Backlash is easy to measure. Set a 123 block or similar on the table. Hold an indicator in the spindle (indicol holder, chuck, whatever) oriented to the axis you are testing. Slide the 123 against the indicator and zero . Have someone hold the ballscrew (it can't turn or the results are invalid), then grab the table and try to move it fore and aft. Indicator reading will reflect the total backlash in the system. Not rocket science.
Joe
Hey Joe, Thank You!!! for the information (Sorry I'm a Novice at best).....However, I checked the backlash your way(only difference is I mounted the dial indicator to the base of my machine) and again I get no BacklashOriginally Posted by scudzuki
Robert
I used basically the same specs as HOSS (with extended Y travel) except I installed a set of the stock X axis thrust bearings in the Y axis Mount which means the Y axis ballscrew was machined to 10mm vs 3/8in.Oh and get their motor couplers (0 Backlash), he gave them to me for free with my order.
I ordered
300mm for Y
500mm for X
500mm for Z
I had them machine the threaded ends (10mm x 1.0 thread)
I ordered the screws long and cut the correct length after mocking them up
I machined the X axis for a end support bearing
Robert
Measuring backlash
The problem measuring backlash that way is that the ball screw is capable of at least 200Kgf (at about 1.5 N.m ) and our hands maybe 20Kgf or so and is by no means repeatable. There is even no way to know what is holding the table, the screw or the dovetail. I don't think you can measure a small backlash like that.
The way I meansure backlash is to fixate the dial indicatior in the table and rest the measuring end in the spindle. Then I make a mark in the ballscrew polley so that I can easely return to the same angular position. After that I bring the pulley to the marked position by a CW rotation and zero the dial, then I turn about 1/4 turn CW and return to the marked angle dy a CCW rotation and read the backlash in the dial.
That way you exactely replicate the way the control system sees the backlash, that is, a difference when aproaching the same point by a CW or CCW rotation.
Besides, if I hear that any ball screw shows "Zero" backlash by using a dial with 10um resolution, the only possible conclusion is that it was not measured right.
I would also like to beleave the zero backlash thing as I bought the same product, but by knowing the real backlash you can use the compensation function in Mach3 which will increase your precision.
I see.....Thanks for the reply.... and valuable information....if you have that much backlash maybe you should send the ballscrews back?or email the guy
Robert
Last edited by rwestbr; 05-26-2009 at 01:16 AM.
44 lbs. of force is more than sufficient to reveal the difference between no backlash and some backlash. The amount of backlash revealed by the aforementioned oversimplified method pretty accurately reflected what I measured with other methods.
BTW I can probably bench over 200 lbs even though I'm out of shape so I'm pretty certain I can muster a push in excess of 20 kilos.
Joe
Well, I looked up the C7 grade spec and it seems that 50um is in the expected C7 range so I think they would not exchange the part.
Anyway, if you say you can't even measure 10um with your screws I guess I should double check mine. I have bought a 3rd screw for the Z axis, so I should check if all 3 of them are consistent with 50um. It could be that they have a large standard deviation in the amount of backlash from unit to unit....
I think this sort of information is not only interesting to us, but there is surely a lot of people considering to buy these parts that might find the information usefull.
Did you try out the methode I described? I tried out scudzuki methode, but unfortunately I am too much out of shape to manage the required 200 lbs pushSo the table did not move at all.
Well, there's always the gym I suppose...
Joe
Does the flange unscrew from the nut? If so why not just machine a new flange (like the Hoss flanges) instead of grinding and shimming the stock one?
Maybe I don't know how much 10um is in inches (Please convert for me).And my indicator has a resolution of .001 inch?As I said before I'm no expert, pure hobbiest who knew nothing when started but have learned alot from gents such as your self.Well, I looked up the C7 grade spec and it seems that 50um is in the expected C7 range so I think they would not exchange the part.
Anyway, if you say you can't even measure 10um with your screws I guess I should double check mine. I have bought a 3rd screw for the Z axis, so I should check if all 3 of them are consistent with 50um. It could be that they have a large standard deviation in the amount of backlash from unit to unit....
I think this sort of information is not only interesting to us, but there is surely a lot of people considering to buy these parts that might find the information usefull.
Did you try out the methode I described? I tried out scudzuki methode, but unfortunately I am too much out of shape to manage the required 200 lbs push
Look at these similar ballscrews from homeshop cnc, read the spec for backlash
http://www.homeshopcnc.com/Ballscrews2.html
Robert
All 1 Piece , I cut my Flange using a angle grinder with cutting disc (easily), Then used the bench grinder too finish up.
Robert
So whats the end results with these screws ?
I am so close to ordering someones screws, if these are holding up I might have to give them a try.
Jess
I think you will find that the ball screw grade has nothing to do with backlash. It is a designation related to the positional accuracy per foot or per 300mm.
Phil
Well, I looked up the C7 grade spec and it seems that 50um is in the expected C7 range so I think they would not exchange the part.
Anyway, if you say you can't even measure 10um with your screws I guess I should double check mine. I have bought a 3rd screw for the Z axis, so I should check if all 3 of them are consistent with 50um. It could be that they have a large standard deviation in the amount of backlash from unit to unit....
I think this sort of information is not only interesting to us, but there is surely a lot of people considering to buy these parts that might find the information usefull.
Did you try out the methode I described? I tried out scudzuki methode, but unfortunately I am too much out of shape to manage the required 200 lbs push
Last edited by philbur; 05-26-2009 at 01:32 PM.
I Love Mine!!! I have about 20 hours on them and all is good....
10um = 0.000393700787 inches
Google will convert anything for ya, give it a shot.
Google: 10 microns in inches
-
Andy
WOW!!!! That's way below what I can measure.........When I first installed the ball screws and nuts, I did have some backlash due to the ball nuts cocking when they were tightened down (maybe because you can only tighten against one side of the flange?), they would turn but not like they do now that they are shimmed at the flange to give proper alignment. And I'm going to stick to my claim of no backlash however, until I can prove to myself otherwise.......
Robert
Last edited by rwestbr; 05-26-2009 at 04:03 PM.
As for unit conversion, for me nothing beats the freeware “A Simple Unit Converter” (http://outrun.com.au/Simple-Unit-Converter.html). In this globalized discussions it always come in handy.
Said that, promise I won’t go of topic again
By the way, I measured the backlash of all my 3 Chinese ball screws just to make sure. Backlash seems to be pretty consistent at 50um (0.001968 inches).
Thing is, backlash seems to be not all. There is also a periodic fluctuation of another 50um or so every turn of the nut, and about 0.2mm displacement at the top of the X2 column due to elastic bending with a 10Kgf pull (for the X2 mini mill).
All that makes me think that 50 um backlash is pretty much irrelevant given all the other factors.
"There is also a periodic fluctuation of another 50um or so every turn of the nut, and about 0.2mm displacement at the top of the X2 column due to elastic bending with a 10Kgf pull (for the X2 mini mill)."
I made a back brace for my X2 , it makes a huge difference in rigidity and flex of the column. Its 5/8" x 5" x 12" steel with a pocket milled for the column to set in and also bolts to the base for added rigidity,and I cut a piece of 2" diameter steel tube to fit inside the column where the shaft goes though so it will not compress the column when the nut is tightened.
Joe, that is not entirely true. Backlash is caused by clearance between the nut and screw. Lost motion is caused by axial movement of the screw (the screw is not held rigidly at the fixed end). For every action, there is an equal and opposite reaction. Not only does the screw push against the nut, but the nut also pushes against the screw. If the screw can move, it will.
Checking for backlash this way will produce a sum of backlash + lost motion . Two different problems. The proper way is to eliminate any movement of the screw before checking for backlash.
Diplomacy is the art of saying "Nice doggie" until you can find a rock. - Will Rogers
It makes a lot of sense what you did. I am thinking about doing the same, but first I would like to trace the flex of every component from spindle dow to the table so I know how much flexibility comes from where.
I only measured on the top of column, mid column and base. From those components I can say that most of the flex comes from that white support that connects the colum big nut to the base. So it does looks like the best place really. I just want to measure the rest so I can quantify the improvement I would get by building a support.
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