Hold down ideas needed

[dealguy11]

I've attached a picture of 1/2 of a paddle that a customer wants made in wood. This paddle is about 7' long or so. The completed paddle is about 1 1/2" thick in the middle and tapers down on both ends to perhaps 3/8" or so. The client wants to make many of these in different configurations specific to user needs, so it's possible that no 2 paddles will be the same.

We tried a prototype, cutting it in 2 halves like the one shown in the picture, then gluing the halves together. For various reasons, including the way the paddles are laminated, wood movement and the difficulty of clamping the thin areas at the ends, we had problems getting the 2 halves to glue together well. This bad glue up is the start of the the problem. One solution would be to change glue and/or try to come up with better gluing clamps and continue to cut them in halves, and we're looking at that as a possible solution

Another thought is to just glue up the entire lamination to begin with and then do a 2-sided carve to cut it out. However, that also presents some challenges. Any tabs that are stout enough to stand up to carving the 2nd side will be so thick that they will distort the part where they hit, especially in the thin areas. That would require a lot of hand sanding to remove. We'd like to avoid that.

Any ideas as to how to hold this thing to the table when flipping it? We thought of making a vacuum cradle of some kind so you could cut one side then flip it and have it held in the cradle, but we'd have to make a cradle for each paddle configuration. Using an rotary indexer is not an option because a) I sold it and b) the part is too thin to cut well in an indexer anyway - would vibrate all over the place. Any other thoughts?

[ElevationCreations]

I would recommend gluing up the entire lamination and doing a two sided job. You could use blocking and hot glue to support the underside of the machined paddle areas or spray foam, but that presents additional prep and cleanup.

You could use a spokeshave or cabinet scraper on the paddles to help reduce sanding.

[LittleGreyMan]

As paddles may not be the same, that's not an obvious challenge.

I'd try this process:
-drill the inner face of the 2 halves for dowels. Try to locate some small dowels in the thinner part
-drill the spoilboard for these dowels and hold the flipped half parts as you did for the prototype. Dowels will help avoiding wood movement
-glue the two halves with dowels using a vacuum bag

No idea if it will actually give good results, so don't blame me

[dealguy11]

Both good ideas so far. Hadn't really considered hot glue as a hold down option. Would need to be very precise on the blocking but could probably deal with that by CNCing the tops of the blocks, assuming they're very firmly attached to a base.

Also like the doweling/vacuum bagging idea...I have a vacuum bag and that could address the clamping problem. Would need to be very short dowels near the ends, but not an impossible thing to do. I could make a universal jig with dowels in standard positions, which would serve different paddle types.

Thanks! Open to any other brainstorms anyone has!

[ohiolyons]

What about the mechanisms used to join pool cues?

Would allow machining of half of each oar at a time and side benefit it breaks down for transport and storage.

Doesn't help much on hold down, but minimizing tiling issues and stock leaning over the edges.

[dealguy11]

Cool idea. The customer is telling me to try to keep the higher-density maple wood laminate layers at the edges (which provide more resistance to rocks) to as thin as possible to keep the number of grams of weight in the paddle down, so I'm guessing he won't go for the metal. Also the profile of the center isn't round...it's more of an oval with flattened sides.

Good thought, though. That is how the paddle for my fishing kayak (the "Manatee") is made, but I get the impression that these guys are a bit more performance oriented. The customer talks about things like attack angles on the blades and speed versus control...has a heavy-duty mathematical model working out the exact shape of the paddle, kind of like those guys who develop sailing ship keels. That's why they're having them CNC'd, I guess.

[mezalick]

Interesting problem Steve.

With my background of mold making I would think a "blanket mold" could be used in conjunction with vacuum pucks to hold the "second" side.

Here's how I see it.
Cut the top as usual.
Cover the cut side with a blanket mold.
Let it harden.
When you flip it over, the blanket mold will create a solid support layer that would follow the shape of the item,,,you would need to have the blanket parallel to the center of the item, but that's an easy jig approach.
Then use the vacuum pucks to hold it all down..

Maybe a whole table vacuum could also work but I'm just thinking out loud.

Michael

[Will Williamson]

I just don't see the problem,You got that vacuum table on your new machine
Setup on vacuum table and apply gasket, to table, Cut ALMOST half way through.
flip it over, land on dowels, located 1x1from the 4 corners of the blank.
Cut ALMOST half way through again.
Take it off the machine and finish the outline on the band saw or pattern route by hand. Woodwork 101
You could also try to center up some tabs in the middle of the blank
I have never used tabs, not even once, so not sure of that application.
,Good Luck

[LittleGreyMan]

Will,

I also thought of this solution, but as Steve stated there were thin parts I don't think it will work. Vacuum handling will probably break the thinner parts and the remaining border in the second step of the process as they are not lying flat on the table. As all paddles will be different using a shaped wedge to support this part doesn't seem possible.

But it may depend on the vacuum value.

Another issue is Steve's comment about tabs in the thinner parts.

[ozymax]

How about this idea...
Make a low profile box that will suit all sizes of paddle and make the box the same size as the blank for the paddle, ie the same size as the grey portion in your pic.
Machine the first side of the paddle and drill perimeter dowel location holes to suit.
Once the first side of the paddle is done, flip it over and mount it on top of the box using the dowels to register it to the box.
Ram fill the box with green sand like they do when making casting patterns.
Smooth the sand off level to the box edge and then flip the entire box/paddle over and mount it on the machine.
Your Z zero should then be at the top edge of the box.
Machine the other side. Because you zeroed on top of the box you shouldn't machine into the sand.
I would imagine the sand would support and hold the paddle, because the paddle has a large surface area and very little draft, or you could leave a wafer thin tab that is easially sanded off.
When the paddle is finished, simply break up the sand and you can re-use it time and time again and you don't have to wait for anything to set.

[dealguy11]

More interesting ideas. Yes, I can gasket the table. I haven't really changed from the gasketing we set up when we got the machine, but it would certainly be possible to gasket an area to focus the vacuum. Regardless of the approach, I'll probably do this.

Also using a spokeshave to address any bumps, whether they be tabs or (this occurred to me as I read the replies) maybe legs that I leave on one side of the carving to support it while the other side is being carved, is a possibility. I would like to carve the edges if possible, as the specific curve of the edge is important to the customer.

Thanks everyone. I have a couple of weeks to work this out before I need to do another prototype. I'll let you know how it comes out.

[rscrawford]

Consider gluing the two halves together in a vacuum bag. You'll get a more even clamp pressure with vacuum than with using clamps, even if the pieces warp a little before gluing up.

[ChiloquinRuss]

I have done a few (very few) two sided pieces. What works for me is to NOT cut all the way through the substrate. I carve a side (45% through), flip the part over, cut the other side (45% through), remove from my 'flip-jig' and take the piece to the band saw or use a jig saw for the final cutting (last 10%). Then all that's left is the cleanup stuff. Pardon the poor illustration but . . . Russ

[Will Williamson]

There you go, legs, with hole drilled in the center, for a screw.
to come up from below, you would need to use a sub plate to vacuum down to your table Iv'e done that before.
Thanks for bringing this subject up I have contemplated cutting some oars in the past.

[LittleGreyMan]

This method requires some extra machining and material.

Machine the first half and the parting surface and drill 2 holes in the parting surface for dowels.

Then machine a negative model of this half paddle and the parting surface in a cheap foam board (insulating foam). Drill 2 dowel holes. You need a way to reference this board on your machine. Use a gasket and hold it with vacuum.

In order to maintain a low cost, use a large roughing mill and an as large as possible finishing mill with a large stepover. Depending of the depth of the "mold" and the DOC of the finishing mill, you may not require roughing. You may require using some allowance to compensate for ripples.

In manual mode, machine large channels in the mold. After machining, make several big holes in the channels through the remaining material.

Put the mold back on the machine using your reference system, put the half machined paddle in the mold using dowels (they are not supposed to hold the part in foam, just help positioning it) and hold the whole set by vacuum. Machine the second half.

This method will work (I use it for some particular parts) if the extra cost is acceptable. The cost may not be higher than other methods requiring machining in two halves and gluing.