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I am rebuilding a park bench - the kind that has a cast steel piece at each end and a dozen or so wooden slats running in between the ends. I plan to use "red grandis" nominal 1-by from a local hardwood store, and stainless carriage bolts.

I plan to make the slats 4ft long, unless that seems too much for this wood. It looks like pretty strong stuff, having a modulus of elasticity north of 2 million psi.

My main question has to do with how to figure out the slat widths. The spacing of the bolt holes (in the steel end pieces) is fairly irregular, being roughly (in decimal inches): 1.75, 3.125, 3.25, 3.125, 3.25, 3.25, 3.5, 3.00, 3.375, 2.375, 1.5, as best I am able to measure through the curved parts.

Using Excel, I first computed it so that, after subtracting 1/2" gap from each of the spaces, a slat was sized to use half of the remaining space on each side of it. For example, the slat that has a space of 3.5" on one side and 3" on the other side came out to be 2.75" wide [(1/2 * (3.5-0.5)) + (1/2 * (3.0-0.5))]. The problem with this is that the bolt hole ends up not being in the middle of the slat's width. Where the spaces are fairly similar (3" or so), this seems reasonable, but towards the end (where the spaces on either side of the bolt hole are very different) the holes are way off center.

I'm wondering if there's a better way to compute the slat widths, so that the bolt holes are nearer the center.

The only table saw that I have access to is pretty crappy, especially for wood this hard, so I'd like to have the hardwood store rip it for me. So winging it as I go does not really work.

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    Maybe I'm missing something, but what is stopping you from custom fitting each piece as you go, allowing for space from the previous piece? There are two issues here: the bolt spacing and the spacing between the slates. Both will be noticeable. Unless I'm not understanding, the best way is to simply cut, fit, and mark the holes for each slat as you go. I could be missing the point, though. maybe provide a picture or sketch?
    – user5572
    Mar 11, 2022 at 21:55
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    Oh, I see. You need this ripped ahead of time. That's not going to be a good time for you. Suggest accepting that these benches need refreshing regularly and just use more copacetic outdoor softwood.
    – user5572
    Mar 11, 2022 at 21:59
  • Use softwood so I can rip on crappy table saw, you mean ? Actually, this saw might be up to it. Mar 11, 2022 at 23:51
  • If you’re desperate, a regular circular saw with a guide would provide decent rips. Mar 12, 2022 at 0:02
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    I'm not familiar with red grandis, however the leaflet for one local supplier says its an easy-machining wood so I don't imagine you'll have too much difficulty sawing it. If you do have trouble replacing the current blade with a dedicated rip blade will always help a lower-powered table saw handle rips that it otherwise struggles with.
    – Graphus
    Mar 12, 2022 at 17:27

1 Answer 1

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I plan to use "red grandis" nominal 1-by from a local hardwood store, and stainless carriage bolts.

I believe in American 'nominal' for hardwoods different to nominal for softwoods. If wood is unplaned 1" is actually 1" or 4/4 (four quarter) and not 3/4" thick. If planed, thickness depends on whether it is surfaced on one side or on two sides.

I plan to make the slats 4ft long, unless that seems too much for this wood.

Many such benches are remade from oak now that teak has become too expensive. Others use a type of mahogany which is much less stiff than oak.

Is red grandis stiffer than oak? Proceed!

My main question has to do with how to figure out the slat widths.

Impossible to help theoretically. This must be done with iron supports in front of maker. I have helped neighbors work on benches, left and right holes maybe not aligned perfectly or enlarged from rusting.

There are YouTube videos that show bench rebuilds/restorations and seeing fitting will help understand usual test fit process.

FALCON390 Workshop
Walkers Woodworks
Wood By Wright
Scott Walker

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