Horizontal Band Saw Blade Pressure Question

[Janderso]

I don't know the equations involved but with a hydraulic downfeed it's more a matter of blade downward travel per unit time rather than static pressure
because if the blade was not being driven eventually the full weight of the saw would bear on the blade and probably break it, so you are essentially clearing a path for the teeth to drop into, bit by bit...I suppose the effective dynamic pressure could be measured somehow-? Maybe a pressure gage on the table with some sort of hinged workpiece? Hmmm perhaps even measuring the temperature rise with an infrared gun?
Mark

All good points. As the blade comes down the geometry is changing and the values along with it. The spring seems like more like a lifting aid to me. The arm is almost neutral at about 70 degrees and falls with increased force obviously.
A challenge to say the least. New equipment works great, this old saw is a far cry from new.
This is a pic of the saw, just not my saw, pretty close though. It weighs in at 680 pounds. 6X12

 

[markba633csi]

Bandsaw designs can get really exotic- I've seen some where the blade descends like a guillotine on two feed columns with all manner
of electronic feedback and servo control- big bucks machines, probably fun to watch

 

[Alan H.]

Each saw's design and geometry is different. I would look for the specific requirements for your saw. Not sure what you saw is so I don't think anyone can give you a recommendation.

My specific experience is with Ellis and the manual tell you how to measure down force and set the pressure. That is independent of the cylinder by the way. There are some counterbalance springs that are adjusted. That has nothing to do with the cylinder.

The cylinder throttles the velocity downward.

 

[Cadillac]

That cylinder is easy to service. Looks like the rod seal is gonna be packing maybe? You might be able to tighten the nut which will compress the packing. The cylinder could be a piston type with a oring which is easy to find and replace. The piston cup seals will take some searching.
My valve wasn't just a typical needle valve. In my picture you can see how the valve has two i.d. Bore diameters. Closest to cylinder has a pin hole opens to bore of .125 then goes to a bore of .250 which then tee's to to tube that goes to top of cylinder and other side is valve handle. Where it goes to the .250 bore there is also a tapered seat. My valve shaft was broke at the end of the .250 taper so when you would open the vale for down feed it would allow to much oil to bypass and basically wouldn't cut straight. After adding the needle to the second pin hole taper it regulates the fine speed drop to where I can crack open the valve a touch and get very slow down feed or give a 1/4 turn and it drops with little restriction. I imagine as it should?
The valve you got should work it's gonna take trying it idk.
I always had a problem with the cut not being square. I don't mean angle between vise and blade. I mean vertically. Saw dropped straight to bed. Bushings were new, everything was gone through except the cylinder I kept putting it off for some reason? Rebuilt the cylinder and haven't had a problem yet cuts perfect. I cut thick,thin,square,round, stainless ,
Steel, aluminum, with not a problem.

 

[projectnut]

I am in the process of customizing a brass needle valve assembly for my hydraulic saw arm blade pressure adjustment.It's been a while but I remember the goal is to have the blade come down at the same speed as the descending cut. If I can fine tune the needle valve to go anywhere from full drop to stopping in mid stream I am providing a way to increase blade life, allow the saw blade to cut at it's own speed and maintain a straight even cutting path correct?
Now, any idea what I should be watching for with regard to blade pressure?
In other words, with a 3/4" blade, cutting a 2" hunk of mild steel square stock with a 10-12 TPI Blade, how much downward pressure?
I'll have to guess as close as I can but say 5 lbs? 1 lb?
Anything published on this?
Thanks

Obviously there are different methods of controlling down pressure depending on the design of the saw. I have a Startrite H175 saw that calls for a maximum of 6 psi measured with a spring scale at the blade tensioning control at the far outboard end of the swing arm. The pressure is set by moving the lower mounting bracket on an extension spring at the pivot end of the arm.

The 6 psi setting is with the hydraulic blade retarding cylinder set at full open. In other words the blade would free fall if not held up by the spring scale. The hydraulic cylinder only lessens the blade pressure on the work piece. Depending on the size and composition of the material being cut the hydraulic cylinder is used to slow the descent of the blade to minimize or eliminate jumping or bouncing while cutting. When cutting aluminum or other soft materials the cylinder restrictor valve could be fully open and the blade will proceed smoothly through the cut. When cutting harder materials like tool steel the blade descent has to be slowed so the down pressure is less than the maximum 6 psi to keep the blade from jumping or bouncing in the cut.

Here are a couple pictures of the saw. The knob on the far left is the blade tensioning knob. The 6 psi is measured at this point with a spring scale attached. The down pressure is adjusted by moving the spring mount on the lower right side. The retard cylinder can be seen on the backside between the blade guides


.

 

[Dabbler]

My saw has about 8 lbs down force, but I use the slowest possible downfeed setting because i'm not usually in a hurry. It isn't optimum for blade sharpness over time, but protects the blade from catching on thin wall tubing, angle or being too aggressive on aluminum and brass. Almost all of my blades over the years weren't dull before I lost teeth, and hence the blade. It was cheaper to cut slower and not strip teeth.

BTW My new bandsaw downfeed cylinder was rebuilt with rubber seals and transmission fluid. Of course in a few months the seals had dissolved! Now I have a rebuild, too, one day. -In the mean time I put on my spare down feed cylinder....

 

[markba633csi]

I was concerned about the seals too, I used light mineral oil:
Febi/Bilstein 6162

 

[Cadillac]

I used regular hydraulic oil should be compatible with about any seals. I’ve seen hydraulics Jack oil to it would work.

 

[Dabbler]

Cadillac, that was my point. Hydraulic oil, such as in a hydraulic jack and power steering fluids eat up regular rubber seals, and quickly. An old time mechanic once told me to be very careful about my lube, because there were 'a bunch' of seal types, some of which are incompatible with various oils.

Anyway, because of the last guy's lapse I have a 10 hour rebuild that was unnecessary.

 

[Cadillac]

You might be right with hydraulic jack oil. Ive been in the same field for 20yrs as a heavy equipment mechanic. I’ve resealed hundreds of cylinder and pumps and motors. Majority comes from foreign debris or mechanical failure. A lot of o’rings came out of my box to seal spools in hydraulic valve banks that never came back. Usually using a vg68 or32 oil so idk about “hydraulic oils” deteriorating seals? I do know some oils do harden and dry out seals. Don’t know what but I haven’t seen for myself. I will have dry hard seals on like a spool assembly. But it’s actuated constantly with a machine with like 6K hrs.