What cutting method for this design

Hi everyone,

I am new around here. I am hoping you are all willing to give me your thoughts on what I am trying to achieve. I have drawn up a design for a mold to be used with gluing up veneer into ply wood cylinders.

There are two main components to the design. One being the "rib", this being the two components at the top and the bottom of the sketch. These will be machined from 25mm ply, 5x 12mm holes which will be used to bolt the components together to have a solid half. The horizontal grooves (which bolted together it will equal a 16mm hole) will be used to bolt the two halves of the mold together using m16 threaded bar.

The 2nd component will again be machined with 25mm ply and have the 5x 12mm holes to line up and bolt the halves together. This second component will be stacked in groups of three to create a deeper mold. I have also added a male V profile and female V profile on each half to help line the two halves up perfectly when gluing up the veneer.

It is crucial that the two halves line up perfectly and the ID of the mold is as close to perfectly round as possible and to a tolerance of +0 -0.1 in diameter.

In your opinion which cutting process is going to have the most consistency and accuracy so when assembled all these components line up properly? I was thinking of going with local company using a CNC router though someone mentioned to me they are not always accurate especially with repeat cuts and that laser cutting would be better.

How well do you the think the Router or laser would go cutting a perfect circle or would you suggest having the ID cut a tad smaller originally and when assembled and the halves are bolted together to have the ID cut to its final diameter on a boring machine?

Any suggestions around the design and effective ways to line the two halves up when the glue up happens are welcome. I will also mention when the veneer is inserted into the mold the two halves will be open about 50mm and then when all the veneer is in position the two halves will be closed pulling the veneer into place.

Also there will be packing spacers in the four corners.

Please refer to the attached pictures to hopefully make things a little clearer.

Thanks in advance

Steve

I think you can get it CNC routed to basic shape, a little smaller than needed. Then glue and assemble it, dry it to reduce warping, varnish etc and chuck it in a lathe and bore the inner bore to exact dimensions.

You do know that if the veneer is springy and needs to be cranked 5cm down to fit, then it is NOT going to be round even though the outside former is round? You probably need an inflatable bladder or similar to fit inside the veneer and expand it outwards to form a perfect round shape against the former.

What are you making? I've seen drum shells made in a similar way but usually around a round centre former then held with outside bands until the glue sets.

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Originally Posted by RomanLini

I think you can get it CNC routed to basic shape, a little smaller than needed. Then glue and assemble it, dry it to reduce warping, varnish etc and chuck it in a lathe and bore the inner bore to exact dimensions.

You do know that if the veneer is springy and needs to be cranked 5cm down to fit, then it is NOT going to be round even though the outside former is round? You probably need an inflatable bladder or similar to fit inside the veneer and expand it outwards to form a perfect round shape against the former.

What are you making? I've seen drum shells made in a similar way but usually around a round centre former then held with outside bands until the glue sets.

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Yea I think that will be the most accurate way. I just spoke to some one and they suggest using perspex sheet. I use between 60-70psi clamping pressure when forming the shells. My current molds have been fabricated and machined from steel. I am looking at this new design for when I expand to keep costs down with the same accuracy. I am making drum shells. I personally think the method you mentioned with a center form and the straps is limiting. When you said you have seen it done before was that in person or on the net somewhere?

Thanks for the reply

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How well do you the think the Router or laser would go cutting a perfect circle

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I think it depends on the particular machine. I've worked with $100-150K routers that wouldn't cut round circles. On these machines, better results were achieved by coding the circles as multiple short arc segments rather than 1 line of g-code.

Do you have a plug or bladder in the center to kep the veneer tight to the form?

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Originally Posted by ger21

I think it depends on the particular machine. I've worked with $100-150K routers that wouldn't cut round circles. On these machines, better results were achieved by coding the circles as multiple short arc segments rather than 1 line of g-code.

Do you have a plug or bladder in the center to kep the veneer tight to the form?

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Thats interesting, so far lathing/boring the ID is looking like the most accurate option. In my current forms I have a special pressure system which clamps the veneer after it has been layed up in the form, I use between 60-70psi of pressure. It will be the same process with the mold design that I am working on.

If I machine the ID after assembly there should be no dramas at all with the two halves lining up right? as the horizontal m16 locking bolts will be on a set track.

Thanks

Here's an idea that might be of use.

Why not forget the hard outer former and make an outer former out of some veneer or other roll of sheet?

Imagine a very long piece of thin flat sheet, just roll it up and it will make a pretty good round form. You could use a fixed or adjustable band on the outside to set its diameter.

It has the advantage of being adjustable in diameter (if needed), being self-rounding, and being very easy to take the form off the drum shell afterward (unravel it!) unlike 2 hard forms which may scratch when they are pulled off. Just a thought.

Quote:

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Originally Posted by RomanLini

Here's an idea that might be of use.

Why not forget the hard outer former and make an outer former out of some veneer or other roll of sheet?

Imagine a very long piece of thin flat sheet, just roll it up and it will make a pretty good round form. You could use a fixed or adjustable band on the outside to set its diameter.

It has the advantage of being adjustable in diameter (if needed), being self-rounding, and being very easy to take the form off the drum shell afterward (unravel it!) unlike 2 hard forms which may scratch when they are pulled off. Just a thought.

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Thanks for the reply. This method will work to a certain extent though it is rather limiting and the results are consistant. The thickness of the shell wall would also vary as the veneer would be overlapping on each pass. No doubt it would produce shells but not really what I want. Thanks for the input though. I am almost convinced the best option is to rough out the Id of my design and once assemble machine the ID. I just need to decide on the most suitable material. Either that or shell out cash for my current design with steel forms.

Thanks

Steve