I was wondering what alternatives there are to a spindle sander or hand sanding when it comes to sanding concave curves.
I have seen artificial spindle sanders created using a drill press/drill, but was wondering if there were any better or different options I am not thinking of.
The drill-based ones are the main alternative to the commercial spindle sander.
They can work very well, despite the usual objection raised by someone that the bearings on drills aren't built to take sideways forces. While that is presumably true enough it doesn't appear to be an issue in practice for most people with most drills, including hand-held drills as opposed to drill presses/bench drills, even when a bottom bearing of some sort is not incorporated*.
was wondering if there were any better or different options I am not thinking of.
Sand by hand
You mentioned sanding by hand and while it's not the ideal method it is an option, particularly if you only have a modest amount to do.
One of the main tricks to doing this efficiently is not to start with too fine a paper — if you're still shaping the curve you're only shooting yourself in the foot by starting with 180 grit..... or 240 as I read someone doing just the other day! When shaping you should be using something like 80 grit to start with, then 100, 150 and then finish at 180 or 220.
Another trick is to use shaped sanding 'blocks', if necessary fitted with a fence of sorts to help ensure the edge is square to the face of the board. The simplest block for sanding inside curves is a piece of dowel with the sandpaper just wrapped around it, and this can work surprisingly well. Remember to finish the sanding in the direction of the grain!
Sanding by hand is of course hard work but if it's the only method open to someone it shouldn't be discounted, it can and does work. But there are better ways.....
Don't sand
Compass plane This is obviously a pretty specialised plane and they can be costly, but if you do enough work to warrant one they'll be worth the outlay. Obviously because of their size and shape they can only do certain tasks, but they do excel at it when they can be used for the purpose. A little more in a previous Answer.
Spokeshave With a very sharp edge, practice in controlling the tool and experience in reading grain you can do a lot of your curved edges, concave and convex, using spokeshaves. This includes shaping, so potentially no need to saw the profile first, and finish-ready surfaces are possible straight from the spokeshave just as with a plane, although they're harder to achieve (more user control required, plus most spokeshaves have a greater tendency towards tearout because they have wide mouths and no means to alter this).
Files Files offer a very useful alternative for many shaping tasks, and doing curved edges is one of them. Much rough shaping and quite a bit of the smoothing can be done using a selection of files, leaving just a minimal amount of sanding or scraping to finish off.
When using files it can be worth doing something similar to when sanding — filing in the direction of the grain. With files this is often done with the file held at 90° to the direction of motion (referred to as draw filing) and this can sometimes leave the smoothest surface the file is capable of producing. But the technique does tend to work best of harder materials than wood.
Scraper You an scrape edges just as you can scrape face grain on boards, and the results can be just as good. Lots more on scraping versus sanding in a previous Answer.
*It's worth noting that sanding drums, wire brushes and other things intended to be chucked into hand power drills have been around since before the 1950s, so there is tons of practical evidence that drills can withstand this type of use. But, cheaper modern drills won't be built to the same standards of the past so proceed with caution and as always don't press hard, let the abrasive do the work.
Mostly I add this for completeness. The practicality depends on how much of this you intend to do and the nature of the work.
If you have a band saw, you can use bandsaw sanding belts on it. This precludes sanding interior curves that are completely enclosed. Also, it of course works better with larger radii. I used this ages ago when making forms for guitar sides before I went "freeform". You might be able to rig a curved backer to adapt to tighter radii. One advantage of this method is handling really thick pieces.
That said, I usually a sanding drumb in my drill press and limit the pressure. I chuck this up high in the chuck and sand as high as possible to reduce the torque on the chuck that puts lateral forces on the bearings.
Inflatable drum sander, generally (but not always) used in a lathe. Unlike a rigid drum, these are adjustably conformable by how much air pressure is in the drum.
Not really that different, but for smaller scale stuff sleeves/drums on a handheld or flex-shaft rotary tool.
The end drum of a belt sander. Some of the knife-makers (metalwork, but similar tooling) cobble up (or perhaps buy, now, from other folks that cobble them up on a larger scale) belt sanders with different sized contact wheels to get different diameters without being stuck on buying pesky sleeves in each size. Can't really get to something that recurves with one of those, though.
Then again, if you have a lot of call for it, a spindle sander might be the way to go.
