Coping with uneven table saw table and inconsistent cut depth

Question

What is the best way to cope with a table saw that has an uneven table?

I noticed inconsistent cut depths with my table saw so did some test cuts on a piece of oak known to be straight and measured the depths. Cuts were like this:

I made the cuts in a random order to rule out the possibility of the blade height drifting. I also noticed no play in the blade position or angle with my hands.

Results were like this (leading and trailing edge depths measured about 0.25" from respective edge, note x axis is mirrored wrt above picture, "throat plate far edge" is where the edge of the wood crosses the far edge of the throat plate):

The blade has angled teeth, and I measured to the center of the cut. I made the full set of measurements 3 times and found my results only varied by at most 0.0005", so I believe the relative measurements to be accurate.

The first thing I discovered was that I did a poor initial job adjusting my throat plate height (as evidenced; it was too low), so I fixed that. However, I closely inspected the table with a square (a friend lent me a Starrett) and found that it is not completely flat. There are large, smooth, ever so slight bends and dips in it, and the bit to the right of the throat plate is a few hundredths of an inch higher than the left side. I am presuming this is the cause of my inconsistent cut depths. I also can feel with my fingers that the miter slide's rail is slightly higher than the table although I'm not sure how this affects things.

I noticed an identical trend with a piece of MDF and a piece of poplar as well (this is actually what prompted me to do this experiment) although I did not make a full measurement set on those.

What is the best way to cope with this (or are my expectations too high; the saw is a Ridgid R4513^†)? I have not yet built any sleds. It seems like a sled with a nice flat base will solve the issue for cross cuts. I did read this great answer, however, I am still unsure what to do for non-through rip cuts, where the material is longer and I want to use the fence.

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^{† After reading grfrazee's answer, I contacted Ridgid and they claim a ±0.016" manufacturing tolerance; further inspection of the table appears to verify it is within their tolerance. So it seems my expectations may be too high for this particular saw.}

Answer 1

What is the best way to cope with this? Or are my expectations too high?

It's hard to tell from your picture, but your issue could be one of a couple of things.

It seems like your throat plate is sticking proud of the table saw surface by some amount. You can easily check this using a known straight edge. Shim/de-shim the throat plate as necessary to make it flush with the saw bed.

If that's not the issue, then I hate to say it, but it seems like you got a lemon of a table saw. Assuming you're setting everything up correctly (i.e., no user error producing poor results), a 0.040" variation over 12" in a table saw bed is pretty horrible. I think most higher-end table saw manufacturers cast the beds and then grind to a flatness of +/- 0.005" to +/- 0.010". I can't tell who made your saw, but if it's a lower-end model, this could be the culprit.

Having a look at this related Lumberjocks forum thread, you might be able to shim the table saw bed to get it closer to flat. How you do this will depend on the model, so I really can't give you any good guidance on how to do so.

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On a different note, let's not forget that we're working wood here. Wood will experience seasonal movements in excess of your 0.040" discrepancy very easily. This might just be a case of "good enough."