Dimensioning Tolerances & Straight Edge Use.

[Raymond Stanley]

Hi All,

After finishing my workbench and a homebrew air cleaner, I'm finally starting my first woodworking project that isn't a another device for woodworking! So, I'm dimensioning up some stock for a trinket box that I want to make as a gift.

To what tolerances do you all plane dimensioned lumber? I've done my best, and I've still found at the end of the day about 1/16" off from equal thickness and 1/16" off from perfectly flat. I got really frustrated in the process of getting that, so I'm not sure that I could get it any flatter without losing my temper I know, I know, patience is a virtue...

Do you think the box will still come together ok with those kind of tolerances?

Also, in the process of doing this dimensioning, I used a 4 ft aluminum straight edge that has a rather thick edge. If I tilt it, I can see more gaps, but sometimes I think light refraction is playing tricks on me. What are people's experiences with aluminum straight edges and how much unstraightness they show? Should I just be using my benchtop (although it is flat, its not quite as flat as a straight edge)?


Thanks as always for your wonderful advice!

-Ray

[Jim Nardi]

Use feeler gauges. Light test is not reliable. If the box doesn't come together just use a pencil and mark the spots and adjust them with your plane.

[Joel Moskowitz]

First of all - get one side flat. There are many ways of doing this but the way I do this is by intentially planing the board a little hollow (even with a shorter plane and then planing long strokes to remove the hollow- when thehollow is gone you got flat. IF your board is convex is is nearly impossible to get it flat without first removing the convexity to a concavity.

To test for flat I Use a pair of winding sticks to make sure it's straight and use one winding stick as a straightedge to see if it's convex or concave. You can also sight along the board for a quick decent reference.

Most of the planing is done with a jack plane with a nice convex blade. As I get closer I switch to a longer plane with a nearly flat blade.

Once the board is flat, I mark the side. gauge thickness and plane the other side to the gauge lines the same way

[Bob Smalser]

Nothing in wood is perfect....especially the woodworker. Tolerances only have to be good enough to please the eye and the fingers. Generally the eye can see a 32nd and the fingers can feel a 64th. And the larger and more complicated the object, the further it can be off and fool the eye. "Perfect" 16' lapstrake boat plank lines are often off an inch from one side to the other. A small box obviously needs to be closer to perfect than an inch.

You can assemble the box and work from there so you can "see" it better. Flatten after assembly using plane and/or scraper and get some practice in hiding any joint gaps in your finish. When the material list is small, it's often best to complete the one you're not happy with and immediately begin another. You'll find a use for them all, and your skills with develop quicker by learning lessons completing projects than practicing flattening boards.

[Raymond Stanley]

Thanks for the info everybody.
Bob, I like the idea of just completing the project and starting another...indeed I don't want to spend all of my time flattening boards. It will also give me an idea of how much flatness is really needed...there is always going to be some error. I'm definately excited to see this project complete.
I'm still curious about other people's uses of aluminum straight edges and how their thickness seems to cover up small deviations in flatness...maybe everybody uses their bench, or a thinner straight edge? Or maybe what I can see with it laying flat is plenty of sensitive enough, and I'm being way too obssessive about flatness.

[Bob Smalser]

I'm still curious about other people's uses of aluminum straight edges and how their thickness seems to cover up small deviations in flatness...

Again, I think you're barking up the wrong tree aiming for some "better" tool that'll do what your eye and fingers will do automatically and adequately. The only straightedge I own is a household aluminum yardstick.



I use winding sticks to gage twist, but have never used anything but the eyeball and occasionally that yardstick to gage flatness. Granted my eyeball is more practised than yours, but it'll be a longer road for you if you don't exercise it.

[Raymond Stanley]

Bob, I think I understand what you are saying now...I didn't realize you were referring to what the eye can detect in abscence of straight edge in the first post.

Just another Jedi Woodworker trick I guess!

Thanks-
Ray

[Wiley Horne]

Ray,

Please go back and read Joel's post. There is a ton of wisdom in there, that may not be obvious on a first read. Especially the part about planing a bit concave in order to get a board flat. Plane from the inside out to get it a little concave. If you do as he said, the plane itself will tell you when the board is flat, just by reading the shaving. No matter how you proceed on this project, that is a sound approach to stock prep.

Wiley

[Edward P. Surowiec]

Ray, there is a good article on "Precision Planing" in the 10/07 issue of Popular Woodworking.
Regards
Ed

[Derek Cohen]

Ray

While I agree with both Bob and Joel (and Joel's sequence of flattening a board is what I would have suggested), I think that the posts so far have not placed the emphasis where it is relevant.

I assume you plan to dovetail the box. If so, the important factor is not that the boards are perfectly flat and parallel along their lengths, but that the internal side is flat (you can square up the outside later) and that the boards are flat and parallel (in thickness) at their ends. It does not matter if the ends have slightly different thicknesses from each other - this will just mean a little more work as you will need to re-set your marking gauge for matching ends.

This Paulownia box was build completely from scratch - from a log off a tree I cut down - all boards dimensioned by hand. Taken from my recent review of the Veritas Small Router Plane. The box is not otherwise special (just used for demonstration).



Regards from Perth

Derek

[Tim Sproul]

To what tolerances do you all plane dimensioned lumber? I've done my best, and I've still found at the end of the day about 1/16" off from equal thickness and 1/16" off from perfectly flat. I got really frustrated in the process of getting that, so I'm not sure that I could get it any flatter without losing my temper I know, I know, patience is a virtue...

Do you think the box will still come together ok with those kind of tolerances?

For myself, tolerances depend on the joinery. I don't have a power jointer so all lumber is jointed by hand. The key to determining how much slop I've got room for is knowing how I'll cut the joinery.

hand done? You have a lot of room for slop in dimensioning lumber except for small items.

large machine to cut the joinery? Say a table saw to cut dadoes or a router table to cut sliding dovetails or dovetail jig and router - very very little slop can be tolerated for good joinery in these instances.

You can tolerate more slop if you need less reference surface area when cutting the joinery so a biscuit joiner or Domino can tolerate more slop in the stock dimensions/squareness than a table saw. For hand joinery, you're normally referencing off very small surface area (marking gauge fence or such) so you've got room for slop, similar to a biscuit joiner.

[Raymond Stanley]

Thanks for all the tips everybody!

[harry strasil]

This will work for a thickness planer.

http://www.sawmillcreek.org/showthre...904#post650904

And with small pieces, just use some spray adhesive or tape a piece of carbon paper to a piece of glass and lay the piece on the carbon and move it a bit, or just put some fine dust on the glass and shake it around for full coverage and dump the excess off and lightly blow using your mouth to leave only a slight coating and procceed as above, all the high spots show up like a sore thumb and then you can scrape or plane the high spots off.