Tolerance

[hopm]

I have a small manual mill and I checked the blade and bed this morning and found I'm off 1/16" from side to side and am off a bit less than an 1/8" on the bed. I know the goal is dead perfect but.....
I have made every effort to get to perfect and Im not sure it's possible. Any suggestion? How much variance can you stand on rough lumber?

Thank you in advance for any and all assistance!!

 

[rarefish383]

Don't have a band mill, but as tolerance goes, does it hamper the operation of the mill, or put undue stress on it? If it just means a board is slightly out, for finish work you'll have to run it through the planer a couple extra times. For rough building lumber , who cares. I'm sure one of the band mill guys will be along, and if there is a way to get it dead nuts, they will help you do it, Joe.

 

[BobL]

I'm not quite sure what you mean by "OFF" and what reference you are using.
How are you doing these measurements
Also how wide is the cut?

I should preface my remarks by saying I a newbie when it comes to BSMing.
While it makes sense to have the bed level it doesn't matter if it is not perfectly level as that is usually the reference surface.
The band being off side to side relative to the bed also doesn't prevent very parallel boards being cut provided you don't flip the logs/cants.
This should be especially the case for narrow boards.
If you are not getting parallel cuts then that will be due to some other problem.
When the log/cant is flipped you should see double any difference across the band and this is the best way to check what is going on.

FWIW the band saw I operate has a 35" wide cut.
I use a digital angle finder (DAF) to check levels and start by placing the DAF across the bed I set the DAF to zero
Then I place the DAF onto the band and this tells me how far out the band is relative to the bed.
Then I set the band to zero.
It's very easy to leave the DAF on the band (it has magnets set into its case) and slowly tweak the band levelling bolt until the DAF reads zero
Each measurement using the DAF has a tolerance of 0.05º so combining two DAF measurements gives a tolerance of 0.1º
Over a 35"wide cut that works out to a max of 1/16".

But like I said above that only matters if the logs/cants are flipped
When I measure the thickness of wide parallel cuts (and provided everything else is working right) the difference in thickness is always less than 1/16"
When the cant/log is flipped on wide cuts I do see differences usually around the 1/16" but sometimes as much as 1/8"
However, it only takes a chip between the log and the bed to cause this sort of problem.

 

[Mike Van]

Sometimes you can spend hours getting everything exact, then turn a 2500 lb log a few times & find it's off again. In our world, "close enough" has to be a useful term for rough sawn lumber. Big commercial mills have strict tolerances, but look at how well built their equipment is, and the $$$$$$ it costs.

 

[hopm]

Side to side on blade is 1/16" in a 19" cut.......check some 2x stuff I cut earlier this week and found it is indeed an 1/8" thicker on one end which is what my bed is off.....but if I'm making parallel cuts would the difference on the bed or blade cause the discrepancy on the flip. I know this is really simplistic but just to see if I have a handle on it.....I wouldn't see a difference if I didn't turn the log?

 

[BobL]

Side to side on blade is 1/16" in a 19" cut

That's 0.2º - this should easily be detected with a Digital angle finder.

but if I'm making parallel cuts would the difference on the bed or blade cause the discrepancy on the flip.

It could be either. To fix it you could adjust the bed to the blade or the blade to the bed, it doesn't matter which one you do.
It does assume the bed is parallel to itself all the way along which it may not be (see below_

I know this is really simplistic but just to see if I have a handle on it.....I wouldn't see a difference if I didn't turn the log?

Sorry I didn't make it clear in my previous post.
Once again I assume the bed is parallel to itself all the way along, and if so, you should get parallel cuts.
In the picture below, the bed is not parallel to the ground and the blade is not parallel to either.
Except for the first and last cuts, the cuts should be parallel.


However you won't get parallel or square cuts when the log is flipped. To do this the blade and the bed must be parallel all the way along the bed.

The way I do mine is I park the mill on one end of the mill rails lay a long piece of rectangular hollow Ally across the bed and set my DAF to zero.
(The bed sits on 6" of concrete so the chances of it moving are small and only one end has moved sufficient to be measured in two years.)
Then I level the blade to the same section of the bed using the DAF.
Then I leave the DAF on the blade and push the mill down the rails to check it's parallel all the way along the mill bed and adjust only the bed bolts that need it.

 

[hopm]

Thank you for your detailed reply. I think I understand and with that understanding I see I've a bigger problem than I thought. My log bed is angle and tubing welded. This bed assembly is welded to a chiller frame. There is no adjustment. I'm going to have to study a bit to see the direction I want to go. I have discovered the low point on the bed is my 1st support which is also where my dogs are......not quite sure what I can do to resolve this easily.
Again, thank you for your response!!!!!

 

[BobL]

Thank you for your detailed reply. I think I understand and with that understanding I see I've a bigger problem than I thought. My log bed is angle and tubing welded. This bed assembly is welded to a chiller frame. There is no adjustment. I'm going to have to study a bit to see the direction I want to go. I have discovered the low point on the bed is my 1st support which is also where my dogs are......not quite sure what I can do to resolve this easily.
Again, thank you for your response!!!!!

Even if the bed is welded perfectly flat if it is not well supported it may still move under the weight of the mill and log so some adjustment capability helps address this. My bed is made of angle acting as rails welded to a combination of angle and SHS cross beams. It is supported at 10 points by 5/8" bolts which can be adjusted up/down.