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  1. #21
    Registered balsaman's Avatar
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    What pitch ball screws?

    Eric

    I wish it wouldn't crash.


  2. #22
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    Pitch is .203.

    Youre gonna tell me Im way too far down on the power curve aren’t you ?

    I was trying to save a few bucs by not going with the reduction, but Im looking again and 20 IPM is only 100 RPM direct so maybe I need to rethink that. Or consider higher pitch Acme screws.

    Wee aim to please ... You aim to ... PLEASE.


  3. #23
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    Well,

    It will work direct I am sure. This is how I run mine and it's fine. On the other hand if you want to fully utilise the motors (which probably go 2000 rpm) a belt reduction would be better. You should aim for 150 in/min at least. What is the diameter? I was thinking maybe you had a 1 or 2 pitch screw.

    Try it direct, but plan ahead for posible reduction (as you said you were doing).

    Keep plowing ahead! you are doing fine!

    Eric

    I wish it wouldn't crash.


  4. #24
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    Hey Eric thanks for the input.

    Are your screys 5 TPI or 8 TPI ?

    Im preliminary thinking 5/8 Screw dia X .203" pitch. If cost wasnt a consideration I would go with 3/4 or more probably, but the price (as was mentioned in another thread) of these screws from mcmaster is pretty darn good. Im going to run a few shaft calculations before I lock it in.

    I intend to use thrust bearings at one end and radial bearings at the other end of the ball screw so I think I can minimize any eccentric loading on the screw. Using the 150 IPM baseline, thats about 740 RPM. Which Im thinking should be no problem on a 24" axis. I will see what the numbers say though. I dont want to buy the screws twice !!! they wont be that good of a deal then.



    Speaking of which, did you isolate your screw vibration problem ?

    Wee aim to please ... You aim to ... PLEASE.


  5. #25
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    Mine are 3/4-6. Your screws will be great on a 24" axis.

    My x axis still is noisy, as the scew is 48" long. It's much better now since I went back to the amco nuts with the saw cut antibacklash system I am using now. The 3/4"-6 acme tap made for very little backlash but there was probably 50 thousands "axial"? play in there which made for some nasty side to side whipping in the nut. Now that I can squeeze the nut a little its fine. I need to borrow a dial indicator because my screw is bent a little. I will put some masking tape over the threads in the center and use the dial indicator to see how much. I think I can tweek out this bend and it will be a lot better. Still working on the limit switches in between cutting some parts, so after the limit switches....

    I was lucky with my direct drive setup because my motors are 115 volts. On 75 volts they go around 1000 rpm so that means 150" per minute is almost the motor maximum. If I would have gone belt drive my max speed would have been limited. Quite by accident it worked out for once! So far the motors stay very cool and have more than enough torque.

    Eric

    Eric

    I wish it wouldn't crash.


  6. #26
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    Eric,

    I have had three bent 1/2" leadscrews that I have straightened. The first one I screwed up and now has a compound bend in it because I wasn't methodical about it. It's scrap now. The other two came out alright.
    Here is how I did it:
    Make sure that you know which way your leadscrew is bent and how much. I put them on v-blocks each end. Then I used some .010 brass shim stock that was attached to another block and sort of bent over the rod. I set the indicator on that. Then I took readings every inch and also marked the high points with a marker.
    Make a graph of your readings. If it is a simple bend your readings will decrease consistantly as you get to the ends. Your marks of the high points should also be in a straight line. If the marks tend to go around the rod then it will be a harder thing to fix.
    After you know how the rod is bent then you can plan on how to straighten it. I used the v-blocks again. I took a c-clamp and a block of wood. I tightened the clamp 2 turns and checked the reading again. Then 2 1/2 turns and checked it again.
    The second rod bent at 3 1/2 turns of the clamp. That was my 10" z axis leadscrew. It was out about .008 and is now at about .002. That's close enough for now.
    I know that this is the long answer but I found that it took some real analysis to get it right.

    Good Luck,
    Chris

    Last edited by cbcnc; 08-03-2003 at 02:42 AM.


  7. #27
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    Part of the trick to straightening bent shafts is to not set your V blocks too far apart.

    We do lots of hydraulic cylinder shaft straightening in my shop, using a hydraulic press. We use aluminum V-blocks to prevent marring of the surface. We use 3 V-blocks, two underneath the shaft on each side of the bend as support, and one under the press ram where the pressure is applied.

    Anyway, the trick is that the spacing of the supporting V-blocks should not be any more than about 8 times the diameter of the shaft.

    A bend in a shaft has a significantly increased stress strength in the long side of the bend. If your V-blocks are too widely spaced, the original bend will have enough strength of its own that it will rebound more than the less stretched areas farther from the bend. This will create a "dog-leg" bend, even if you are bending in the proper direction at the proper place.

    Compound or dog-leg bends can be removed as well, but you will have to overbend one side of the bend, so that you get the shaft back to one simple bend. Then, moving your V-blocks back to the recommended seperation, and working your way both sides of center of the original bend will get you back to straight.

    Use a straight-edge for guidance until you have it very close to straight, then use the dial indicator. Using the dial indicator too early in the process can be a source of great frustration.

    Last edited by HuFlungDung; 08-03-2003 at 04:07 AM.
    First you get good, then you get fast. Then grouchiness sets in.

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  8. #28
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    Hu,

    Thanks for the reply. I was hoping to get some professional feedback. It was quite an exercise for me to figure out how to straighten my leadscrews.
    I was wondering as I did it about how far to space the support blocks. I guess I managed alright but I could see that there was alot involved.

    Chris



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