Loosing steps or backlash

[ranchak]

I have a series I ridgid ram that appears to either be loosing steps or has excessive backlash. I'm not too sure where to start. I am machining multiples of the same part, but the inconsistencies vary. One part will be off on the "top" of the part the other on the "bottom". I attached some pics to show the problem. It's hard to see, but in the first picture the bottom is smaller than the top and in the second the opposite, the top is smaller than the bottom.

Here's some further information. I removed the drive belt and installed a dial indicator on the y axis. I then moved the axis and compared the dial indicator reading to the DRO in Mach, typically I had about a .0003 - .0006 difference. But the difference didn't compound, meaning it didn't keep adding up, the axis was always within this difference. Of course I wasn't able to see what happens when the table moves a few inches. I am wondering now if maybe my motors are loosing steps?

[Hood]

If it is backlash then really each part should be the same amount off, that is presuming you are starting at the same point each time.
Sounds like your problem is more likely steps being lost or some intermittent mechanical issue such as pulley slipping occasionally.
To test backlash just set a dial on the table and have the point against the quill or head, move the table one way until the diaal moves a bit then stop, zero the DRO in Mach and change to Jog Step mode and move back the other way at 0.001" steps, as soon as you see the needle move read the DRO and subtract one step and you have your backlash for that axis. You can repeat at other points of the axis and also on the other axis.

What type drives do you have? what size motors? Do you have gearing between motors and ballscrews?
Hoodl

[ranchak]

Hi Hood, my machine is a BP Series I CNC, I am using a PMDX-122 BOB, Gecko 203s and the original BP motors. The axis are belt driven. The pulleys are held by a key and taperlock pulleys. I checked and my lube pump is lubing the ways. I am going to try and reduce my acceleration and see how that affects my overall cut times and if there is a difference in the finished part. I will check to see what my backlash is.

[Hood]

It could well be the steppers, they are likely getting tired  with the age of them. I have a Rigid ram as well with G202's and PMDX 122 but I fitted new steppers. Mine is a Boss 6.1 which was the UK version of the Boss6 I believe. The gearing on the UK ones were 2.5:1 where I think the USA ones were 2:1 and I think earlier versions may have actually been 1:1. The ballscrews on mine ar 5mm pitch where yours if a USA one will be 0.200"

Hood

[ranchak]

Here is a picture of a similiar machine. You are right about 2:1 ratio and .200 ballscrew lead. I don't think that my machine is worn, the ways are well lubed and you can still see the scraping. I tend to agree with you that the steppers may be getting tired, but right now all I am doing is taking stabs in the dark. I really don't have any hard evidence to back any of this up. What are you using for motor tuning settings? I was using Velocity = 70, Acceleration = 4, but I spoke with Steve at PMDX and he thought that my Acceleration was a bit too fast. I am now trying my Acceleration = 2. I'll see what time difference this makes.

[Hood]

Mine was originally running 2500mm/min with accel of 40mm/s/s so that would equate to 100IPM and 1.57inch/s/s
After a while however I was not so hell bent on rapids but looked at increasing the accel, I dropped the rapids down a bit, think to about 2200mm/min(86 IPM) and was able to get the accel up to I think 120mm/s/s (4.7 I/S/S)
But as said that is with modern steppers of 916ozin.

I havent used the Bridgeport for probably six or more months as since I finished my Beaver NC5 mill going back to the Bridgeport would make me cry

Hood

[Hood]

Oh BTW that pic is a series II is yours like that? Mine is a series 1, heres a pic of when I first did it, its dirty now
Hood

[ranchak]

No Mine is a Series I, I didn't realize that picture was a Series II. Also I have 1:1 belt drive. I removed all of my cabinets and installed a smaller cabinet to the back of the mill. I'll take some pics of it in the future, right now it's a mess and I am going to move it later this week. I am now running some test programs to see if I can replicate my problem. I really need to figure out where to concentrate my efforts. I'm getting mixed opinions about the motors, some people say replace them, others think the problem is elsewhere.

[ranchak]

Mine was originally running 2500mm/min with accel of 40mm/s/s so that would equate to 100IPM and 1.57inch/s/s
After a while however I was not so hell bent on rapids but looked at increasing the accel, I dropped the rapids down a bit, think to about 2200mm/min(86 IPM) and was able to get the accel up to I think 120mm/s/s (4.7 I/S/S)
But as said that is with modern steppers of 916ozin.

I havent used the Bridgeport for probably six or more months as since I finished my Beaver NC5 mill going back to the Bridgeport would make me cry

Hood

You can ship it over to me if you want, I wouldn't want you to be upset and break down in tears!

[Hood]

LOL well you supply the packing case and its yours.
Really though I am keeping it as a backup and maybe a test machine. Have a new back board board made up with AC servo drives etc but not sure whether to go ahead or not.
Hood