bandsaw downfeed force

How much force should I be able to apply to a horizontal bandsaw blade?

In reworking my bandsaw I got curious about how much pressure I should be able to supply to the blade so is there a good rule of thumb for this? The blade is 1", 8-12 pitch and the main materials are 4140 or 6061, everything from thin wall tubes to 3" solid rounds. the hydraulic pump will develop ~650psi and it has a 4" bore cylinder with a 1" rod so in theory I could develop about 7500lbf at the cylinder, but with the ratios involved would likely be about 1/5 of that at the cut. The blade arm casting itself probably applies 250lbf to the blade just by it's weight.

Any ideas?

Usually, it's gravity. The "downfeed" cylinder is simply to retard the force, not enhance it.

Try pushing 1" steel plate (3tpi or more teeth) through a vertical bandsaw. It's doable, my guess about 75-100 lbs.

Gravity should suffice, or perhaps air assisting the typical bandsaw "downfeed" cylinder.

SteveBausch said:

Usually, it's gravity. The "downfeed" cylinder is simply to retard the force, not enhance it.

Try pushing 1" steel plate (3tpi or more teeth) through a vertical bandsaw. It's doable, my guess about 75-100 lbs.

Gravity should suffice, or perhaps air assisting the typical bandsaw "downfeed" cylinder.

Click to expand...

It definitely helps to have the down force and the saw was tubed up for that from the factory. The root of my question is where to set the relief valve for that circuit of the hydraulics.

Start as low as possible, create a chart as you cut various materials.

Are you sure 8-12 tpi for 3" round??

If your cutting thin walled sections and you don't want to strip teeth, its more about controlling feed rates than it is force, blades don't need much to cut, its only when you get into large solids - wide flats that you start to need some force, even then its not much normally under the weight of the arm.

Chances are the plumbing was more for a auto blade return at end of cut though.

You need to think of downforce in terms of number of teeth in the cut. Forces have an equal and opposite reaction force. In this case, that reaction force is delivered through the teeth of the blade. If there is to much force per tooth, the tooth digs in and either stalls the blade or the tooth is broken from the blade. Even when starting on a large piece of square, there is only one tooth in the cut for a brief period.

How much down force is necessary? Not much. I have cut 10" rounds with my saw which has a manual lift head, so probably no more than 40# at the lifting end.
I agree with setting the relief valve as low as possible.

While other saws may be different. The manual for my Startrite H175 calls for 6 psi. As mentioned the cylinder doesn't power the feed, but rather retards it. I can't imagine teeth lasting very long on the blade with more than about 10 psi of down force.

On many bandsaws you have a reliefvalve with a spring on top of the saw in the back guide housing
You can adjust that spring for blade pressure and set the downspeed with a separate valve
You do not set the hydraulic pressure

Peter

I'm guessing that it probably tells you in the manual. My Wells does. You want no more force than necessary to make the teeth take a cut. Excessive force will lead to premature blade failure.
JR

SteveBausch said:

Start as low as possible, create a chart as you cut various materials.

Are you sure 8-12 tpi for 3" round??

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Yes, I very rarely cut heavy bars so I just let the thinwall blade grin and bear it.

adama said:

If your cutting thin walled sections and you don't want to strip teeth, its more about controlling feed rates than it is force, blades don't need much to cut, its only when you get into large solids - wide flats that you start to need some force, even then its not much normally under the weight of the arm.

Chances are the plumbing was more for a auto blade return at end of cut though.

Click to expand...

Good point.

projectnut said:

While other saws may be different. The manual for my Startrite H175 calls for 6 psi. As mentioned the cylinder doesn't power the feed, but rather retards it. I can't imagine teeth lasting very long on the blade with more than about 10 psi of down force.

Click to expand...

I'll keep that in mind.

JRIowa said:

I'm guessing that it probably tells you in the manual. My Wells does. You want no more force than necessary to make the teeth take a cut. Excessive force will lead to premature blade failure.
JR

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While I am fortunate enough to have the original drawings and manual from 1971, it's in German. I looked through and didn't find any units of pressure.

The saw is a Rusch HBSA2. Currently I have a dedicated relief valve for the downfeed circuit and needle valves to control the blade speed in each direction. I think I'll make some test cuts with the relief valve backed all the way off and see what happens. When I tubed it up I put a tee anywhere I thought I'd want a gauge, now I can perform a fair level diagnostic to see what it does and does not like.

I have a Wells No 12 twin post horizontal band saw. It has a needle valve that's used to adjust the feed rate down like most large horizontal band saws. The needle valve regulates the oil flow out of the hydraulic cylinder that raises the saw. It also has this:



It has a roller that runs on top of the blade connected to a microswitch. If the down feed rate is too high it tends to push up on the blade. The microswitch clicks and shuts a valve in the oil line so the saw can't proceed down. After the cutting catches up the blade goes down just a bit and the solenoid opens up again allowing the saw to proceed. In practice if the down pressure is too high you can hear the solenoid going click-clack as it keeps shutting off to allow the saw to catch up. Adjust the needle valve closed a bit and it stops doing that. One of the simple but great features that I like about this saw. I might add this is a very old saw, seems they forgot about this feature on newer saws.

I hope you will post pics of your saw. That German band saw that you have is very scarce in this country and I suspect a really nice saw.

Alan

I agree with the "no more force than necessary" except if you happen to be cutting stainless (or other work hardening materials). In that case a heavy feed is necessary to keep the blade cutting, letting each tooth penetrate enough to get under the work hardened layer.

Bob