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Need Help With Bent Lamination Jig

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Forum topic by Th961605 posted 04-09-2017 03:52 AM 447 views 0 times favorited 9 replies Add to Favorites Watch
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Th961605

9 posts in 1481 days


04-09-2017 03:52 AM

Topic tags/keywords: jig dry bend laminate form jig

I could really use some help with a bent lamination form I’m trying to get working. Here’s the deal: The shape is a half-ellipse like a stretched out semicircle arc. I glued and screwed 3 MDF sheets together, cut the arc, and taped the edges, but the pieces keep getting pinched at the end. I attached a picture of a similar form, but mine is longer (~65”) with a radius of 30” to the form bottom. Here’s what I think my issue may be: Take the image below and imagine the curve much steeper, to the point where the curves reach the “bottom” of the jig. This is where my laminations are pinching. I tried taking off 12 or so inches from each side on the bottom jig, and they still won’t fit. Is my issue the fact that the arc runs all the way to the bottom of the jig? The cut-off idea I got from here, but I still can’t get them to fit. For reference, I’m using 12 1/8” strips, which are bending fine, but I can’t the jig to fully close! Any help would be much appreciated!


9 replies so far

View jbay's profile

jbay

1857 posts in 737 days


#1 posted 04-09-2017 04:13 AM

Did you only make one cut for the arch?

You have to cut the thickness of the piece you are bending out of the form.

In other words if the piece you are laminating is 1 ” thick, you have to cut out 1” thick slice out of the form.

-- If anyone would like to see my Portfolio, PM me and I would be glad to send you the link.

View jerryminer's profile

jerryminer

805 posts in 1279 days


#2 posted 04-09-2017 05:56 AM

If you’re doing a full half-ellipse, I don’t think you’ll be able to do this with a one-piece form. The very ends need to be clamped inward, toward the center of the ellipse, not perpendicular to the major axis. I would put some holes in the inner form to provide clamping purchase, then either:

1. Cut the outer form into pieces, or

2. Use a “back band” to distribute pressure along the outside of the laminates.

This way, you can apply clamping pressure in the right direction.

-- Jerry, making sawdust professionally since 1976

View bondogaposis's profile

bondogaposis

4479 posts in 2189 days


#3 posted 04-09-2017 01:25 PM

How about a picture of the actual form?

-- Bondo Gaposis

View hairy's profile

hairy

2586 posts in 3370 days


#4 posted 04-09-2017 03:08 PM

Could you make the jig longer than the final piece you need, and then cut off the excess?

-- My reality check bounced...

View Loren's profile

Loren

9631 posts in 3486 days


#5 posted 04-09-2017 04:23 PM

As the laminates get further away from parallel
to the edges of the jigs you’re effectively
making them wider. Stack 12 up and measure
45 degrees across. The measurement is a lot
more than straight across. The jig needs to
compensate.

This sort of issue is one reason I prefer steam
bending but that’s not helpful to you here.

Another approach would be to drill clamp holes
in your lower caul and make the upper caul in
3 or more parts so you can compensate for
the difficult-to-calculate stuff.

Another way to do it is cut off and throw away the
ends of the upper caul. Glue the middle
section using the caul and use a lot of clamps
for the ends.

If you don’t have clamps and the laminates can
be glued, taped together and wrestled into the
curve by hand, the jig can be arranged to use
wedges to close the joints.

The FWW book on bending wood has some
useful articles about glued laminate bending.

View Th961605's profile

Th961605

9 posts in 1481 days


#6 posted 04-09-2017 06:23 PM

Thanks for the replies so far!

jbay: I thought about making another cut, but this would just shift the problem to a gap at the top of the arch, if that makes sense.

jerryminer: Is the back band something like a piece of metal flashing? Is the purpose of this to even out clamping pressure, or is there something else to this?

View MrUnix's profile

MrUnix

6006 posts in 2037 days


#7 posted 04-09-2017 06:58 PM

Almost every lamination I’ve made that had that extreme of a curve starts by clamping one end, then working your way to the other end – clamp by clamp.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View runswithscissors's profile

runswithscissors

2564 posts in 1863 days


#8 posted 04-09-2017 07:34 PM

JBay and Loren have it right.

A back band is to prevent stretching of the outer lams, which can lead to splintering. It forces the compression of the fibers into the inner lams.

But I don’t see the need for this with 1/8” laminations in that shallow a bend. I always used a metal strap when heat bending (not laminating). The steel binding straps from sling loads of lumber work very well for this. Check the dumpster at the lumber yard.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

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jerryminer

805 posts in 1279 days


#9 posted 04-09-2017 10:47 PM



jerryminer: Is the back band something like a piece of metal flashing? Is the purpose of this to even out clamping pressure, or is there something else to this? – Th961605

I don’t mean a metal bending strap, as runswithscissors is alluding to. That would be more appropriate for steam bending.

I mean a few extra layers of material (I often use masonite for this) to distribute the clamping pressure.

In your case, though, since you’ve already made an outer form, I would cut the outer form into several pieces so the clamping pressure can be distributed and directed in the right direction—- something similar to my drawing above (left side).

Loren’s suggestion of cutting off the ends of the outer form and clamping the ends of the laminations—or using wedges instead of clamps—is also viable

-- Jerry, making sawdust professionally since 1976

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