molding knife profile

Having recently acquired an EDM machine, and having a RBI 816 molder,
the idea struck me is if it is possible to create my own molding
knives...
Knife stock is not terribly difficult to source. I think I have a few
methods of cutting the electrodes that should be effective(CNC,
surface grinder, etc).
What I am wondering is how the profile is cut into the knife. It does
not appear to be a direct tangential from the knives(and templates) I
have. I think this is due to the cutting angle of the knife. I THINK
that if a direct profile was introduced into the knife at the same
angle as the knife is held in the cutter head I should get the profile
I want in the resulting cut. Correct?
Explaining that again(but differently) the knife on edge does not
match the template(ie the knife and the template are on plane with
each either). But if a grinder wheel(or EDM electrode) had the
desired profile and the blank was held at the cutting angle used in
the molding head would the resulting profile create a molding that was
the same as the grinder or electrode?
JW
Reply to
jw
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I've been making custom molder knives for an older 12" Belsaw Planer/Molder. I suspect the RBI uses the same knives. (Mine are 1/4"x 1-1/2" for 3/4" stock) I also do some mini kives that mount in the center of the head, max of 1" wide, made from .134" (??) stock. These are the same knives used in the Sears molding head.
IIRC, the knife is held at 30 degrees off of tangential, the back needs at least 15 degrees of clearance so the cut angle needs to be at least 45 degrees, more is helpful for chip clearance.
To make a profile the width will be exact. The knife depth will need to be increased by the desired depth / cos 30 degrees. I am usually copying old molding so I just cut it off at 30 degrees, trace around the stock onto a piece of knife stock with layout dye, and hand grind it. If you try and EDM it, you could pluge through at the 45 degree angle but you would still need to have the EDM profile slightly different than the original since the original will be based on the 30 degrees, not the 45 degree angle.
I'm using .250"x1.500" A2 steel from Crucible. Hardens up to about 62 Rockwell, tempered back to 56 to 58.
jw wrote:
Reply to
RoyJ
So are you using knife stock, or just "plain" A2 bar stock and then grinding an edge?
I think what you are describing makes sense to me. I should just do the same. Take a small piece of knife stock and try to copy an existing profile and see how close I come out on it. Would be a worthwhile experiment.
My molding head uses the corrugated knife stock. I am not sure, but I think it can hold plain back with different gibs, but I like the positive holding of the corrugated.
Thanks
Jeridiah
Reply to
jw
My head uses just plain bar stock, smooth on both sides. I usually put larger knives in the surface grinder and take a finishing cut of a couple thousanths to take out any heat treat warpage. And I use just plain A2 bar stock. I've been pleased with the hardness, toughness, and low warpage. And I really tighten up my gibs!!! The knives weigh around 6 oz for the larger ones, pretty scary thought about them coming out.
For just occasional knives I rough grind in my chopsaw. I stick a slide table made of 3 pieces of 3/4" boads in the vice, and use a 4x4 wood block with a 1/4" slot in the top. Everything is sized so the angle of the blade is correct and I can clearly see the marks of the profile I want. I then head to the bench with a die grinder, air cutoff tool, V files, etc for cleanup. On a good day I can start with a sample piece of wood, be running a test piece in a couple of hours. If I only need 50 feet, I don't even bother heat treating. If it works out, I bring it over to the lab and run it thourgh the furnace and surface grinder.
FWIW: I can usually get a good knife or a good copy but not both on the same day. Sigh. My machine is set up to use a single knife with counterweights in the other 2 slots. No way can I get 3 knives the same, let alone set them up so they all cut evenly. I just finished off several rooms that took 190 pieces of various moldings. Some was stripped and reclaimed, some new. Ceiling cove, baseboard capstrip, and some of the backbanding was done with my own cutters. All paint grade pine so I'm not pushing the cutters.
jw wrote:
Reply to
RoyJ
Sounds encouraging. Mine is the same. When molding it uses a single knife. When planing it uses all three.
I will order up some knife stock and take a whack at it. Probably the cheapest stuff I can find. I dont' see need for the extra special stuff available. Not looking to run several thousand feet of anything. At least at this point. For sure plan to make a few scrap knives anyway.
JW
Reply to
jw
The A2 in .250" by 1.500" is $29 for 18" at Mcmaster, about 1/2 that from my local supplier.
jw wrote:
Reply to
RoyJ
You are correct in knife shape not being tangental from patern, although this does give you a close approximation on some contours. To create the knife shape from original pattern the knife pattern has to be drawn by projection taking into account rake angle of cutter head. Another consideration is that each time you mount the knives in the cutter block if they protrude a little less or more than previous time the shape is slightly different, although not enough to be a problem unless you are butting the two pieces together.
Reply to
Paul D

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