Thread: 6X4 Bandsaw acquisition.

Hi Guys,

Earlier in the week I managed to find an old car rear shock absorber. A "Monroe" branded unit. Complete with a stamped into the casing warning, about not attempting to open or disassemble. Naw, don't take notice of that

Anyway based on various comments and suggestions from you Guys, I decided that this was going to become a hydraulic down feed damper for my 6X4 bandsaw. I put a flap disc on the small angle grinder and gave it a good fettling. I removed all the rust and other crap that it was encrusted with. It was at this point that I found the warning stamped into the case. Mmm, should I stop here... No ! what's the worse that can happen.

I could hear the oil inside splashing when I shook it, I pulled the piston rod right out to make sure that it was clear. So playing safe I wrapped it in several layers of cloth, and drilled a 2 mm hole a couple of inches from the bottom through the cloth. All that got me was a little hiss, after that zilch, nothing bad happened, and I'm still here in one piece.

Pushing the piston rod back down caused a dirty thin oil to start squirting out of the hole that I had drilled, pulling the piston rod back up caused more oil to come out. I pushed the piston rod back down, more oil. When that had stopped I used a hacksaw to remove the top two inches, cutting carefully round the case, being careful not to damage anything inside.

Removing the top got me more oil ! It also got the internal cylinder and piston free. Pulling on the piston rod got me some more oil. In total about 200 mil of it.

Unfortunately I didn't take any pictures of me pulling it apart. However I did take pictures of the other parts that I obtained.

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This picture is of the internal parts of the shock absorber. A family shot and pictures of the piston and top guide for the rod. Notice the small vent hole in the top guide. Oil squirts out of there if you pull on the rod whilst holding it upside down and squirts out of the bottom when you push the rod in. You can't win with this, the oil spreads all over very rapidly.

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A close up of the piston, notice the vents round the outside edge. For all intents it is the same either way up.
A bit of thought and I don't have to set to and make a suitable piston in order to use this for my damper. Assembling it dry, that is oily but not wiped clean. The piston moves smoothly up and down without any sign of sticking, bearing in mind that the rod doesn't have the guide on it.

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These are all the washers and shims that I removed when I took the piston off the rod. Note the very interesting spring. That was fitted to a non return valve located in the bottom of the cylinder. Basically it allows oil to be sucked in to the cylinder from the reservoir of oil in the outer cylinder. Thus keeping the inner cylinder and piston full of oil. Those shaped shims are actually flow control valves and they are placed on either side of the piston.

So only needing a bypass valve and tube, I think that I have all the components to build my damper.

So at this point I'm looking for suggestions and advice.
Thanks Guys.

Hi Baron,

I'm watching with interest to see how you do this. I have opened up a few shock absorbers, I just nick them with a hacksaw until I just gets through the case.

The chrome rods are very accurately ground and if cut up into short pieces can be used for spacers for measuring or setting up etc. to measure dovetails etc.

Simon

Hi BaronJ
Here is the link to my version of a simple down feed control for my band saw, it may help you with a few ideas.Post 943 . I found that you need a large quick return valve in the piston so that you can lift the blade up easily after the cut, I put two fair sized holes in my piston. You will need to find or build a fine needle valve to control the down feed, I added a stop cock as well so that you can lock the blade up without having to fully lift it.
Bob
https://metalworkforums.com/f65/t121...+modifications

Hi Simon

Originally Posted by simonl

Hi Baron,

I'm watching with interest to see how you do this. I have opened up a few shock absorbers, I just nick them with a hacksaw until I just gets through the case.

The chrome rods are very accurately ground and if cut up into short pieces can be used for spacers for measuring or setting up etc. to measure dovetails etc.

Simon

Thank you for your notes ! I've never taken a shock absorber apart before It was a very educational experience. Of course I had read the generalised information about how they worked without any real world experience of them.

I actually found it quite difficult to get hold of one, garages and service centres simply rejected my request for a scrapper out of hand, and vehicle dismantlers wanted silly money for a second hand one. It was actually a neighbour clearing out his garage that gave me that one.

I'll remember about the rods, they do look very nice.

Hi Bob,

Originally Posted by krisfarm

Hi BaronJ
Here is the link to my version of a simple down feed control for my band saw, it may help you with a few ideas.Post 943 . I found that you need a large quick return valve in the piston so that you can lift the blade up easily after the cut, I put two fair sized holes in my piston. You will need to find or build a fine needle valve to control the down feed, I added a stop cock as well so that you can lock the blade up without having to fully lift it.
Bob
https://metalworkforums.com/f65/t121...+modifications

Thank you very much for that very interesting link to your bandsaw down feed damper.

One of the things that I discovered was the great multitude of very thin shims and washers on either side of the piston. laying them out as I took the piston off the rod, I began to understand how the shock absorber actually functioned. That pile of washers is basically a two way flow control valve and the variable rate behaviour due to the shaped cutouts in those very thin washers acting as reed valves. The spring and its plunger only allowing the cylinder to remain full of oil and the excess ejected through the small hole at the top.

It struck me that all I need is that spring and one or more of the washers to behave as an automatic one way valve on the bottom of the existing piston. I did note that the piston is a very close fit in the tube and does not have any "O" rings, just very sharp edged discs on both sides. The core is some kind of moulded material, but definitely not a plastic.

I'm not entirely sure how I'm going to construct the device at the moment. I do confess to looking at yours and others to borrow ideas.

Hi BaronJ
I think the valve arrangement in shock absorbers is way more complicated than what you are after and I feel you will have a lot of trouble getting it to work as you want it to, they are far more restrictive in controlling the flow and are designed for a much higher working pressure. The valve arrangement on my piston is simply two holes through the piston with a thin disc of polycarbonate covering them held in place buy the two round head allen screws. One of the pictures shows this setup but it is hard to see the poycarbonate disc.
Bob

John I agree with Bob in regards to the piston and the need for a needle valve, I also like Bob's idea of the non return valve on the piston.
I put my valve in the base opposite to the needle valve and there was a bit of work involved but it has worked well.
My piston was made out of 1020 and I put one o ring at either end of it (so 2 o rings) and so far it has had about 10 years of use and still works well, I put SAE 30 in it.
I gave it a bronze bush for the rod and used the seal out of the shocker.
The other thing you might want to consider is the side you situate the damper, I went against the norm and put mine on the left side of the saw, my reasoning from memory was to put it on the left side it was less likely to interfere when cutting 45's, it was further away from the cuttings and coolant and I could mount it further away from the saw head pivot. Being further away from the pivot also meant less pressure in the cylinder and also less force on the pivot as the leverage is much less, so hopefully less wear.
I rarely adjust my needle valve as it is pretty much set and forget and cuts thin to thick without touching it, the only time I might have adjusted it is on a very hot summers day when the oil has been a bit thinner and the blade down speed is a bit to quick for thin stuff.

cheers, shed

Hi Bob, Shed, Guys,

Thanks for your comments, they are much appreciated.

I've laid out the shims as I took them apart,
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This is approximately the original assembly. I've missed out all the plain shims that were between the various others.

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This is the layout that I'm having thoughts about. Particularly after reading about the use of a polycarbonate disc. Those two shaped steel discs are exactly 2.5 thou thick and the piston has quite large apertures in it. I wonder just how much pressure it would take to cause those steel discs to lift and allow oil to flow past them, certainly they would not allow oil to flow the other way, or would they ?

Including the spring in the assembly would make it harder for the oil underneath to get past, forcing it through the needle valve in the bypass.

There is also a full washer without any apertures in it that is also 2.5 thou thick that could be used on its own.

Hi Guys,

I've done a drawing for a damper tube clamp.

Drop Damper 1.pngDrop Damper 1a.png
This is how I propose to both secure the damper cylinder and create an access point for the bypass tube.
Two steel blocks are clamped together to form a ring that will both hold the cylinder and allow a hole to be drilled for the bypass tube without anything protruding into the cylinder itself.

Anyone see any problems with doing this ?

Hi Guys,

A few posts back I said

I've also found a source for the 1" square section tube that I want to use for the stand. Surprisingly enough from the place that sold me the bandsaw. They have also said that if I let them have a copy of the drawing they will cut all the pieces for me. So that is next weeks task.

They got the material in and had cut the pieces according to the drawing. I hope that I didn't make any mistakes. Well I've now got all the pieces of 1" square 2.5 mm wall steel tube, thanks to the very kind people at the engineering company.

All I need now is to weld it up.

I haven't done any welding for over 50 years so this would be a whole new ball game for me. As luck would have it, a chap who works for another of the various firms that I've done work for in the past was visiting and the foreman introduced me to Peter, explaining who I was and what I was doing. Peter has invited me to go to their place and they will help me re-learn to arc weld and let me use their equipment.

It was never like this 40 years ago ! I had to wear protective clothing ! Boots, I never wore boots or leather aprons in my day. Then a protective eye shield plus the self darkening headgear. Leather gloves to protect my hands. They felt very clumsy and not at all comfortable. All this had to be done inside a tinted plastic curtained enclosure. To prevent people outside from the arc flash. Health and safety was never like this when I was taught to weld.

Anyway I've had the instruction and done the practice and managed to produce what I've been told are quite decent welds. I've had a go at MIG as well as TIG, I’ve never done any of that, only stick and gas welding. TIG is a very interesting method, a bit like gas without the flame.

Most of the easy bits of the frame are now together. Getting the angles done was quite clever. I would never have tacked and blocked up pieces like I was shown. The guy instructing me was brilliant. I found out that he had been an instructor in another firm that went bust and now is a manager and partner in this one.

I'm back there tomorrow and hope to finish the stand ready for painting. I'm quite looking forward to it.

Hi Kryn,

You are going to have to wait for pictures ! I'm not allowed to take any photographs inside any of the buildings or on any part of the premises. I can't even tell you what work they carry out. Part of being allowed into the works was an NDA. Its been at least 15 - 16 years since I was last in there and there was only gatehouse security and visitors passes then. I can say that an awful lot has changed since then.

Sorry about that...

Not a problem John, sounds like it's all national security stuff now.

Hi Guys,

Well I finally got the bandsaw frame finished and brought home ready to paint it. I had considered getting it powder coated but the wife said that I might as well use the rest of the tin of paint and it would all be the same colour. She is right, of course, no point in wasting about a third of a tin of expensive metal paint.

Kryn here are the pictures you wanted

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Well here it is nearly four days of rather enjoyable learning. Would I do it again ! I sure would.

All the pieces were very accurately cut both to length and angle, which did surprise me a little. The feet were laser cut from 3 mm plate. I was tempted to say that I filed the screw slots, But I didn't, my filing isn't that good. Originally I was going to put "L" shaped pieces on the top and drill through the body of the casting then thread the holes M8. But the instructor suggested that since absolute accuracy wasn't needed, offcuts of 1" inch square tube turned through 90 degrees would allow for any deviation in screw hole position. As it happens he was dead right. The holes in the casting are about 2 mm offset from each other between left and right sides. In addition it gives me a few mm to play with sitting the base casting square on the frame.

Now to get on with painting it and starting re-assembly.

Has any one mentioned.
Having a strut that is just fixed at the top or bottom i suppose
https://www.camloc.com/products/vari%E2%80%90lift/
something like this.
As im typing this i feel i should read the whole feed. .

If im right the object is to control the speed of down motion of the saw but be able to lift it quickly after the cut.

emm

aaron

Hi Aaron,

I think that they are only adjustable once ! The gas is let out to set a particular force. You would have to find a way to re-gass them to get a higher force.

When I've got the bandsaw up and working I have a piece of steel to make a start on building a drop damper for it. I've also got the brass elbows and "T"s for the oil bypass system.

Watch this space...