# Milling lumber how flat is flat enough



## ajthomas5009 (Dec 21, 2013)

Earlier this week I completed a planer sled for milling lumber. For the past couple days I was practicing with it and today I felt it was time to finally mill some of my own hardwood. I started out with some 3/4 maple and found it's not too difficult to get one face completely flat but on the second face I usually have a .015 gap in a couple spots. Is this flat enough? Is it practical to think one can get a board completely flat on both faces?


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## RogerM (Oct 31, 2011)

.015" is flat enough for almost any project


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## TheWoodenOyster (Feb 6, 2013)

First, we have to define what you mean by "one face completely flat". If that means completely and utterly coplanar for its entire length, then you should be getting a better result than .015" error. I would say that a jointer is really the only true way to get a board dead on completely flat, as in coplanar, for its entire surface. If you are using your sled to flatten the reference surface, I highly doubt that reference side is "completely flat". I say this speculatively, and by no means intend to insult your sled, I actually have wanted to build one for a while. Personally I don't think any planer sled will ever measure up to a jointer in making faces of boards completely flat and coplanar. Somewhere or another on that sled there will be inconsistencies. Once those inconsistencies are planed off of the reference side, they are going to be transferred to the other side when you send the board through with the "completely flat" reference side down. That is my guess on what is going on.

OK, I'll step off of the soapbox now. In many projects, that .015 doesn't really matter much. This question is at the core of my milling process every time I take on a new project: "Do I need to joint faces of boards for this project, or will planing the face do?". In other words, "Is pretty flat good enough, or do I need coplanar flat?" Sort of dorky, but I really do consider that on almost all of my projects because it affects the performance and abilities of the pieces you end up with, in particular their ability to be crosscut to a perfect 90 degree angle.

So, to answer your two stated questions, I'll just leave it at this:

Is this flat enough? 90% of the time, yes. It all depends on how the piece of wood will be used

Is it practical to think one can get a board completely flat on both faces? With a good jointer and planer and practice, yes it is. With a sled in place of a jointer, I personally don't think it is likely, especially if the board is relativley large.

Hope this helps


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## timbertailor (Jul 2, 2014)

Just depends on what you are trying to do.

That is plenty flat for most things, as Roger pointed out.

Boards usually have far more warp in them than that to begin with.


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## ajthomas5009 (Dec 21, 2013)

My current projects are the rails and fence for a nice crosscut sled. Both out of the same 6" maple boards. I planned 4 3/4×42" x 6" boards and glued them up to make (2) 6/4×42" x 6" boards for the fences. I plan on planning the waste for the rails. My other project is an outdoor cedar table similar to the one Marc Spag did a lil while back.


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## ajthomas5009 (Dec 21, 2013)

That is very true, I never though about the unavoidable inconsistencies thAt a planer sled will have vs a jointer. But for what it matters my sled is as flat as one can make it. It consists of 3/16" hardboard. 3/4" mdf, 3/4" bbirch, 3/4" mdf followed by 3/16" hardboard all glued and screwed with counter sunk GRK washer head screws. Unchecked it with an18" machinist straight edge every way possible and was hard pressed to find many inconsistencies.


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## TheWoodenOyster (Feb 6, 2013)

Yeah, it is hard to get a sled to match up with a machined jointer bed. Sound like your sled is way more accurate than I could ever make, but making a perfect one is a serious challenge, if not impossible. Again, I was by no means trying to discount anyone's planer sleds, I was just saying it is dang near impossible to replace a jointer with one.

One other thing that usually factors in (though I don't know if this is the case in your particular situation) is the fact that planer sleds are usually used for large pieces of wood. Large pieces of wood are just so much harder to get flat than small and medium pieces. In my opinion, once you get up to about a 5 to 6 foot board, it is really hard to get it perfectly flat, on a sled or even on a jointer. Pieces just get so heavy and unwieldy at that point and it gets really difficult. I can usually manage to get pretty good results with 6 foot plus boards, but I am much more comfortable and accurate when my boards are in the 1.5 foot to 5 foot range.


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