C7 Ball Screw Quality, Bent Screws

[starion007]

Hello everyone, I need some advice.
I purchased a CNC conversion kit on eBay for a G0704 mill/drill. I got the mill yesterday and started checking over things. I found the ball screws for the X and Y to be bent over .025" almost wavy actually.
I told the eBay vendor and then talked to him on the phone last night, he stated that all China ball screws would be bent because of their low quality standards. He assures me that it will be fine once I install them because then they are captured between the end cap bearings and the ball carrier. He is willing to replace them but states that the ones I get back will also be bent to some degree.
I had a small harbor freight mill/drill in the past and had a funny "shake" in the X axis, I traced that to a bent Acme screw, I was able to straighten that screw and the problem went away. I not sure about this problem and would like some feedback before I proceed, I don't want to tear down the machine and have it apart while sorting out the screw issue.
Thanks, Mark

 

[JimDawson]

I think you could straighten the screws if needed, but my concern would be that the ball grooves would not be concentric with the OD. That could cause some problems. You should be able to check this with a dial indicator and a flat surface.

 

[RJSakowski]

Bent ball screws or lead screws create an inaccurate tracking. As the ball screw rotates, the nut will wobble causing small variations in the lead. Depending upon the curvature of the bend and your desired level of accuracy, it may not be an issue. The variation will be cyclic in nature with a period equal to the pitch of the ball screw. Precision ball screws sometimes are accompanied by accuracy reports. Here is a white paper concerning ball screw accuracy. https://tech.thk.com/en/products/pdf/en_a15_011.pdf

 

[RJSakowski]

I told the eBay vendor and then talked to him on the phone last night, he stated that all China ball screws would be bent because of their low quality standards. He assures me that it will be fine once I install them because then they are captured between the end cap bearings and the ball carrier. He is willing to replace them but states that the ones I get back will also be bent to some degree.
Thanks, Mark

First off, every machined part will have some inaccuracy. The question is how much and is it tolerable. I don't believe that all Chinese ball screws are "bent" but those sold on e-Bay may be the seconds that don't pass q.c. for a particular product.

For your particular use, the proof will be in the pudding, as they say. Once installed do a critical inspection of the movement. As far as the checking effect of the bent screw, I would make a position measurements at distances corresponding to about 1/10th the lead screw pitch and extending several times the lead screw pitch and at the location(s) you noticed the bend. For instance, if you have a 6mm pitch, make measurements at intervals of .6mm. Plot the measured position vs. the commanded position. The ideal curve will be a straight line with a 1:1 slope. If the average slope of the curve varies from the 1:1, it means that the screw pitch is not accurate. If there is variation above and below the ideal line, there are localized inconsistencies, one of which is a bend in the screw.

How much variation you are willing to tolerate depends on you. In most situations, variation of less than a thou is most likely sufficiently accurate for hobby needs and any effort to improve it may be swamped by other machine shortcomings.

A final note. In a company that I worked for, we mapped out the variations and made software corrections for them. This allowed us to use a lower grade of screw which was more economical and still reach our desired accuracy goal. It similar in principle to backlash compensation. I don't know that this is possible in existing controllers though.

 

[The Liberal Arts Garage]

I think you could straighten the screws if needed, but my concern would be that the ball grooves would not be concentric with the OD. That could cause some problems. You should be able to check this with a dial indicator and a flat surface.

I think you could straighten the screws if needed, but my concern would be that the ball grooves woul with the OD. That could cause some problems. You should be able to check this with a dial indicator and a flat surface.

We pft. We often hold a bent sc.rew up to.
the light decide whether or not to save it (I do , anyway). Any way you do it, it will
wobble; you still have to decide if the locating error is tolerable........BLJHB.

 

[starion007]

Thanks for the reply's, RJ I started reading that white paper and there's a lot of good info in it. I'm going to use that to see how bad my screws really are. I was hoping someone here had used screws that were bent and could say what issues they had. I did some YouTube searching and found it is a common problem. When you roll them on a flat surface and can slide a quarter under one end I think that may cause a problem. I don't think they can be straightened either, I place them on V-blocks on the large mill table and put a piece if aluminum rod in the collet to act as a ram and could not move them with moderate force. I'l be working on it today and see how it goes.
Thanks, Mark

 

[TomS]

Thanks for the reply's, RJ I started reading that white paper and there's a lot of good info in it. I'm going to use that to see how bad my screws really are. I was hoping someone here had used screws that were bent and could say what issues they had. I did some YouTube searching and found it is a common problem. When you roll them on a flat surface and can slide a quarter under one end I think that may cause a problem. I don't think they can be straightened either, I place them on V-blocks on the large mill table and put a piece if aluminum rod in the collet to act as a ram and could not move them with moderate force. I'l be working on it today and see how it goes.
Thanks, Mark

I bought Chinese ball screws and found the X axis screw bent about .025". The Y and Z axis screws checked out OK. I straightened the X axis screw by supporting it in V-blocks on my mill drill table using the spindle to put downward pressure on the screw. I monitored the downward pressure with my Z axis DRO. The downward pressure was low enough that I could have used my hand to straighten the screw but the DRO was a way to monitor the process. Anyway, after about 30 minutes I had a ball screw that was straight within a couple of thou.

Keep in mind these are C7 screws and not "top-of-the-line" parts. This is reflected in the price. With a little TLC they can be made to work satisfactorily. And you have a few bucks in your pocket to buy more parts.

Tom S.

 

[starion007]

Thanks Tom, I did go out today and worked on the X axis screw and got it much better. Down to about .012" overall now.
I will work on it more tomorrow and go from there. I had to put a considerable amount of pressure on it to move it, the Y axis might be harder since its shorter. I paid $610 for the whole kit including the 425oz motors, controllers, c10 board and power supply so hopefully it will work out. Not a very well manufactured kit really but you get what you pay for i guess the saying goes.
Thanks, Mark

 

[TomS]

Thanks Tom, I did go out today and worked on the X axis screw and got it much better. Down to about .012" overall now.
I will work on it more tomorrow and go from there. I had to put a considerable amount of pressure on it to move it, the Y axis might be harder since its shorter. I paid $610 for the whole kit including the 425oz motors, controllers, c10 board and power supply so hopefully it will work out. Not a very well manufactured kit really but you get what you pay for i guess the saying goes.
Thanks, Mark

I also bought a Chinese kit. It's worked out well for me. The only part that needed to be replaced was the breakout board. Bought another better featured China board which has worked out with no problems. When you get a chance post pictures of your conversion. We like to see what others are doing so we can learn too.

Tom S.

 

[MachGeek]

The kit that I bought for my g0704 off of EBay had Chinese C7 ballscrews as well, but my vendor did straighten them. They work beautifully or at least within my ability to measure.