Belt sanding across the grain

[HonkyLips]

I found this forum while looking for a way to get poorly joined boards flat... think table tops etc. This is obviously a very common problem and thanks to the feedback on this site I did not buy an electric planer, but bought a belt sander instead.
I have had great results - far better than expected - but I found that it worked best for me when I used the belt sander across the grain, at 90 degrees. However all advice and documentation strongly recommends to use a belt sander WITH the grain of the wood.
Just today I bought some new belts and the packaging on the belts again stated to make sure I used the belt sander along the direction of the grain... and so I'm wondering - Why? I'm happy with the results I got going across the grain, and considering that I had 5 pieces of timber unevenly joined, this worked well for me and I avoided any grooves or channels I was expecting. The panel is now perfectly flat and I'm very happy.

So is it as mortal sin to belt-sand across the grain? If so, why?

[Cliff Rogers]

G'day.

What sort of timber is it?

Most timber shows a lot of unwanted scratches across the grain. :confused:

Across the grain will give you an even board height with an evenly rough
finish. IMHO
I've seen floor sanders used diagonally across the boards to level an
uneven floor but then they usually turn it back in line with the grain to give
it a nice finish.

[Dean]

I go 45 degree across for flattening first, then go with the grain once flat to smooth it up and get rid of any nasty scratches and marks caused by step 1

[HonkyLips]

Cliff - It's rock-hard Jarrah, and I haven't got to the finishing stage yet so perhaps the rough looking surface I have will be harder to smooth than I expected. Yes, there are sratches on the surface but they're nothing compared to what I was expecting.
When I do get around to sanding it smooth, I'll be sanding with the grain using fine sandpaper first, then to an orbital.

Dean - I tried the 45 degree approach first, and when I was approaching flatness I was even getting good results by using a "hands off" approach and waving the belt sander over the surface using the power cord. Probably sounds a bit weird, but worked well.

[echnidna]

I am generally very careful aligning boards so I dont have much problem sanding.
But if my job needs a lot of levelling I sand across grain first.
Then at 45 degrees from both near corners.
Then along grain to eliminate the cross grain scratches.
This is basically the procedure floor sanders use on a rugged floor.
It is also more or less the same stages that I was taught to use a hand plane.

[John Saxton]

As a point of interest, the video for the Performax 16/32 Drum Sander recommends placing the timber on a 30/45 degree tilt whilst feeding on the moving table for best results...cant fault it yet.

Whilst this is not directly related to a hand belt sander it does demonstrate that there is no problem with it...not at this juncture in time.

Cheers

[jimmyjames]

I've had this trouble with joining boards to make flatpanels as well. I don't have hand planes or a jointer so use DAR from timberyard. Problem with that is that it twists and bends on the rack or in transit which makes for inconsistancies. The kids table I made has more waves on it than bondi.....

It seems obvious to me that a belt sander across the grain will flatten the boards. Any scratches should then be worked out by finish sanding with the grain. I guess you'll end up taking a lot of wood off so the boards should be a little thicker to compensate.

Now all I've gotta do is convince SWMBO that I really need a belt sander to go with the disk and ROSander.....

[Dewy]

When I firstst started making coffee tables I wasn't getting the board lined up correctly so there was a lot of sanding needed to flatten the surface.
I used a belt sander at 45° until the ridges were removed and then finished off with a ROS. With bread board edges the top looks perfectly flat now. So much so that it looks as if the top was made out of a single board instead of the five it was really made from. Matching the board colours before jointing was half the battle.