Choosing a Moulder and Shaper Knife Grinder
Advice on how to get into knife grinding for your own shop. August 31, 2010
Question
I need to start making my own knifes for my shapers. I am running 7.5 HP shapers with 30mm spindles and can mount up to a 140mm tall tool, but I plan to buy 4" knives/heads. I think I want corrugated knives.
My shop is in SE Asia so there are no suppliers, and no one will sell knives to competitors. Actually none of my competitors know how to make good knives. I am open to insert tooling options, but I do not think there is anything that will do what I want. The profiles I am cutting are the usual stuff for cabinets; crown, light mould, fluted boards, beaded boards, coves, base mould and etc..
I assume the old fashioned knife grinding setup is what I need. We are not making that much moulding, and later on if I need to I will buy a five or six head moulders. I know nothing about these things other than I have seen them used. It looks easy enough. Make a plastic template, dress the wheel to match the follower pin, and grind away. Like a key copy machine. I am hoping I can get a setup for under 2K. Any suggestions on brands or models?
Forum Responses
(Cabinetmaking Forum)
From contributor U:
You are correct in that you just make a plastic template and then use that similar to a key copy machine. There are many options. The first question is how do you plan on making your plastic templates? Most people in the US use a small CNC router to cut these out for their grinder. The most popular and heavy duty grinder I tend to sell is the Weinig Grinder. The prices for used machines start at around 5k plus here in the US. I don't know what machine you can find there for 2k. If you don't mind quality problems and the machine not holding up over time, then that is the way to go.
From the original questioner:
Thanks for the info. I spent some time researching and I see that these machines are a lot bigger that I thought. At those prices I think I will have to consider ordering my knives for now. I only need ten or so profiles. I am selling cabinets, not moulding at least for now.
From contributor L:
Back in the olden days before I had a molder and profile grinder (I had a good 1 1/4" spindle shaper) I hand ground knives on an 8" bench grinder. You will only have one knife that does the finish cut but that's ok on a shaper since feed speeds a relatively slow. You can easily determine the proper angles by drawing circles on a paper to match the head and cutting circle. Balance should be close but it doesn't have to be as good as on a high speed molder. You can buy a relatively cheap balance (two pans) and use the grinder to first balance the blanks and then again after you have them profiled. You need soft grinding wheels for high speed steel. Get several 1/2" wide wheels and profile them with a diamond tipped wheel dresser. Cut the end of a sample molding at the angle you have found by drawing the circles. Put layout dye on the steel and scribe the pattern from the sample. If there is not a sample you will need to elongate the pattern to compensate for the radius. It's really pretty easy once you've made a few (mistakes). With a shaper and a power feed you have to feed slower to keep from having the knives pound and bounce the wood off the soft feed tires. On a molder the wood is supported by the bed plates.
From the original questioner:
I don’t think I am willing to do it all by hand like that. For now I can order my knives from Australia or the U.S. I still need to choose a cutter head. I think I want a 100mm (4") tall (knife capacity) cutter. I see there are also different diameters, and some that hold 4 knives. With a 7.5 HP shaper what are the practical limits for cutters? Obviously it will take multiple passes for a larger profile, but this is not a big concern for me. Does this mean it will hold two knives at either 10 or 15 degrees or does it hold four knives?
From contributor L:
That head has two slots for one hook angle and two for the other. I might be inclined to go for a greater spread between the two. The lower angle is for very hard or difficult woods and offers more of a scraping action. It reduces tear out but requires higher feed forces and power. An angle of 20 or 22 will give freer cutting in easier to machine wood. If you get dual angle head be sure to get some filler bar for the unused slots. This is needed to keep from distorting the bore when you tighten the gib screws on the occupied slots. I'd be inclined to get at least two heads - a shorter one (60mm) for doing edge work and a wider one for face molding. Molder heads are meant for mechanical feeding. They have no chip limiting so can be dangerous to free hand. Be sure your power feed is set tight. If you are inclined to climb cut do it only with the feeder and taking small bites that are limited by the fence.
A long time ago I had a stick get away from me while climb cutting with a feed, it went through an oak panel door in the shop! It would have gone through me if I'd been in line. We turn big column bases for a local company using a molder head on a heavy shaper. Jig mount them on an auxiliary table with a heavy bolt as a pivot. Clamp the table to the shaper with spacers to the fence that you can take out to move the table in after each pass. Tilt a power feed so only one wheel feeds and it will turn perfect matches time after time. Hand grinding really isn't that tough of skill to acquire.
From the original questioner:
I only use power feeders on shapers. You said "more spread between the two." Do you mean a bigger diameter tool or greater difference between the two angles? I have seen heads with 12 and 20 degree slots. As for hand grinding, It seems that getting the two knives close would be near impossible. I’ll give it a try for some smaller profiles. Also, is it ok if we mount 50 mm knives in a 100 mm head? Or do you have to fill the slot? What effect does the tool diameter have on the cut/feed and finish?
From contributor L:
"You said "more spread between the two" do you mean a bigger diameter tool or greater difference between the two angles? I have seen heads with 12 and 20 degree slots." Between the two angles, most of ours are 12/20 or 15/22.
"As for hand grinding, it seems that getting the two knives close would be near impossible. I’ll give it a try for some smaller profiles." It’s not really necessary to get them identical as only one knife does the finish cut on an un-jointed head. The second knife is more of a balance than anything. To tell the truth I used to (when I was hand grinding) only make the one profile and set up a second knife that was close to the same weight and a little less protrusion in the second slot. That isn’t maybe the best but as long as there wasn’t detectable vibration from out of balance it worked. Keeping in mind these setups were usually for short run custom moldings. They were usually done with lock edge steel.
"Also, is it ok if we mount 50 mm knives in a 100 mm head? Or do you have to fill the slot? What effect does the tool diameter have on the cut/feed and finish?" The important thing here is to be able to tighten all the gib screws uniformly. The knife steel doesn’t have to go all the way to the end of the slot (but you will have a reference issue if it doesn’t). The gib screws should always press on a filled area of the slot. If two of three screws would fit over the knife area and you removed the others it works, may not be fully recommended! Use caution! The larger the head the less the scallop in the knife marks and the smoother it looks for a given rate of feed. Not a big deal with typical 4 to 6" heads and 30-40'/min. feeds. Be sure to keep the mating surfaces clean. You don’t want to have gunk further compress while running and allow the assembly to loosen!
Quality: we have a variety of heads from different manufacturers and have used several brands of steel. Weinig is the most consistent for being able to remove the knives and reinstall them (not necessarily in the same slot) and have them be really close to the same cutting circle. Other heads are quite a ways off. If we have much to run we always touch up grind (profile grinder) before running. For custom runs, typically 1,000’ or less we use M2 steel from European Tooling, seems to work fine. The steels that will run longer are typically harder and have the drawback of chipping the edge if they hit an old knot. You could say we should always defect the blanks before running but for straight knives we use the double back system and grind the carbides in house. Our grinder is not suitable for profile grinding carbide. Speaking of straight knives - if you have a profile that has a lot of straight and then a smaller amount of bumps you can mount the straight knives in one pair of slots and the profiled knives in the other and not have to grind away a lot of steel. That may only be applicable to a moulder where we run from hit or miss surfaced or even rough lumber. On a shaper you will normally surface to thickness first, another step over running a molder. Modern molders with their digital readouts or even electronically located spindles are fast to set up. The heads are measured at the grinding bench and marked with the deepest part of the knife radius before going to the molder. The side of the head that rests against the spindle shoulder always uses a 10mm offset, so reference stays the same head to head, profile to profile.
From the original questioner:
You really know your tooling. Did you use a low speed bench grinder or just the ordinary one? It seems like heat would ruin the steel on a regular grinder. So the angle of the grind (back bevel) is dependent on the tools diameter? In my case I not usually have a piece of sample molding to use. I guess I can draw in CAD and manipulate the drawing there to get the correct "elongation". I never even knew there was such a thing. Now I understand why. I know that the knives on my tooling never exactly lined up with the cut profile unless I angled the knife. Thanks a lot. With my next tooling order I will order some blanks and give it a try. Is there an internet reference for these technical details that you can point me to? Or should I just keep asking you?
From contributor K:
The advantage to having a grinder in a small shop is being able to re-sharpen your knives in the head and right back into the moulder. We rarely grind from scratch because you need to do it all the time to be any good at it.
From contributor L:
Yes there is a technique that needs to be learned for doing a good job of grinding but you can do it. Being able to sharpen a nicked knife and then continue with the job is a great benefit. I'm not familiar with his tooling suppliers. Weinig tends to be higher priced than most. You only need to buy the 54 grit wheels and then just shift to a higher speed for the finish grind. I looked to see if Weinig published their grinding manuals online but no luck. The trick is to keep lots of coolant flowing right at the point of contact and not force cut more than what can be cooled. A lot of guys use a kind of bump cut process: rapidly grinding then backing off then grinding again. It’s impressive to watch the automated grinders work, really fast!
