Feed speed and knife life

      How the former affects the latter. January 15, 2003

Question
I have read a lot about feed rate and chip removal and how it affects knife life. Does anyone know the feed rate for the longest knife life? I mold maple, cherry, ash, walnut, etc. into picture framing, 2000 - 3000 feet at a time. M3+ steel.

Forum Responses
Assuming the lumber is clean, your feed speed should dictate the finish more than knife life.

Aside from that, remember that the knife needs to remove a certain sized chip in order to cool the knives effectively. Running too slow will not only cause the knife to overheat, it will also cause a "glaze" on the surface of the wood. This is even more a factor with the cherry and walnut.

I would run a little slower (just a few F.P.M.) for the ash and maple, just because they are harder than the others.



From contributor M:
Our company runs various species as you mentioned. As a general rule I train operators to 30 FPM per knife. On a conventional run this would be considered single knife. This would be a minimum feed rate. Obviously, the faster you can run, the cooler the tool will be, resulting in extended tool life. I have also run into problems during high speed runs on resawn material. Taking minimal amounts of material off the top head resulting in premature knife break down. Cutting angle plays a lot bigger part in the tooling process than most people think. I would consider 10 degrees to be max with the various species. I would look more to 7-5 degrees. Understand it is more of a scraping effect but it also leaves more steel towards the tip edge of the knife to dissipate heat. At 2-3,000', the drop in tool life from 10-5 degrees would not be a factor. Setup is critical - it is important to keep the piece moving through the machine while the cutters are on. It does not take a lot to fry these knives to the point of break down. Tools are often ruined during the sharpening process. The grinding is the first place I look when having problems with tool life. Just two cents from an experienced operator.


From the original questioner:
Here's a little trick I found when I took over this moulder/grinder position. The last operator did a 25 degree rough grind then a 20 degree finish grind. I went to a single 23 degree grind. It doubled the knife life and reduced chip out about 90%. Needless to say, that alone saves my company a boat load of money every run.


From contributor C:
Cutterheads with a 12 degree hook angle require 30% more horsepower than similar heads with a 20 degree hook. The knife life on a 12 degree head would be reduced by 30% as well in comparison. Just more food for thought.


From contributor M:
I believe you are talking about back clearance angle. I was suggesting changing your actual cutting angle. To do this would require you to purchase new tooling designated to the specified angle. Back clearance angle will generate adequate air flow behind the tool to help keep it cool. An additional drop in back clearance angle will further add to this and add greater accuracy toward your grind due to minimal grinding wheel wear and metal removed. Keep the tools as cool as possible. Keep your wheels from getting clogged and don't force the grind. I think this is what played more of a factor in your success than changing your back clearance angle by a couple of degrees. Heat is used to form the steel and it will also destroy it.


From contributor J:
This is a huge question in my books. If you are running poplar, oak or cherry, m3 works great at around 36 to 40 ln ft a minute at a 14 degree grind ("ouch" say the experts). I have found it keeps the heat in the heel of the blade instead of the tip, depending on two knives or four on a 20 degree hook head. I have found most grinding is done at a much steeper angle to get a sharper tip but it seems to burn out easy and sooner. Try sharpening an ax to a fine point, chop some wood, then get a blunter edge and see what you get. Just a simple theory that has worked great for me for 14 years and many thousands of feet.


From the original questioner:
I thought I had a secret weapon. Does a 14 degree grind in a 20 degree pocket really work good? For hardwood? Have you tried it in the twelve? I have duel angle heads plus a rondamat grinder. I get really good knife life but I'm always looking to do better. We are a high end company as well.


From contributor J:
Yes, I get very good results with both 12 and 20. But then again I have both in one head (Weinig heads). I try to run four knives as much as possible in runs 1500 lin on up. Of course you should listen to the experts but I have found wood can be unpredictable considering kilning, minerals, knife steel, etc. But I like to try different angles and speed rates on different runs. I grind many heads a day on 2 moulders (high speed and slow). I am starting a new molding plant with a new 960 Weinig grinder and Weinig blue line moulder and probably will get different results on them too!


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Feed speed and knife life are two totally different subjects. The feed speed depends on many factors, such as general quality expectations, knife marks per inch, wood species, moisture content, quality of the raw wood, and much more. Generally speaking, maple is not as forgiving as oak and other species. Tiger wood, wavy grain, etc. are typical for maple, and usally require to reduce the feed speed in order not to generate a bunch of scrap.

Knife life depends on the cutting speed of the cutting edge through the lumber. In other words, the higher the cutting speed, the higher the cut force. That is causing an increase in pressure on the cutting edge, reducing life expectancy for the tool. The quality of the finished part will be improved as well by choosing a higher cutting speed.

The cutting speed has nothing to do with the actual feed speed, and depends solely on current tool diameter and spindle RPM's (cutting speed = tool diam [m] x 3.14 x RPM's/60). Recommended cutting speeds for hardwoods is 35 to 60 m/s (meters per second).



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