Thread: Saving some cupped Redgum?

I had have had Tops and shelves from antiques and new constructions that were Dry and straight , which then developed a cup , and I have straightened these with water and sunshine . But mostly these were 19mm thick . And it works most of the time .

Slabs that I use like 35 to 50 mm Oak, sawn through the log that have dried out with a big cup like you have . I wouldn't bother trying . I don't think it would stay straight for long if it moved back in the right direction.
What I and others do is saw it down the middle . Re joint it with a good glue. Butt join or Biscuit or domino . A good job and its invisible because the grain is such a good match.

Rob

Thanks for the quick reply Rob, and please excuse my ignorance but I need an explanation re: sawing down the middle.

Apologies if this is painfully obvious to everyone else but I admit I'm lost. If I cut down the middle of the slab I'll just end up with two cupped pieces of timber except one? Or am I completely missing something?

Cheers mate.

Sent from my SM-G920I using Tapatalk

Putty, the net effect of the two cupped pieces rejoined will be a tiny amount of cupping/lost material compared to where it's at now, probably about 1/4 of the loss. Kind of a maths thing; if you draw an arc and then join the ends with a straight line and measure the distance from the straight line to the top of the curve, then cut that curve in half, do the same thing, the height of that line reduces dramatically.

So having glued them back together, looking at the rejoined bit from the end, it looks like a shallow m. I'd actually have a go at flattening each piece separately though before re-gluing. Your losses will still be greatly reduced.

Got it. I'm a very visual person - the most explicit, basic verbal descriptions can sail completely over my head, but I'm starting to see it now.

Cheers guys!

Sent from my SM-G920I using Tapatalk

I always feel that it is such a pity to have to rip a wide board to narrower strips to remove cupping. There is however an alternative to this that involves relieving stresses that cause the board to cup or warp.
This video explains the process very well.

You want to look at the end of the plank and see where the end grain circles show where the heart of the tree is or which direction it was . The plank is contracting back and its related to that. Its complex to describe but its simple when you get your head around it .

So lie the plank on a table with the cup facing Up like it would catch and hold water, and when you find its heart or where it was below that by looking at the end grain growth rings , its not always in the center of the plank . lay a straight edge across what you see as the usable width of your cupped plank . And measure the depth of the cup to the center based on the growth rings showing you where the heart was. Write it down .

Next rest your straight edge on the center over the heart out to the edge of usable wood and measure the deepest dip .

This is what I do to get an idea if its worthwhile cutting it down the middle .

Ill draw it and put it on . Its hard to describe .

Just to clarify the extent of the cupping. You say if you used your router sled to flatten it, you would lose 30mm of material. which means if you laid a straight edge over the width of the board, there is a 30mm deviation between the high spots and low spot. I ask because a lot of people put the straight edge over the width, and see there is 15mm deviation and immediately double that figure because they "need to machine both faces" and therefore lose 15mm x 2. But you actually only lose 15mm once.

Secondly, you say the slab is seasoned. It is a thick slab at ~70mm and if it has been left out in the rain, it may be holding some water. If you lay the slab concave side down on some plastic, this is often enough to correct any problems, though with such a thick slab of a dense species, a bit of weight on the crowned side wouldn't go astray. Personally I would avoid using any water as it will only end up being a temporary fix.

If you can get it flat to a reasonable level and then make it into a dining table, 90 x 45 side aprons along with 90x45 rails inside the frame (no idea what these rails are called, probably 'rails') will do a good job of keeping the top flat until eventually the top relaxes and just stays like that.

Originally Posted by Kuffy

Just to clarify the extent of the cupping. You say if you used your router sled to flatten it, you would lose 30mm of material. which means if you laid a straight edge over the width of the board, there is a 30mm deviation between the high spots and low spot. I ask because a lot of people put the straight edge over the width, and see there is 15mm deviation and immediately double that figure because they "need to machine both faces" and therefore lose 15mm x 2. But you actually only lose 15mm once.

Secondly, you say the slab is seasoned. It is a thick slab at ~70mm and if it has been left out in the rain, it may be holding some water. If you lay the slab concave side down on some plastic, this is often enough to correct any problems, though with such a thick slab of a dense species, a bit of weight on the crowned side wouldn't go astray. Personally I would avoid using any water as it will only end up being a temporary fix.

If you can get it flat to a reasonable level and then make it into a dining table, 90 x 45 side aprons along with 90x45 rails inside the frame (no idea what these rails are called, probably 'rails') will do a good job of keeping the top flat until eventually the top relaxes and just stays like that.

Excellent point Kuffy and something I didn't assess properly. First I'll try letting it drain some moisture naturally, then I'll run it through the sled.

Cheers.

Sent from my SM-G920I using Tapatalk

Slab 1 will cup this way on drying .
Slab 2, right through the middle will be closer to straight across but because it still contains the heart will be cracked around the heart area.

Picture of no 1 slab shows how to check with a straight edge and compare the depths of cup . Watching and measuring from the heart or above where it was.
M1 is across usable wood but you have to machine off a lot , and the under side ?
Cutting through 3 and re machining then re joining , machining from M2 saves a lot of wood.
Its a natural thing for a slab to dry this way .
looking at the way slab 1 and 2 is changing shape relates exactly to how wet timber dries and why it cracks when left in wet log or branch form. Thats a nother step away from what I drew because cracking is because its still in the round form . If you get your head around that , you then understand what dry boards in your timber rack are capable of doing in different humidity conditions.
I didn't understand it completely until I read a few pages from, Woodwork in Theory and Practice . By J A Walton.
Once I did that, walking in the bush and looking at my timber rack made complete sense and I under stood all the steps from forest to table top. For Me a memorable moment in my learning .
IMG_3592.JPGIMG_3593.JPG

Originally Posted by auscab

Slab 1 will cup this way on drying .
Slab 2, right through the middle will be closer to straight across but because it still contains the heart will be cracked around the heart area.

Picture of no 1 slab shows how to check with a straight edge and compare the depths of cup . Watching and measuring from the heart or above where it was.
M1 is across usable wood but you have to machine off a lot , and the under side ?
Cutting through 3 and re machining then re joining , machining from M2 saves a lot of wood.
Its a natural thing for a slab to dry this way .
looking at the way slab 1 and 2 is changing shape relates exactly to how wet timber dries and why it cracks when left in wet log or branch form. Thats a nother step away from what I drew because cracking is because its still in the round form . If you get your head around that , you then understand what dry boards in your timber rack are capable of doing in different humidity conditions.
I didn't understand it completely until I read a few pages from, Woodwork in Theory and Practice . By J A Walton.
Once I did that, walking in the bush and looking at my timber rack made complete sense and I under stood all the steps from forest to table top. For Me a memorable moment in my learning .
IMG_3592.JPGIMG_3593.JPG

That's awesome info Auscab, thanks heaps for the detailed explanation, and pics help. I figure I have nothing to lose by sitting on it (figuratively, at least) for a few months and seeing how much it reduces by itself, then I'll make the judgement on whether to saw it up for small projects or do the dining table idea. It seeks like such a waste not to make it into a table top given the size and natural shape of the slab, so here's hoping.

Sent from my SM-G920I using Tapatalk

No Probs .
The re sawing and machining is based on knowing that you have a dry slab .
If its come from a dry stack and been there a few years it shouldn't be moving much more .
Rob

Before investing a lot of time in it I would definitely see if it is still moving - especially if it is moving back and forth (cupping and uncapping) over time.
I have seen so called dry redgum slabs that move +/- 15 mm over 12 months and have continued to do that for over 20 years.

Ideally you'd put it in the room where you are going to use it, horizontal up against and internal wall is fine, and leave it for 12 months to see if and how much it moves over that time. If it moves too much, even with a split and reglue it might mean its not suitable as a table top.

Do you have a moisture meter to stab into it?

How about a picture