CNC machining with multi-axis equipment is all the rage today, and it should be. Shops of all sizes are learning to make extremely complicated parts ultra-efficiently with single setup manufacturing. So it’s only natural that owners of such equipment would be interested in optimizing their investment by doing some grinding on their machines at the tail end of their manufacturing processes.
For these reasons, this was one of the more frequent questions we fielded at IMTS in September. When questioners learned that grinding on a multi-axis machine had a number of critical pre-requisites, some immediately decided it would not be worth the effort. Surprisingly there were several others who responded with something like “I could do that!”
The first thing you need to consider is your volume. Is the integration of grinding with your CNC manufacturing equipment something you are going to do a lot of? If so, then consider purchasing one of the new hybrid machining systems that incorporate a grinding spindle along with milling and\or turning. If you can keep a system like this in operation full-time it will do a better job for you and will be far more profitable. On the other hand, if you don’t have that kind of volume and you still want to use a grinding wheel on your machining center, here are some of the things you have to consider.
Surface Speed: Cutting tools don’t typically spin at the speeds ideal for grinding. For grinding ceramic or carbide you will need to generate surface speeds of 20 – 30 meters per second and for steel 30-50 meters per second. Depending on the proposed grinding wheel diameter, you may need a spindle that can operate at 60 – 90 thousand rpm, and those can be quite expensive.
Guarding: If you use any kind of bonded wheel, you have to consider what might happen if the wheel comes apart. So your machine has to be well guarded and this is not just a safety issue. Your ways and slides have to be protected because your CNC was not designed to prevent tiny abrasive particles from infiltrating its wear surfaces.
Coolant: Except in special cases, dry grinding without coolant is a deal breaker if you want to get the most out of a vitrified wheel. If you are using coolant, great. Most people use oil, which works fine for grinding. However you need to be filtering it. The oil used in most mill and lathe operations is returned to a settling tank where the chips settle to the bottom. When you have tiny abrasive particles in the oil, they need to be removed with a paper filtration system, otherwise they will return to the process and cause surface finish issues on your parts and potentially damage to your expensive machine.
Dressing: There is little point in having a magnificently automated process that has to be interrupted for manual wheel dressing and conditioning of vitrified bonded wheels. So there should be some sort of dressing device mounted on the machine or on a nearby chuck.
Wheel Type: You can avoid the need for dressing by going with a plated tool, which consists of a single layer of diamond or CBN that is nickel plated to a steel wheel body. The advantage is that you don’t have to worry about dressing because it’s just a single layer. You don’t have to be concerned about guarding either because nothing is going to come apart. However, if you are trying to achieve very repeatable smooth surface finishes, we strongly recommend dressable wheels like Meister Abrasives’ vitrified CBN or diamond products.
There, in a nutshell, are the pros and cons of grinding on your multi-axis machine. If this is something you are considering or are already doing, post a comment on this blog. We would be very interested in learning about your experiences.