Cant get threaded rod straight.

[Fodder1]

Hi All.

Right my JGRO is all together, not as acurate with the Table saw as i hoped, but oh well. I had planed on getting the Jgro to cut its self out again anyway.

The major problem I am having at the moment is trying to get the all thread rod straight. If I dont have the bearing on the end it goes every where. I know there is whip but at the speed i am traversing it at should not be a problem (very slow).

I tried runing it backand forth through a block of wood, say 100mm thick with a hole drilled init and a "T" nut but i dont thinkit has the contact area to straighten the threaded rod.

And due to this movement I think its binding up the z axis. the other two work axis but you can see the rod is all over the shot. I think they work cos they have a lot more lenght to flex and move, if that makes sence.

But the z axis just binds. I have removed the anti backlash cos i thought that was the problem. but no joy.

I am running these screws via some HP steppers with the Hobby CNC 4 axis board.(great board highly recomend it) I was reading the hobby cnc forum it seems they would like to see 36v driving the board and steppers. For testing i have it running via a PC power supply at 12v.
I know there is a big differeance in performance when i upgrade the PS to 36v but could this be the problem?


So the questions?

how do I straighten the damm all-thread rod/lead screws?

Should I step up to the 36v PS now? that would give the steppers more grunt, but this sounds bad.

Should I change the drive plates to the "HPDE block" (threaded with a slot cut down it etc etc) I dont even know if I can make them.

See all you melbourne-ites at the BBQ!!!!

CHris

[wbegg]

Doesn't sound like an effective allthread, sounds like an alignment issue. The Z probably binds towards the top or bottom of travel, right? this would suggest that the rails are on one plane, and the screw is slightly off this plane.

The Z axis binds because the least "give" is at the ends of the screw.

[Fodder1]

Thanks for taking the time to think about this.


And i have to agree, but it binds in the middle and the z axis Pipe is parallel down to the +/- 0.5 mm (smallest i can messure).

I think the z axis bearing carriers are a little out of alingment, as the z axiz as a "bit" of movement when you try and twist it. Beause the Z axis (like you said) has the least amount of travel, i suppose this could be binding.

I will pull down the z axis and check the alignment of the bearing carriers against the center line. and then check pipes against the centerline of the Yaxis carrier. but as these are dead parallel it will only be off to one side of the other.

The only other thing i could be is that he bearing block in the z axis is not square. As i did this with hand tools that would not surprise me.

If this is the case i will use the baring block as the center line and run the carriers at 90 degrees to it.


CH

[Jason Marsha]

I have never managed to remove a bend in threaded rod. The best thing is on picking the threaded rod from the shelf in the store roll it on the ground (hopefully it fairly flat). If the threaded rod is straight it will roll uniformly. If it is not straight you will see a high spot every turn.

Jason

[Fodder1]

Hi,

Hmmmm, there is a way, but it is too hard for me to do.

There was a post here that went on about using a long bit of HPDE (50mm) and thread it. Search for "cheap lead screw".

Once he threaded it he cut a slot along the time to lossen it up and allow for mosture etc. He ran it back and forth though the block. he said it straightend out the rod. But 1. I can get the HPDE and 2. I dont have a long enough tap.

So, after thinking about it, I figure the next best thing is to make sure that the bearing carrier, Stepper & drive nut are in alignment.

Sounds simple. but with a bent lead screw.......



So I was thinking over a cup of coffee, and my thoughts wandered over to a laser level sitting in the garage like I was talking about in my last post. But as I was thinking it occured to me that I could get one of the tinny toy pointers and attach it to a small bit of all thread. This way I can screw it into the drive nut and measure off the beam and find the deflection from one side. take it out and screw it into the other side of the drive nut and measure off the beam on that side.

With a bit of work and two of the little pointers, and a short bit of all thread..... one virtual, dead straight lead screw to allign the pipes against, the motor and bearing carriers.

COOL that will work.

Gotta go and find two little laser pointers

/rambling

CH

[fyffe555]

Might be simpler to just put the screw under tension, that will significantly reduce whip and straighten the screw somewhat.

Over here we can get 'joiner' nuts for allthread. Regular nut just four or more times the diameter long. Intended to butt join sections of threaded rod. maybe try one or several of those?

In extreme cases put a bearing right next to the A/B nut to take the screw whip/binding load off the nut.

Does sound more of an alignment issue, the leadscrew would have to be visibly bent to bind hard. That is, alignment between screw and bearings is off, not bearing to bearing, assuming the axis moves freely without screw attached...

Andrew

[NC Cams]

Rolled thread rod will be hard to get and/or keep straight. Here's why:

All thread is used to hang stuff or fasten non critical devices that need clamping. Thus, it is usually rolled cold and the residual stress tends to self relieve over time. As this occurs, the stuff tends to bend/warp. Since the stuff is fairly low alloy material and NOT necessarily heat treated or stress relieved, it is pretty much NOT going to be stable.

You can TRY to have it stress relieved which MAY help stabilize it. Usually HANGINg it vertially and baking it at 250-300F for several hours MAY help. If it moves, this is where it will move to and tend to stay. HOwever, if it does move, it will go where it is molecularly "happy" which is usually NOT straight which is what you want/need.

Sadly, if you really want good straight high speed stuff, threaded rod (as in rolled thread hardware grade stuff) is NOT what you should bet your life on. HIWIN makes something that will suffice but it is at a far higher price and accuracy level than what you'd get from Fastenal - they call the stuff "ball screws".

[Fodder1]

fyffe555

Does sound more of an alignment issue, the leadscrew would have to be visibly bent to bind hard. That is, alignment between screw and bearings is off, not bearing to bearing, assuming the axis moves freely without screw attached...

That is what I think. As this Jgro was built by me, and i have ade a few mistakes allong the way. I beleive that it is an alignmetn issue for the most part.

I am unable to get the collar type bearings over where I am at a reasonable price $20aus each is all i have been able to find them for. So I am using a standard skate bearing in one of the PIPE adausting blocks.

Although i have not put one at the motor end. I am not sure of the advantage of doing this. I also looked at mounting some more bearings either side of the drive nuts, but with out some major redesign on those carriges and the parts i have that is not going to happen in the short term.

I am tracking down some of those little laser pointers ATM so i can align every thing. I think, like you said it is more an alignment issue.

Over here we can get 'joiner' nuts for allthread. Regular nut just four or more times the diameter long.

This is a nice idea, might solve a few problems. I will have a look around and see if I can get some. I know exactly what you are talking about too, i was staring at some in a parking lot this moring and did even click on what i could use them for!!!!

NC Cams

they call the stuff "ball screws".

Yea I know about this stuff, big $$$$ over here. But I really need to get this one going so I can do some bits for the wife. Then I can build a better one, maybe with ball screws and linear bearings.

Thank you both for your time.

Does anyone for Australia know where I can get the collar bearings that are called for in the JGRO design at a reasonable price. If I could get these I could , slightly, tension the threaded rod. This would virtually eliminate the whipping , once I have alinged everything.

CH

[fyffe555]

Fodder

instead of using flanged bearings you could use skate and not cut all the way through the mdf so putting the flange in the mdf rather than needing one on the bearing. Or just make a small plate with a through hole for the screw and screw that to your gram so putting the bearing outside the new plate and a nut outside that will tension the screw. I've done that and the previous suggestions on a small mdf/allthread cnc for pcb's and it works well and runs fast.

[Fodder1]

HI,

the plate behind the "PIPE Adj" plate sound like a goer.

BUT (there is always a but)

1. wouldnt the center of the skate bearing get pulled out? Or do we not put that much pressure on it. I ASSUMED that the flange bearing was a 'thrust' type, were it was designed to take that sort of load.

2. If the "backing plate" for the PIPE carrier that holds the skate bearing has a hole through it, wouldnt the center of the bearing rub on the backing board? I guess this relates to the above question. If the bearing can take a small amount of side load, enought to tension up the lead screw, without pulling the center collet out of the bearing then this would be a great and simple fix.

the idea of not drilling out the hole in the PIPE adj block completely has merit as well.

CH

[fyffe555]

Fodder,

1/ The flange on a bearing outer race is to locate the bearing and in regular groove or deep grove beaings don't provide any increased capacity to carry thrust loads. A flange on these type of bearings is usually just another option, like, shielded, sealed etc and doen't change the load ratings.

2/ The bearing can take a small side load, even though that's not the intended use for them. For home / hobby cnc, skate bearings which are not thrust or angular alignment types can carry enough thrust loads to do what a mdf router needs. Locate the screw, put it under a bit of tension and carry the loads of the gantry and cutting heads motion. Sure they're not angular contact types but a deep groove skate bearing's thrust rating is probably more than many mdf router gantrys can stand and cheap enough you can quickly and easily replace them if need be. EDIT: just looked them up and a typical skate bearings static load ratings start around 1kN or very roughly 100kilos or 220lbs.

For the 'backing plate' just drill the hole large enough so it clears the screw AND inner race but traps the outer race only. It's the outer that needs locating and you can use the nut against the inner race on the scew to put the screw under tension.

[Fodder1]

Hey thanks for all that info.

from what i have read, putting the Leadscrew (allthread rod) under tension solves a lot of the whiping issues and my issue of the rod not beaing straight.

Coools

thanks

CH