Straight Line Rip Saw Accuracy

A discussion of ripsaw cutting tolerances (most variation is caused by natural variation in the wood). June 28, 2006

Question
If I was to take a 12" board and rip 3/4" strips out of it with a straight line rip saw, how accurate would all the strips come out if there was no tension in the board? Can I expect less than .010 inch deviation from 3/4"... or would it be more? I haven't been able to try a saw yet, so for now I can only go by word.

Forum Responses
(Solid Wood Machining Forum)
From contributor C:
Depending on the species, you should probably expect more like .06" on some pieces. .01" deflection is almost imperceptible and hard to achieve consistently. Are you talking about straightness or consistency of thickness? I thought you were asking about straightness.



From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
It would be rare to see a saw blade have that little variation, due to straightness of the blade and bearing runout. Also, the teeth themselves have thickness and alignment variation, so even if the piece is the correct thickness (do you measure thickness using the high spots only, which is what happens with a caliper?), you will find lots of small variations in surface smoothness. If you need wood this close in tolerance, you had best use a planer, or a jointer-planer combo.

Specifically, the standard deviation of thickness on an operating SLR would seldom be under 0.020, including variation from the holding system, saw blade wobble, bearing runout, etc.

What also confuses the issue is that some wood cells are compressed when they are cut and they will spring back as soon as they have exposure to moisture. Further, with a piece that is 3/4" thick, a 1% MC change will cause a 0.002" size change. A 1% MC change will occur with a 5% RH change. Now, if the wood and the air are not in perfect equilibrium when you start sawing (especially considering the MC inside the piece) you can easily get a 1% MC change as the wood adjusts to the air. Note that normal kiln drying easily has a vspread of final MC of 2% or more.

In summary, close size tolerances with wood are extremely difficult to establish and maintain.



From the original questioner:
Maybe I should rephrase my question. Will a SLR saw hold a better tolerance or the same as a cabinet saw? Right now we are ripping everything to 3/4" and this is good enough for what we need, then it goes straight to glue-up. Our product gets a 1/16 trim once complete, so there can be some variation. Pieces just can't be wider than 3/4". Narrower from blade deflection can be okay as long as the board is straight enough for a glue joint. A glue joint from end to end is also important.


From Professor Gene Wengert:
Thanks for the clarification. It does change things a bit. It is my impression that a SLR is not necessarily made to cut against a fence, but will cut a straight line at an angle or at a direction which can be not parallel to the other edge. So, your best tolerance for a continuous, constant width will come from a system that holds the piece being cut against the fence the best. I think either saw system has that potential. The best system for very close width control from a saw would be from a gang saw - 2 saws or more.


From contributor J:
Did you read the SLR post from a couple weeks ago? All the experienced SLR operators that responded were very accurate with their information. We have been using our Oliver SLR for 2 weeks now and from what I see it is more accurate and consistent than a table saw for ripping. We have not tried anything shorter than 24 yet.


From the original questioner:
Good to hear, and yes, I have read every post regarding a SLR... Just making sure I didn't make a wrong decision. I bought a general 50-600 SLR. It looks similar to the Grizzly G0524. It should be here within a few weeks since it is a single phase model and a special order.


From contributor B:
There is no question that the SLR will exceed the accuracy of a hand fed table saw. I use my SLR to cut to width against a fence with good results. However, if the board is a little stressed, each piece can bow, resulting in width differences. I don't have the best faith in boards as short as 12" as my fence is short. A 24" board, no problem. My guess is that coming from a bad rip situation, you will be ecstatic with your new saw.