Page History: In-place Planer Knife Grinding and Jointing
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Page Revision: 2008/04/06 15:33
Some planers come with a knife grinding and jointing attachment that allows the cutterhead knives to be sharpened in place. It is a two step process, although there is a question which to do first.
- The knives are jointed simultaneously using the jointer head, with the machine running(!).
- The knives are ground one at a time using the grinding head.
_thouser6943_ gives a description of the process here:
http://owwm.org/viewtopic.php?t=14882To bring all knives into the same cutting circle the jointer stone is run across until a silver line is seen on all knives. Then switch over to the grinder and slowly grind to the silver line. At this point you should have all knives sharp and cutting the same amount of material. This never takes into consideration bearing play and such but will should give you a very nice cut pattern.
Phil Clement poses a number of questions related to setting the correct bevel angles on the process here
http://owwm.org/viewtopic.php?t=18221, and Bob Vaughan suggests it is a lot easier just to replace the blades with sharp ones rather than joint/grind them.
Bob Vaughan also commented here
http://owwm.org/viewtopic.php?t=22396It's way faster to set the knives by hand and have someone else sharpen them. The "jointing process" after the knives are sharpened is, IMO, doing damage to the knives, I don't care what the manuals say. To get a knife grinder to work will take some precision setup with a dial indicator to make the bar EXACTLY parallel to the center of the cutterhead.
The Powermatic 225 24" planer manual here
http://www.owwm.com/mfgindex/pubdetail.aspx?id=1298 and the Yates American J-180 18" planer manual here
http://www.owwm.com/files/PDF/YatesAmerican/J180Manual.pdf provide descriptions of the process.
Phil Clement provides more detail on the process he uses on his PM 180 here
http://owwm.org/viewtopic.php?t=22396The grinding bar does need to be aligned precisely as Bob has pointed out. On my PM 180 the bar is precisely located with taper pins. I initially verified both vertical and horizontal alignment with a dial indicator across the cutter head. Alignment is critical for proper results.
For grinding, the cutterhead is indexed to the proper angle for the first blade and the cutterhead locked into place. The grinding wheel is lowered onto one end of the blade, until sparks just start to fly, and then cranked across the blade. Without changing the height of the grinding wheel the head is next indexed to the second blade and the process repeated. Same for the remaining blade/blades. Blades are ground stationary, one at a time, and all blades are ground before the grinding wheel is sequentially lowered. This process is repeated until all the blades are properly ground. My cutter head has three knifes. I generally have to make 3-5 passes on each knife to get a proper grind. So I may make 15 passes with the grinder, total. Each pass takes 15 seconds or so. Pretty fast process.
The jointing is faster than the grinding. Also, you don’t absolutely have to joint the knifes, you could just grind. To joint, the grinding attachment is changed for the jointing attachment and the cutter head is brought up to speed. Now here is where it gets intimidating, the first time. With the cutterhead spinning at speed the jointer attachment/stone is lowered into the spinning head. Idea is to very slowly lower the stone until just a faint whisper of stone on metal blades is heard. If the crank on the grinding bar is properly cleaned, adjusted and lubricated, this goes very smoothly. Although it sounds intimidating, in reality it’s quite slick. The jointing stone is cranked across the length of the spinning cutterhead and the results checked. You don't really want sparks here, it's more a whisper with a hint of a spark. The jointing stone is then lowered in steps until just a thin line appears at the leading edge of the knives. This is the joint and is in essence a blunting of the bevel edge. This process leaves all knifes at the same height and adds strength to the bevel.
First time I ground and jointed my knifes in the machine there was a bit of a learning curve. I can now do this in under 20 minutes, including dragging out the equipment and setup. It really is very efficient. I will probably be faster with this as I gain experience. I would imagine that perhaps Bob could change a set of knives in the time it takes me to go through this process. I can’t.
Advantages for me are:
- No knife send out.
- Little down time
- All knifes are at precisely the same height
There are other theoretical advantages and disadvantages but I’m not the one to discuss the theory of grinding versus jointing. There are proponents of only grinding and of grinding and jointing. Good arguments in both camps. I started with just grinding in the machine. I subsequently added a jointing attachment and much prefer the grinding/jointing process.
I use a lot of salvaged lumber, heart pine and such, for our 1890’s house restoration. I have a really good metal detector. Even so it’s tough to remove all the metal. Salvaged wood is notoriously dirty, gritty, and hard to clean well before processing. Also some of the heart pine that I’m working with is just really, really dense and resin filled. All that to say that the wood I’m working with is quite hard on planer knifes. For me being able to quickly grind and joint the knifes is a huge time saver.
Someone else mentioned the advantages of getting all the knives to the same cutting height. You can get the knives pretty close by hand setting with a dial gauge. Even better IMHO is to grind the knifes on the machine. A level above this is to joint the knifes. Some people feel that the jointing process also gives better finish results with the cut. Others disagree and as Bob stated feel that the process damages the knife edge. Truth is, I think, it depends.
It’s also true as Bob has stated that this process is time consuming initially. However, for me, once set up it is way, way, faster than sending out and hand setting the knifes. If I was Bob and working on other folks machines I would change the knifes by hand, as he does. Not worth the setup for each individual machine serviced several times a year with each machine setup being different.
For me and for what I’m doing, grinding and jointing in the machine has a clear advantage.
Later in the same post
http://owwm.org/viewtopic.php?t=22396, Phil Clement adds:
This starts to get confusing as you read more. And it can't be that complicated. No two manuals write this up the same. Even the PM manuals have different procedures at different points of evolution. Lot's of differences of opinions. There's also the debate of which do you do first joint or grind? Some folks say to grind to the joint and some say you absolutely must joint after the grind. Personally I don't think it matters so much. But I'm pretty far from an expert. I think there was never one established, absolute method. The books/manuals seem to reflect the opinions of whoever was consulted by each company. Not a lot of consistency.