Benchtop Flattening

Re: Benchtop Flattening

Adam Cherubini

>All I have is a 3 foot straightedge and the bench will be 8' by 32".

Dear Tom,

You are going to need a much bigger straight edge. In order to check diagonals my calculations indicate that you will need a straight edge approx 101.19" long. Now that will get things straight, but flat is a different matter. If you could find a really wide straight edge- something resembling a surface plate maybe.....

Okay that's a stupid idea. Forget that one.

When you attach the strings, I'd use fishing line to prevent sag caused by the stretch in the string. Otherwise, you'll end up with a bench with a hollow in it that perfactly matches your string! Now maybe if you inverted the bench top so the string would sag the other way....

Okay that's a stupid idea. Forget that one.

Tom, what you need is a long plane. A 3' long plane wouldn't be too long, but a 2' long plane (or there abouts will suffice). Make sure the iron is ground to form a curve so the corners don't leave gutters. I recommend scraping after that- not smooth planing since your smoother can dig holes if you're not careful. Also, I think I prefer a rough bench (as opposed to a finished waxed furniture like top). The rough surface stop boards from slipping so much while planing. Forget the straightedge or use it if you like- but learn to trust that plane. I call it a try plane, but other folks might call it a jointer or some such.

Adam

Re: Benchtop Flattening

Todd Stock

>I guess I've been doing things the hard way, but in the past, I just went at it with scrub, jack, jointer and winding sticks. Running monofilament corner to corner and end to end might be useful in initially surveying the top's condition; however, a couple of winding sticks will do the job as well, and you won't need an assistant (or two) to get the job done.

The eye is an amazingly accurate tool - I think you'll find that sharp planes and frequent checks with winding sticks and your existing straight edge will get things close to flat.

If you wish to get down to the .005-.003 range, you might consider making a couple of master bars in various lengths (2', 4' 8'). FWW has instructions in one of their machine set-up articles a few years back, or use John White's shop machinery book as a guide.

I don't think that .005 accuracy is either very practical or likely to be achieved for more than a few minutes given wood movement, so somewhere between 3/32" and 1/16" of an inch over 8' is probably a reasonably challenging target without resorting to Adam's giant surface plate.

Just my opinion - good luck on the bench.

I disagree

Adam Cherubini

>I disagree with Todd's seemingly sensible approach. I think the way to go is a giant surface plate and some machinist's blue dye!

Or how about this one: Giant surface plate with loose grit. You could hone it flat!

So I was just coming back from lunch and one of my friends was complaining about Philadelphia. He said "Its dirty and the people are rude". So I said, "Shut up @sshole!"

Adam

smacked with a sarcastic stick this morning

Re: I disagree

Dean J in MN

>Adam is in rare form, quick ask him questions about all of his micro-adjustable marking gauges!

Back on topic. I'd also just use the longest plane you can find and some winding sticks.

-Dean

Re: I disagree

Joel

>You could also make three bneches at one time and rub the tops against each other.

Joel's New Product

Todd Stock

>And I thought bench grit was the stuff that accumulated in the corners of the tool tray...turns out to be Joel's latest addition to his abrasives inventory.

Comes in economy 5 lb bags for leveling those longer benches, right?

Re: Joel's New Product

Tom Wolf

>Hey All,

Thanks for the good humor and good information. I've got a 6 1/2' by 24" tradidtional style bench that I use for all my rough milling, joint cutting, etc... and I have been able to get it pretty flat with the standard winding stick, straigtedge approach.

Anyway, I build guitars and that will be the primary assignment for the new bench so I do want to get it pretty flat. The reason I'm thinking of this string method is that I'm thinking of going with a torsion box for the top, skinned with 1/2" plywood. I have had problems with the ammount of material you can remove while flattening a plywood surface without running into voids, knots, etc...

With the string technique it seemed you could get localized reading of high spots and attack those very spots just like you would do if you were scraping a plane sole instead of lapping it on an abrasive surface.

Thankyou for the information,

Tom Wolf

planing plywood?

James Watriss

>Yeesh! You're a brave soul.

If you're going for a torsion box, I might think about using something a little thicker than 1/2". But that's just me. If I remember correctly, torsion boxes involve cutting grooves, or dadoes, or whatever the technical term is when using wacky wood, (the grain runs in all directions, so what's what? Is it a stacked dado and groove set?) What that means is that you'll be dealing with a bench that is less than 1/2" thick in those areas. While I suppose for some things that's fine, for a workbench, I'd want something a little more stout.

Actually...(links on Torsion Boxes)

Christopher Fitch @ Memphis

>1/2" for the outer skins is perfect...

Here is a link to the episode where David Marks makes a Torsion Box top for his assembly table.

Torsion Box

Here's another link:

Making a Torsion Box Table

Ian Kirby has a detailed article on making Torsion boxes on FWW's pay article section. He shows examples of using them for shelves, desks, etc.

I plan on making a torsion box top for a storage cabinet/assembly table I'm going to make eventually...

Re: Actually...(links on Torsion Boxes)

Pam Niedermayer - Austin, TX

>With all due respect, a bench top is quite different from desks, assembly tables, shelves, et al.

Pam

Re: Joel's New Product

Clay C in Miami

>Well, if you've already got a traditional style bench, and can use that for planing, a torsion adjunct may well be worthwhile - but it just seems like it's not a 'bench.' You can't put dog holes in it, you can't mount a vise without some pretty elaborate planning inside the box before it goes together, etc.

I use a torsion box assembly table, and it is ideal for that use - just be sure this will never need a vise, dogs, or other torsion-inappropriate items ...

Clay

(Who has recently gained even more respect for luthiers, watching DIY's series called 'Handmade Music.' So far, I've watched 'em make a violin, guitar, mandolin, [maybe a dulcimer?], gourd banjo, and 2/3 of an old-style open-back banjo. Makes me wanna put purfling into my next appropriate piece of furniture.)

Re: Actually...(links on Torsion Boxes)

Christopher Fitch @ Memphis

>With all due respect, nice way to jump to conclusions Pam.

I was responding to the comment that 1/2" was to thin for the outer skins, not that a torsion box was appropriate for a planing bench.

Luthier's Bench *LINK*

Todd Stock

>A bench for guitar construction is a different animal than our general purpose woodworking benches - much more an assembly table with vise and provisions for holding jigs and fixtures. Flatness is pretty important, as luthiers routinely work to very tight tolerances, so a torsion box design might be a good bet.

Kathy Matsushita, an amateur luthier with a great on-line presence, has a very functional shop with several good shots of adequate benches for building instruments.


An Amateur Luthier's Shop

Re: Actually...(links on Torsion Boxes)

Pam Niedermayer - Austin, TX

>It makes little difference if 1/2" skins are OK for torsion boxes if torsion boxes aren't OK for benchtops, as pertains to this thread. I suspect that torsion boxes could be OK for benchtops, depending on the space between skins and skin thickness.

Pam