Machine Shop / Locomotive Drive Work

[bob turner]

That looks significantly more massive than Sherline. Here is a more reasonably priced Atlas - $400 would be my mark.

https://www.ebay.com/i/143437898304?chn ... gKrqPD_BwE

Also - these things are not large. Kitchen table size, but twice the size of Sherline.

[up148]

Thanks Bob. Looking for a mill 1st and then maybe a lathe. Used is okay, unless it has issues (abused, dropped, etc.) and then I wouldn't know how to fix it and might not even know there is an issue unless the tolerances were sloppy. I don't have your experience with machinery (or aircraft) so believe new might be the way to go for me.

[up148]

Jim, do you mind me asking what model you chose? Choices of Ballscrew, Leadscrew and CNC and multiply choices within the main styles.....looks like I need to do some research as I have no idea what the difference is or which I need.

[R.K. Maroon]

No problem, Butch. I bought the model 2019ER, which is the manual mill with the longer table. I did buy it with the upgrade to the 1/4 HP motor. I bought mine in April of 2015, and as I recall it was just south of $1000. They did not have the ball screw option back then, so mine has the lead screw, which is just fine for what I do.

Jim

[up148]

Thanks Jim. I doubt I need the ball screw either, but I don't know the difference.

[R.K. Maroon]

A lead screw is probably familiar to everybody -- a threaded rod and a nut. A ball screw is a wild contraption -- one I certainly never would have thought would be practical. The video here gives you an idea of how it works:

https://www.youtube.com/watch?v=kl6qNn9-nkk

It the idea of a return path for the balls is the part that strikes me as improbable. But I would be wrong.

With a lead screw, there is sliding contact between threaded rod and nut. With a lead screw, the contact is rolling. This is a lot like the difference between a sleeve bearing and a ball or roller bearing. Sliding contact always has friction and lubrication issues, rolling contact much less so. Also, any sliding contact device has to leave some gap for machining imperfections and for lubricant. So these are generally not as precise. Rolling contact can be metal-to-metal (which is what happens when you tighten those wheel bearing nuts on your car).

Bottom line: Lead screw is higher friction, slightly less accuracy. Ball screw is lower friction, slightly higher accuracy.

More information is available with an online search
Jim

[ScaleCraft]

Same idea as a recirculating ball steering box.

https://auto.howstuffworks.com/steering3.htm

Been around for what, 60 years or more?

https://en.wikipedia.org/wiki/Recirculating_ball

[ScaleCraft]

The Saginaw recirculating ball steering gear was introduced on the 1940 Cadillac, so close to 80 years.

[big bad jim]

I'm mostly still a rookie when it comes to machining, but I would submit that a lead screw isn't necessarily less accurate than a ball screw, as long as you always account for the backlash. If anybody isn't yet aware, backlash is the little bit of turning of the hand wheel before anything actually moves, when you change the direction of wheel rotation from clockwise to counterclockwise . The rule of thumb I learned was to always approach the desired position via a clockwise turn of the hand wheel. Sometimes you would need to move past your desired position so that you could come back to it via a clockwise turn. This may seem wasteful, but in this way, you would have always taken up the backlash. It becomes habit and second nature pretty quickly.

Backlash is quantifiable, enough so that a digital read out device can be calibrated to account for it, in a relatively simple manner.

[up148]

Thanks guys, this is great information and tells me what I needed to know. As explained by BBJ, the play with the lead screw is like knowing you always have to compensate for the end play (1/16" + -) on most retractable tape measures. Like buying tools, cars, etc, you have to decide if the "supposed" improvement is worth the up charge. Looking very forward to getting a mill.

[R.K. Maroon]

Jim, your point about backlash is well taken. However, an internet search on the differences between a lead screw and a ball screw consistently indicates that the ball screw is more accurate. Again, I believe this is analogous to the difference between a sleeve bearing and a ball or roller bearing. With no sliding contact, the rolling contact devices can be built with tighter tolerances, as they don't have to worry about seizing up.

Note for the record that I am perfectly happy with my Taig mill (and lathe), both of which have lead screws. If I were going the CNC route though I might choose the ball screw, not for its accuracy but for its lower friction values, which would reduce the load on the table stepper motors.

Jim

[ScaleCraft]

More of a zero lash due to the recirculating balls taking up[ any slack, I think.
Why it worked so well for so long in steering boxes.

[bob turner]

We don't need such tight tolerances, except for crankpin throw and quartering. Neither of these require a lead screw; they require jigs.

[R.K. Maroon]

Back to the PA Project. The roof of the cast PA has a hollow under the dynamic brake unit. I don't know if this was to save a bit of brass during casting or to keep the casting approximately the same thickness throughout to better control warpage and shrinkage during cooling. I measured the depth and calculated that the roof would be very thin (maybe .030") with the dynamic brake milled off. That got me worried about the roof breaking out during milling or otherwise leaving it susceptible to damage down the road ("Jim, you won't believe this, but I picked up your PA and dented the roof with my thumb"). I decided to fill the hollow by soldering in fitted brass bar stock. The before and after are seen on the left and right:



I had to be very careful with the torch less I unsolder the nose on this one, which does not have a bolt flange to the roof to hold it in place. I used a wet rag on the nose as a heat sink. Perfecto!

Bob T may wish to note that the roof on the left is pristine new-old-stock, which somehow wandered into the shop as an orphan.

Jim

[bob turner]

Noted. All we need now is two nose halves. We means I don't care which of us winds up with all the pieces - we will have saved one of 600.