concave shape in wood or mdf

[travis]

Is there an easy way to create a concave shape in wood or mdf. I'm looking to create 7cm diameter circle with a concave dip of about 6 to 7mm on a flat board. I've been down to the local library and had a look at the wood working books but joy yet. Any type of work like this seems to be done using wood lathes to create the concave dip.

I've posted up an illustration to give a better idea of what I'm wanting to do.



direct link to image - http://img516.imageshack.us/img516/6291/concavepicpf3.jpg

[ScottS]

You need one of the following tools: an adze, an inshave, a pullshave, or a collection of hand carving gouges. Or all of the above. Try to find information about chair making, since chair seats are usually shaped in a concave fashion. The Windsor chair, in particular, uses many of these tools to shape the seat.

Edit: all of these tools are designed to be used in solid hardwood. Which means that plywood, particle board, and MDF are probably out. If you purchase new versions of the tools, I wouldn't expect them to be sharp enough to cut well. Be prepared to sharpen them before and during usage. If you don't know how to sharpen curved tools well, you'll probably become frustrated very quickly.

[shardian]

Technically, you could do this by making a bunch of circle templates, and then stepping in and down each time using a router to get a very rough shape. Then use a buttload of sanding to finish it off. Theoretical of course, because I wouldn't ever have the desire to attempt that myself.

[ChadTower]

I saw a wooden bowl a guy made the other day.  It was heart shaped.  He used a single heart cutout, then used the scroll saw to cut the same shape from the center of the wood several times, each a half inch in from the last, at a 5 degree angle.  Then he reglued them slightly offset.  A buttload of work but it came out really well.

[karlpenney]

I made a couple of similar templates last weekend for routing the rounded inside corners of my side panels.  Here's how I did it:

- took a short piece of 1x2 (6 inches or so), and used router to make a notch in one end for top bearing of pattern bit to sit in
- measured from end of notch the radius i wanted my circle to be, minus the cutting width of my pattern bit and made a mark
- drilled pilot hole on this mark and used screw to attach 1x2 to scrap piece of mdf so that notch hangs over edge by a couple inches
- used jigsaw to cut some material away from the mdf under the 1x2 to give clearance for a starting point for the router bit
- drew line of cut I want to make on template workpiece and used jigsaw to cut away material just outside this line
- clamped workpiece and jig I just made to my workmate bench and lined them up so I could cut my arc
- place router over jig with top bearing sitting in notch in 1x2 and then cut arc buy moving router with 1x2 around center of arc
- trim straight edges on either side of the arc you just cut to get whatever depth you want

see pictures attached

[shardian]

I made a couple of similar templates last weekend for routing the rounded inside corners of my side panels.  Here's how I did it:

- took a short piece of 1x2 (6 inches or so), and used router to make a notch in one end for top bearing of pattern bit to sit in
- measured from end of notch the radius i wanted my circle to be, minus the cutting width of my pattern bit and made a mark
- drilled pilot hole on this mark and used screw to attach 1x2 to scrap piece of mdf so that notch hangs over edge by a couple inches
- used jigsaw to cut some material away from the mdf under the 1x2 to give clearance for a starting point for the router bit
- drew line of cut I want to make on template workpiece and used jigsaw to cut away material just outside this line
- clamped workpiece and jig I just made to my workmate bench and lined them up so I could cut my arc
- place router over jig with top bearing sitting in notch in 1x2 and then cut arc buy moving router with 1x2 around center of arc
- trim straight edges on either side of the arc you just cut to get whatever depth you want

see pictures attached

Owww. That made my head hurt... I'm still trying to figure out WTF you did.

[scotthh]

On pages 198-199 of the paperback version of  Woodworking with the Router there is a discussion of how to form a concave (or convex) surface. You use a surfacing platform with curved tracks, allowing you to hollow the workpiece. This is used to make the area for you butt on a wooden chair. It's essentially a jig that the router moves up and down along. This is a complicated set up: You need the surfacing platform, a carriage to hold the router, and the tracks.

Also, you'd probably want to reseal your MDF with thinned down waterproof PVA wood glue.

[bfauska]

Here's a potentially screwy solution:

Make an auxiliary base for your router that can be hung from three lines (120 degrees apart around the perimeter of the base) using 3 equal length lines going to a single point suspend the router from a tripod or sawhorse or whatever you want. The length of your string will determine the radius of your hollowed out area.  Clamp your workpiece in place under the suspended router and rout away the wood, the router's range of motion is limited to the sphere defined by the string.

[ChadTower]

I'd be afraid of the router jumping on the string if the bit hit something.  Uncontrolled router bit bouncing near your arms = bad bad bad.

[ScottS]

I'd be afraid of the router jumping on the string if the bit hit something.  Uncontrolled router bit bouncing near your arms = bad bad bad.

Agree. That contraption sounds like a recipe for a nasty kickback accident!

[ChadTower]

Engineering wise, it's a reasonable solution, but just not a safe one with a power tool on the pendulum.

[travis]

Certainly interesting reading the replies, I had the idea of using circle templates that shardian mentioned but wasn't sure if i was making too much work for myself.

I've got some free time next week to try this out, I'm designing the jig at the moment but I'll be sure to post pics when I'm done.

Thanks for the input guys, appreciate it

@ scotthh - thanks for the book recommendation, I'll have to order that I think.

@ bfauska - maximum point for creativity, seems rather risque

@ karlpenney - nice work


travis

[ChadTower]

How many of these things are you going to make?  If it's only a couple, I'd just use hand tools.

[scotthh]

I took both the aforementioned Woodworking with the Router and The Router Book out from my library:

Pat Warner's The Router Book is loaded with jargon and tells us about things he made and things he does. I thought it was completely useless. It has been returned.

The Hylton & Matlack book (Woodworking with the Router) is loaded with information, definitions, and plans so you can make and do the things the authors do. Due to all the information, it's not an easy read but it will make an excellent reference. I will be buying a copy.

As an aside, both books were originally printed in the 1990's. It's amazing how far routers have come since then. There is no mention of routers that come as kits with both fixed and plunge bases. Stock baseplates weren't see-through--the tools look primitive. Soft starts were a luxury 10 years ago. Also, the authors say a toggle switch is bad because you can't switch it without taking your hand off the knob. My hands aren't big, and I can easily reach the toggle switch on the Dewalt DW618PK while maintaining a firm grip on the handles. The version on Amazon was reprinted in 2006, hopefully they've updated the chapters on equipment with the newer stuff. That said, the jigs, techniques and bit shapes are all still very relevant, and I'm no longer in the market for a new router.

[bfauska]

I suppose the pendulum router could be risky, but I have to mention that when a router kicks back in general it can go anywhere it wants, when a router kicks back on a leash it can only go where the leash allows it. 

Mostly I'm just saying that to be argumentative, I don't necessarily recommend the "tool" I described for anybody, I just think it's a novel idea.

[ChadTower]

Your arms and possibly head are both well within the pendulum's range... otherwise you wouldn't be able to hold the tool or see the cut.  Ow.

[Dudeman]

You've got to give it to the guy, that's a VERY inventive solution.

Then again... I think I'll wait for someone else to try it first and let me know how it worked out.

[prOk]

There are three ways to do this..

one, find a local company with a CNC router and it's easy..   

Two, build a jig that allows the router to follow a curved path that matches your concave line.. think of it as a lazy suzan with to vertically mounted pieces that have a curve in the tops that matches the deepest cut you want for the concave feature.   Make a pass, turn the suzan 4 degrees, make another pass..rinse, wash, repeat..  http://www.diynetwork.com/diy/ww_tables/article/0,2049,DIY_14446_3877238,00.html

Third option, put it on a lathe and turn it.

/b

[NiteWalker]

Pat's book is more or a "what it is and what it does" kind of book. It outlines the form and function of the router. It helps you understand what it is and what can do.

Hylton's book is more of the "how to do it" book with tons of illustrations and directions. I love both. The 2006 update of Hylton's book is a complete redo. Not much of the text is the same, it's in full color, hardcover, etc. It's almost an entirely different book. But it's the best router book I've ever read.

You have a really good router.

I took both the aforementioned Woodworking with the Router and The Router Book out from my library:

Pat Warner's The Router Book is loaded with jargon and tells us about things he made and things he does. I thought it was completely useless. It has been returned.

The Hylton & Matlack book (Woodworking with the Router) is loaded with information, definitions, and plans so you can make and do the things the authors do. Due to all the information, it's not an easy read but it will make an excellent reference. I will be buying a copy.

As an aside, both books were originally printed in the 1990's. It's amazing how far routers have come since then. There is no mention of routers that come as kits with both fixed and plunge bases. Stock baseplates weren't see-through--the tools look primitive. Soft starts were a luxury 10 years ago. Also, the authors say a toggle switch is bad because you can't switch it without taking your hand off the knob. My hands aren't big, and I can easily reach the toggle switch on the Dewalt DW618PK while maintaining a firm grip on the handles. The version on Amazon was reprinted in 2006, hopefully they've updated the chapters on equipment with the newer stuff. That said, the jigs, techniques and bit shapes are all still very relevant, and I'm no longer in the market for a new router.

[TroyO]

Hmmm, what if you made a jig that used a circular saw mounted on a circular disc of MDF that sat inside a hole in another piece of MDF. Take shallow cuts turning the saw until you got the depth you wanted.

You would want the saw offset so the blade was centered in the MDF disc.

I've seen a similar concept of using a sawblade "sideways" used for cutting raised panel doors. It wouldn't be dangerous I don't think.... the blade is mostly buried in the work, and kickback would be controlled because the saw can't really move laterally when locked in by the outer jig.

Just a thought... not sure it would work but I'd try it in a pinch.

(My verbal skills suck, so I'm attaching a pic... which also sucks. LOL)



 

[ScottS]

I've seen a similar concept of using a sawblade "sideways" used for cutting raised panel doors. It wouldn't be dangerous I don't think.... the blade is mostly buried in the work, and kickback would be controlled because the saw can't really move laterally when locked in by the outer jig.

Give it a try! I'm sure we can get your username changed to "Lefty", "Stumpy",  or "OneThumb" when the inevitable accident happens 

Seriously, though, I'm not sure I understand what you're suggesting. But I'd make ---goshdarn--- sure you don't: 1) allow the saw blade to contact wood before it is completely up-to-speed and 2) allow the back half of the blade to come into contact with the wood before the front half. Not saying your "solution" wouldn't work, but I wouldn't try it myself. And I own the only circular saw on the planet that was specifically designed to make plunge cuts!

[TroyO]

The key to it is making shallow passes.... just like when using a table saw to make raised panel doors or cutting a cove. Both of those techniques are tried and true, this is just a modification of it. Plus, the blade is enclosed in the work and the jig.

The intention was to hold the saw about 1/4" off the surface (Which still leaves a 1/2" MDF ring to hold the saw in place) start the blade and then lower into the work for a 1/16-1/8" pass by rotating the saw in a circle. Lower the blade about 1/16-18" and repeat until finished. A plunge cut saw would probably help, if you could limit the depth of the plunge.

I don't think I'll be called "Lefty" anytime soon. :-) (Not the least of reasons is that I don't have a need to try this technique in the first place, LOL.)

[Chris]

You know, we're only talking about a circle less than 3 inches across and a bit more than a quarter-inch deep.  It seems like a lot of these ideas are serious overkill.

What about just getting something to attach to a drill?  Isn't there some sort of abrasive like a paint stripper that can be shaped to a slight convex form and attached to a drill?

[scotthh]

I did a fair amount of searching for a 2.75" or 3" drill bit or countersink bit. I thought maybe you could drill and then sand. But I came up empty.

[TroyO]

I don't know that anything would work out of the box in a drill. A panel raising router bit would be close, but there's usually a space between the blades and the bearing that wouldn't let it go very deep. I guess you could drill the depth with a forstner bit bit enough to clear the panel raising bit blades and do it on a drillpress that way.

A lot of woodworking has to do with the question of "How many of these do I need to do?"

If you need to do a lot, it's worth investing more (Money, Time, Research) in figuring out the best way. If you just need to knock out one sometimes your just best off doing it the hard way.

If I just had to do one dish like that I'd probably just do it with a coarse grit sanding pad on a grinder or even one of those ones for hand drills. If a few more than that I'd try the MDF/Circular saw jig or maybe regrinding a spade bit to use in the drill press. (Although the biggest spade bit I've seen was about 1 1/2 inches, there might be bigger ones available somewhere.)