How to add a solid wood edging on MDF?

[atgcpaul]

I'm making a round table top that will be veneered.
The substrate is MDF but I want a solid wood banding
around the outside edge that I can route a profile
into. It's a 12 piece sunburst, so I want to cut
12 facets on a square MDF blank, then glue 1 1/2"
elongated keystone shaped strips to that facet.
Then cut the 12 faceted polygon to a round using a
router.

What's the most efficient way for me to cut the 12 facets?
The final diameter of the top will be 26". I have a 24"
square blank right now.

Thanks,
Paul

[LarryG]

That's kind of a toughie, Paul. First problem is to do an accurate layout -- not rocket science, but tedious, with many places for errors to creep in due to the many angled cuts.

But beyond that, there's physically making the cuts, and I'm guessing that's what has you stumped. The SMT on the BT won't have enough travel (I'm pretty sure). A sliding CMS might reach, if you have one. Or you could maybe use the rip fence and a 15deg tapered wedge.

How thick is the MDF, and how thick is the edge banding? Another approach would be to make the facet pieces wider (maybe 3"), glue them up into a dodecagon, use your router and trammel arm to cut an inside diameter of about 22", then cut a rabbet with an outside diameter of about 23", and finally cut the MDF blank to the same 23" outside diameter and drop it into the rabbet. If the faceted edge strips aren't thick enough to allow this, you could cut half the rabbet's depth into them and the other half into the bottom of the MDF blank.

I'm just making these diameter values up off the top of my head so I'd need to draw it up to check the dimensions, but in principle I think that's how I'd do it. Everything except the 15deg angled cuts on the ends of the facets would be done with the router, with no need to figure out the layout or devise a way to hold the MDF blank at the correct angle relative to a saw blade or fence.

[meika123]

BISCUITS-BISCUITS-BISCUITS..

Dave in NC

[atgcpaul]

That's kind of a toughie, Paul.
First problem is to do an accurate layout -- not rocket
science, but tedious, with many places for errors to creep
in due to the many angled cuts.

But beyond that, there's physically making the cuts, and I'm
guessing that's what has you stumped. The SMT on the BT
won't have enough travel (I'm pretty sure). A sliding CMS
might reach, if you have one. Or you could maybe use the
rip fence and a 15deg tapered wedge.

Duh! A 15deg wedge! Now I just need to go make one.
The school actually has a vertical panel cutter. Maybe
I can use the wedge and the cutter to do this. I'll probably
have to fine tune each cut for the banding to make a no-gap
fit. But I figure as long as I have enough material, when
I cut the final circle, and lopsidedness will be covered up.
I hope.

How thick is the MDF, and how thick is the edge banding?
Another approach would be to make the facet pieces wider
(maybe 3"), glue them up into a dodecagon, use your router
and trammel arm to cut an inside diameter of about
22", then cut a rabbet with an outside diameter of
about 23", and finally cut the MDF blank to the same 23"
outside diameter and drop it into the rabbet. If the
faceted edge strips aren't thick enough to allow this, you
could cut half the rabbet's depth into them and the other
half into the bottom of the MDF blank.

I read about this in one of our handouts and it sure didn't
seem Paul-proof at the time.


Thanks Larry,
Paul

[LCHIEN]

with a lot of angled segments your accuracy will have to be outstanding.
A consistent .25° error on each angle will end up with a 3° gap or overlap depending on whether you were over or under.

One way to take care of that is to make the table in two halves,
each haf of 6 15° segments, but make the 6th segment 2-3 degrees oversize. Each half should be 180° if perfect plus the 2-3 extra degrees. YOu can them use a straightegde and a router to make it perfectly straight, taking any extra off the wider segment on the end.

With two halves perfectly 180° you can then glue up with no gaps.
If you keep your angle errors small then you'll only be off about a degree in 6 segments or each half, and I don't think an degree off in one segment will be noticed.

[Wood_workur]

Lorings has got it spot on. That is the best method. As for the edge, you will want to make templates, and use a router to cut it into round.

[atgcpaul]

The deed is done

Flipping through an old Fine WWing mag last night gave me the inspiration to
finish the dodecagon this morning and apply the wood trim. This morning I
glued a piece of straight MDF to a piece of hardboard. I chucked up a 1/2"
straight bit in a router and butt the router against the straight edge and ripped off the overhanging hardboard.

I carefully laid out the dodecagon on my MDF substrate with a compass and a
straightedge. I cut on the outside of the 12 facets with a bandsaw and
then clamped my router jig with the ripped edge flush with the lines I drew.
Quickly enough, I cut the MDF substrate to finished size.

The next step was tedious but it worked. I ripped my edges to 1 3/4"
and set the miter gauge to 75deg. I cut out a keystone oversized to one
of my facets and nibbled away the bevel until it was flush. Then I fit the
next piece to the one I just cut and worked my around the facets until I was
all done. Everything is glued in place and waiting to be rounded with a router
trammel jig.


Thanks for everyone's ideas,
Paul