Problem with JET PB-85 metal bandsaw

[crazybrit]

I have a Jet PB-85 bandsaw. Initially I wasn't cutting anything that required precision but now that I'm cutting larger square stock for welding it's a pain.

Issues like this after often because of the belt, it's alignment or it's tension but I don't think so in this case. I contacted JET technical support and the only suggestion they had was to replace the bushing (red arrow in parts diagram) but there is absolutely no slop in this part of the assembly.

The issue as you can see from pictures is that the blade arm (saw not running) doesn't track perpendicular to the base so by the time the blade is at the bottom of the 1-2-3 block, it's 2mm further out than at the top.

Using a 90 square the vice is almost but not quite square to the base (very slightly off at bottom of 1-2-3 block) so the vice isn't the issue.

I'm assuming the arm assembly (#119 on the parts diagram) is just defective. It's $170 for a new one.

Curious if anyone has any suggestions on fixing this, else I'd rather invest $170 into a better saw. The problem is I have very little space so I'd want something similarly portable

 

[NC Rick]

If I’m thinking correctly, the issue has to be in the hinge right? Have you taken the pin out to look at it and the bushings? That is off by a lot! If I understand correctly, as the travel comes down, the blade is touching the block but by the time your at the bottom of the block you are off by 2mm (that’s 2 mm in 50mm of blade travel). Unless I misunderstood, the bushings would have to be so fried that you would surely feel the slop. Something bent of bored incorrectly?

 

[crazybrit]

Yes, you're correct. 2mm in 50mm of travel. It's way off.

I think the issue is with the "pivot arm" #119 circled in green in the attached image.

The entire blade assembly attaches to the pivot arm (through the bushing, red arrow).

The pivot arm attaches to the base at a single point (blue arrow). It rotates around the axis of the blue arrow so you can set the angle.

If you grab the blade assembly and move it with your hands you can cause the pivot arm to twist slightly against the base at the blue arrow. I've tightened the #107 bolt down as a test (so it's now hard to adjust the angle) and it doesn't make the issue go away.

All I can think of is that either the bottom of the pivot arm isn't correctly parallel to the base, or the 90 degree transition formed into the pivot arm is a bit off, or the bushing is indeed bad, or that the design is just bad and the weight of the blade assembly just causes the pivot arm to twist off the base.

I'll remove the bushing #118 and post some pics of it and the mounting pin.

 

[OccupantRJ]

I think the base of the pivot mount is not parallel to the pivot pin. Shim underneath as a test, and if this is the case, have the pivot mount base machined parallel to the pin, the amount of the shim.

 

[NC Rick]

It’s a miter saw... my 4x6 is all cast together, vise through to the hinge in one part. You have many things that can be wrong. You have to keep checking backwards for the error. If you set it at 45 deg. Is it the same. I like the shim idea but I would want to know why the problem exists.

Is the base stamped steel? I suspect the base as the problem.

 

[whateg01]

It's junk! Send it to me to dispose of it!

I've been wanting to build a swivel head saw since the new ones are so expensive. That much error should show up with a good square set next to the base of #119. You should be able to sight along the edge of the square and it should line up with the edge of the horizontal pivot up top. If that's good, check that the bed of the vise is parallel to the stamped base of the saw next to the swivel. Set a good straightedge on the vise and put a 123 block next to the swivel, or as well as you can. I've always been advised to avoid these saws with stamped bases since they are "flimsy". They look decently heavy to me, but I'm sure they could be damaged.

Last, figure out whether it's easier to shim the vise or the swivel. Might be able to get by with a simple shim under the vise vs. disassembling the swivel to make it work. If the side and bottom of the swivel are square, don't machine anything off of it or it will be out when swiveled to the side.

Dave

 

[crazybrit]

I removed the pivot arm.

The base is fairly flat. The underside of the pivot mount is not.

In the pics below the blue arrow is where the pivot mount bolt goes into the base. The green arrow is where the locking lever goes (locks the pivot mount 0-45 setting).

I need to do some more measurements to verify that securing the pivot mount in the milling vice as pictured is getting me correct measurements. If it is, then the far side (by the machinists square) is considerably lower (like 2mm lower) than the near side (by the end mill).

 

[NC Rick]

That’s cool crazybrit, it will be easy to fix but difficult to measure. Can you use a feeler gauge or shim stock on the low spot to verify your suspicion before making the commitment to material removal?

 

[crazybrit]

That’s cool crazybrit, it will be easy to fix but difficult to measure. Can you use a feeler gauge or shim stock on the low spot to verify your suspicion befor commitment to material removal?

Yes, I'm going to proceed carefully. Measure three times, cut once
I did install a shim before I took it all apart. About 2mm. Where the blue line is in the attached image. It cured most of the issue but it also raised the pivot arm off the base at the green arrow which tells me the issue is that the opposing side to the shim is too high. This was confirmed by moving the end mill around.

My gut is that there is less material to remove on this opposing side than there would be material to add (by laying down a tig bead) on the parts that would need building up but I need to check very carefully before I make any mods.

 

[OccupantRJ]

Clamp a square bar in the vise and measure down to the base mounting points of the pivot arm to see if the vise floor and the pivot mounts are coplanar. Some of the issue may be in that region of the base. You may can even mill the base mounting points of both the vise and pivot flat in one shot, then move on to the pivot arm.

 

[PCustoms]

I apluad your in depth measuring....

But my ancient Dayton is off in a similar way. A couple pics of shim stock behind the blade guides and adjusting the fixed side of the vise took care of it.

When I install a new blad I sometimes have to adjust a little. For welded joints, it's more then close enough. 45's are dead on with almost no gap.

 

[American Locomotive]

So something to keep in mind, is that the base is just thin stamped steel. It's going to bend and tweak out of alignment again eventually. It's very likely you'll have to make more adjustments again after some use. It may make more sense just to shim it, instead of machining it.

 

[whateg01]

As long as your vise is square to the bed (jaw face is vertical), then your front to back measurements should be correct. If you can run a coaxial indicator up and down the pivot hole, you might gain some confidence in your setup, but the only two things that matter really are the face against the jaw of the vise and the side facing upward. Those should be perpendicular. Other faces may or not be square or flat since it is a cast part.

Did you say that you checked the flatness of the top of the base?

Dave

 

[crazybrit]

So something to keep in mind, is that the base is just thin stamped steel.

It's actually aluminum,

Clamp a square bar in the vise and measure down to the base mounting points of the pivot arm to see if the vise floor and the pivot mounts are coplanar. Some of the issue may be in that region of the base.

I will check this but I think most of the issue is the base of the pivot arm, but yes it's definitely worth checking before modifying anything.

If you can run a coaxial indicator up and down the pivot hole, you might gain some confidence in your setup

Agreed.

My thinking is. Use the pivot hole as a reference. Attach a dial gauge to the mill chuck and level the pivot arm such that I get a consistent reading measured all around the pivot hole. At this point, consider it level. Then figure out which is less, removing some material or using the tig torch to build up some material. I don't think the actual height of the pivot arm is that critical so removing material is an option.

I'll have to construct some kind of jig to secure the far end of the pivot arm to the table as I don't think the vice alone will hold it securely enough for milling operations.

 

[whateg01]

...My thinking is. Use the pivot hole as a reference. Attach a dial gauge to the mill chuck and level the pivot arm such that I get a consistent reading measured all around the pivot hole. At this point, consider it level. Then figure out which is less, removing some material or using the tig torch to build up some material. I don't think the actual height of the pivot arm is that critical so removing material is an option.

I'll have to construct some kind of jig to secure the far end of the pivot arm to the table as I don't think the vice alone will hold it securely enough for milling operations.

I agree that the actual height isn't critical, and removing material will be the fastest way to get there. Even if you add material, you'll still have to go back and face it again. Might take a couple of clamping setups to do but shouldn't be that difficult. Light cuts. Some 123 blocks, a jack, and a strap clamp positioned right ought to work. I've had some pretty crazy, scary-looking, setups but as long as the work is solid, it'll be fine.

 

[crazybrit]

I agree that the actual height isn't critical, and removing material will be the fastest way to get there. Even if you add material, you'll still have to go back and face it again. Might take a couple of clamping setups to do but shouldn't be that difficult. Light cuts. Some 123 blocks, a jack, and a strap clamp positioned right ought to work. I've had some pretty crazy, scary-looking, setups but as long as the work is solid, it'll be fine.

Thanks, I appreciate your comments, confirming I'm not out in the weeds