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Quickie question about torsion box assembly and repeatable cuts

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Forum topic by Holbs posted 12-26-2015 05:13 PM 1459 views 0 times favorited 37 replies Add to Favorites Watch
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Holbs

1512 posts in 1636 days


12-26-2015 05:13 PM

Topic tags/keywords: torsion box mdf bosch 4100

I cut strips of 1/2” MDF for my torsion box assembly table. They measure 3” high. During a non-glue rough draft placement of all pieces, I notice some pieces are slightly taller by 1/16 to 1/32” or so…enough to register by the finger tip. With this being an assembly table (roughly 40” x 40” table real estate), should I be worried of such a difference? I am using 3/4” for the outside framing, 1/2” inside webbing, 3/4” bottom and top panel, 1/8” hardboard on top.
I was thinking of a 4’ sanding bar using a sanding belt or something if there was a need.

2nd question: repeatable cuts. I am using the Bosch 4100 saw and fence. I you cut 50 pieces at 3”, would you expect the pieces to always be exactly 3” with a cabinet saw and quality fence? I wonder if the small real estate on table and wielding 3’ or 4’ wide pieces when the piece engages the blade is my problem.

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter


37 replies so far

View Fred Hargis's profile

Fred Hargis

4330 posts in 2100 days


#1 posted 12-26-2015 05:25 PM

1:, Yes! what gives the torsion box it’s extreme rigidity is the tension created between the skins and the web…for that you have to have a good glue bond between the two. Do something to make the web pieces identical.

2: Yes (again). I find the pieces to be identical whenever I cut strips like this ( on my old contractors saw and the cabinet saw that replaced it).

-- Our village hasn't lost it's idiot, he was elected to congress.

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Holbs

1512 posts in 1636 days


#2 posted 12-26-2015 05:30 PM

Then I’ve been doing something wrong for 3 years with my table sawing technique. If I cut 50 pieces of anything, there are always 5 or 6 pieces that are larger or smaller by fingertip telling. Now to figure out if it’s the contractor saw design (not exactly made for precision) or my own technique.

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter

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Ger21

1061 posts in 2737 days


#3 posted 12-26-2015 05:32 PM



If you cut 50 pieces at 3”, would you expect the pieces to always be exactly 3” with a cabinet saw and quality fence?

Only if the saw is set up properly and the operator knows what he’s doing.

I would set up a featherboard just in front of the blade to hold your boards tight to the fence, and recut them all, removing maybe 1/16”, to get them all even.

-- Gerry, http://www.thecncwoodworker.com/index.html http://www.jointcam.com

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MrUnix

5103 posts in 1805 days


#4 posted 12-26-2015 05:33 PM

Ripping boards without moving the fence should produce pieces the same width every time. As for your first question, if you are not going to have a bottom (usually not needed for most), then just attach the top to the stringers so they are flush… won’t matter that the bottoms are a little out of plane.

Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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Holbs

1512 posts in 1636 days


#5 posted 12-26-2015 05:37 PM

Good idea Gerry. I can do that. I have featherboards for the miter slot and I still have that option to do just that. Still bugs me with the doubt that I can’t cut pieces like all of ya :) But then because I am using the Bosch 4100, I should of rough cut the pieces first and used featherboard for the final rip in the first place. Ahh..lessons learned. Of course now comes in the question of the blade alignment, fence alignment, etc. I have doubt so will tackle those questions today (it has been 8 months since I have done those alignments).
Brad..it will have a bottom 3/4” piece. I thought the whole purpose of a torsion box was upper & lower skin to hold it all together.

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter

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Harry

77 posts in 786 days


#6 posted 12-26-2015 05:43 PM

I also have a 4100 that was my primary saw and had a problem once that resulted in bad cuts. Turned out to be the fence had become slightly loose and got knocked out of square with some MDF. Check to make sure it is tight enough when locked down.

Harry

-- Harry - Professional amateur

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jumbojack

1672 posts in 2231 days


#7 posted 12-26-2015 05:46 PM

Cutting exact pieces from four foot wide material is tough. Cut them a bit over size and THEN set up for your exact precision cuts. Leave enough to take a bit from each side 3/8 to 1/4.

-- Made in America, with American made tools....Shopsmith

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Holbs

1512 posts in 1636 days


#8 posted 12-26-2015 05:47 PM

Harry… my first cut, I did take calipers to the piece. It was exactly the same width and height, perfectly square. I did not do such in the middle or near the end after 50+ cuts. Could of easily nudged the fence ever so slightly. Time to play with fence alignment and adjustments. Maybe, my fence is looser than it should be.

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter

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jbay

1241 posts in 506 days


#9 posted 12-26-2015 05:48 PM

Are your pcs smaller than 3” or bigger? Bigger would indicate your fence moving, smaller would indicate the material not riding fully against the fence. Easier to start with 12 3/4” rips then break them down to 3”

-- If anyone would like to see my Portfolio, PM me and I would be glad to send you the link.

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MrUnix

5103 posts in 1805 days


#10 posted 12-26-2015 05:50 PM

Depends on what it’s for and what you need.. in many cases, a top and bottom skin isn’t necessary. For example, here is a torsion box type extension table plan (from the Beisemeyer commercial fence manual) that only uses a top skin:

And here is a table that only uses a top skin:


Cheers,
Brad

-- Brad in FL - To be old and wise, you must first be young and stupid

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jbay

1241 posts in 506 days


#11 posted 12-26-2015 05:52 PM

I would use a top and bottom. Being that it’s made with mdf, It will sag overtime if not.

-- If anyone would like to see my Portfolio, PM me and I would be glad to send you the link.

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Holbs

1512 posts in 1636 days


#12 posted 12-26-2015 05:55 PM

It will have a bottom. The assembly table basically sits ontop of a moving cart that has 3/4” plywood top. I could of use the moving cart top as my bottom, but… had to use up the rest of the 3/4” MDF :) Hate having that stuff laying around.

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter

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Holbs

1512 posts in 1636 days


#13 posted 12-26-2015 06:00 PM

A quote from Festool forum group: I called Bosch and was told that a misalignment of 0.010” was within their specs, because the saw is considered a contractor’s saw.

Never knew this. I would assume a cabinet saw would be down near 0.002” or less. Hmm…..gives me more ammo to get a cabinet saw asap.

-- The Carpenter Bee is derived from the Ancient Greek word wood-cutter

View Fred Hargis's profile

Fred Hargis

4330 posts in 2100 days


#14 posted 12-26-2015 06:19 PM

I’ll disagreee with Brad about not needing the bottom skin. By design an assembly table will have clamp pressure (when you clamp long assemblies to it) and still need to be perfectly flat. The only way to do that is with both skins and the good bond to the web…it’s just the way they work. Now, I’m not saying a table isn’t more stout with an egg crate under it, way more that without the egg crate…but now where near as stiff as one with a top and bottom skin.

-- Our village hasn't lost it's idiot, he was elected to congress.

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Ger21

1061 posts in 2737 days


#15 posted 12-26-2015 06:32 PM


As for your first question, if you are not going to have a bottom (usually not needed for most), then just attach the top to the stringers so they are flush… won t matter that the bottoms are a little out of plane.

Cheers,
Brad

If there’s no bottom, then it’s not a torsion box. So don’t go through the trouble of building it like one. Withoput a bottom, there’s no rigidity, and nothing to keep it flat.

-- Gerry, http://www.thecncwoodworker.com/index.html http://www.jointcam.com

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