Tolerance for bed rails on sawmill?

[Jim Timber]

As a machinist, I'm used to "dead nuts" precision. I've dealt with many projects where the window between success and abject failure (scrap) is a fraction of a human hair. The equipment I use needs to be precise to get those jobs done - if my vise is off .001", the parts made in it will not work. Wood is a whole different animal.

So while I'm doing the mental gymnastics of designing my sawmill, I've come to realize I'm not sure what my tolerances should be. Guys use ladders, dimensional lumber, and off the shelf angle iron for saw tracks - none of those are overly precise. I know precision ground ways aren't needed (not even close), but I'm not sure at what point things go from acceptable to needing attention? Is it 1/16" from a given plane (plus or minus), or 1/4" from one end to the other? What's sufficient for a bandsaw mill to produce useable boards and beams?

 

[rwthom279]

You sound about as picky as I am!

If precision is the goal, I'd shoot for less than 1/16". If you have a 1/16"+ hump on a side, then turn 180 deg. and do the same....well you're looking at a 1/8"-3/16" swelled area on your cant/board. Or as you're sawing thru-n-thru, the change in the track will be present in each piece that you saw. Chances are you'll be able to fine tune the frame once setup and correct any variances that are not acceptable with whatever support system you use for the frame. The choice of materials will determine the flexibility to adjust the frame post construction, as you well know. Its all in what tolerances YOU want. What one person says is "close 'nuff" may not be anywhere near to the tolerances of the final product you find acceptable.

Keep in mind....It's still classified as ROUGH CUT lumber.

 

[Jim Timber]

Yeah, I know it's still rough cut. The thing that humors me in all of wood working is that even if you got it perfectly flat today, when the temp or humidity changes tomorrow it'll be off again.

1/16th sounds like a good upper limit. I hadn't thought about tolerance stacking, but I should've.

 

[BobL]

Most milling and stacking is not absolute but relative i.e. a suface is cut and the the mill can generally be made to cut, and stacking made, parallel to that surface within a 1/16" or better.
Even a chainsaw mill can cut 4 ft widths thicknesses to within 1/25" (thats about 1mm for us metrics) - above 4ft the bar sag starts to become an issue.
The thing to minimise for mill rails is twist. This can be done by checking the rails are parallel to each other by eye to as good as I can set them with a digital angle finder (+/-0.1º).
Across a 4ft wide cut this is +/- 0.08" but they all stay like that due to internal tension and drying probs.
For the length wise dimension care taken in stacking and drying the timber is often far more important that how true the length the were made.

 

[Jim Timber]

I'm looking forward to getting this beast underway. I haven't been this excited about manual labor since I finished building my current shop. I think sawing my own wood is going to be tremendously satisfying.

Thanks for the insights.

 

[BobL]

I'm looking forward to getting this beast underway. I haven't been this excited about manual labor since I finished building my current shop. I think sawing my own wood is going to be tremendously satisfying. Thanks for the insights.

It is. I started out cutting up a few 1-2 ft long pieces of firewood to use for the small woodworking tool making projects and before I knew it I had 9 chainsaws, 4 mills and over 100 logs sawn up. I rebuilt my shop over the last 3 years because Ialso like doing the metal work and have been a bit distracted this year attending a couple of blacksmithing courses and building a town gas powered forge.

 

[Dave Boyt]

Across a 4ft wide cut this is +/- 0.08" but they all stay like that due to internal tension and drying probs.
For the length wise dimension care taken in stacking and drying the timber is often far more important that how true the length the were made.

That's one of the challenges of working with wood. There are a number of places where close tolerance are important, but the wood is going to shrink, sag, twist, cup, and move in all sorts of ways. On the other hand, the set of the blade teeth, angle of the blade, and having the blade parallel to the crossbunks are all critical to smooth cutting.

 

[luckydozenfarm]

I rough out lumber as best I can with the Alaskan, then after drying it's off to the jointer or planer, where I get it down to precision. I'm with you on the precision with metal, I run an automotive machine/mechanic shop and you can imagine my frustration when I first realized a 2x4 wasn't actually 2"x4". I find the longer you can allow to dry and keep from warping, the better it will "hold" its final shape.

 

[Jim Timber]

I'm ok with wood's movement - part of my educational journey has been learning the various ways of dealing with working with green wood and how to compensate for it's shrinking over time.

I want to do a timber frame entry room to my new house: two stories with an exposed beam ceiling for the 3rd floor above it and loft on the sides with stairs going up the two flights. I'd like to cut my own wood for the rest of the house too, but I doubt that'd be cost effective, so I'm just planning on the flooring and trim outside the entry. I'll build a solar kiln as one of my first projects.

 

[rwthom279]

I want to do a timber frame entry room to my new house: two stories with an exposed beam ceiling for the 3rd floor above it and loft on the sides with stairs going up the two flights. I'd like to cut my own wood for the rest of the house too, but I doubt that'd be cost effective, so I'm just planning on the flooring and trim outside the entry. I'll build a solar kiln as one of my first projects.

WOW, I can only imagine what that could look like. Sounds like a rewarding endeavor... yep, you're gonna need a mill

 

[VA-Sawyer]

To the OP.... The acceptable cutting tolerance is mostly dictated by the use of the wood. If I am cutting fence boards, or rough outdoor siding, I have no problem with 1/8" to 3/16" variation in thickness. If the wood is headed for a light duty planer than can only handle 1/32" per pass, then I try to keep my cuts within 1/16" thickness variation over the length of the board. If the mill itself is built to maintain a cut within 1/32" , then it should be able to maintain 1/16" with a good blade in the real world. I don't see any advantage of building a mill to a tighter tolerance. Like others have said, the mill is just a support system for the blade.

 

[poleframer]

Design adjustability into it. I decided on inverted angle iron- A up- on my box tube rails. I only welded it an inch every 4 foot or so, that way it both sheds sawdust, and if it needs adjustment I can cut the welds with a zip disk and shim or move them and reweld.

 

[BlueRider]

Being a patternmaker I'm probably one of the few people around here that has done what some have called precision woodworking, but as accurate as patternmaking can be it is a far cry from the precision of machinist standards. We measured everything with dial calipers rather than micromiters and while we would work to a few thousandths of an inch, tenths just don't exist in a patternshop.

So the way I transition from the accuracy I'm used to in the shop to the accuracy appropriate for milling is to work to the tolerance of "by eye". I don't bring any straight edges or levels or squares when I mill, though if you are milling beams a framing square would be appropriate. I've used a lot of different methods of making the first cut and have mostly settled on using a 2"x 12". I screw it to the top, and then by eye I use wedges and screws to get the board flat. A practiced eye can see to within about 1/32" for straightness and twist.

Years ago I made an 8' mahogany straight edge that I store by hanging in my shop to keep it straight. It is straight by eye not by machinist standards. I use that and a level for setting up concrete blocks and dunnage for the foundation of where I sticker my milled lumber. I spend way more attention to the accuracy here then in milling but it is still straight by eye and level by a framing level not a laser or machinist level.

 

[Mandres]

In my experience it really doesn't matter very much. The slabs are going to cup, crack, warp and twist while they're drying so there's no such thing as precision until then. Just eyeball it, and plan to spend some time with a jointer/planer afterwards


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[Franny K]

After typing I see this is an old thread. Maybe it is complete by now.

I am thinking of a rectangular tube with a piece of flat stock somewhat larger on top and then a round element or the inverted small angle on top of that for the rollers to ride on. The flat stock can be used to keep the carriage from lifting. The rectangular tube and the flat stock should be straight enough. The problem will be keeping the sides from being skew. Skew=not in the same plane. A problem without some sort of jig to me would seem that all the welds for the cross pieces are on the inside. And probably more toward the top. That will make the main elements want to distort. It may be hard to be sure to know how much deviation until it is all welded up. I guess you can always put some weld on the other side to attempt to bring it back. If the intent is a single sided like some Woodmeizer or a bolt together things will be different. You might want to make your log stops adjustable. I guess if you only have two they will always be in line but when you get more adjusting will be easier than metal work.


I would think the log bunks should be as precise as possible to the rails. If you want to quarter saw off the bottom then turn end for end and rotate having otherwise probably will end up with taper.

Which way are you thinking of setting it up. A, blade pulls sawdust toward stops, log dog pushes in direction of blade travel, blade leaves movable roller before entering log, B, blade pulls sawdust away from stops, log dog pushes against direction of blade, blade comes off fixed roller before entering log. B has less blade entering bark if quarter turns are used. If electric I guess you could go both ways.

 

[Jim Timber]

The mill got put on hold again. Haven't given it much thought in a while, and won't for a while longer.