[TommyB]

Hey guys, just need a little input from everyone. Ok, so i'm up to the point where I am ready for machining/installing the ball screws on my machine. I ordered the 5/8 thomson ball screw/ball nut and when I mounted the screw in a lathe to turn the journals, I find the screw to be bent. I grab the other one I ordered and same story. I looked at the nook website (Thomson had nothing on this matter that I could find) and they say that their rolled screws may be crooked and need straightening before and during machining of the ends. I was wondering if anyone else has had this problem of new screws not being straight. The screws are 36 inches long. When I spun them in the lathe at 50rpm, it looks like the runnout on the other end is about 0.25". How would I straighten them? Any suggestions will be appreciated.

[HuFlungDung]

I'd straighten them in a press. But, you need to be sure of the chucking accuracy as well. A 5/8 screw 36 inches long is fairly flexible.

You might get away with using a hydraulic (or mechanical) jack beneath something heavy (like your car). Cut some vees in some blocks of hardwood or aluminum to keep the part from getting away on you. Rotating the part in the vees is a better way to determine straightness than chucking it in a lathe chuck.

Place the supports about 4" or 5" apart (at most) and press midway between them. Work your way along the screw, to remove the bends gradually. If you have your support vees too far apart, you will create a series of "dogleg bends" which is two bends with a third bend in the middle that is really nasty to figure out later

[Bloy2004]

As an added suggestion, to determine where the bend is, or if it is a gradual arc along the whole length, lay the screw up along a known straight edge and rotate checking gaps between the screw and straight edge.
Bloy

[samualt]

I know I'm no help, but if I were you I'd demand a refund. I would want them to say on the phone that they it was OK for the product to be bent, and then watch the sparks fly! There is really no excuse for that. Unacceptable!

Perhaps we should start a black-list or slam-book. We could make a list of companies who have done one of us wrong! Of course, it would have to be public and show lots of details about each incident.
Or, perhaps a rating system much like Ebay has for its users. We need something to let others know of unfair practices.
:boxing:

[ger21]

I think slightly bent ballscrews (or acme) is normal. If you but them with the ends machined, then they should be guaranteed straight. If you're machining them yourself, it's your responsibility.

[BigDaddyG]

Hello,
Thompson states that thier screws will be Straight .010/ft. and not exceed .040" in any 6 foot section. You would have been required to "find" the problem PRIOR to machining or doing anything to the screw! At this point the straightning thing is your only option. A "Spinning Screw" will bend and flex in 36" if the excepted bends are as stated, you will see about .25 in all.
Good Luck,
Glen

[hobotek]

The screws are 36 inches long. When I spun them in the lathe at 50rpm, it looks like the runnout on the other end is about 0.25". How would I straighten them? Any suggestions will be appreciated.

Are you turning these between centers? A steady rest may be needed to support the center of the screw while machining. I would place the screw in a pair of v-blocks and use an indicator to check runout on the outer ends.
If it is bent it's easy to straightened by jacking between centers.

[trubleshtr]

Quick question, are you worried it will affect acuracy when positioning? If you are using soild precise ways (linear guides) you will obtain your accuracy from them, a-lot of ball screw assemblies allow one bearing end to float to accomodate the screw assembly moving around..All my machines at work are designed like this and we hold tolerances to microns............

[TommyB]

Thanks for all the replies guys, but the bent screw is my fault. When I was machining the journals, the other end of the screw hanging out the other end of the lathe caused a slight wobble on the end I was machining. I made a bushing that holds the screw centered coming out of the lathe on the other side. I checked the runout and its 20tho off, at the same spot every time i rechuck it. When I put the preloaded bearing on and in the block, the rest of the screw bowed and thats where I got the wobble from. Always learning. I can still salvage the screw by using it on my z axis cause it will have smaller journals. I appreciate all the responses. I'll get some pics up of my machine later on today, probably start a new entry in the projects log. Once again, thank you all.