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I purchased a router table mounting plate to go inside a custom made table. The insert includes some crown in the middle of the plate. The height of the crown is roughly 0.030".

At fist I thought this was a mistake, but after reading the back of the package it says this is by design and that it insures the work is at the highest point. The consensus here http://lumberjocks.com/topics/39338 seems to be that the reason for the crown is so that when the router is mounted on the plate, it sits flat.

The primary use of the table would be for joinery. I am a novice woodworker, but it seems that the crown imparted would do more harm than good in this scenario. I have 3/8" aluminum plate on hand and am considering using that to build the insert.

Will having the crown on the pre-made plate be better for my purposes than the flat custom plate?

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  • You didn't say what the material of the plate is. There are certainly flat plates available from other suppliers (Rockler, for example), so it isn't at all clear that the crowning is a good idea.
    – keshlam
    Commented Sep 2, 2014 at 23:15
  • It is the Bosch Ra1250 plate made of phenolic.
    – Brian
    Commented Sep 2, 2014 at 23:43
  • I thought the crown was to keep the piece flush tight against the table where you need it most - right AT the bit. That way the rest of your table doesn't have to be precisely flat and you can be assured that your routing will work well. That said, I'd suspect that the best thing is to have a flat table and flat insert. He you tried cutting some joinery with it?
    – aaron
    Commented Sep 3, 2014 at 1:41
  • @aaron I have not because you need to drill holes for your router - I would test it out first if I could. I guess I can always return it and buy it again if the homeade job doesn't do the trick.
    – Brian
    Commented Sep 3, 2014 at 2:03

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I know the theoretical answer to this because I read it here, in reference to a different router table top: http://www.leevalley.com/en/Wood/page.aspx?p=41793&cat=1,43053,43885 It's twofold, as suggested earlier in the comments:

  • it helps the table to sit flat, or at least not concave, with the weight of the router.
  • it ensures that the workpiece contacts the table right next to the bit, even if it is not quite touching out at the edges.

I also have seen this in practice, with my own makeshift router table top, made of too-thin plywood:

  • If the table sags, the accuracy goes horribly awry, because the depth of cut changes according to the workpiece's position on the table, as well as with the amount of sag (which tends to fluctuate with weight).
  • If the table is bowed up slightly, the error is not appreciable. Technique helps here too--you want to apply downward pressure on the workpiece near the bit, not near the edge of the table.

The key here is that the curvature here is very slight. The amount of tilt the workpiece might get with a convex table doesn't generally add up to appreciable error, but the depth variation with an equivalently sagging table is unacceptable for most joinery.

My experience suggests that the convex table is a good solution. Of course the other answer is to get something perfectly flat and sturdy, but I would be careful about that, because a lot of materials (like MDF) will sag over time.

EDIT: For a convex inset plate, install it so that it is flush with the rest of the table at the edges, and you should be in the same situation as if the whole table top was crowned.

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  • I think you're answering a different question than the one asked. Crowning the table slightly might make sense, if it isn't supported adequately from underneath. Crowning the drop-in mounting plate...? With an adequately stiff plate, I'm really not convinced that's needed. The possible deflection within the space of the plate is minimal, especially given that the router base mounted under the plate will help brace it against flexing. This still sounds to me like a matter of a manufacturer trying to reclassify a bug as a feature. But I may be wrong.
    – keshlam
    Commented Sep 6, 2014 at 7:39
  • That is an interesting distinction to draw, but I think it amounts to the same thing. It's still a hedge against sag, it still puts the bearing surface next to the bit. Where you could go wrong would be insetting it so the peak of the plate is level with the rest of the table, then you would be much worse off. But if you inset it so the edges are flush with the rest of the table surface, I think the trade-off is exactly the same as with an entire table that is domed. I will edit my response to point this out.
    – skiggety
    Commented Sep 6, 2014 at 11:56

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