Grinding Your Own Moulder Knives

      Here's a long, rich thread covering many aspects of moulder knife grinding. October 2, 2005

I am interested in purchasing a used knife grinder. We're beginning to use our moulder more and more and outsourcing our knives is becoming costly. How long does it take to grind a typical 3 1/2" crown knife, for instance? Has anybody found it more cost effective to buy semi-finished knives and just do the finish grind yourself?

Forum Responses
(Solid Wood Machining Forum)
From contributor J:
I am an ex-production manager of high end mouldings and trim work and am now grind tooling for moulding manufacturers. Your question is asked almost daily. If you are a high volume producer, yes, you should grind in-house due to volume turnover of product and the quality control. If you are a small shop with minimum runs, purchase your tooling and add that cost to your customer's run. You should know there is a lot of expense in just buying a grinder and doing your tooling - there's steel cost, electric, wheels for grinder, a way to cut steel, template manufacturing, scales, and the biggest expense and overhead… who's gonna grind?

There is a learning curve to grinding and I would suggest going to a grinding school for proper grinding and balancing (it will save on your equipment).

My formula would be cost of employee + downtime on moulder while grinding and machine expense for grinding. Compared to scheduling a run, purchasing tooling, moulder setup, product run, customer happy! Of course there are many other variables that come into the picture, such as if you have other equipment that would help in your own grinding, like shapers, rosettes, etc. And, of course, there are nicks.

From contributor F:
It would take me about two hours to grind a crown cutter that length by hand, so the machines must be ten times as fast or more.

From Dave Rankin, forum technical advisor:
For most grinder operators, a 3-1/2" crown of normal depth would take about an hour to grind. I do a lot of profile grinding, both for customers and during training of new operators, and have many operators that can grind this size profile in well under an hour. This time includes cutting the knife stock, squaring the stock, balancing the stock, inserting it into the head (2 knives), grinding and being ready for the moulder. For example, when I ran the Grinderman's Association trade school, each student would grind on at least 3 grinders and set up the moulder 5-8 times. This was done in a 3 day class.

The method of grinding and the use of new fixtures can save you a lot of time.

As for used grinders, there are many available. Check out auctions as well as your local dealers. To give you an example, at present I have 3 profile grinders available from $2500 to $8500.

If you calculate the cost of doing in-house grinding versus out of house, it is almost always less expensive in-house. If you need to sharpen the tools or make a new tool in under 48 hours, then in-house grinding. Training on a grinder takes 1-1/2 to 2 days.

From contributor J:
I have ground on most grinders and when it comes down to it, you should buy a grinder if you can afford it. I also know you will have to have side knives or one bottom for your crown (add another hour and a half) plus template work. I have run production on high speed, too - you better know what you're doing! I can set up moulders, grinders, finger joint (and grind the heads), gang rip, saw mill, etc. You need to research. Are you doing high-end work or custom trim, what is your volume, what type of equipment and what skills do you or employees have?

By the way, kudos to the knife grinder by hand. I'm not sure I would load that into my $25000.00 moulder! Did I hear a few tooling scopes go on?

From contributor Y:
Where can I get grinding training? Just got a used Wadkins nx grinder and no manuals. I have a good idea of how to do it, but need formal training.

From Dave Rankin, forum technical advisor:
There are a couple of schools that offer training. Fox Valley Tech in Oshkosh, WI is one of the best. University of Kentucky in Quicksand, KY also offers training.

If you are looking for training on your machine in your shop, then you have other choices. Howard Hayes out of OK offers this. Ron Hassell from NC (former Wadkin) offers this. Moulder Services from NC offers this.

From contributor F:
Included in that 2 hour free hand grinding time is polishing the back of the corrugated high speed steel knife blank to mirror finish (220-1500 grit). After the profile is ground, all facets of the profile are honed with Arkansas Files. The honing is also included in the 2 hours.

The machine these knives are run on cost me $450.00 used. The grinding equipment I use and modifications to it made by me cost me about $75.00. The moldings that are produced by knives processed in this fashion shine like they were made with a molding plane. I did have to buy a triple beam scale on E-bay for $50.00 to get my knife/gib weight to specs. Over the past 5 years, I have ground over 35 profiles for my personal use. Many are antique molding reproduction. Some are my own design for my furniture making.

Although my machine couldn't compete with the volume that yours can produce, I can assure you that you wouldn't be ashamed to load one of its knives into your machine.

From contributor J:
I work mainly on high volume machines and have purchased many machines as well as ground thousands of knives and have found in the wood industry a lot of frustrated machine owners that are not told enough info on machine setups and tooling. That is why I said take some classes and talk with other people that have the equipment locally (also read your manuals). Our industry is changing with faster equipment and there's not much room for mistakes unless you don't mind spending big bucks on repairs or personal injury.

From contributor F:
My original post on this topic asked about how long it takes to grind knives. I was amazed to find out through Dave Rankin that my freehand grinding is only about 3 times as slow as automatic grinding. I had assumed it would be 10 times as slow or more. I was not suggesting that the poster of this thread try my way. It takes a lot of hand/eye skill and it's not for everybody. My second post was to point out to you that, contrary to what some modern folks have been led to believe, handmade things are usually made better than machine made things.

From contributor E:
Does polishing knives really make the finish on the wood better? I sharpen everything with a 46/60 grit wheel. The knives are quite shiny and the finish (all hardwood) is great. I have heard of filling the wheel with wax to polish in the end. Does it make for a better finish on the wood and if so, for how many LF before you can't tell anymore?

From contributor F:
I am not really a lineal feet kind of guy. What I mean by that is, although I will run trim (usually Victorian style) for a house sometimes or reproduce an antique molding profile for restoration work, I mainly do short runs of molding for the furniture and built-ins I make. Polishing the back of any knife or chisel or plane iron makes an incredible difference! The key to understanding this is to realize that the bevel on any edge tool is only half of the cutting edge. The other half is the flat back of the tool. So, if you polish (hone) the bevel of your molding knife with Arkansas files, and polish and make the back perfectly flat and remove all scratches and mill marks, it will mold even the nastiest grained woods with nary a chip out. Yes, the polished look on the wood diminishes with prolonged use, but I can't really quote you any lineal feet quantities.

From Dave Rankin, forum technical advisor:
Many operators like to finish grind their knives. I like to use a 46/60 grit Magic ceramic wheel that I helped develop several years ago. Other techs like the blue ceramic wheel. When used with the correct coolant, you will get a very high quality finish. The use of other wheels may or may not provide the same finish. A little secret that I have taught for years is to put wax into the wheel for the finish pass. This fills the wheel and provides lubrication during the finishing process. I do use 100 grit wheels when I do thin wheels. If you need to dress your wheel to 2mm or 1/16", then you need at least 100 grit to prevent wheel breakage.

From contributor R:
Wax works really well. 100 grit wheel works great, but if you really want the best obtainable finish, use a 120 grit borazon wheel to finish grind. It's like finishing a fine piece of furniture. You wouldn't use 60 grit sandpaper to sand just before you put the finish on. 100 grit would look better, and 120 grit would look better yet. Be aware that you must shim the borazon wheel to align itself with the rough wheel, and it is only to be used in finish grinding. If you get too aggressive with it, you will burn the steel, and you will loose the temper. Also, it will flatten the leading edge, making it square. Use lots of side clearance and this will help in keeping the 1mm radius edge. They are expensive, but your knives will cut like no others. High speed guys like these wheels because there is no wheel wear between knife #1 and the last knife, thus making jointing easier because the cutting circle is almost perfect.

From Dave Rankin, forum technical advisor:
Just a quick technical correction. When tool steel is being ground, the temperature reached does not exceed the tempering temperature. The burning of the tool will, however, cause a fracturing of the cutting edge. Use good coolant and a proper motion and you will not damage the tool.

From contributor R:
What temperature does knife steel get tempered at? My point is that it is easy to burn the knives with a borazon wheel because of the fineness of the wheel. I know that if the steel gets brown or black, this is not good for the knife.

From Dave Rankin, forum technical advisor:
Most tool steels are heat treated at around 1150 degrees F. When grinding, the maximum temperature that is normally reached is 850 degrees F. When grinding, the result of getting tools too hot does cause tool problems. You will see nicks and have a shorter tool life. If you are jointing, it is harder to hold a joint.

From contributor E:
I understand all that, but my question is quite simple. After 500 lf of hardwood, and I'm tailing the moulder, could I tell the difference if I polished my knives or not? (This is a non-jointed Weinig moulder.) It just seems like a lot of extra time that might not be needed for production runs. Maybe if I was grinding a customer's knives for them… an appearance thing.

From contributor J:
I have a saying - keep it simple, stupid. I have run millions of feet and never found the need to polish knives! I just ground them with the right wheels, vitrified and borazon, etc. with the correct angles side relief, etc. and voila - moldings! Must have done something right, as our molding sales were around 15 million a year. But I could see doing that for the small shop planer-molders where you have less production and the machinery has more vibration, etc. Polishing the knives might be of benefit where it counts.

From contributor R:
I learned how to grind freehand over 25 years ago from an old timer who was an expert. I was not very good at it as a young apprentice. Then we purchased a profile grinder, which made my life a lot easier. It is quite an art to freehand knives for mouldings.

From contributor S:
Moulder and grinder training is available that can help you design and grind knives on your profile grinder. I suggest you look at having someone come to your facility to do the training, because you would then be trained on your specific grinder and can learn how to get the best out of it.

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

Comment from contributor A:
I have had good luck grinding my own knives using my home-built profile grinder. King's Landing is a government run historical site consisting of many restored buildings here in the province of New Brunswick.

Back in the seventies when the restoration work was being done they used a table saw with a Craftsman moulder attachment which held 2 knives. They used the metal from chainsaw bars to make the knives, and by holding 2 pieces together with vice grips and grinding the profiles out by hand on a grinder.

They also used a small moulder machine and used the same method for knife making. They ran miles of moulding that way. I was surprised when I was told of their methods, as I was picturing a government contract with some big company using up-to-date modern methods with big expensive machinery.

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