If all parts were perfect, tolerances would not be needed. However, the reality in manufacturing is that every part produced has and will have some level of deviation, even the more precise part.
When designing a product, one of the main tasks of the designer is to define what level of deviation is acceptable for each dimension without compromising the part functionalities. With this objective, the designer sets the total amount a specific dimension is allowed to vary, this acceptable field is called TOLERANCE.
When set correctly, tolerances can also help to reduce costs, since parts are more expensive to produce if they have tolerances that are tighter than necessary (for example, it is a lot more expensive to produce a part with dimensional tolerance of ± 0.005mm than a part with tolerance ± 0.05mm. In this case, it means that it would be needed an additional grinding process in order to guarantee ± 0.005). In other words, if something works well with ± 0.05mm, there is no added value to produce it with ± 0.005mm as it would only add costs.
In this tip we are going to talk about the diameter tolerances of Lamina cutter bodies. The correct understanding of this subject will help you to manage customer expectations about the precision of these products, as well as to address right away occasional questions.
According to norm ISO 1832 , K tolerance (e.g. APKT 100308) is ±0.05mm
For this dimension, with any value in between 6.65mm and 6.75mm the insert is considered in good quality.
On the other hand, with dimensions 6.64mm (or smaller) and 6.76mm (or bigger) the insert is out of tolerance and has unacceptable quality.
Cutter diameter tolerance
The diameter tolerance of the cutters varies according to insert size and cutter concept. For instance, we are going to see below the tolerance of a cutter LT 741 D40
This cutter diameter dimension is controlled over master inserts (master inserts are inserts with exact nominal dimension). To be in good quality, this dimension should be in between D39.875mm and D39.775mm, which means D39.825 ±0.05mm.
Tolerances in assemblies
When we have parts that work combined with other parts as an assembly, we need to think about the cumulative effect of each part tolerance and make sure that the assembly will work properly with every possible combination allowed by the tolerances defined for each part.
As the cutter works rotating, every 0.01mm deviation in the insert width (plus or minus) will count double (0.02mm) affecting the cutter diameter.
The assembled cutter diameter
The assembled cutter diameter needs to be smaller than the cutter nominal diameter.
Imagine that a customer buys a milling cutter D50mm to produce a slot with 50mm width. After the first pass, he measures the slot dimension and if the slot is smaller than 50mm, he can do another pass and adjust the slot dimension. But if after the first pass, it is already bigger than 50mm, then the workpiece might be scrapped.
An assembled cutter body should always have diameter over inserts smaller than the nominal cutter diameter. E.g. a cutter body D50mm, in fact has its diameter over inserts varying in between D49.68 and D49.98mm.
Diameter tolerances of Lamina cutters
Note: the table above is based on more popular items and was created for explanatory purpose. Consult Product Management if you need to know exact tolerances for a specific item.
When we understand the concept of cumulative tolerances (inserts + cutter body), in addition to the fact that the cutter diameter over inserts can never be bigger than its nominal diameter, we know how to explain why a cutter that is 0.3mm smaller in the diameter is perfect accordingly to the specifications.
How to measure the cutter diameter?
After talking about cutter assembly tolerances, it is also important to clarify how a cutter should be measured:
Wrong way to measure cutter diameter: Manually with a caliper, results are not reliable, nor repeatable.
Reliable measurement of a cutter diameter needs to be made with a tool presetter machine.
In summary, dimensional tolerances are needed in order to ensure that a product will work well and at a reasonable cost. With regards to our milling cutters line, the knowledge about its precision can help us to address ocasional questions, manage customer expectations and confidence on the high-quality tools we deliver everyday to many companies all over the world.