On 1/26/2018 6:20 PM, Jim Wilkins wrote: > "Bob La Londe" wrote in message > news:p4ghjn$lcv$ snipped-for-privacy@gioia.aioe.org... >> >> On 1/26/2018 5:14 PM, Jim Wilkins wrote: >>> "Bob La Londe" wrote in message >>> news:p4ge4j$h0a$ snipped-for-privacy@gioia.aioe.org... >>>> On 1/26/2018 10:26 AM, snipped-for-privacy@whidbey.com wrote: >>>>> On Thu, 25 Jan 2018 18:05:06 -0700, Bob La Londe >>>>> >>>>> wrote: >>>>> >>>>>> I am sure somebody can tell me why this is a stupid idea, but I >>>>>> needed >>>>>> to get this ball nut off the ball screw without dumping the >>>>>> balls >>>>>> all >>>>>> over the floor. I don't know if the screw is salvageable or >>>>>> not. >>>>>> Its got >>>>>> a bend in it somewhere. If its just in the machining in the end >>>>>> I >>>>>> might >>>>>> be able to turn it down and machine some bushings. If the screw >>>>>> is >>>>>> bent >>>>>> in the main span its done. I might be able to straighten that, >>>>>> but >>>>>> it >>>>>> wouldn't be very accurate anymore. Normally guys screw the nut >>>>>> off >>>>>> onto >>>>>> a piece of cardboard tube, but I have bad luck with that. This >>>>>> hand made >>>>>> stub of aluminum "ballscrew" mates up with the end of the >>>>>> ballscrew I >>>>>> need to check, and I was able to just screw the nut off onto >>>>>> the >>>>>> stub. I >>>>>> had to hand grind a radius tool out of HSS in order to thread >>>>>> the >>>>>> holder. It took me a couple hours, but it worked out perfectly. >>>>>> Of >>>>>> course that time is wasted except for the learning experience >>>>>> if I >>>>>> can't >>>>>> save the screw. >>>>>> >>>>>> Anyway, now I can roll the screw on my granite surface plate >>>>>> and >>>>>> determine where its bent. >>>>>> >>>>>>
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>>>>> Greetings Bob, >>>>> If the screw is indeed bent but the bend is a large radius I >>>>> think >>>>> the >>>>> screw might still be OK after being straightened. I used to >>>>> straighten >>>>> shafts used in piston to turbine engine conversions for small >>>>> planes. >>>>> The shafts would need to be straight within a couple thou over >>>>> about >>>>> 40 inches. After straightening I had to check the shafts pretty >>>>> much >>>>> completely along the length of the shaft. And I never saw >>>>> localized >>>>> small radius distortions, they were always over long distances. >>>>> So >>>>> maybe the same thing would hold true for your screw. As I see it >>>>> a >>>>> sharp bend would cause the ball groove(s) in the screw to be >>>>> compressed in one spot and expanded in another, opposite spot. A >>>>> large >>>>> radius bend would cause the same effect but it would be much >>>>> less. >>>>> If >>>>> the shaft is then straightened I think the distortions in the >>>>> ball >>>>> groove(s) would be lessened to the point that it wouldn't >>>>> matter. >>>>> Eric >>>>> >>>> >>>> The screw doesn't seem to be significantly bent. I got maybe >>>> 0.00075 variance from one end to the other by resting the screw >>>> on a >>>> granite surface plate and passing an indicator over the top of >>>> the >>>> thread. I checked on multiple lines along the screw. Then to >>>> double check myself I grabbed a piece of .002 thin shim stock and >>>> tried jamming it under the screw all along the screw at 4 >>>> different >>>> rotational positions of the screw. I know the indicator is >>>> better, >>>> but I can get a feel with the shim stock. A sort of >>>> confirmation. >>>> >>>> Then I ran the indicator over to find the high point of each >>>> turned >>>> step/shoulder. The results were about the same. I double >>>> checked >>>> myself by using .001 difference gage block stacks as improvised >>>> go-nogo gages to slide under the turned surfaces. I confirmed >>>> it's >>>> within .001 all the way around. >>>> >>>> I'm at a loss. >>>> >>>> The thing is I could see the screw bind and release before it was >>>> removed, and I could feel it bind and release while I was moving >>>> the >>>> machine back and forth by hand. I'm totally at a loss. >>>> >>>> The only thing left I think is to go ahead and dump the balls out >>>> of >>>> the ball nut and check it out. The thing is it felt perfectly >>>> fine >>>> once the screw was out of the machine. Maybe it has alternating >>>> ball sizes and at some point in the past somebody packed the >>>> balls >>>> without alternating them creating the same affect as a bent >>>> screw. >>>> I think that is a shot in the dark, but I don't think it can hurt >>>> anything to check. >>>> >>>> I'm just guessing now. >>> >>> Did you notice a pattern in where it tightened? >>> >>> >>> >> >> Yes. It was anywhere along the travel. I think it was every turn. >> That's what made me think bent screw. > > I asked because I think a problem with a ball or bearing might not > repeat at the same frequency as a bent screw. > -jsw > >
Oh the bearings were definitely bad. I think they were what allowed it to work at all before. I'm putting it back together, but I drilled out the bolt holes and pulled the alignment pins for the front cover that holds the thrust bearings for a little slop. I ran the saddle forward and snugged down the cover mounting bolts. (Kind of like the machine to fit approach you have to do when fixing a Chinese mill.) So far its smooth, but the ball nut mounting bolts are only t-handle tight. I have not gotten in there with a long handle L key to get them good and tight. Maybe tomorrow. I'm done for today. I can't turn the screw by hand, but almost. If I could get inside and get a good grip with both hands maybe I could. The table and saddle are heavy and the the balls are still nice and tight in the ball nut. It spun all the way up and down the screw, but I only get about 1 to 1-1/2 turns on the spin test. Like I said. Nice and tight. I have the end of the shaft wrapped with leather and gripped with locking pliers. I can turn the screw with two fingers, and its not pulsing or binding so far. The cover did snug down shifted over very slightly. There had to have been a little continuous bind on the screw before.
It wasn't a super productive day, but I did get a mold finished and shipped, and I got the CAM done for three more steel embossing dies. I'm not counting anything on the Hurco as an accomplishment until I'm sure I don't have to tear it down again. If I can get it going tomorrow I might reward myself with a day out on the river trying to drown some rubber worms.
Well, I think its about beer:30. Maybe even beer:45.