Grinding Moulder Knives with a CNC

Good info on the advantages and limitations of CNC-based knife-grinding setups. October 3, 2009

What are your thoughts on grinding moulder knives with a CNC grinder? I don't know much about it, as we have 2 Weinig Rondamat profile grinders and I think they are great! It's just I was talking to someone and they think CNC is the way to go. Can CNC grind 90 degree angles? Is it in any way limited at what it can do?

Forum Responses
(Solid Wood Machining Forum)
From contributor D:
Our P20CNC Profile Grinder can grind everything that a manual machine can grind. This includes 90 degree angles, normal corrugated steel, router bits, raised panel cutters, inserts, carbide inlaid, etc. One difference may be wheel wear. The machine doesn't know how much the wheel has worn down during the grind. With CBN or diamond, wheel wear isn't enough to matter. But if you're using vitrified wheels you sometimes need to run a final finish pass to make up for wheel wear.

A CNC grinder will grind a knife in about the same amount of time as manual grinding, so it's not a production decision. The real benefit is no one has to stand there while the machine grinds. So there is a huge labor savings. Other major benefit is safety... all the grinding is happening in an enclosed cabin.

They aren't cheap, though. Investment is $160k. This works down to $18/hr on 5 yr financing and 1 shift operation. People find that the labor savings justifies the machine at about 8 hours a day of manual grinding. (Meaning if you have 2 machines and they're both used 4 hours per day, you should look into one.)

From contributor K:
I don't know much about CNC grinding, however we have a 75-12 profile grinder from SCM and it works perfect for 2 shift production and has since I have been there, for 3 years.

From contributor J:

One thing to consider when grinding corrugated knives on a CNC grinder is that if it is a machine that picks the knife from a magazine rather than grinding in the head, it is going to reference the profile off one edge and off the back of the knife. The head references off the corrugations. Bar stock is not necessarily ground perfectly along the back edge, so you will always have problems with one knife doing all of the work. It is not the way to go for high-feed applications. If your CNC machine grinds the knives in the head, it is a good way to go.

From contributor D:
Contributor J makes a good point. The P20CNC machine grinds the knives in the moulder head or in a dummy head that references off the corrugations. Pick and place autoloading is not the most accurate way to grind corrugated knives.