Question
I need to replicate some very old style molding and windows. I would like some advice on how to grind custom cutter blades. What kind of grinding stones should I use and how should I do all the fine tuning?
Forum Responses
(Solid Wood Machining Forum)
From contributor V:
I have been grinding my own molding cutters by hand and eye for many years. I need a bit more information. What type/model of machine will you need to grind the knives/cutters for? To get you started, you need grinding wheels that are 1/4" thick for most moldings and you will need to dress the wheels in certain ways during the making of a knife/cutter. I always lap the back side of my high speed steel knives flat and polished with silicon carbide paper on 3/8" thick glass. To get a really nice surface on the molding I also hone the profiled bevel with Arkansas files.
Copy of old post follows, note it primarily addresses beveled edge steel.
1. Balance the knives. Unbalanced tools can cause finish problems, bearing wear and, in the case of the slick steel you have, can cause a knife to be thrown. A lot of the old timers used to grind 1 profile knife and fill the other slot with a balance knife, usually an old knife with no profile that weighed the same. As the projection of the two knives cannot be the same in this situation, this guarantees an unbalanced assembly, even though the knives are the same weight. Avoid this kind of logic.
2. Choose the correct thickness of steel for the job. Beveled edge steel is available in 1/4”, 5/16”, and 3/8”. Maximum safe projection of the knife beyond the collar is 3-4 times the thickness of the stock (verify with supplier). There is nothing wrong with overkill in this area, but remember the thicker the steel, the harder and more time-consuming the grinding.
3. Inspect the grooves on the collars for dirt or damage, to ensure a good seat for the knives. Discard collars with any sign of damage or excessive wear.
4. Don’t mix steel of different manufactures in the same pair of knives. Some say not to mix different bars from the same manufacture, but I would not go that far.
5. Cut the stock long enough to fill the slots at least to the center of the spindle/cutterhead. I consider this a bare minimum and usually cut to fill about 80% of the groove.
6. Make sure corrugations are engaged (if using lockedge) and knives are properly seated in the groove before tightening the spindle nut.
7. Do not project the knife out the open side of the slot (the side the screw threads into) on lock edge collars.
8. Do not use lock edge knives in smooth collars. Note: I do not recommend the slick collars and steel.
9. If you use the 2 cap screws, run them down with the assembly on the spindle for alignment and carefully tighten equally. Personally, I don’t use them unless I wish to keep the knives in place when removing from the spindle for later use. I normally rely on the spindle nut for clamping.
10. Do not over torque spindle nut, as in--no wrench extensions or persuaders--this is not good for the machine and can put unnecessary stress on wide, thin knives as well.
11. Start small and at relatively low RPM and work up as experience is gained. Large cutterheads, small spindles and high spindle speeds on light duty shapers are recipes for disaster.
12. Use feather boards, jigs, etc to guard cutterhead. I generally grind the profile so the heaviest part of the cut is below the work if possible. A power feeder is one of the best safety devices around for a shaper. It will also increase quality and production with a smooth, even feed.
To supplement information on the hand grinding of knives, the following are some additional tips:
1. Transfer the pattern carefully to the knife stock, use the long edge for alignment, and remember there is no independent vertical adjustment of beveled edge steel. I use machinist blue ink and a carbide scratch awl or scribe for this.
2. When grinding, always hold the knife parallel to the wheel, the grind marks on the knife should be perfectly vertical and parallel on the bevel. If you don’t observe this rule you will end up with a mess!
3. I grind to my layout line at a 0 degree clearance angle, as in no bevel. You will grind quicker and cooler this way, as less surface area is presented to the wheel. Once the profile is perfect to the layout line, adjust the tool rest for a bevel of about 45°. Grind bringing the line of the bevel to the top edge of the knife. At this point, you no longer need the layout line, you simply need to bring the bevel to the top edge of the steel, forming a cutting edge and turning a minute burr. If side relief is needed, I stop short of the cutting edge at points where side clearance and back clearance intersect, grind the 5° side relief first and then finish up the back clearance. Rub out the burr and the bevel and you are done.
One final note: don’t let anyone tell you it is impossible, unsafe, inaccurate, or even impractical to hand grind your own profiles. It is a skill like anything else and takes practice and commitment. For the serious amateur or small shop professional it is well worth the effort.
So grinding out a sill and rail set with 90 channels I'm guessing can’t be too easy but possible with time? How do you get those inside edges looking right? Also the head I'm looking at can hold four blades. I personally asked the distributer and he said it only needs two for balance. Would I need to fill the others with balanced weights? Again, thanks for all you help.
This is where CNC knife grinders shine. I highly recommend making a template or even scale drawing and having them made. Either that or use a simple tongue and groove set of cutters for the stile and rail cuts and go with applied moldings. Then you can grind the knives to duplicate the molding.
You grind as precisely as possible and run test cuts. The test cuts show you which knife needs to be changed to improve the fit of the parts. Removing steel from a convex area on the knife adds wood in a concave area of the molding. Removing steel from a concave area of the knife adds wood to a convex area of the molding.
What is probably unclear to those who have never tried is that high speed steel grinds very slowly with the type of grinding wheel you use when you are near to the finished edge of your knifes profile. This makes it fairly easy to grind perfectly to a pattern line as well as to refine a shape.
I recommend that you look into Radiac brand abrasive wheels. They have a line of grinding wheel called Por-OS-Way. These wheels remove stock quickly and stay cool for roughing out. Then finer and denser wheels are used for the finish cuts. I also use run of the mill cutoff wheels to to do my initial rough steel removal. These will burn the steel but with quenching as often as possible the burning does not seem to adversely affect the knives.
If hand grinding is something you are determined to do, buy the first set of knives from a quality grind shop and use those as a guide to how a set of knives should look. Buy the book "Knife Grinding and Woodworking" by Charles Monnett to gain a good understanding of the principals of cutting tools.
