Hand Grinding Cutters to Make Reproduction Mouldings

Detailed advice on how to grind your own custom knives in order to match unique profiles. March 28, 2010

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.

From contributor F:
First you have to decide if it's worth it to grind them yourself. There are many reasonably priced knife shops out there already that can do it for you.

From contributor R:
Following is a copy of a post I made several years ago on this forum which includes some tips on grinding by hand. One thing to note is the profile as ground will not match the profile that is cut ( its cutting depth must be elongated), due to the knives mounted on a cord of the circle that is the cutterhead and not through the center. Do a search for "hook angle" on this site for further info. You can develop the knife pattern using the scale tool in a cad application, geometrically by graphing it on paper or cutting through a sacrificial piece of an existing profile at the hook angle and then tracing the cut edge directly. As contributor V pointed out you will need some narrow wheels about 5mm and a dresser with a diamond nib is also very useful. In good conscience I should mention that using loose knives of any type should not be taken lightly. Do your homework or seek out someone who knows what they are doing for some instruction. If this is not going to be an ongoing endeavor for you, you are probably better off getting a grinding service to make them for you.

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 1/4Ē, 5/16Ē, and 3/8Ē. 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. Donít 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 donít 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 donít 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 45į. 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 5į side relief first and then finish up the back clearance. Rub out the burr and the bevel and you are done.

One final note: donít 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.

From contributor V:
The other big factor for me is that it is very satisfying to make molding for my projects with cutters I have made myself. I don't do it exactly like you and I have never worried about the small difference in the profile due to the cord of the circle. The main thing is that it is one of the most fun and satisfying things I do in the shop. Also, although I could probably buy the cutters for around the same money that it costs me in my own labor, I can have a brand new cutter in my machine about two hours after the need for a new cutter presents itself. Try to beat that with a knife service.

From the original questioner:
You guys are awesome, thanks. The cutter head I'm looking at is an FS Tool Corrugated Toolhead.

So grinding out a sill and rail set with 90 channels I'm guessing canít 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.

From contributor V:
Not sure exactly what a sill and rail set is but if you are talking about something like stick and cope or two different cutters that make pieces of wood to fit together I would not advise that as a first attempt to grinding your own knives by hand. As far as your cutterhead, I think most grind-it yourself guys would prefer a two knife cutterhead to save on work. It is possible to get a nice molding with a single knife and a dummy knife in the second slot for balance. Corrugated is the best choice.

From the original questioner:
Thanks again contributor V. My first project is an old door circa 1900. It has an ogee profile but all that I've found anywhere are a much deeper profile, more than an 1/8 of an inch which is too much for my liking. I do have about a year to complete this part of the restoration, so I have time to practice. So it's a raised panel door, 1-3/8 inch stock, stile and rail. I guess my question still is, how to fine tune the inside edge?

From contributor F:
I hate to be the doom and gloom guy but I think it's an awfully ambitious project if youíre doing what it sounds like. Youíre talking about making a stile and rail set for passage doors right? Meaning the ogee profile has to be the same on both front and back of the door, and an exact match between cope and stick? And this will be your first time grinding knives?

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.

From contributor V:
If you have that kind of time and don't mind the chance of failing I see no reason why you shouldn't attempt any cutter you want. Just be sure to get any available specs for your cutterhead so as not to exceed any safe cutting diameter meaning maximum safe projection of a knife from your head with regards to a raised panel knife etc. I can understand why someone who has never hand ground knives would think it is next to impossible to make stick and cope that fits. At one time I thought it was all pretty near rocket science too. The fact is, human beings are capable of much more than they know. Hand grinding knives for cuts that fit together is like anything else that must be fairly precise, you have to have a procedure and an ordered process.

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.

From contributor M:
I have hand ground a fair amount knives myself, mostly reproduction mouldings way back when and yes there is a fair amount of satisfaction in doing so. However, there would have to be some serious underlying circumstances before I would consider a set of cope and stick knives even with the added years of experience I have now. Heck it took a long time to master a really good set of door cutters using a profile grinder! There is a fair amount of protrusion beyond the cutterhead for cope and stick patterns and good accurate relief grinding is paramount for a quality finish, and balance is of the upmost importance. The investment alone in steel, grinding wheels and associated goods will cost well in excess of a bought out set of knives, so you really aren't saving money if that is your motivation.

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.

From the original questioner:
Thanks again for your continued suggestions and help. I think I might take the safest route for the time being and have these blades shaped with a local CNC shop. But I will learn this art by hand when time allows. Safety first.

From contributor V:
I agree strongly that buying a set of pro made knives as a model is a must.