Thread: Sanding red gum slab, belt then orbital

Originally Posted by davep

It went through a thinknesser but has an overall high area along it's middle..

Do you mean length wise or width wise, and how long ago was the thicknessing done?

Being red gum it could have just moved. It might be worth leaving it for 6 months and seeing if it bends back the other way. Are you intending to see it or use it. If you intend to use it I would store it in the room you intend to use it in, that way you can assess the extent of the movement over a year.

Obviously I'm aiming for a flat surface.

Good luck.

Hi Bob,

Thanks for your help.

The rise along the middle is length ways although it also has a rise width ways, which I'm wondering is due to my having it on 2 saw horses. It's been near two years since the thicknesser, and since has been in my enclosed garage on the saw horses.

We presently use an older slab on our bar, hence we've not had the new one in place.

Maybe it should be put in place and allowed to settle first as you suggest.

The along the middle rise I assume will still need to be sanded down ??!!

Ta.

Originally Posted by davep

The rise along the middle is length ways

Is there a dip on the other side of the rise? If so the slab is cupping. This can happen esp with red gum but any (even dead flat) slab that is thicknessed - the thicknesser removes a bit more harder drier outer shell of timber on one side of the slab which allows residual moisture to escape preferentially on one side.

I have seen redgum swing back and forth over 12 months for many years. A friend of mine has a 700 mm wide red gum counter to with a dip of about 4 mm on top in summer and a rise of about 2 mm in winter. The difference between the two sides is probably due to the the time of year he put it through a thicknesser!

I'm wondering is due to my having it on 2 saw horses. It's been near two years since the thicknesser, and since has been in my enclosed garage on the saw horses.

The length wise rise could be due to the saw horses but if you space the saw horses such that the weight is balanced this effect should be minimal

We presently use an older slab on our bar, hence we've not had the new one in place.
Maybe it should be put in place and allowed to settle first as you suggest.

If you have time to do this I would do it. Then you would know if there is a seasonal effect. Some people find if they seal the timber completely with multiple coats of poly when the slab is at its flattest that it then stays stable or moves much less than before.

Hi again Bob,

Yes the slab is cupping, I just checked the underneath of it.

I've just moved the saw horses a little, but had them sitting where I thought would give even weight distribution.

I'm thinking as you have pointed out, we'd best put it in situ for a while to see how it behaves, before doing any sanding.

It's a big slab 700mm wide, 2650 long and 50mm thick.

First thing I'll need to do is square the ends up. I'm intending to use my 210mm circular saw for this, unless there is a better way?

This slab has been a real non work in progress due to an initial error https://www.woodworkforums.com/f9/newbie-stuck-red-gum-slab-55203

Many thanks, Dave.

Hi Dave,
I have done a fair bit of this and will share my method.
You have had advice on the cupping, so I won't discuss that, other than to say it is a real pain and patience is required.

Sanding- I start with 100mm belt sander 40 grit (have used 16 grit) and use a steel rule and pencil to mark high or low spots. Then I go to 60 grit on the belt sander, again checking with rule. In between sands I may run cloth soaked in water. When it dries you can feel little low spots.

The onto random orbital 40 grit on the high orbital eccentric mode. I may wet again. Then ROS on 40 grit on low orbital eccentric mode. It's at this stage the piece must be flat and level. Then I work through the grits on ROS not jumping grits. I have on occassions used urethane at 180 grit as a filler before going to next grit.

As for the edges I use a router with a long straight bit which has a pattern follower bearing on the top. The bearing runs on a factory made straight melamine edge with the router sitting on the melamine. I get superb edges this way, even when I am jointing thick heavy pieces.

On a recent project where I joined 2 slabs together, after some cupping (long story and stress) which when sorted, I routed 5 slots on the bottom about 19mm deep x19 mm (router bit) across the grain over 2.4m and then glued pieces of same timber into the slots. I reckon the slots released some pressure??? The table has now endured a Brisbane summer and winter with no movement at all. (see thread https://www.woodworkforums.com/showth...ight=satinwood)

Hope this and other forumite tips assist you.

Regards,
conwood

Many thanks for all the detail you've provided Conwood, I was wondering about the change of belt to ROS. May I ask what speeds to your run these at?

Again really appreciate your help, Dave.

Hi Dave,
The belt has only one speed- flat out. With the ROS I vary and change according to my instincts. Fairly fast on the course 40 then I slow down a bit. The timber and condition of the discs dictate terms here.

cheers
conwood

Many thanks indeed. We'll be going for the wait first up, moving it into position for 6 months to watch for cupping changes. Also still to decide how to manage the polyurethane that was wrongly applied.