Sawmill Log-Dog Hydraulics

      Sawmiller working on a self-built bandmill gets advice on hydraulics for his log dogs. November 25, 2008

Question
We are about to finish our homemade bandmill. We have finished the hydraulic head raise, hydraulic blade engage and brake, and the carriage movement. The problem is the hydraulic log dogs. The mill is 35 feet long, so we want 3 dogs. Is there a way to control the 3 clamping cylinders with only one control lever? The third dog will not be used all the time, being at the end of the mill. Do I just let them all close, and let the last cylinder close on nothing for pressure to build up on the other two? Or is there a valve that lets a cylinder build up to a certain pressure before feeding the next cylinder, and so on? Hydraulics are not my strength, so any help would be appreciated.

Forum Responses
(Sawing and Drying Forum)
From contributor M:
I attached a short length of chain to my log dogs and welded a hook to the bed. When I don't want to engage a dog, I simply hook the chain to prevent it closing on the log or empty space. My cylinders are all hooked in series with no issues. The one closest to the valve closes first. Speed is gained by using small diameter cylinders and you don't need much force to clamp the log. I use 1.5" x 24" cylinders and have a 7 gpm pump and still run my waste conveyer and dogs at the same time. My final tip is make sure your dogs and cylinders are open underneath, otherwise you will pack full of sawdust and find yourself redesigning (after you spend the day with a screwdriver picking oil coated sawdust out from around the damaged cylinder).



From contributor T:
I am not a hydraulic genius, but another idea would be to put a shut-off valve between the second and third dogs.


From contributor B:
You could use hydraulic connecters like on-air hose to disconnect the dog you aren't using.


From contributor S:
Actually, the simplest approach is to do nothing. The log dogs move at different rates and therefore they never reach the logs at the same time. Pressure will build up as all three dogs reach a solid object to press against. It doesn't mater where the dog is, it only matters that you continue to move the dog until good clamping is obtained.

Are you setting up two types of dog? One for the log's rounded surface (about 4-5 inches above the table), and the other to clamp a rectangular part - these need to be much lower, under 1 inch high (assuming your saw cuts down to 1 inch).



From the original questioner:
The saw has a 30" x 30" throat and the lowest it will go is 1". We are going to make the dogs out of two cylinders each, one for clamping and one that moves with the clamp for log height purposes. I think that is how Wood-Mizer does it. We are taking ideas from different saws to build our saw. Our saw bed is 35' long, so how many dogs would be required? Where is the best place for them to be placed? If someone out there has experience as a sawyer, maybe they can tell me their likes and dislikes of the dogs on the machines they ran.


From contributor S:
My saw has a 24' log capacity which means it has about a 30' bed. The log handling machines are in the first 16' of the working area of the bed.

It is set up this way:
Log loader 3 arms 8' apart.
Two log turners with integrated cant dogs, just past the first and second log loader.
Round log dog just in front of the second log turner.
Tilt past the second log turner.
The last 8-10 feet is just the table.
I also have an 18' long variable height fence on the off cut side.

The idea is to be able to handle short logs - as short as 5' with the first set of parts, 10' with both sets of parts - and the long logs have the wider loader. The table has variable spacing to do the same thing. Big stuff is plenty heavy, so it doesn't need much to hold it (but getting it set up the way you want it is the great contribution of the hydraulics).

The log dog is very handy. I can use it for all kinds of stuff. Using the Wood-Mizer's dual dog should be similar. (I looked at their advertising video - they use them to rotate the log, very clever.) I just don't see how they keep from bending the vertical cylinder's rod.



From the original questioner:
We have a permanent 3/4" high fence welded on down the back of the bed, and 4-12" hydraulic tilt up fences on the back for logs (3 in the first 16' and one at around 24'). We don't have a log loader set up. We have a 420 crawler with log grapple to load the logs. The saw isn't portable, so we are setting everything up to work around the saw. It has 28" balanced cast iron wheels from an old resaw, and is powered by a 10 hp 3 phase motor (I hope that's enough.) The hydraulics are powered by a 5 hp motor.

Would I need any dogs at the end? Like you said, any log that big will be heavy enough, and will be held by two dogs already. How much pressure are the clamping cylinders set at? I don't want to crush the lumber, just hold it.

Can you explain your log turner some more? We were going to do the chain turner, but if there is something out there easier and cheaper to fabricate, I'm all ears. Thanks! I'm receiving a lot of useful information.



From contributor S:
Okay, my fence is a piece of 3" angle about 18' long. It is supported by three or four (I am not sure which) arms that pivot together to set the height from 1/2" to 18". I work the log against the fence to rotate it and also cut into it, since the blade is moving towards the fence, the fence set the 90 degree angle. The fence really works well to control the log while the turners are working, but my turners work better in one rotation direction than the other (I would prefer this not to be true). I wouldn't want my fence any shorter. Large logs need a high fence to control rotating and leveling.

Most folks use skids to load to a permanent mill. It is better for the mill to role the log on. 10 hp at 3600 rpm, or 1750 rpm?

The dogs or clamps are set by you. You can watch them work to control how hard they push. I think mine use a 2" cylinder and run on full system pressure.

Wood-Mizer is the one using the log dogs to turn the logs. I think you can watch a video of it working online. I did receive a DVD from them. It sounds like the description of your log dog; it can move a rod up and down 0-18" and at the same time in and out. By using these two motions you can push the log up and over at the same time. This assembly uses a small foot on top that had a flat on the log side to be used as a clamp.

My machine has a chain type log turner. It has a single bar much like a chainsaw. It can rotate the chain up, move the arm in and out. When the chain turner is rotated up, the log dogs (three, one behind the other 4" OC mounted on the chain turner frame) are rotated down so they do not contact the log. When the turner is dropped below the table, the dogs pop back up and the in/out motion will set the dogs. The in/out on both turners work together. The raising/rotation on each is independent; the chain turn motors run together.



From contributor W:
You should look into getting a bigger motor. I'd say 30 hp at least. Five or six years ago I bought a brand new 30 hp Electrim for about $700. I don't know how much they cost now. For the log dogs, just put a "T" in the line and run the hoses to the two cylinders. Put an extra valve for controlling the third dog. What size (width) blades are you going to run on this mill?


From the original questioner:
We will be running 1 1/4 blades, mostly carbide. We mostly cut reclaimed Douglas fir bridge timbers, so lots of grit and steel (if for some reason we miss it with the metal detector). Before we took the resaw out of commission for the wheels, we cut 1500 bf of 8/4 fir with 1 blade (even cut through eight 3/8 spikes) and the blade still cuts decent at the end. They cost more ($250) but last forever and grit and steel does not affect them much. They have a thinner kerf (there is no set), so I don't know how they will cut green lumber.

The 10 hp motor is 1760 rpm. We have it pulleyed to run at 5500 fpm. The old resaw only had a 5 hp on it and it went through 14" oak at a good clip. Any input would be interesting. We have never cut green on the old resaw, so we might get a big surprise when we do on the sawmill.



From contributor S:
Sounds like you will be working DF cant-like logs. You should be able to get at least 300 bf with a HSS blade, if not more like 500 bf since you are working cants. The old DF should be hard, but you wouldn't have much pitch to cut through. I have cut some DF that was more than five years old. I thought it cut okay, just harder.

By the way, used 3 phase motors are inexpensive. They pack a lot of power for their size compared to single phase. You could also implement a vfd (variable frequency drive) and then you could run the saw at a higher frequency (or lower). It would simplify your drive line.



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