Gravograph IS400 Head Positioning Issue

[Bill George]

Steppers only. Not sure about encoders or whether it just sends out steps with no closed loop. But definitely only steppers.

If a stepper motor gets overloaded in some way, it starts losing steps. Since it has no feed back like a servo with an encoder bad things can happen. The controller just sends out steps or pulses, does not have any feedback.

[Phil Vernon]

If a stepper motor gets overloaded in some way, it starts losing steps. Since it has no feed back like a servo with an encoder bad things can happen. The controller just sends out steps or pulses, does not have any feedback.

Exactly.
But I've just realised (yep that's how we spell it over here) what it is, sort of What Gary Hair referred to. The IS200 uses a rubber coupling between the stepper and the lead screw. This is where I have seen this error before. The coupling perishes over time and the connection becomes flexible hence the position changes. Imagine 2 small toothed gears, 1 on the motor and 1 on the leadscrew, they are inline with each other (they don't mesh) and these are then both encased in a rubber sleeve with the inside face moulded to suit the gear teeth (this handles the drive part). The sleeve splits allowing 1 of the pinions to sort of freewheel a bit and then grab again. Hence the position change. The IS400 must be the same, I've only ever replaced them on the IS200, The IS6-7-8000 all use toothed belts for drives.
Hence when it is under more load the situation is exacerbated as half of the coupling moves inside the sleeve more.
Ask for a replacement coupling.
Phil

[Crystal Hurtle]

Thanks for the responses, everyone. It may be a few more days until I can get back to the machine with the space and time to remove the cover and work on it.

Update. See my above post for the cause

Hi Crystal,
Ex Gravograph Employee here and still getting contacted by old customers to fault find, seems Salesman with half a brain don't actually grow on trees after all.
Sounds like the machine may be binding up on the leadscrew and could be improved simply by spraying some silicone cleaner, or you need a new stepper motor. Why do i think this? You mentioned that the problem becomes worse when you are actually touching the surface, the machine will be under more load at that time. In the air there is no load.
Does it happen when the machine is cold or hot? Are the fans working in the control box? Maybe overheating and doing funky things.
Taking the top off the IS400 is honestly dead easy, just a couple of little tricks is all. If your still having an issue let me know and i will try and help. What country are you in?
Regards,
Phil.

I am in the US. Sometimes I imagine the distortion gets worse if the machine has been through a few runs, but I may be making this up. The differences are small. The fans in the control box sound like they're running, at least.

Two questions:

1) What kind of silicone spray should I use? Is this preferred over other oils?
2) Anything I should know about ahead of time about taking the cover off? What are these "couple of little tricks"?

is that image actually what it looks like? If so, I'm betting you have an encoder and it has some dirt on it and you are losing steps. Even if it's not that precise that it shifts, I'm still betting on an encoder issue. It could be a loose gear or belt between the motor and leadscrew, but that wouldn't typically display such even shifting and it would probably shift differently depending on the direction.
Have you tried running the job from bottom to top? If so, does it shift about the same amount?

I haven't tried running a job bottom-to-top yet. It would be a good test. The machine also has a good amount of dirt and metal shavings inside the case from the previous owner. The whole thing could really use some good cleaning and lubrication.

I'll post an update once I get back to it.

(Edit for mixing up "bottom-to-top"...)

[Gary Hair]

The machine also has a good amount of dirt and metal shavings inside the case from the previous owner. The whole thing could really use some good cleaning and lubrication.

Take care of this first and you may just solve your problem. I had a cnc router I built that was loosing steps on the X axis and it turns out I completely forgot to lube the leadscrew. I cleaned it really well with denatured alcohol and put a very thin coat of lithium grease on it along with a few drops of 3-in-1 oil and it never lost a step again. If it's really bad you could use WD-40 to clean it then clean off the WD-40 with denatured alcohol, then apply the grease/oil combo to lubricate it. Don't leave WD-40 on it as it really attracts dust and dirt and will clog things up pretty badly over time.

[Phil Vernon]

Two questions:

1) What kind of silicone spray should I use? Is this preferred over other oils?
2) Anything I should know about ahead of time about taking the cover off? What are these "couple of little tricks"?



I haven't tried running a job bottom-to-top yet. It would be a good test. The machine also has a good amount of dirt and metal shavings inside the case from the previous owner. The whole thing could really use some good cleaning and lubrication.

I'll post an update once I get back to it.

(Edit for mixing up "bottom-to-top"...)

Hi Crystal,
So I assume that you have already removed the top plate on the Y-Travel, this lets you see the Y-Axis lead screw. This is on top of the beam (Y-Axis) that sticks out and holds the spindle. The Y-Axis is where your trouble will be as I am certain it is the coupling that connects the stepper to the leadscrew. I can't exactly recall where the position of the motor is but there are 2 ways of getting further access into the machine.
1. Either manually pull the spindle as far out as it can go, you need to do this so the large cover can come off. Or drive it ou with the power connected and then disconnect everything. The back of the machine cover has a bulge that the Y axis moves into when fully retracted, if you dont move it forward it stops the cover from coming off. Now undo and remove the 4 screws on top and the 5-8 (not sure how many) screws across the back at the bottom of the cover, below the aforementioned bulge. Then simply lift it off. Then you will see that there is not much to it. You now have access to the X-Axis leadscrew, but this is not where your problem is. This drives side to side motion, not front to back.

Not sure if you need to do this next one. The motor and coupling that you need to get at may be accessed by taking the top off the Y-Axis.
Maybe take some photos and either post them here or message me and I'll give you an email.
But if you want to get more access then continue on.......

2. Remove the bellows/dustcover at the front of the Y=Axis beam underneath behind the spindle. To do this you will need to hang the machine off the front of a table and get access underneath, or tip it on it's side or back (but I recommend the first way)


Regards,
Phil.

[Kev Williams]

Hey Phil, just wondering if you know my rep Bruce in Salt Lake?

And while I agree with your diagnosis, I'm having trouble understanding why her machine is GAINING Y-steps on the way right and down the plate rather than LOSING steps?

[Phil Vernon]

Hey Phil, just wondering if you know my rep Bruce in Salt Lake?

And while I agree with your diagnosis, I'm having trouble understanding why her machine is GAINING Y-steps on the way right and down the plate rather than LOSING steps?

Hi Kev,
No, I don't know Bruce mate. I used to be the Victorian Territory Manager looking after Melbourne but I no longer work at Gravograph.

Gaining Y-Steps?
If you go back to the image you will see that it is a matrix engrave. It does the first 2 lines in the top left corner and works its way across, when going from the first line to the 2nd line it is not running fast it is at engrave speed (not ramp speed or reposition speed) nor does it have far to travel. But when it finishes the 2nd line and re-positions to the top of the next grid to the right it hits reposition speed in the case of the IS400 I believe is 60 or 35mm/s. Now if you imagine the coupling as I explained above, at this speed the connection freewheels a bit as the rubber connection is damaged. It cant hold itself together on the loads applied at acceleration (Re-position)
So as it engraves across it does Line 1 > Line 2 fast Reposition (Coupling slips?), Line 1 > Line 2 fast Reposition (Coupling slips?), Line 1 > Line 2 fast Reposition (Coupling slips?), etc.
And when it slips if it slips it does not go all the way back to what it believes is the top, but of course when it goes home at the end it homes to a limit switch so you would be none the wiser.
I put the question mark in because looking at the sample it sometimes slips and sometimes doesn't, may just slip enough to move 1 tooth or might just stop short of 1 tooth and then drop back in the tooth.

So if I have explained it clearly enough you can see how it moves down, its not really gaining steps but more not re-positioning at high speed as the coupling is Kaput! This is a image similar to the rubber casing the 2 pinions one on the leadscrew and one on the motor
LR001471.jpg

With the coupling damaged it will also cause slanting text as the coupling is not rigid but sloppy.


Cheers,
Phil.

[Kev Williams]

Makes sense-- I was giving most thought to steps changing while engraving, not so much thought about the move UP to the next line from the bottom of the previous line.

[Crystal Hurtle]

Phil --

Sure enough! Take a look at this:

IS400-Couplings.jpg

The y-axis is on the left. X-axis on the right for comparison.

Does anyone know the best place to find a replacement coupling (or how they are typically sized so I know what I'm getting?)

[Kev Williams]

Not only does that coupler look like it's toast, that screw looks to be severely worn down too..?

Some couplers are in 2 pieces, but I don't think that one's supposed to be? And if that screw is really as worn down as it appears, its likely been putting up quite a fight just to move at all. Could explain why the coupler is in half, those stepper motors are pretty strong...

I'm guessing you're going to be needing a new lead screw assembly too, these 2 screws should look the same, and they definitely don't...

[Phil Vernon]

Does anyone know the best place to find a replacement coupling (or how they are typically sized so I know what I'm getting?)

Hey Crystal,
Sorry for delay in responding. Was Unemployed for a while but landed a new job and have been a tad busy.
I would just ring up Gravograph and send them the picture.
The picture on the left shows the grub screw on the shaft side and i'm guessing that the other grub screw is behind the motor flange. There are 2 size couplings (different shafts) and it doesn't really matter which one they send as the rubber sleeves comes off. So you keep the existing aluminium parts, look at my picture above and attached (the black bits are the Aluminium and the white bit is the black rubber), you can take the rubber sleeve off as its knackered. I have seen peeps do a temporary repair with some strong tape...but it is only temporary.

Sorry to disagree Kev, but I would be very surprised if you needed a new leadscrew, I have serviced a heap of these and never seen a worn lead-screw. One looks lubed one looks dry. You can turn it by hand to check for binding. They are great machines it's a shame that the new Management in Australia breaks Employment agreements, otherwise I may still be selling and servicing them.

Any questions let me know, maybe with a pm also :-) I will try and be more diligent!
Regards,
Phil.
flexible_curved_tooth_gear_coupling.jpg

p.s. I am the father of 4 boys and 3 of those boys just landed in the states for a 3 month visit across the whole USA with a stay in Atlanta for 6 weeks.
So if they get into any bother I may need to reach out through here...hope not though. :-)

[Phil Vernon]

Hey Crystal,
Did you get this sorted in the end?
Cheers,
Phil.

[Crystal Hurtle]

I did get it working! I improvised by clamping the broken coupling together while I finished a job, but an entirely new part is installed now and working great so far. Engraving is just a small part of what I do, so a lot of time passed in between...

I owe everyone here a lot of thanks!