Stack of MDF sheets = flat reference surface? - Woodworking Talk - Woodworkers Forum
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post #1 of 20 Old 08-06-2020, 08:10 PM Thread Starter
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Stack of MDF sheets = flat reference surface?

I'm gluing up an torsion box style auxiliary fence for my table saw, like this one (https://www.woodmagazine.com/tool-re...iary-rip-fence). So I need to clamp to a F-L-A-T reference surface when I glue it. I am stunned how hard it is to find a flat reference around my house. MDF has a reputation for being flat. I suspect I can't just clamp my glue-up to a single sheet and hope my auxiliary fence is flat. BUT, I'm wondering if a stack of 3/4 MDF, like 4 sheets, would work as a temporary flat reference surface.

I read through a bunch of old posts and didn't see it mentioned, so before I go buy a bunch of MDF I'm bringing it up here to see if anyone has tried this.
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post #2 of 20 Old 08-06-2020, 08:28 PM
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Your table saw has a flat table top.(at least it should be flat) That would be a good place to use.


George
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post #3 of 20 Old 08-06-2020, 08:38 PM Thread Starter
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< Your table saw has a flat table top. >
Good idea. But actually, I have a Dewalt jobsite saw. So it ain't flat. I also want a longer fence than my table. My fall-back plan is to find someone w/ a good & large table saw.

There's nothing wrong, it just doesn't work.
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post #4 of 20 Old 08-07-2020, 07:12 AM
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I use a fake marble tile threshold for a flat surface. It was from a tile job where the guy cut it too short and had to get another. It comes in super handy when I need a flat surface. I use it as a clamping caul for gluing banjo fingerboards to the neck. They come out dead flat. A tile or granite store might give you a leftover from a job or one with a blemish.
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post #5 of 20 Old 08-07-2020, 08:05 AM
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I would think that if all the spacers are exactly the same length and you glue and screw the faces to the spacers that it would be flat without clamping as shown in the article. If any of the spacers are of a different length then clamping the whole fence to a flat surface would not make it flat anyway.
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post #6 of 20 Old 08-07-2020, 09:11 AM
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I made mine using the fence as a reference surface and clamped to it. I clamped mine upright then after glue set... trimmed the bottom to square. I made mine with a taller blade side face, so I can make raised panels if needed. I used 3/4 mdf. I also use zero clearance inserts on my saw.

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post #7 of 20 Old 08-07-2020, 10:00 AM
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MDF appears flat and consistent but it's not. I use MDF for a spoilboard on my CNC router and even though it appears flat I still have to surface it to be really flat. I've also mic'd it a few times and found it to be as much as 0.005" from one area to another on the same sheet.

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post #8 of 20 Old 08-07-2020, 10:10 AM
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David, may I ask ...how did you type 0.005".....The plus minus in your example?

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post #9 of 20 Old 08-07-2020, 10:26 AM
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Not sure if if is how David did it, but if you use a word processor, you can get the symbol and then copy and paste it into your reply.

In Word, you go to the Insert Menu and then to the far right is a sub-menu call Symbols, click on it gives a couple of options, one of which is symbols, the is listed there. Clicking on it will insert the into your document. This might be different in other versions of Word or other word processors.

I've lost a couple of replies while typing in the reply box so I generally type my reply in Word and then cut and paste the entire reply to post.
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post #10 of 20 Old 08-07-2020, 10:37 AM Thread Starter
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Good ideas all around.
- I would clamp to my existing fence, but I want this fence to be quite a bit longer, and the stock Dewalt fence is... pretty flat.
- I know just the place to get scraps of countertop. I'm on it.
- I'm thinking a metal stud might be another option to clamp to.
- FWIW, I now know my own granite countertops are not flat. Even the 'flat' 1.5" thick MDF planer sled I made last year is not flat anymore. And I'm not talking about +/- 0.005. That sucker is headed for the scrap bin. The table on my saw definitely has a low spot on the left backside. I'm pretty sure the 'flat edges' on my squares are not perfect. One thing I've learned through this experience is that almost nothing is flat and to not ever assume it is. I'm definitely going to stick to wood, I think metal working would drive me nuts.

My inner scientist is screaming to build multiple fences, one w/ Jimmeim's suggestion of not clamping to anything, one with a granite tabletop scrap, one with whatever else I can come up with.
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post #11 of 20 Old 08-07-2020, 11:40 AM
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While on the subject of auxiliary table saw fences, have you considered the 'L' fence? There is an article in Fine Woodworking Magazine and probably some info on the net.
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post #12 of 20 Old 08-07-2020, 11:41 AM
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Quote:
Originally Posted by gmercer_48083 View Post
David, may I ask ...how did you type 0.005".....The plus minus in your example?
Hold the Alt key and type 0177, when you release the Alt key you'll see . If you want the degree symbol do the same with Alt 0176 to get .

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post #13 of 20 Old 08-07-2020, 02:54 PM Thread Starter
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Quote:
Originally Posted by JIMMIEM View Post
While on the subject of auxiliary table saw fences, have you considered the 'L' fence? There is an article in Fine Woodworking Magazine and probably some info on the net.
That is a pretty cool idea. It's simple enough to make another auxiliary fence. Dude cut a perfect trapezoid in 2 minutes. I already see myself adding trapezoids for no particular reason other than using that jig. Or you could clamp an regular height auxiliary fence a bit higher I suppose. https://www.finewoodworking.com/2014...blesaw-l-fence

There's nothing wrong, it just doesn't work.
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post #14 of 20 Old 08-07-2020, 02:54 PM
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post #15 of 20 Old 08-07-2020, 02:56 PM
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for ensuring something is dead flat, I use braided fishing line stretched super tight.
spacer blocks at the ends, third space block to run along the string and check for dips/high spots.
shim as needed.
check along the length, width and diagonals....


probably not going to get you within 2 thou, but usually good enough for assembly.


strings do straight quite well - and the framing square + Pythagorean theory will get you a 90 right angle check right quick.
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post #16 of 20 Old 08-07-2020, 06:45 PM
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Set up a couple sawhorses with 2x4 stretchers (on end).

Set up 2 diagonal strings, one string raised the think ness if the string higher than the other, so that where they overlap they just barely touch.

With levels and using shims between 2x4 and saw horse. when the strings just touch, screw then 2x4 s noto saw horse.

You have a perfect flat reference surface to lay a sheet of MDF on.

Robert
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post #17 of 20 Old 08-07-2020, 09:54 PM
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Much to my wife's dismay... I often use our granite kitchen counter top when I need a near perfectly flat reference. We have one continuous section that is one just under 10 feet long.
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post #18 of 20 Old 08-13-2020, 07:01 PM
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The King's Fine Woodworking channel on Youtube has an interesting technique to make a flat surface for their extreme torsion box assembly table build. They joint one edge of a bunch of 2x4's, then rip the other edge to match, then set up sawhouses and use a level and shim them, then put the 2x4's across between the two horses to get a large flat reference to build the table on. It's a trick to keep in mind when you're starting with no flat surface. And if you watch the rest of that series you won't question the extreme in the title. The technique I mention is in the second video of the series: https://youtu.be/-Y_0l3ap2ls?t=137
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post #19 of 20 Old 08-13-2020, 08:33 PM
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Quote:
Originally Posted by gmercer_48083 View Post
David, may I ask ...how did you type 0.005".....The plus minus in your example?
Just hold down the alt key and type 241. You can google "alt codes" and find hundreds of codes. I found a free download from www.usefulshortcuts.com .

Ron
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post #20 of 20 Old 08-14-2020, 11:06 AM
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In windows look at the character map in the accessories folder (win 7) you'll find everything there for all your fonts. Cut and paste.

Don't call me, "The Butcher"
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