Template sanding bumps

[brad hays]

I'm brand new to making templates and I'm finding out making a good one is not as simple as it looks. Luckily I've got the Ridgid oscillating edge belt/spindle sander to make it easier for me. My problem is making smooth inside arcs. After about an hour of practice my outside arcs or curves are nice and smooth, but my inside curves feel lumpy when you run your finger down the edge.

What I figured out that got my outside curves smooth out was that almost all sanding passes must be made as long as possible. I zeroed in on flattening down the proud parts by doing long passes and keeping a steady speed, but then doing a gradual application of pressure at those sections. I applied this technique to my inside curves but because I have to work on the curved part of the sanding belt I'm not able to get the same effect. I'm sure it's because the flat part of the sander is more forgiving than the curved part which has a much smaller sanding 'foot,' or contact patch I'll say. I've tried to lessen this effect by keeping my approach angle as shallow as possible but this still isn't producing the smooth edge I'm after.

So what am I missing? Do I just need to keep refining my sanding technique to be even lighter and sneak up on the template edge even slower doing more long and shallow passes? Or is there some kind of technique out there I haven't run up on yet?

I'm using 3/4 melamine for making this piece. Paper template is on the bottom side for the pic.

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[David Stone (CT)]

My experience is the same as yours: making a template with fair curves is not as simple as it looks. I've found that a spokeshave is a useful tool for fairing template curves, particularly concave ones. (At least assuming the template is made from a spokeshave-able material--I've used 1/4 ply.) Using fingers/eyes to judge where to concentrate the removal effort is necessary since a spokeshave sole isn't long enough to naturally find the high spots, but it's better than a sander. Even still, some small bumpiness may remain and translate to the workpiece: the realistic goal, as I see it, is to reduce it to the point where whatever bumpiness exists on the workpiece can be eliminated relatively easily with a hand sanding block, card scraper or both.

Edit: Re-reading your post, I see that you're using 3/4 melamine for the template. I think that's part of the issue: too thick and particle board core is hard to shape. Absent a compelling reason to use this material, I'd switch to something more easily shaped and refined, like the 1/4 ply I mention.

[Kevin Jenness]

Making fair inside curves is hard with a spindle sander. If you are trying for a true radius it's easier to use a router on a trammel, and you can use more than one router-generated curve to make a more complex shape. Otherwise use thinner, more easily shaped material, saw close to the line and use a spokeshave or a sanding block or spindle as close to the template radius as possible.

[Mark Gibney]

I make the initial template from 1/4" material, then use that to make a 3/4" thick template, which hold up to repeated use.
I might reinforce the edge (if it's MDF) with CA glue depending on how much use it's going to get.

[Patrick Kane]

I also like to use a spokeshave whenever possible. A flexible sanding pad with a 'shoe' thats decently long should also help in correcting your peaks and valleys. I assume your problem areas are measured in the 64ths or 128ths, right?

[Thomas McCurnin]

+1 on making thin templates first (1/4" MDF), which usually don't have the humps or are easier to remove them.

+1 on using a spoke shave.

[Carl Beckett]

In general, I find sanding a smooth curve using a sanding drum that is a smaller diameter.... pretty much impossible. Using the largest dia sanding drum as possible 'helps'.

Then often I glue a piece of sandpaper on a thin piece of stock so it flexes. Then sand it. Doesnt create an exact profile since the amount of flex impacts how it sands out - but it sands out all the bumps.

Doing it on thinner stock first then using that to make a thicker template could be pretty good trick.

[Keith Christopher]

I like using the flexible sanding strip. take thin scrap glue a couple of blocks you can hold on to on each end, then adhere sandpaper to the bottom and sand flexing the board to match the curve.

spokeshaves work also as stated above, a fine rasp can help also.

[Rick Potter]

As mentioned above, using the largest spindle that will fit is good advice. In addition, note that direction of travel is important when using spindles. Going against direction of spindle grinds a bit differently than going with direction. Practice doing it both ways with MDF (it's soft) and it might help. Going with the direction of rotation with a light touch might help in certain inside curves.

[Tom M King]

Flexible sanding blocks for auto body work.