Sanding a convave curve smooth

Hi all

Long time lurker first time poster but I have a question for you.

I am making a grandfather clock from one of Carbatec's plans and everything is going smoothly so far. The problem I have is how to finish the semicircle's around the dial area.

I have only cut out one semicircle so far, it is roughly 40mm thick and has a radius of about 140mm.

I have gotten it reasonably smooth but it is not as smooth as I want. The variation from a circle is fairly small but can still be seen.

How do other people finish off this type of cut, I am also going to have the same problem later on in doing the arch on the top of the door.

My current thinking is to turn up a circle the same size on a lathe then attach sandpaper to the outside and use this to sand the cut.

Since I don't have a lathe at this time this is the expensive answer.

Gday mate, & welcome.

A sanding drum, either solid or pneumatic, in a drill press comes to mind.

You could do what you said, turn a disc on the lathe (a bit smaller than the radius of the curve), & attach sandpaper, something between the wood & the sandpaper with a bit of give could be worth considering, inner tube or somesuch?

What about a router & template to do the actual truing up of the curve, or even cut it out with the router & a circle cutting jig?


Cheers.......................Sean

Thanks Sean

I am using a sanding drum around 60mm diameter and got it to this stage but it is not as good as I want. The imperfections are small but can still be seen if you look. Since this is a hobby I am not concerned over the time I spend I just want to make it as good as I can.

What I am concerned about is explaining to shmbo that I need to buy a lathe just to finish off an edge, I am not certain she would understand.

Good idea of the give under the sandpaper I will keep it in mind if I go that way.

I thought of the template but don't have anything smooth enough of the correct size and to make it gives me back the original question. I did think of making a template using the lathe method and then keeping it for future use.

is that you can give it several tries. It's made from cheap material and isn't part of the finished product. If the first template isn't perfect enough, do another, and another, etc.

I had the same problem with finishing the arch in the sides of a step stool I made. Firstly I found it difficult to cut tight enough around the head of the arch and then my cut wasn't right on the line.

I used a rasp to take the wood down to line and then hand sanded with various shaped blocks to cleanup the cut, however I wasn't entirely happy with the result. The main problem using the rasp was tear out on the arrises.

I think trying to use a power sander rig to take the cut down to the line isn't going to work because my experience of trying to do detail work with sanders is they always manage to dig in where and when you dont want them to.

I think for a perfect circle using the router circle jig would be the way to go. For my next attempt at a parabolic curve I think I will try a curve bottom spokeshave. I think a sharp spokeshave might provide more detail control than a rasp or sanding.

I'd use a router jig. I made a simple one out of 4mm MDF removed the router plate and fix the jig using the same screws. Then I just drill a hole at the desired radius tap a small brad through to the material and away I go. It works well and will cut a near perfect circle, arc every time. I haven't bothered to make it out of metal as I find that it's more useable to set the diameter where I want and when needed I simply make a new one. It's extremely hard otherwise to fix up a botched curve if it's not cut symmetrical.

Good idea, somehow I missed the idea of a router circle jig.

I will try and have a go this weekend.

Unfortunately the wife has something to say about tomorrow and today I am mowing the grass.

I think the idea of using a template with a router is a good one. You will probably still need to sand the curve.

For this you don't need a lathe, as you had suggested, to turn something the same diameter. Just find something with a large diameter and wrap some sandpaper around it. A large drink bottle, for instance, or a tin can, even.

Thanks aspectdisplays I made up a simple router circle jig, made a template the correct size. Then used the template to finish up the radius I was working on.

Came up nice, still needs a bit of sanding for the final finish but it worked great.

Thanks for all the help

Shame I thought I had a 'reason' for getting a new toy (lathe) now will have to wait for later.

Wrap a bit of coarse paper around the outside of the scrap cutout and use it as a black to hand sand.

It won't take long then just work up through the grits to a fine finish

I've seen Terry Gordon use one of his spokeshaves to do just this job. It was a beauty to watch. Can't guarantee that denncarm or myself would have the same skill in doing it.

CP