Cupping wood

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Jeremy Nako

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I've started to play with inlays using hardwoods such as Purple Heart, Padauk and mostly Walnut.

To-date I've been cutting down (thickish) off-cuts but I need to order some more stock, so I'm looking at 12mm thick stock, which suits me as my inlays are never deeper than 7mm.

On the suppliers website it warns of cupping when using such thin stock.

So.. two questions if I may..
Firstly.. is there an optimum -and practical - way to store the pieces to avoid as much as possible any cupping ?

Secondly.. should it occur, is there a sensible solution ?

This may sound daft, but would wetting the wood and then drying it under pressure (I have an old heat press that was used for T shirt printing) work ?

The inlays (very thin) would be glued in under pressure when used, so I'm not concerned with that (unless someone knows different !)

Any advice gratefully received.
 
If you properly glue a thin piece, 7mm as you mentioned, to a stable backing substrate, it will remain flat. Thicker than 7mm it may have enough strength to lift.
 
If you properly glue a thin piece, 7mm as you mentioned, to a stable backing substrate, it will remain flat. Thicker than 7mm it may have enough strength to lift.
Up to a point ... depends how thick the substrate is. If you're inlaying into a 20mm substrate you should be fine. Into 10mm, leaving only 3mm below, I think it might distort.

If you look at the end grain, glue down the inlay so it would cup upwards. The other way round, the edges might lift. Ideally, use vertical grain boards, but I suspect you'll have to put up with what you're sent.
 
Up to a point ... depends how thick the substrate is. If you're inlaying into a 20mm substrate you should be fine. Into 10mm, leaving only 3mm below, I think it might distort.

If you look at the end grain, glue down the inlay so it would cup upwards. The other way round, the edges might lift. Ideally, use vertical grain boards, but I suspect you'll have to put up with what you're sent.
I thought that was obvious, apologies. The substrate is typically thicker than the inlay.
The point of inlay is for aesthetics, you can't simply turn a piece over just for grain orientation you think is less likely to cup. Use the appropriate thickness of substrate/inlay and you don't need to worry.
If the OP's inlay's are 7mm or under a 20mm would be my suggestion. Roughly 1/3 inlay-2/3 substrate
 
Thanks for your comments.

I'm less worried about the inlays themselves (most of my pieces are 40mm deep with 4-6mm inlays) and more to do with the cutting of those inlay pieces.

I'm cutting complex pieces on a CNC machine so the source wood needs to be flat.
 
I've started to play with inlays using hardwoods such as Purple Heart, Padauk and mostly Walnut.

To-date I've been cutting down (thickish) off-cuts but I need to order some more stock, so I'm looking at 12mm thick stock, which suits me as my inlays are never deeper than 7mm.

On the suppliers website it warns of cupping when using such thin stock.

So.. two questions if I may..
Firstly.. is there an optimum -and practical - way to store the pieces to avoid as much as possible any cupping ?

Secondly.. should it occur, is there a sensible solution ?

This may sound daft, but would wetting the wood and then drying it under pressure (I have an old heat press that was used for T shirt printing) work ?

The inlays (very thin) would be glued in under pressure when used, so I'm not concerned with that (unless someone knows different !)

Any advice gratefully received.
Keep them covered and they’ll stay flat indefinitely.
 
5 -6 mm is quite thick for an inlay. If your thicknesser will get down to 3 mm they will have structural integrity and stay in place. Making your own inlays is quite wasteful of wood and I tend to make my own out of veneers. I might suggest having a look at www.rockler.com who sell loads of exotic woods in 1/2" thickness. Importing is not as difficult as you might think but be prepared to pay customs and vat. Just avoid couriers as much as possible and use the US Post Offfice..
 
There are quite a lot of variables. The movement in the pieces will be different across the grain both radially and tangentially and along the grain. Your substrate is also critical
In general if using a totally stable substrate like MDF or plywood, do not go over about 3mm thick as the applied inlays/veneer will want to move and will crack. This is not as much of an issue if applied onto solid wood provided the grain is in the same direction
Why do your inlays have to be so thick? As recipio said 3mm is plenty and is still thick enough to edge glue
You also must balance what you do on one side with the same on the other side otherwise you will get differential stresses and the piece will bend
 
In answer to 'why so deep', I create the inlays (inlaid into hardwoods) using a CNC using Vcarving.

Many of the inlays are very intricate and as such I've found that anything less than around 4mm doesn't carve / inlay properly using that technique.
 
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They tend to go straight out the door as soon as they're finished, but here's one in progress.

Just to clarify.. I use my own designs for the inlays - this one is for our own use and the Betty Boop image is no longer copyrighted or trademarked.

And.. I know that the grain of the inlay is running the wrong way.. I cocked up but still planning on using it for our own purposes to see what - if anything - happens.



Bettys.jpg
 
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With thin inlay pieces, there is no need to keep the grain running in the same direction.
At the 7mm and under thickness, any wood movement is minimal at best and their isn't enough force to create joint failure. Nothing more than a thick veneer
 
As a relative newbie to woodworking and a very newbie to inlays that's very interesting and helpful - thank you.
 
At the 7mm and under thickness, any wood movement is minimal at best and their isn't enough force to create joint failure. Nothing more than a thick veneer
Wood movement with change in moisture content is the same regardless of thickness. The difference is that thinner sections exert less force so the glue is more likely to hold. As previous the general rule of thumb for veneers or inlays onto solid wood is less than 3mmm. If onto solid groundwork then keep the grain direction the same and there shouldnt be a problem. As Jeremy has said the inlay is the wrong orientation. It doesnt mean that there will be problems, it just increases the risks
 

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