Profile Grinding Bent Wood
The developer of an innovative method for extreme-bending hardwood explores options for profiling his bent pieces using abrasive wheels. March 12, 2014
Question (WOODWEB Member) :
I need to create a custom grinding wheel to grind a simple profile. Knife shaping won’t work for this application because of a type of engineered wood I'm using for the moulding. I'd like to turn the profile on a plywood disc on the lathe, apply adhesive and then coat it with 60 grit (silicon carbide). What adhesive could work for this? I'm comfortable with UF, Epoxy, PVA, PolyU and etc.
I understand that this may not be very durable and realize I'll probably just have to test a bunch of adhesives to see if any will work. A commercial disk like a Larick with flexible SiaFast paper on it was my preferred method for this job, but, the grinding/shaping operation may overheat the SiaFast and melt the velcro backing. This grinding disc will be used on a Larick style variable speed profile sander, but will grind the profile instead of sand it, so it is a much more aggressive operation. Other ideas to do this are greatly appreciated.
Forum Responses
(Adhesive Forum)
From contributor J:
This seems very improbable. Can you post more info about what you're trying to accomplish (molding shape, material, quantity and etc.)?
From the original questioner:
The molding is a small glass stop moulding with an ogee profile like the one shown below. I would be making something like a few hundred feet of it a month, so not a lot of production, but I'll use the concept once developed for other wood grinding jobs. The moulding will be re-sawn to approximately a small triangular cross sectional shape, just oversize, to minimize the amount of wood to grind off. The grinding wheel will be a fairly large diameter, about 10 to 14", so that it will wear better and the speed will be reduced due to the increased diameter.
I am thinking about applying the adhesive to the wheel blank, then rolling it through the silicon carbide grit to pick up as much as it can. Of course, a lot of it will fall back off, but some will stay attached and be bonded. The glue will need to be able to handle heat buildup, and not melt. The finish will be coarse, but then it can be sanded with a normal Larick style wheel with SiaFast flexible sandpaper.
From Contributor R:
I know you said that knives won't work, but have you tried to create this profile using a shaper or router table? Is there too much tear-out or chipping using a custom knife on a shaper to be effective?
From the original questioner:
The shaper cutters and router bits will perform the same way as the moulder. The issue is cutting with the grain on my engineered wood. I engineer hardwood for extreme wood bending. It is solid hardwood, but subjected to intense longitudinal thermo-mechanical pressure. That is just a fancy way to say that I take 10' planks of hardwood and after many sets of conditions are met, I compress those planks into 8' long planks. The cell walls form a bellows, and the wood is then extremely flexible. It is like the difference between a hospital straw and a straight straw. One is flexible, the other rigid.
Once bent, this wood is dried to fix the shape. Cutters running with the grain of this wood cause terrible tear-out. I can cross-grain plane or route it, but that isn't how routers and shapers are set up to work. Any type of abrasive method works great for shaping it. By shaping it first, the moulding can then be bent, dried and installed.
From contributor J:
My experience with grinding wood to shape has been in a pattern shop, where disc and spindle sanders are standard tools for final shaping. Both of those tools spread the wear over a broad area; flat and convex part surfaces can be ground anywhere on the disc, and the spindle sander oscillates. The wood used in pattern work is virtually always fairly soft - pine, mahogany and jellutong. I can't see the approach you're contemplating being at all practical, given that you want to shape hardwoods with a tool that inherently can't spread the wear around. I think your abrasives are going to wear off or load up almost instantly.
Do scrapers work with the grain on your material? I'm wondering whether a shaper tool with a less aggressive cutting angle (more radial, less tangential) could work.
From the original questioner:
To contributor J: That tooling idea is very interesting. I'll see if I can find a way to test it. It could very well be the angle of the knife that pulls out the grain. A steep angle might work. I can't think of a way right now to adapt a shaper head that way, but I'll work on it. I recall coming across some adjustable angle heads, but will need to rediscover where I saw them. It could work. Now you have me thinking about how saw blades cut, and those blades can have positive, neutral or negative angles on the teeth depending on the application. This wood responds very well to all types of saw blades without tear-out. Any ideas on shaper or router heads like this would be greatly appreciated.
By the way, the engineered hardwood is very soft when bendable. It is at a 20%MC at that point, so not that wet, but definitely soft like pine. After drying, it is as hard as kd hardwood.
From contributor D:
I think you may be hung up on the abrasive thing though. Have you approached any tooling companies? Their engineers always seem to love a challenge. I suspect some sort of hogging tool would work. Look at a solid carbide spiral hogging bit- the edges are serrated like a tiny sine wave. There are fine and coarse hoggers, and hogger finishers. Seems a custom set of shaper knives could be ground with your profile slightly wavy, two knives cutting in the shadow of two others, leaving a fairly good surface. This could be tried pretty low cost before investing in a custom cutter with scraping or negative hook angles. Hoggers have a great reputation in the CNC world for not tearing out.
From the original questioner:
Great lead on the hoggers. I tried some serrated cutters without much success, but certainly didn't exhaust that possibility.
From Contributor W:
Have you tried a very low or even slightly negative hook angle cutter? Have you tried climb cutting? Machine the wet wood while frozen and it cuts much nicer.
From Contributor W:
If your wood was positioned with the OG running horizontally, that is machined with the future point at the bottom, your profile is quite shallow and there are applications where a sanding belt is run over a shoe giving the smooth OG to the top surface of a square plank which then gets processed further to create the 90 degree back. Research profile sanding equipment. It seems routine for them.
