Tolerance, or engineering tolerance, is the permissible limit of variation of a given value. In the context of tolerance of CNC plasma tables, “tolerance” refers to how much the final product can deviate from the initial dimensions.
Parts need to work together, and for that, they need to be mutually compatible dimension-wise. Incompatible tolerance of different parts will lead to dysfunctionality.
It’s better to have finer and more accurate cuts, but at the same time, it’s more expensive. That’s why there should be a balance between the tolerance and cost of the cutting process. The end product needs to be of quality but also cost-effective.
In the article, we will cover the process of determining tolerance and the various levels it can be set to.
Tolerance of CNC Plasma Table
CNC plasma cutting is a process of cutting metals using a plasma torch. It is a fast and precise procedure, but still, it might be imperfect. As long as that imperfection allows the part to function, we call it tolerance. The tolerance can be over or under the nominal value, meaning the item can be bigger or smaller than projected.
Managing these deviations to construct functioning parts is important for the functionality of the product, as well as its quality and profitability. Executing more precise cuts with lower tolerance requires better machinery, which costs more money, and the process takes longer.
It is important to remember that it’s impossible to create two parts that are exactly the same regardless of the quality of the machine.
Why is tolerance important?
Having adequate tolerance is essential because of the concept of interchangeable parts. This means that one part of a machine can seamlessly fit onto another machine. It allows for malfunctioning parts to be replaced faster and at a reduced cost because there would be no need to make each part with customized dimensions.
This concept allows for automation as it removes the need for fitting or adjusting individual parts. Industrialist Eli Whitney presented this idea in front of Congress in 1801, and its implementation further amplified the industrial revolution.
In CNC plasma table cutting, tolerance is important as it determines the cut quality, efficiency, and functionality of each part.
Regardless of what tolerance demands you have received from your clients or engineers, SquickMons can provide the best CNC plasma table options to satisfy any requirement. Visit our website and choose your optimal tool to elevate your business to the next level.
Source: shutterstock.com / Photo Contributor: Mark Stewart
Standard tolerance of CNC plasma table
CNC plasma tables can produce interchangeable parts with consistent measures and quality. The standard acceptable deviation ranges at 0.005 inches. This means that the part’s dimensions can vary up to that amount and nothing more.
According to DIN ISO 2768 standard, there are four general levels of tolerance:
- Medium
- Fine
- Coarse
- Very coarse
Medium has a permissible deviation of ±0.004 inches. The fine tolerance level allows for ±0.002 inches deviation, coarse has a ±0.008, and very coarse is at ±0.02 inches.
Improving the accuracy of cuts
There are various factors that contribute to the execution of more accurate cuts.
The smoothness of the CNC table
A smooth and accurate motion control of the CNC table is guaranteed to contribute to more accurate cuts. Moreover, the table has to have the capacity to handle the weight of the material.
The CNC table needs to be flat for the torch to follow the design. If it’s not flat, the torch will move astray, and the end product will be bad. Make sure that the table is not bent in any way, and take several other precautions, like cleaning and lubricating the table.
Appropriate cutting parameters
Cutting parameters like speed, amperage, voltage, gas type, pressure, and kerf width have to be adjusted to suit the type of material.
- “Current” determines the thickness and speed of the cuts.
- “Cutting speed” is the tempo at which the torch moves along the path. The “optimal speed” is relative to the type of material.
- “Arc voltage” determines the cut quality, as an improper amount can cause bad-quality cuts.
- The “type of gas” determines the energy of the plasma arc and the chemical reaction it will cause with the given material. The most common types used are air, oxygen, and nitrogen.
- “Gas pressure” affects the velocity of the plasma jet and the stability of the arc.
- “Kerf width” determines the width of the cut.
Torch height control
THC, or “torch height control”, is another important factor when getting a CNC plasma table and the accuracy of its cuts. It measures the arc voltage and adjusts the length of the arc accordingly. It is a device that automatically adjusts the distance between the torch and the item that’s being cut.
THC can be standalone and integrated, i.e., it can either operate independently from the CNC controller, or it can be integrated and controlled alongside the torch. The standalone THC is useful for hobbyists and beginners as their CNC plasma tables likely won’t have it built in.
Consumables
The parts that wear out are called “consumables”. The quality and efficiency of the cuts depend on the quality of the consumables on the plasma torch. The consumable components of the plasma torch are the swirl ring, electrode, nozzle, retaining cap, shield cap, and drag shield.
Each of the consumables affects the quality of the cuts and the tolerance levels of the CNC plasma table. The swirl ring affects the temperature and stability of the plasma arc; the electrode affects the current, the nozzle affects the velocity of the plasma jet, etc.
This is why for optimal cuts with the desired tolerance, it is imperative to change them when they show signs of damage. Common signs of them being worn down include cracks, chips, erosion, enlargements, cracks, dents, and burns.
The consumables have to be replaced before they are completely worn out because otherwise, they can cause damage to your cuts.
The applications of tight and loose tolerance
Referring to tolerance as “tight” and “loose” is just another way to describe the amount of tolerance. Tight means that the tolerance is low, and loose means that there’s higher tolerance allowed.
Tight tolerance
Tight tolerance means that the tolerance is low and that the cuts need to be as accurate as possible. This is implemented when there is high demand for quality. Having tight tolerance guarantees high functionality and quality of the product.
When the objects have delicate geometries and intricate shapes, they may be required to have a low tolerance. It can be for cutting out small holes or making very sharp corners to look like spikes, etc.
Aerospace and medical industry machines tend to require tighter tolerance. Manufacturing some mechanical and electrical parts may require tight tolerance in order to fit with other components. Examples include bolts, nuts, gears, flanges, gaskets, etc.
Loose tolerance
If you provide tight tolerance measures to all elements of a given project, the cost may go through the roof. Not everything is worthy of applying tight tolerance, and there are elements where loose tolerance can be applied as long as the functionality remains high.
For artisans and hobbyists, the tolerance of CNC plasma table is looser. Works such as signs, logos, sculptures, ornaments, etc., tend not to require tight tolerance.
Inspection
Inspection of the cuts is necessary to determine the quality and adherence to the designated tolerance levels. It’s first performed visually using a magnifying glass, and the inspector looks for obvious flaws.
After that, there are various methods for measuring the dimensions. Caliper and gauge measurements use simple tools that help determine the size and shape of the item. There are more advanced ways of measuring, like CMMs, that record the coordinates and compare them with the original design.
- Caliper measurement helps determine the thickness, width, length, diameter, or depth of a part.
- Gauge measuring allows you to determine if the item fits into a certain shape.
- CMM stands for “coordinate measuring machine”, and it gives the highest degree of accuracy. It is used to inspect the dimensions, cut angles, and the overall quality of the cuts.
Summary
Tolerance is the acceptable range of deviation from the initial dimensions of a given part. The amount of tolerance allowed depends on the design and the functionality of the part in spite of the deviation.
Having a universally accepted concept of tolerance is important as it allows for parts to be easily replaced and mass created.
In order to adhere to tight tolerance standards and demands, the CNC plasma table needs to be well and adequately equipped with working consumables.
After finishing the cuts, an inspection is performed in order to determine the level of tolerance of the CNC plasma table and its performance.
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