drills are more efficient and penetrate hard surfaces better and faster.

Coring drill bits are also useful for making large holes in concrete walls. These are much bigger than typical bits and come with diamond-tipped teeth blades to be as tough and sharp as possible. These need to be paired with a heavy-duty hammer drill or impact gun to really put the power into your bore. For deep core drilling, a lubrication system may also be necessary to keep the bit cool as it cuts into the concrete.

In equation eq6, the calculation is performed based on the Normal approximation for a symmetrical Nonnormal case. It assumes there is no mean shift. A simulation method can also be used as outlined in the following case.

Therefore, tolerancing and tolerance analysis are integral parts of the engineering process and product lifecycle management in order to produce high-quality products at reasonable prices.

Traditional methods of tolerance analysis include 1D, 2D and 3D Tolerance Stacks, and statistical methods like Monte Carlo simulations. Tolerance Stacks calculate the accumulated variation across a set of dimensions. 1D stacks do a single directional linear stack, while 2D stacks and 3D stacks include multiple directions and influencers.

Tolerancing directly influences the cost and performance of a product. A piece of sheet metal that is quickly stamped using a stamping die is much cheaper to produce than one that needs to be machined to more precise dimensions. The same applies for plastics, composites and any given part. The tighter, as in the smaller, the tolerance, the more difficult the part is to produce, and the more expensive the part is. In the same regard, the performance of a part and product is influenced by tolerances. An automobile door will not close well if the tolerances are very large, and may have additional road noise from a poor seal. Aircraft wings may need large amounts of shims if the tolerances are incorrect in order to fit properly to the fuselage. This costs time, money and increases the weight of the aircraft, reducing its fuel efficiency.

Let Ms be an output, Ms0 be the nominal value, λsi (i = 1, 2, … Nt) be the constant parameter, Ti be the tolerance, and Nt is the total number of tolerances.

Drill bits made to drill through concrete are called masonry bits. These are typically thicker and sturdier than carpentry bits in order to bore through thick concrete, brick or stone without warping and snapping. They're best paired with a specialized hammer drill, which moves the bit in an in-and-out hammering motion as well as the typical rotation. This allows you to punch through stonework more quickly and efficiently than a typical electric drill allows.

The most common type of drill bit is known as the twist bit, which is great for tapping into soft wood and metal. In a pinch, it can be used on brick or concrete, but a specialized bit will always give faster results and be safer for big home improvement jobs.

It is important to account for manufacturing variation, as there is no such thing as perfect parts. What's more, those tolerances and variation can greatly affect production costs.

It is often applied to manufactured parts in general to determine the impact of manufacturing processes on the final dimensions of those parts. Tolerances are determined by a variety of methods, from standards such as ISO or ASME, or from the use of geometric dimensioning and tolerancing (GD&T), a method of annotating and marking tolerances on parts.

Any type of drill can be used with standard drill bits for masonry work, but regular drills may not be the best choice for particularly tough jobs. Bits made to punch through tile are very similar to masonry bits, but they're even sharper in order to cut through the brittle materials without chipping them.

Tolerance stackcalculation

Where sti is the adjusted sigma based on the distribution type for Ti. For example, a uniform distributed tolerance Ti will have sti 2 = (∆Ti 2 /12).

Each part is modeled using a statistical distribution for its tolerance range (variation) which are then summed using the Root Sum Squared method to predict the distribution of the assembly measurements. This process describes the variation as a distribution instead of only showing the extremes of variation, which gives more design flexibility by allowing the design and engineering team to account for varying levels of quality, instead of just 100 percent of all variation, which is statistically rare or impossible.

A well performed statisitical method can reduce the manufacturing costs by accounting for acceptable levels of variation, designing what is called a Robust Design, which 'loosens' (ie. increases) the tolerances on non-critical areas where it does not affect the overall build, and focus on the critical and sensitive features of the product.