Best practice for getting exact thickness wood needed

[Paul K. Johnson]

The parts I make are within 0.002 in height and width. I have the tools to make that happen and have been doing it for years.

The problem I've had is that when I order wood the tolerances are all over the place. For example, I ordered 1/4" wood several years ago and what I got was from 3/16" to 5/16". At that time I had no way to accurately thickness wood. I needed it to an actual 1/4".

So this time I ordered wood that was supposed to be 5/16" and I would finish the thicknessing myself. Again I got a lot of wood that's well under 1/4" thick. At least a quarter of the wood is too thin.

From here on out I'm going to be buying thick slabs, resawing it, planing it (Dewalt thickness planer) and finally sending it through my thickness sander.

I've never done all this before so this is the first plan I came up with but my real plan is to do some research before I start anything. This is just what makes sense to me.

Saw off a slab on my band saw. Send the slice and the rest of the slab through the thickness planer to true them up on the cut face. Repeat until the entire slab is cut into planks.

Next saw the planks into workable sizes and send them through my thickness sander.

Carry on as usual from there - shaping, cutting into individual parts, drill, tap, etc.

I can't see any way to avoid putting the slab back through the planer after every board is cut from it or I'll have to take each board to the jointer to get one good face and then send it through the planer. I guess either way works.

Assuming that's all ok then my actual question is how much extra thickness do you give yourself when resawing to ensure you have a good board that's not too thin? Let's pretend I can resaw a relatively straight line - say worst case would be 1/32 deviation.

If the first face is good and I don't have tear-out or other defects then I just need to bring the second face to true and then thickness so I'm thinking 1/16" too thick to start would be good. In other words, if I want 1/4" thick finished planks then I'd mark my slab at 5/16" for resawing.

Does that sound good or am I missing something?

Thanks,

- Paul

[George Bokros]

You need to face joint the stock on a jointer first to get one face that is flat and true. If a piece has a banana shape it will have a banana shape when you are done surfacing it in the planer. You cannot rely on the face cut on the band saw to be true and flat.

[Paul K. Johnson]

Thanks George. So after that's done is the rest of my plan reasonable? Is there a better way?

[George Bokros]

Yes. Milling your own stock is the only way to have the exact thickness you desire. I know of no better way.

[Larry Frank]

I think you need a better place to buy wood. You did not say what species and amounts. I get very good wood from Ocooch Hardwood.

I buy kiln dried hardwoods from a small sawmill company and order my wood about 1/8" oversize. I then plane it to my dimensions. I get it close and then let it sit for a few days and then to exact size.

[Jim Barstow]

I do exactly what you describe. And yes, you need to run the newly cut each edge or face through the joiner, depending on whether you are cutting strips or resewing, after each cut. I also mill oversize and let the wood sit for a couple days before final dimensioning in order to make sure it is stable. (I've had newly milled pieces turn into pretzels overnight and thin pieces stay perfectly flat and stable. You never can tell.)

After milling close to the final thickness with a planer, I take a couple hundredths off with a drum sander using 150 grit. This removes any milling marks, minor snipe, and leaves the wood ready for finish sanding.

After taking a couple classes on metalworking, my woodworking standards have become much, much more exacting. I use a high quality digital caliper now and to me, 3/4" thick means that it needs to be within 0.005 of 0.750.

It really isn't much more work and it makes the downstream tasks much easier when I know that something really is the dimension it is supposed to be.

[Lee Schierer]

IMO trying to hold metal working tolerances in woodworking is going to be a frustrating experience. Wood dimensions change with with moisture changes so trying to stay within .002" over any length of time is going to prove difficult.

[Paul K. Johnson]

IMO trying to hold metal working tolerances in woodworking is going to be a frustrating experience. Wood dimensions change with with moisture changes so trying to stay within .002" over any length of time is going to prove difficult.

The parts are so small that any wood movement is negligible. I hold those tolerances so the parts fit in my drill jigs without so much play that the hole ends up off-center. Holding the tolerance isn't the issue though. It's ordering wood 1/16" over-size and getting wood that's too thin. I guess some sawmills have a different 1/4" than I do. Wood that thin shouldn't vary 1/8" in the same batch.

[Paul K. Johnson]

I think you need a better place to buy wood. You did not say what species and amounts. I get very good wood from Ocooch Hardwood.

I buy kiln dried hardwoods from a small sawmill company and order my wood about 1/8" oversize. I then plane it to my dimensions. I get it close and then let it sit for a few days and then to exact size.

I order months in advance. The wood I'm cutting up now was ordered in September or October.

The parts I make are 1/2", 7/16", 3/8" and 1/4" thick. I use good domestic hardwoods - usually cherry and maple. Every once in a while I use walnut. Whenever I buy wood I ask for a chunk of something I've never used to I can play with it.

The last time I ordered I asked for 5/16" and 7/16" to make the 1/4" and 3/8" parts. A whole lot of it was about 1/8" thinner than what I asked for.

[Paul K. Johnson]

These are the parts I'm making. The clamp heads are 3/8" square by 2+" long.


http://www.airfieldmodels.com/store/...aper/20704.jpg

http://www.airfieldmodels.com/store/...aper/20705.jpg

http://www.airfieldmodels.com/store/...aper/20694.jpg

http://www.airfieldmodels.com/store/...aper/20902.jpg

http://www.airfieldmodels.com/store/...aper/20902.jpg

The truss-work is about 3/32" cherry.

http://www.airfieldmodels.com/store/...aper/20695.jpg

[Bryan Lisowski]

If the wood you have been buying is not dried, I could see them drying out if you are not using for 6-8 months after receiving. If kiln dried, I would return the ones not to your specifications. Resawing would probably reduce the frustration, face joint before first cut and after each cut.

[Bill Space]

Hi,

Have you thought to discuss the issue with your supplier, and explain your needs?

Seems like getting something you specify should be easy, if you specify what you need specifically to begin with. OR the supplier should tell you that they cannot meet your requirements and that you need to go somewhere else.

Just ask if they can supply what you want, and if not perhaps they can refer you to someone who can (perhaps not if they should feel they are sending you to a competitor).

[James Zhu]

What you really need is a jointer. The airfield models you make are all small pieces, so 6" jointer is good enough for you.

Jointer is a must have tool for processing lumber. Get one face side of the stock flat, then one edge side square, then you can go to your thickness planer or thickness sander if the final thickness is too thin for the planer.

[Bill Dufour]

The box stores sell metric plywood and call it some inch dimension for thickness. I think it is all undersize for thickness. Why they do not tell you the metric thickness I do not know. Instead on the label in small type they claim some weird fraction like 49/52 inch thick. Maybe your supplier is selling metric thickness to you?
I found this out after I made all the rails and stiles for a workbench. I could not buy 1/4" plywood to fit the grooves. I had to buy it a fraction over and rabbit all edges. The 1/4 plywood was either too thin or too thick. I think they were one or two mm apart in thickness.
Similar problem at OSH were they try to sell a few ball bearing and label gives English decimal size instead of the true metric size. like 0.9448" instead of 24mm I am sure no one shopping there has an inch size precision bearing. Maybe a lawnmower wheel bearing could be English but that is about it.
Bil lD.

[Paul K. Johnson]

Bill, I was very specific with the man I talked to and he's the one who actually cuts the wood. I specifically told him what and why and that it could not be any thinner than specified but it could be thicker. This is the second time I've spoken to this company about that. The wood was good quality and I don't want to publicly bash them but even when specified he couldn't give me what I asked for. And I paid a lot for it. He told me he had to charge me per board foot even if I got fractional thickness.

In other words I paid for 1" thick wood but got 5/16" thick. I agreed to that so that's on me. But I was very clear about thickness requirements.

Bill - I'm talking about hardwoods, not plywood. The plywood comes from a completely different place. I don't recall the metric thickness but it's marketed to model airplane builders and we work in imperial so they round to the nearest multiple of 1/32" or 1/16" depending on the thickness.

I don't know if most model-builders are aware that the plywood isn't the actual thickness specified or not but I'm aware of it. For what I'm doing it doesn't matter. It's just a flat plate that other things bolt to and those other things are made of hardwood.