Disc Sander for scarf joints leaves radius

algale · Post by **algale** » Tue Jul 15, 2014 7:54 pm

I am joining some narrow strips of cedar for my long-delayed canoe project. The strips are 6 to 9 feet in length by 0.25 inches thick by 0.75 inches wide with bead and cove on the long edges. The scarf is supposed to be across the 0.25 inch thickness of the strip and is about 2 inches long (1:8 ratio).

The canoe book shows the scarf joint being made on a disc sander with a temporary fence set up at a shallow angle. The blunt ended strip is simply fed along the fence into the disc standing on its 0.25 inch edge with the 0.75 inch face pressed into the disc.

Looked easy in the book, but when I tried this on my Shopsmith, the result was a scarf with a very pronounced radius on the 0.75 inch face both on the leading edge (knife edge) and trailing edge of the scarf. (I did this with the 12 inch disc sander, the table set as low as possible and the scarf starting just inside the outer edge of the disc.)

Is this unavoidable on the disc sander? Would the radius be minimized if I tried to get the point of the scarf as close to the disc's center as possible (as opposed to its edge)? Would raising the table increase or decrease the radius?

Btw, I was able to do a similar set up on the belt sander and got a nice straight scarf joint. So that may be the solution.

JPG · Post by **JPG** » Tue Jul 15, 2014 8:20 pm

This doesn't make sense to me unless you are feeding the workpiece from the back side of the disk which makes even less sense.

So what IS your angle of attack?

algale · Post by **algale** » Tue Jul 15, 2014 8:37 pm

JPG40504 wrote:This doesn't make sense to me unless you are feeding the workpiece from the back side of the disk which makes even less sense.

So what IS your angle of attack?

I am feeding from the front of the Shopsmith toward the center of the disc. The fence angle I haven't measured. I set the fence set by making a mark 2 inches from front of fence. Front of fence touches disc and a 0.25 inch gap is left between the disc and the fence at the 2 inch mark. Thus strip tapers from 0.25 to 0 in a two inch span. Should have taken photos.

BuckeyeDennis · Post by **BuckeyeDennis** » Tue Jul 15, 2014 8:46 pm

If I understand correctly, you have the workpiece positioned such that the sanding disc is sanding the strip from top to bottom, across the grain. This will generate a downward force on the strip edge, making it want to twist counterclockwise. Such a thin strip would not have much stiffness to resist this twist, and may well "chatter" torsionally, resulting in the radius. Convex, I assume, and exaggerated at each end due to the steep scarf angle.

I'd try raising the table, and feeding the strip into the top edge of the disc, directly above the spindle centerline. This will get the sanding action almost directly in line with the grain, and pretty much eliminate the downward/twisting force on the workpiece. Hopefully giving you a nice flat scarf.

algale · Post by **algale** » Tue Jul 15, 2014 9:00 pm

Here's a photo of the set up, transferred to the belt sander.
[ATTACH]25634[/ATTACH]
Imagine this same set up on the disc sander and you will understand.

algale · Post by **algale** » Tue Jul 15, 2014 9:03 pm

BuckeyeDennis wrote:If I understand correctly, you have the workpiece positioned such that the sanding disc is sanding the strip from top to bottom, across the grain. Yes. This will generate a downward force on the strip edge, making it want to twist counterclockwise. Such a thin strip would not have much stiffness to resist this twist, and may well "chatter" torsionally, resulting in the radius. Convex, I assume, and exaggerated at each end due to the steep scarf angle. See clarification, below re radius.

I'd try raising the table, and feeding the strip into the top edge of the disc, directly above the spindle centerline. This will get the sanding action almost directly in line with the grain, and pretty much eliminate the downward/twisting force on the workpiece. Hopefully giving you a nice flat scarf.

You are correct about the set up but I think I need to better explain what I mean by radius. The disc is leaving a sanding line on the face in the shape of an arc and that translates also as an arc at the knife edge of the scarf rather than a straight line.

JPG · Post by **JPG** » Tue Jul 15, 2014 11:04 pm

The sanding disk will leave arc shaped 'swirl' marks since it is a rotating disk.

The disk should however create a straight boundary at the ends of the sanded area unless the workpiece is not flat(cupped).

rcplaneguy · Post by **rcplaneguy** » Wed Jul 16, 2014 1:01 am

Put a coarser grit on the sanding disk, then use the quill to gently bring the sandpaper to the (stationary) wood piece.

rcplaneguy · Post by **rcplaneguy** » Wed Jul 16, 2014 1:02 am

Disk or work piece flex? Put a coarser, more efficient grit on the sanding disk, then use the quill to (gently) bring the sandpaper to the (stationary) wood piece.

Ed in Tampa · Post by **Ed in Tampa** » Wed Jul 16, 2014 9:30 am

Raise the table above the center line of the sanding disk. In fact you might want to sand on very top with the table set just low enough to let you engage the width of the wood you are sanding.

Also if you are using the velco system there is flex in it that could be causing the problem.