Z Zero is slip slip slipping away

[jim_rutske]

Here in Albuquerque I cut 5" x 7" ovals with different 3D art in them for an urn company. I have cut hundreds of these with nearly all mistakes attributed to operator error...that is until recently. It appears the Z while cutting slips down. Sometimes it is abrupt and happens instantly while like this morning gradually slips down with each pass. In the attached pictures you can see from left to right during the roughing pass my Z slipped .483 down. I can not seem to figure this out. I have had engineers and electricians look at this with no apparent answers. I have traced every wire and every ground. I even set a new ground rod last week in hopes that it was a solution.

The bot is a PRT 2002 model with a 4g board. A new clean computer runs the bot with plenty of power and a 74 to 76% transfer rate. Well maintained and lightly oiled rails. All pinions were replaced 8 months ago. Grounded throughly with no ground loops. Vacuum is well displaced from power and data lines.

In picture 1 you can see how gradually the Z slipped down with each pass. In a correct cut that background is totally flat with no pitch.

Picture 2 shows how the Z kept getting deeper as it cut from left to right. It also cut the top of the board when it was returning to 0,0. Thats when I noticed it was slipping and stopped the cut before it cut into my spoil board...again.

In picture 3 you can see the damage in just the last week of the wayward Z.

I really would appreciate any advice on resolving this issue, it is has grown from an occasional nuisance to a nearly daily occurrence. I am losing a significant amount of time and lumber. I have contemplated installing the new upgraded Z but I am hesitant to sink any more money into this machine until I can pinpoint the problem. Has anyone had and solved this problem before?

[ken_rychlik]

A few things to try.

Measure where the bit is in the collet before and after a run to see if the bit is slipping.

Switch geckos with another from the y or whichever one you want to swap.
Make another run.

Then I would be removing the z motor and checking to see how smooth the z slide goes up and down with no motor in it.

If it is loosing steps it is loosing them on the way back up, so if it goes down smooth and has hangups on the way back up, that would be what you are looking for. Are your springs working to counter balance the weight of the router or spindle?

Your motors are the same on the y and x also. The z gets the most abuse, and I had to switch my z with one of my x motors once. Everything was fine after that. I am pretty sure I had to do a countersink on the z motor to make it work on the x. You will see what I mean when you take them both off.

[ron_churches]

Jim

Awhile back, I had the same issue with my 2005 PRT. My problem turned out to be a worn collet. After a number of broken bits, and plenty of stress about having a major repair to do, I started a thread here for help. It turned out to be a $14.00 part and I was back in business the next day.

( http://www.talkshopbot.com/forum/showthread.php?p=98536#post98536)

Ron

[zeykr]

Like Ken says, When you zero, measure how much of bit is sticking out of collet and distance from collet to table top when z=1. Then when it slips remeasure and see what has slipped, the z axis or the tool in the collet. Then you know where to start looking.

Like these guys, I suspect tool slipping in collet is more likely as I've had it happen too.

[jim_rutske]

The collet slip was my first guess too. I have had it happen in the past, but it wasn't it this time. What I have been doing the last six months or so is drawing a line with a thin sharpie around the bit. Then I chuck the bit to the line and see at the end of the cut if it has moved. I replace my collet about every four months. This one is about three months old and I disassemble them about every two weeks and clean them good and check for wear.

I have taken off my Z motor and the router drops pretty easily to the table. It doesn't have any bad or grinding friction going up but I would guess about 5 pounds of downward weight. I assumed it would be neutrally buoyant but it is not. Does anybody know if this is normal or is this a sign that my spring is shot and causing an issue?

[ken_rychlik]

The closer it is to neutral weight, the less problems you will have. depending on how fast your z lift is in your file, if It can't lift it and misses a step on the way up, then that would explain the deeper plunge on the next pass.

I would work on that before going to the other areas. You may be able to just take a little off of them and bend some new hook ends to get it closer. Making them shorter will increase their strength.

[jim_rutske]

I agree the weight of the router is too much. Once I removed my vacuum hose it made the router even more negatively buoyant. I have attached a photo of the type of spring I'm up against. Not sure how to tighten this monster. I plan on calling Shopbot in the morning. I have a healthy respect for springs under tension. Has anyone successfully adjusted this spring before?

[tmerrill]

Jim,

ShopBot sells an upgrade kit that will replace that with two coil springs. You will just need to drill two holes.

Talk with Frank and he can provide details and determine the springs you will need.

Tim

[ken_rychlik]

Yep, Tim is right about the kit.

Throw that spiral spring away and get the two spring upgrade kit.

I was thinking you already had the two coil springs.

Tell them what all you have on it, so they know how strong of springs you need.

[hespj]

"A new clean computer runs the bot with plenty of power and a 74 to 76% transfer rate."

Clean computer? Have you got the settings right in the SB software? I had a similar problem a few years ago after fitting a new Z and I think it turned out the Unit Values weren't correct - "Z unit value" and "Z resolution multiple". What was happening was that whilst Z moves were correct, Z jogs weren't, so whilst it would move down to cut correctly, it would jog up out of a cut incorrectly.

John

[jerryk]

Jim
Another possibility is the upgrade to the 4G board. I had a similar problem when I upgraded a 2002 vintage machine to the new controller. Contact shopbot. Frank was able to solve the problem after about 1 1/2 months of hair pulling to trouble shoot the problem. My Z axis would gradually or abruptly drop, eg go deeper into the carving. The problem was much worse on 3D carvings and almost non existant on 2D cutting. Problem solved by decreasing the current to the motors. Ted Hall sent out a warning letter to purchasers of the new controller in early to mid November.
Jerry

[jim_rutske]

Thank you everyone for all the great input, the intrinsic customer support in the forum is a main reason why I have stuck with Shopbot over the years. The big iron machine companies just can't hold a candle to the Shopbot Army :)

I spoke with Ryan at Shopbot this morning and ordered the double spring upgrade kit ($39.50). Now that everything is apart it is very apparent that the old spring is totally shot. That put way too much weight on the Z motor and was causing the Z to loose steps. I can't believe I didn't burn up the motor; I'm still swapping it with my new spare motor anyway. So in theory the new springs should make the router close to neutral and I should be 3D cutting without error.....right? ;)

I will update in a few days when I the springs are in place and I see the results.

[jim_rutske]

After a bit of drilling and Z disassembly I installed the upgrade spring kit. It made the PC router float a little above neutral buoyancy. I leveled the table to ensure the PC was still square and today I put the upgrade to the test. I ran three 3D carvings back to back and the results were the best I have seen in a long time. I didn't see any of the wavering Z depths in any of the cuts. Praying it holds up;)