Comparison Of Processing Costs For Different Tolerance Levels

Comparison of processing costs for different tolerance levels

We all know that there is a situation in machining. If the tolerance level is one level different, the price may increase many times. Many people often blindly choose IT6 or higher levels out of the pursuit of high precision. As a result, processing fees soar, but the products simply do not require such high precision. Let’s make it clear based on actual data how much cost different tolerance levels correspond to.

The higher the outer diameter machining accuracy, the more expensive it will be.

Let’s take a look at outer diameter machining first. The tolerance level that can be stably achieved by ordinary turning is between IT8 and IT9. At this time, the cost is the lowest, marked as No. If you require to reach IT7 level, ordinary turning will take a lot of effort. You will have to switch to a hexagonal lathe or an automatic lathe. The cost will become -, which is about 2.5 times more expensive than ordinary turning.

Once the tolerance requirements reach IT6 level, or even further to IT5 level, ordinary turning is completely incompetent, and a cylindrical grinder must be used. In this case, the cost is marked as… and is 5 times the normal turning cost. For example, to process a shaft with a diameter of 50 mm, if the tolerance is IT9, it may only cost 50 yuan to use an ordinary lathe. However, if the tolerance is IT6, grinding technology must be used, and the price will suddenly rise to 250 yuan.

Centerless grinders have higher precision and are suitable for mass production situations above IT5 level. However, it requires heavy investment in equipment, long debugging time, and the unit cost will be higher in small batch production. Many factories insist on adopting high-precision methods in order to accept orders, but eventually find that equipment depreciation and grinding wheel consumption have exhausted all profits.

The inner diameter machined hole is more difficult to service than the shaft.

The tool has poor rigidity and difficulty in chip removal, making the inner diameter machining much more complicated than the outer diameter. Ordinary car boring can only reach IT8 to IT9 levels, and the cost is average. If IT7 level is required, reamer or precision boring will be used, and the cost will increase, which is 2.5 times more expensive than ordinary lathe boring. For a sleeve with an inner diameter of 50 mm, IT9 level boring costs 80 yuan, while IT7 level precision boring costs 200 yuan.

When the tolerance requirement reaches IT6 level, the operation must be performed on an internal cylindrical grinding machine. The efficiency of internal cylindrical grinding is extremely low. The time required to grind a hole may be several minutes, and the cost directly soars to 5 times. Some precision hydraulic parts require IT5 level, and then a grinding process must be used. The cost of grinding a hole is 5 to 10 times that of ordinary boring, and the consumption of abrasive paste and grinding rod is very fast.

The highest accuracy level that can be achieved by a drilling machine for rough machining is IT11 level. Although the reamer can achieve IT7 level accuracy, reaming requires very high requirements for the bottom hole, resulting in a high scrap rate. Although precision boring has good precision , the cost of precision machining is different from that of ordinary machining , but even every adjustment of 0.01 mm takes a long time, making the machine adjustment fee more expensive than the processing fee. When performing inner diameter processing, you must carefully consider what purpose this hole is used for, and whether the first-level tolerance can be relaxed.

Length size Many people ignore its cost

In terms of length processing, it seems relatively simple at first glance, but in fact there are many problems. For ordinary turning, the length tolerance can reach IT8 to IT9 level, and the cost is moderate. However, the length tolerance marked on many drawings is IT7 level. In this case, the positioning function of a hexagonal lathe or CNC lathe must be used, so the cost becomes a negative number. The processing costs of different tolerance levels are compared, and it is 2.5 times as expensive as the original.

In actual production situations, length tolerances are often excessive. There is a situation where there is a stepped shaft with a length of 200 mm. Its length tolerance reaches IT9 level, which is actually fully satisfactory for use and can meet normal production needs. However, it must be marked as IT7 level. This resulted in a series of complicated matters. As a result, each piece had to be measured using a digital ruler, and multiple infeed correction operations were required. Some factories have made statistics on the cost difference between precision machining and ordinary machining , and concluded that if the length tolerance is increased by two levels, the time required for processing will at least double.

When performing hexagonal lathe processing length operations, IT8 level stability can be achieved by relying on dead positioning stops. However, if it is expected to reach IT7 level, the wear of the stopper needs to be checked frequently, and the tool setting operation must be performed again after every 50 pieces of processing. Although automatic lathes are highly efficient, several cams must be replaced to adjust the length once, so it is not cost-effective to produce them in small batches.

The truth about cost ratios at different levels

According to the symbols in the table, the cost gradient can be clarified. The number refers to the baseline cost, which is generally IT8 to IT9 level. The – number refers to 2.5 times the cost, which corresponds to IT7 level… The number refers to 5 times the cost, which corresponds to IT6 level and higher. This ratio is not determined arbitrarily. It reflects the comprehensive difference in equipment depreciation, tool consumption, working hours, and scrap rate.

Taking the processing market in the Yangtze River Delta region in 2025 as an example, the hourly cost of an ordinary lathe is 80 yuan, while the hourly cost of a cylindrical grinder is 200 yuan. The same processing of the outer circle can be completed in two minutes with an ordinary lathe, but four minutes with a grinder. Including the time for clamping and measurement, the per-piece working time of the grinder is five times that of an ordinary lathe, which is completely reasonable.

With different precisions, the difference in scrap rates is very large. The scrap rate of IT9 level precision is usually controlled within 1%, while the scrap rate of IT7 level is about 5%. As for IT6 level, the scrap rate may be as high as 15%. The material fees and front-end processing fees for those scrapped workpieces are all wasted, and the costs need to be calculated. Many factories will include the risk of scrap products into the unit price when making quotations, so the quotations for high-precision items will be higher.

Accurate selection saves hundreds of thousands a year

There is a real example here. There is a factory in Ningbo engaged in the production of auto parts. Originally, all its drawings were marked with IT7 level accuracy. Later, after the technical department re-evaluated, they determined the cost difference between precision machining and ordinary machining , and the accuracy of non-fitting surfaces was reduced. Processing, down to IT10 level accuracy, while the mating surface still retains IT7 level accuracy. The final result is that the factory's annual processing cost is reduced from 1.8 million to 1.1 million, saving 700,000 yuan. At the same time, there are no problems with the performance of the products it produces.

It's not that the higher the accuracy, the better, but that it's enough. Just like bolt connection holes, IT11 grade is enough, and IT7 grade is a waste. Another example is the positioning pin hole. IT7 level is necessary, but do not bring other irrelevant dimensions to IT7. When designing drawings, you need to ask yourself a question for every tolerance: Is this accuracy indispensable?

When the processing factory makes a quotation, it must be clearly explained to the customer. If the customer insists on IT6 level, then the cost difference should be displayed and let the customer decide whether it is worth it. Many customers actually don't know much about processing and believe that high-precision products are not expensive. If you calculate the accounts clearly, they will trust you more.

Reasonable accuracy ranges for different processes

For ordinary turning, the most economical range is IT8 to IT10. Hexagonal lathes and automatic lathes are suitable for mass production from IT7 to IT9 levels. The drill press can only reach IT11 level, and the reamer can only reach IT7 level, but the efficiency is low. Boring machine and precision boring, suitable for IT7 to IT8 grades, internal grinding and grinding, suitable for IT5 to IT7 grade high-precision parts.

A good way to efficiently finish the outer diameter is centerless grinding, which is suitable for IT5 to IT7 levels. However, the equipment is relatively expensive, and it is more cost-effective only if there is a certain batch size for processing. The highest grinding accuracy can reach IT4 level, but its processing operation is extremely slow. It takes about ten minutes to process one part. This method is only suitable for a few fields such as aerospace and precision instruments.

Choosing a processing method requires comprehensive consideration of accuracy, batch size, material identification, and shape clarity. Both are IT7 grade, but there are differences in the processing costs of steel parts and aluminum parts. The grinding costs for the same outer circle with steps and without steps are different. Don’t just focus on tolerance levels, but also pay attention to the specific structure of the part.

When you are processing parts on a daily basis, have you ever spent more money than you should have because of extremely strict tolerance marking? Welcome to share your experience in the comment area, like this article and save it. Look it up and check it out before making a quotation next time. Don't be deceived by accuracy again.