Help with ball screw calculation

Inferno

Inferno

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I need a ball screw for my lathe modification.
I want to retain the factory dial indicator.
The leadscrew is 10 TPI so each revolution is .100
I would like to have a 10TPI ball screw but could live with a 5TPI
It looks like ball screws are all metric. Am I wrong?
I need one that's 5/8 or 16mm in diameter.
 
Rockford makes a 10tpi ballscrew, but the smallest dia is 1", so probably much too large. A 5tpi ballscrew can back drive under load. I have them on my manual mill. I can work around it on the mill pretty easy but on a lathe you wouldn't want the hassle, unless you're doing a cnc/els setup
 
Inferno said:
I need a ball screw for my lathe modification.
I want to retain the factory dial indicator.
The leadscrew is 10 TPI so each revolution is .100
I would like to have a 10TPI ball screw but could live with a 5TPI
It looks like ball screws are all metric. Am I wrong?
I need one that's 5/8 or 16mm in diameter.
Click to expand...
Not sure if I can help but not sure why you want to convert without going CNC.

From my understanding for manual work taking up the backlash helps with accuracy which you don’t do with ballscrews.

John
 
matthewsx said:
Not sure if I can help but not sure why you want to convert without going CNC.

From my understanding for manual work taking up the backlash helps with accuracy which you don’t do with ballscrews.

John
Click to expand...
The conversion is toward eventual CNC. Baby steps.
I should have said it's a lathe/mill combo. I want to use the stepper most for the milling. With a 6" travel it's 60 hand cranks. Multiple passes got to be a pain.

What do you mean "you don't do with ballscrews" in relation to the backlash?
 
Inferno said:
The conversion is toward eventual CNC. Baby steps.
I should have said it's a lathe/mill combo. I want to use the stepper most for the milling. With a 6" travel it's 60 hand cranks. Multiple passes got to be a pain.

What do you mean "you don't do with ballscrews" in relation to the backlash?
Click to expand...
When I take a cut with my manual lathe or mill I have to take up the backlash (the amount I turn the screw before before the carriage or table moves) before I start cutting. Most textbooks will describe this but I’m sure you’ve experienced it with your machine.

With my CNC I use a stepper motor that’s coupled to a ball screw where the motion is nearly instantaneous. That’s what allows the stepper to “count steps“ and know where it is relative to a zero point. On a manual machine with standard acme lead screws, the zero point is measured from where the backlash is taken up. That’s why the dials rotate independent of the screws and allow you to “zero” them where your cut begins.

Hopefully that’s a good enough explanation.

When I first started I had a similar machine to what you have. On this forum we try not to discourage folks from doing whatever they want with whatever machine they have. However, knowing the limitations of any particular machine is an important part of the learning process which we do encourage.

I understand and applaud the desire to do as much as you can with the tools you have. But, I would encourage you to do whatever you can with the machine in it’s current configuration. You’re unlikely to increase the accuracy or efficiency of this type of machine by converting it to ball screws since it probably doesn’t have the necessary rigidity for any future CNC conversion. There are many worthy machines out there, often for less money than you think. You can build/add a power drive for the x axis without changing the screw if cranking by hand is an issue.

Stick with us here, many have gone down this path before you and you can learn much by reading about our experiences. Here’s one path I’ve been going down for a few years now. I’ve learned an awful lot and I can assure you I would have done it very differently if I had know what I do now when I started.

www.hobby-machinist.com

Homemade vertical mill drill

Well, I guess I'm gonna try to make myself a mill for cheap. I found this column on Craigslist for $80. And I ordered this spindle I can power with a VFD I already have. And I'm going to get one of these cheap x-y milling tables. And probably look for a really flat piece of C channel...
www.hobby-machinist.com www.hobby-machinist.com

Cheers,

John
 
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I understand backlash. I think that was during the first week of Machine Shop. I didn't mean for you to have to explain that part.
I was questioning if you thought there'd be some backlash with a ball screw.

I'm under no delusions that my machine will ever be a super amazing mill. I know it has limitations.

Most everything I do has a +/- .005 anyhow but if that gets better, it won't hurt my feelings. I've done .001 repeats on this machine so I know it's tighter than many hobby mills. I've already done a few upgrades and tightening to get it closer. The weakest link for accuracy is the quill on the mill head. It's not the best machining job out there and has a lot of limitations.

The DRO part is going to be done "Just because I can" and not to serve any important purpose. I might use it to hog out metal and then finish it manually. I could send the code to rough it out and sit back and watch.

I actually enjoy reinventing the wheel sometimes. That's why my current builds are a 3D printer, a 30X52 CNC machine, the Stepper mods to the combo and I have a car that never came with a 5speed transmission converted to a 5 speed using the original engine. (not as easy or hard as it sounds)

I will say this, though. The backlash is stupid loose and that's after tightening it up. Even if all I do is get that gone, I'd be happy.
 
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dfloen said:
Rockford makes a 10tpi ballscrew, but the smallest dia is 1", so probably much too large. A 5tpi ballscrew can back drive under load. I have them on my manual mill. I can work around it on the mill pretty easy but on a lathe you wouldn't want the hassle, unless you're doing a cnc/els setup
Click to expand...
Rockord's finest lead is .125" and that is on a 3/8" shaft. 10 tpi would be difficult to accomplish on a lathe lead screw as the balls would be tiny and not capable of carrying the load. I would suggest that the OP look at a larger lead, e.g., .20" or .25" and use appropriate gearing or software division to to achieve his goals.

My Tormach mill commands steps and assumes that the commands are received and executed properly. Backlash is controlled by mechanical means but still exists. It is on the order of .001" and there is software compensation available. A better option would be to have a separate and independent linear encoder system which controls motion. This would in theory limit backlash to the resolution of the linear scale resolution.

Tormach uses a term called "lost motion" which in addition to free play, also includes things like windup of the lead screw under load. Adjustments like increasing thrust bearing preload can actually increase the lost motion. Increasing cutting load will also increase lost motion. An example of this is the spring pass commonly used when turning on the lathe. A small diameter ball screw will be more prone to twisting due to load induced torque.
 
Inferno said:
I understand backlash. I think that was during the first week of Machine Shop. I didn't mean for you to have to explain that part.
I was questioning if you thought there'd be some backlash with a ball screw.

I'm under no delusions that my machine will ever be a super amazing mill. I know it has limitations.

Most everything I do has a +/- .005 anyhow but if that gets better, it won't hurt my feelings. I've done .001 repeats on this machine so I know it's tighter than many hobby mills. I've already done a few upgrades and tightening to get it closer. The weakest link for accuracy is the quill on the mill head. It's not the best machining job out there and has a lot of limitations.

The DRO part is going to be done "Just because I can" and not to serve any important purpose. I might use it to hog out metal and then finish it manually. I could send the code to rough it out and sit back and watch.

I actually enjoy reinventing the wheel sometimes. That's why my current builds are a 3D printer, a 30X52 CNC machine, the Stepper mods to the combo and I have a car that never came with a 5speed transmission converted to a 5 speed using the original engine. (not as easy or hard as it sounds)

I will say this, though. The backlash is stupid loose and that's after tightening it up. Even if all I do is get that gone, I'd be happy.
Click to expand...
If your screws are good you can easily make new nuts of modify the ones you have.

How much backlash are we talking here?
 
RaisedByWolves said:
If your screws are good you can easily make new nuts of modify the ones you have.

How much backlash are we talking here?
Click to expand...
.020 backlash. It was closer to .050 when I first got it. The leadscrew is still pretty low miles.
 
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