Replies

1. 
   esch5995 | Sep 24, 2021 05:07am | #1

   What you are describing relates more towards wood turning using lathes, although traditional woodworking occasionally creates curves by bending wood or in the case of some traditional chair seats that are carved slightly, objects with large concave or convex surfaces generally are created by turning.

   Sanding objects on a lathe is usually quite different than sanding a piece of furniture. Sanding on a lathe usually involves holding a piece of sandpaper relatively stationary while the work piece spins, a pretty simple and effective task. Sanding furniture on the other hand involves moving the sandpaper across a stationary work piece. For the most part sanding large radius concave or convex surfaces can be easily handled by existing tools, specially the round Random Orbit Sander that every woodworker owns.

   While I applaud your interest in woodworking and engineering I'm not sure the problem you are trying to solve exists. If anything the problem is just the opposite, sanding small radius objects and curves is much more difficult than large ones. That is where we could use some innovation.

2. 
   user-1118538293 | Sep 24, 2021 08:22am | #2

   While I generally agree with esch5995 with respect to smaller/tighter radius curves, to your question specifically I use RO sander when possible, and also the Ridgid combo drum/belt benchtop oscillating sander for sanding to shape, and then hand-sanding (usually with semi-hard blocks) to get a final surface. My method does rely on feel to get a curve that is fair, and sometimes through inattention or whatever it ends up with a bump or two--plus multiple copies of the same part will not be truly identical. There are the pattern-duplicating sanding accessories for spindle sanding (esp on the drill press) but I have never used one. And of course in that case the curve on the workpiece will only ever be as good as the curve on the pattern.

3. 
   elmaduro | Sep 24, 2021 11:08am | #3

   The Large surfaces you refer to is very subjective. Generally cylindrical objects are sanded on a lathe freehand with strips of sandpaper.

   Concave and convex surfaces are usually sanded by hand with a custom built sanding blocks. This block is custom cut to match the curve and sandpaper is attached in a removable fashion to allow running through grits.

   Best with your project!

4. 
   pantalones868 | Sep 24, 2021 04:09pm | #4

   I take a section of 3/4" plywood and score across it every 1/4" ish with the table saw or radial arm saw. I can rip a piece off to custom make a flexible sanding block to the size I think I need and glue the appropriate grit sandpaper to it. I find I need to sand to curved surfaces quite a lot.

5. 
   EdWeber | Sep 24, 2021 05:12pm | #5

   The main approach to sanding a large curved surface it to maintain equal pressure as the surface contour changes. This is difficult by hand but can be helped by the use of contoured sanding blocks. For a mechanical approach you need something with a floating and/or flexible head that will keep constant pressure between the abrasive and the wood without making flat spots.
   Craftsman used to make a model called the 3D sander, it did a fair job of negotiating smooth curves.

6. 
   [email protected] | Sep 27, 2021 06:04pm | #6

   I use various methods using rubber and flexible blocks.The rubber sanding blocks have a curved back that i use rather then the flat side.Body fender workers have a two handled sanding block with a one foot base that is flexible .You set it to the curve you require and sand away

7. 
   mikaol | Sep 27, 2021 07:25pm | #7

   Someone may have covered this, but there is a brand of sandpaper with backing made to be hand-held (nonslip.) Should make a tedious task a trifle easier. I used it to sand down a round stair handrail.
   Mikaol

8. 
   rob_ss | Sep 27, 2021 08:05pm | #8

   As you identify, sanding these shapes is very difficult.
   The problem can be encapsulated as "how do I make this smooth without changing it's shape"
   How this is done will depend on the circumstances.

   In turning, sandpaper is held against a rotating workpiece. Sometimes this is a strip of sandpaper held at each end, the middle against the piece, sometimes it is fixed to a block, sometimes on a powered rotating pad and sometimes just rubbed on with hands. always though the aim is to take away unevenness whilst not damaging the crisp corners and detail left by the chisels.

   In furniture making, the usual process is to use a resilient block with paper wrapped around it, or a soft abrasive faced foam pad. On a concave surface, it is difficult for a flat piece of paper to make even contact, so contact tends to occur at the edges. This helps to fair curves though and a convex pad can more easily result in less fair curves as it can quickly eat away wood where it should not.
   On a convex surface, the flat block will always be at a tangent to the surface so contacts at only one point. Great care is needed to avoid flat spots. This problem is often addressed by starting at higher grits and taking longer.

   Where absolute precision is less important, both convex and concave surfaces can be smoothed with a random orbit sander, again usually with finer paper.

   Over time a great deal of work has been done to address the problem of curved surfaces - have a look for instance at the compass plane - but these have largely fallen out of use because they are very complex and difficult to use, and frankly tend to make things worse more often than they make it better.

   Good luck!