Do You Need The Balls?
There is a common conception in the amateur machinist world that ball or roller bearings are always the preferred selection to bushing or solid surface bearings. That is not always the case. Machine engineers, of all people, should know it always depends on the application. There are many applications where ball bearings are contra indicated, meaning they should not be used. Some of those reason can be as simple as cost over performance, excessive noise or the possibility of bearing contamination in harsh environments.
The primary benefit of using a ball bearing over other types is the greatly reduced drag due to its small rolling surface contact point. Their use is preferred when low friction is a high priority and other factors like low noise is not. They will reduce power requirements, help reduce friction heat and provide long periods of operation. If they are properly sealed, they will reduce maintenance. The best, high quality ball bearings can reduce run out and take thrust loads. These are worthy goals but sometimes are higher goals than necessary.
To quickly get to my point, I’ll examine the bearing needs for the lead screws on all the axis of a small manual mini lathe. First, do radial ball bearings “increase machining (position) accuracy” on any of the calibrated axis including the Z axis main feed screw over a bushing type bearing? The answer is an absolutely no.
This is because the linear slide position accuracy is a function of axial (not radial) screw loads affecting the amount of compression/expansion of the lead screw, drive nut thread engagement, drive nut mounting rigidity, thrust surface material and screw shaft bearing end play. This as combined is commonly called backlash. Minor drive screw side play (run out) is not a factor.
Radial bearing loads are produced by the slight vector rotational driving force (and a human hand) pushing sideways (radially) on the shaft hand wheel or end gear. A bushing bearing can easily distribute this force with little wear. I’ll say it again; radial run out of the drive screw bearing is not an accuracy factor in any practical example of carriage position accuracy.
On manual machines there is always some axial backlash because removing it all requires use of a special anti-backlash drive nut. Removing all thrust movement at the shaft bearings and using standard drive nut adjustment will eliminate most all backlash but gets into a preload condition that will make the drive screw more difficult (sometimes impossible) to move. This causes substantial and premature bearing and drive nut wear, very undesirable.
Most if not all of mini machines use a single ended (one end) drive screw bearing. The far end of the cross feed and compound screws have no bearings at all. The end of the screw can “wiggle” when turned. The main lead screw on a lathe is usually double ended to prevent whip and bending. (Wiggle)
A low friction axial (thrust) bearing could be considered desirable on the powered end of the main Z axis to reduce friction, but even here there is only a low RPM intermittent duty load and it is machine powered to overcome friction. An axial (only) ball bearing will not increase accuracy or improve performance. On some lathe machines a simple rack and pinion provides manual Z axis movement.
The critical accuracy parts are the thrust surfaces. Ball bearing thrust washers and radial ball bearings with thrust load capabilities can be used to reduce turning drag on the hand wheels and drive screws of larger machines. In this application (of ball bearings) a very slight preload can be applied to the shaft to remove end play while still providing the ability to rotate the knob. High quality sealed bearing surfaces are required because of the small point contact of the ball bearings. Cheap “soft” bearings can easily flatten the ball with too much preload creating a “lumpy” turning action. If you feel lumps it is already too late.
On a small mini lathe I believe the complexity and cost benefit of miniature ball thrust bearings and especially radial (only) ball bearings can not entirely be justified. Well finished, properly adjusted and oiled flat thrust surfaces and bushing radial bearings will provide equally long service and help reduce cost in this application. Most manufacturers and customers seem to agree.
I like some light friction of movement of the hand wheel so the setting is less likely to shift because of “back driving” or vibration while making a roughing cut. Of course for making critical finishing cuts I always lock or tighten the axis gib lock screw to prevent this shift movement. That’s the correct way to do accurate machining. I never rely on constant axial load forces against the lead screw nut and bearings to maintain an accurate tool position.
When I think of nearly continuous CNC machining, none of the above applies. That is a different beast and I do want reduced friction and zero backlash wherever I can find it.
So what makes expensive machines expensive? Larger high quality commercial machines and very expensive hobby +$10,000 machines (like the Myford Super 7) use ball bearings wherever they can. So it is true such bearings are a sign of quality. (The maker has to do something to justify that price.) Only you can decide if it is a requirement on your mini machine “must have” list.
I’ve asked myself: Will ball bearings make the manual mini machine with inherent back lash more accurate? Will they make me a better machinist? Is “nothing but the very best” my equipment goal? Can I afford that goal? Is there a hidden, perfectly designed, with every feature, at the lowest cost (or even medium cost), all ball bearing, zero backlash mini lathe machine just waiting for me and the world to discover? If so, why has its existence been so well hidden? Wouldn’t everyone be using that machine?
I have answered my questions. Would I like ball bearings? Sure. Do I need them? Read this again.
Oh, and yes, my larger machines (like the X3) do have ball thrust bearings. I do like ‘em. I’ve decided I don’t always need them to make one perfect part at a time.