Calculator for spacing slats/spindles - input welcome

[Swingman]

I am frequently having to do this particular calculation, months apart, and have to rebuild the wheel each time. Today I attempted to write the damn thing down as a formula so I could make an Excel spreadsheet for my own use.

The following, in a spreadsheet, worked twice for me today, but there is likely a better/easier way?

A formula for spacing a given number of same width slats/spindles evenly between two points:

X = (S - (W*N))/(N + 1)

where:

X = spacing in inches S = Space to be filled with slats of a desired width W = Desired width of slat N = Number of slats desired

Your corrections/better ideas are more than welcome ... the older I get, the more I am just looking for a way to getting back to making sawdust faster.

[Luigi Zanasi]

On Sat, 2 Oct 2004 18:32:56 -0500, "Swingman" scribbled:

Instead of calculating W*N, measure it. If you've got lots of spindles/slats, it's easy to screw you up if there is the slightest inaccuracy in the measurement of the width of slats/spindles. Put all your slats/spindles next to each other & measure the width. Then divide by the number of slats/spindles plus one to get the spacing.

Luigi Replace "nonet" with "yukonomics" for real email address

[Greg G.]

Or calculate Centerlines for mounting the slats, rather than spacing between slats. That way, variations in individual slats will not matter.

I won't modify your formula for that - you're a smart guy! ;-)

FWIW,

Greg G.

[Swingman]

"inaccuracies" in woodworking measurements are inevitable regardless of the method ... you just do your careful best as in all woodworking. The idea is to be able to calculate without having to cut a bunch of expensive wood to do a trial and error method like that.

Not to mention that it's convenient to have the option to change your slat width as your design may change.

Thanks, in any event.

[JeffB]

In addition to the considerations expressed about inaccuracies and measuring, you should modify your formula to divide by N - 1 (not N + 1). For example, with

5 slats, there are 4 spaces between them...

JeffB

[Luigi Zanasi]

On Sat, 2 Oct 2004 20:16:49 -0500, "Swingman" scribbled:

Absolutely true.

Ah! I see! You're talking about the design, not the installation. Then using your formula makes absolute sense. And you get to try out different #s of spindles. But when it comes to installing them, I would still use my method. A 1/64th error in fabricating the spindles means a gap that is off by 1/2 inch if you're using 30 spindles or so.

Luigi Replace "nonet" with "yukonomics" for real email address

[Greg G.]

Kinda depends on whether you leave spaces at the ends - like a baluster.

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Greg G.

[Luigi Zanasi]

On Sun, 03 Oct 2004 05:53:59 GMT, JeffB scribbled:

Not if he wants spaces beyond the two end spindles. So with 5 slats, there would be 4 spaces between them and 2 more at each end of the row of slats.

Luigi Replace "nonet" with "yukonomics" for real email address

[Robert Bonomi]

"Close, but no seegar" applies.

change the denominator to "N-1", and you've got it.

"Close, but no seegar" applies.

change the denominator to "N-1", and you've got it.

Note: with N slats, there are only N-1 spaces _between_ the slats.

[Edwin Pawlowski]

[Swingman]

"Close, but no seegar". :)

Note that (n + 1).is the goal.

When positioning spindles between two legs, or bars in a window, you generally leave a space between the first post/ and the first spindle, and a space between the last spindle and the last post, therefore you need one more space than (n-1).

[Swingman]

You're right, I actually do use it for the "design" stage, particularly when doing a shop drawing with a CAD program, which I generally do for every piece I make since I am usually buiilding custom and rarely use plans ... then it is a matter of implementing the design.

Most of the time I am looking to accurately mark the location of mortices for a row of spindles/slats on both a top apron and bottom stringer.

Yes. that's the beauty of having a spreadsheet, you can just plug in the different options/design.

There's a better, and more accurate way then that, actually.

I generally take the results of the calculation and make a "story stick" with the measurements on it, then use that the "story stick" mark the location of the mortises for the spindles/slats ... that way there is very little measurement error, particularly if you reference the same post or leg as the starting point, and it is much faster.

Thanks for the discussion, Luigi.

[Swingman]

Exactly ... AAMOF, I don't think I've ever seen spindles/slats between two posts or legs that started with a slat/spindle next to each posts instead of a space?

(n+1) is definitely correct for the task at hand.

[Edwin Pawlowski]

I take the top and bottom stringers and clamp them together when making the marks. That way, if there is an error, the spindle will still be straight even if it is off by 1/64. I also do the math rather than mark say, every

2". The pencil line can be a huge error when marking 14 slats. Or so I'd told. I'd never do that myself would I?

I guess I could use a spreadsheet but I take pencil and paper and draw out (crudely) the spaces and slats: --0--0--0--0--0-- The I make marks as needed. 1 1/2" spaces with 2" slats is 1.5, 3.5, 5, 7, 8.5, etc. This is the place where the Lee Valley Saddle Square is the perfect tool for layout. The only important rule is simplicity. Don't make 1 13/64 slats and end up with 2 11/16 spaces. The math will drive you crazy and you will be blind looking at all those little lines on the ruler.

[Randy]

[Swingman]

Great minds think alike? ... right now I am looking at six "stringers" clamped together (top/bottom/top/bottom, etc) that I started marking mortises on before I quit the shop last night. ;>)

AAMOF, here's a picture of the very operation (incomplete) you describe:

you can see, I keep an electric pencil sharpener on the wall hung workbench where I do most of my marking.

The "story stick", which I make carefully and keep for future use, also clamped to the stringers, makes marking less error prone.

I use an engineer's square ... I've used a hinge to do the same thing in the past, but I've been meaning to get one of thse LV Saddle Squares, but just haven't gotten around to it. Sounds like you like it?

That's _precisely_ why I spent the time yesterday doing the formula and making a spreadsheet. That, and here lately I can't remember the next time how I did it the last time ... you probably haven't got that 'advanced' yet (in age). ;>)

[Swingman]

While I agree, IME, you will hardly see anything but the last with spindles/slats in woodworking.

However, it would not be a bad idea, and easy, to add that to the spreadsheet in the event you would want to do 1 and 2 above ... options, options!

Thanks!

[Randy]

[Swingman]

A distinct possibility, and a good point.

However, said CAD program is not available in the shop, the formula works quickly with my shopbench calculator which does woodworking fractions, and the spreadsheet works on a PDA, which is easily available in the shop.

Last but not least, I've just in the recent past gained enough proficiency on my meager CAD program (QuickCAD) to make good shop drawings ... IOW, I am more interest in making sawdust than in being a proficient CAD jockey. ;>)

[bridger]

classic fencepost error....