Cutting MDF panel problem

Cutting some MDF panels. They are 18x16.5, 3/4 MDF.
After I cut, I checked the diagonals. NOT even.
I checked the blade to the miter slot - dead on. I checked the blade square to the table - dead on. I checked the fence to the miter slot - dead on. Fence was locked down tight.
Each of the measurements were done using a dial gauge or a Wixey angle measurement.
The only think I can think of is that the fence is flexing. It's a Jet contractor saw with the original (not an Xacta) fence. Aluminum box type frame to the fence.
Any thoughts? I cut some spare 1/2 plywood and came out dead on to the diagonal.
I am thinking of just re-cutting from a fresh sheet and see what happens.
MJ
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Sawdust or other debris between the board and the fence or miter? Art
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There's a good chance that the sheet was out of square before you started.
I would rip 1st (1/2" oversize, 1st cut, then trim to size) and use a sled for crosscutting.
--Woodson

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wrote:

How much off?
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1/8 of inch. Building a box that needs to be square.
MJ
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wrote:

When you make your cut, does your kerf stay the same width or does it either widen or narrow? I have run into MDF that had some 'reaction' problems with internal stresses. Hasn't happened often, but I have had kerfs close up and widen. Uneven absorption of humidity will do that as will temperature either in your shop or on the factory floor.
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snipped-for-privacy@gmail.com wrote:

Are the two edges *parallel*? If so, your sheet wadn't square to begin with and you have no cutting problem.
--

dadiOH
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A perfectly set up fence will cut perfect parallelograms all day, all you really need is one perfect 90 corner.
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says...

If you need it square, you really need to make up a sled big enough to hold the size sheet that you're working with, or at least the part. Or make up a panel saw and make sure that _it_ is square.
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------------------------------- Check your technique.
Are you keeping your eye on the fence/material joint or the blade?
If blade, you'll have problems.
BTDT
Lew
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RE: Subject
Properly built, this sled insures square panels.
http://preview.tinyurl.com/2ct6a6w
Lew
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On 9/30/2010 7:08 PM, snipped-for-privacy@gmail.com wrote:

Remember, your fence only insures a cut parallel to the edge against it. If the perpendicular edge to that cut is not 90 degrees, your diagonal will be exactly as you describe.
Try a fresh sheet that you've checked for square on at least one corner before making your first cut, the use only those two edges for your cuts.
--
www.e-woodshop.net
Last update: 4/15/2010
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My suggestions;
1) Make sure that the saw blade is equal to the task. Is it sharp. Is it straight, etc? 2) Make sure that the area is clean. That there is nothing between the stock and fence. 3) If the fence is flexing, clamp something behind it to make sure it does not move. I had a cheap contractors saw that I had to do this with. Something small and thin, no problem. Anything 3/4" or more, I had to clamp the fence so it would not flex. 4) Make sure you have a straight edge to begin with. Then make all cuts using the straight edges as a reference. 5) Check for square after the first cut. Don't cut everything, then check. I learned that one early. You will save money on materials. 6) How secure is the stock when you are cutting? Any slippage or??? One problem with cutting panels on a table saw is that by being safe, sometimes you do not get the stock up against the fence all the way through. That is where some good rubber based grippers can come in handy. 7) And last, but not least, you can break down and make up a sled or two.
Good luck. Let us know what you figure out and your solution.
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The things you checked will ensure parallel, not square. What are you doing to square the board? Did you measure the width of the board? Is it consistent? Explaining your procedure step by step will make it a lot easier to give advice.
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If you are assuming that a previous 90 degree corner was square, it probably wasn't.
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