Planer Knife Grinding and Jointing¶
Some planer manufacturers offered as an option with their larger machines a knife grinding and jointing attachment to allow the cutterhead knives to be sharpened in place. A grinding bar is mounted on top of the jointer, to which the grinder attachment or the jointer attachment is affixed. The attachment is then lowered (e.g. using 19 in the photos below) to the correct location above the cutterhead. Typically a handle or wheel (e.g. 17 in the photos below) is used to manually turn the threaded grinding bar, causing the jointing stone or grinder to move across the cutterhead blade(s).
Powermatic 225 with Grinding Bar on Top
Powermatic 225 Setup with Jointing Attachment
Powermatic 225 Setup with Grinding Attachment
It is a two step process, although there is some controversy which to do first, or even if both steps are necessary. Some folks say to grind to the joint and some say you absolutely must joint after the grind. In either case, you need courage to lower that jointing stone onto the spinning cutterhead the first time, it can seem very intimidating.
The Powermatic 225 24" planer manual
here recommends jointing first, then grinding. The Yates American J-180 18" planer manual
here recommends the opposite order, grinding first then jointing. No two manuals write this up the same. Even the PM manuals have different procedures at different points of evolution.
Grinding First Process Details
(The grinding bar does need to be aligned precisely. 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. On a cutter head that has three knifes expect to have to make 3-5 passes on each knife to get a proper grind. Thus it may need about 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. {Reference:http://owwm.org/viewtopic.php?t=22396|http://owwm.org/viewtopic.php?t=22396}
Jointing First Process Details
The steps are as above but in reverse order. Knives are jointed first, until a silver line is seen on all knives. The knives are then slowly ground one at a time to the silver line using the grinding head.
Advantages and Disadvantages¶
First time through this process there will be a learning curve, indeed it is time consuming initially. With practice, it can be done in under 20 minutes, including dragging out the equipment if required and setup. It really is very efficient, being able to quickly grind and joint the knives is a huge time saver. Plus, it is possible to joint the blades quickly several times before needing to regrind (or vice-versa!).
Advantages are:
- No knife send out.
- Little down time
- All knifes are at precisely the same height
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.
Using the jointing attachment ensures that all blades are at the exact same height, meaning they will all share the work equally rather than relying most on the highest blade, which should result in a smoother cut. This does not take into consideration bearing play and such but will give you a very nice cut pattern. {Reference:http://owwm.org/viewtopic.php?t=14882|http://owwm.org/viewtopic.php?t=14882}
You can get the knives pretty close by hand setting with a dial gauge. Even better is to grind the knives on the machine. A level above this is to joint the knives. Some people feel that the jointing process also gives better finish results with the cut. Others disagree and feel that the process damages the knife edge. Truth is, it depends on how well it is done.{Reference:http://owwm.org/viewtopic.php?t=22396|http://owwm.org/viewtopic.php?t=22396}
Some note that when the blades become dull it is much quicker to simply replace them with a set of sharp spares, and that the jointing process after the knives are sharpened will damage the knives, despite what the manuals say. To get a knife grinder to work will take some (time consuming) precision setup with a dial indicator to make the bar EXACTLY parallel to the center of the cutterhead.{Reference:http://owwm.org/viewtopic.php?t=22396|http://owwm.org/viewtopic.php?t=22396}
A number of questions related to setting the correct bevel angles on the process have been raised, but not completely answered in
this thread.