Grinding vs hard turning

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Which process has the edge?

When it comes to precision machining, classic grinding and machining with a defined cutting edge have always been neck and neck in the market. There have been plenty of discussions about the pros and cons. Dr. Frank Fiebelkorn, head of product development, research and technology, and Mathias Gerber, precision machining product manager at Swiss Fritz Studer AG, a United Grinding Group company, discuss the subject in this Q&A, conducted by Frank Pfeiffer.

Frank Pfeiffer: How have customers’ behaviours changed the competition between hard machining with a geometrically defined cutting edge and grinding?

Frank Fiebelkorn: It has led to a relativization of the previously strict positions held by the advocates of both processes. Fifteen or twenty years ago there were many publications which predicted that, due to the increasing potential of hard turning, the requirement for grinding would be much reduced. Precision tool manufacturers reinforced this with references to the increasing capability of their cutting materials. However, at the same time there was also an increase in customer requirements, which could best be met by means of grinding. Today these two processes coexist happily.

Mathias Gerber: As you can see from our business development, cylindrical grinding has anything but disappeared, as some people predicted. On the contrary, it is in demand as never before.

FP: What are the requirements that have given grinding a new impetus?

FF: In the first instance, one surprising fact should be mentioned: precision tool manufacturers have been processing increasingly efficient cutting materials, such as carbides or cermets, which are almost exclusively machinable using grinding. And we have been helped by an increase in the consumption of these cutting materials of around 40 per cent between 2010 and 2015. More and more materials must also be precision-machined for medical technology, especially ceramic materials. Once again, grinding is virtually the only possibility here. Sources for North America show an increase of more than 20 per cent in this field from 2010 to 2016.

FP: What role is played by the users’ requirement for higher precision?

FF: The tolerances for precision components have become ever tighter. When manufacturing high precision toolholders, for example, today, an ATI tolerance class is often used instead of AT3 for a standard toolholder. This means that roundnesses and straightnesses of less than 0.5 micron must be achievable at the toolholder cone.

MG: In principle, it is possible to precision-turn these tolerances, but process reliability can not be attained cost-effectively. Often, the absolute tolerance of the customer part has remained the same, but the process capability and machine capability have increased, so the window in which you may utilize the tolerance has become smaller. Here too, grinding is the user’s first choice.

FP: Is grinding always preferred in cases where high precision is required?

FF: Despite what has already been said, this should not be our message. Whatever benefits the customer is preferable. And if hard turning is the best process for the application, then that’s what we recommend. We have sufficient experience in both areas to make a neutral assessment of the respective manufacturing task. Our starting point is the advantages of the process group.

FP: Which processes in particular?

MG: Well, machining with a geometrically defined cutting edge is very fast, can produce contours very flexibly and requires only little set-up effort, if you need to perform a number of operations. However, as the cutting edge wears you get a gradual loss of precision, culminating in complete replacement of the cutting edge, with the inevitable interruption of production. This also brings disadvantages for production with reduced staff. The situation with grinding is different. Here, as we have mentioned, you can permanently ensure the values for diameter, length, surface and shape. You always have an easy-cutting, geometrically accurate tool available, which can be dressed in an autonomous and process-integrated manner, so that it once again resumes it initial state. With a 500 grinding wheel for example, this can be done up to 15 000 times. And it should also be borne in mind that grinding is less susceptible to material in homogeneities such as hardness fluctuations or inclusions, than hard turning or milling. SMT

This article is an edited version of a Q&A supplied by United Grinding Group.


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