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About LinkBacksThis is true and the minimum useful size depends on surface tension and viscosity of the resin. I do not know what the optimum minimum grain size is for common epoxies.Originally Posted by Robert Steinwandel
This is true and the minimum useful size depends on surface tension and viscosity of the resin. I do not know what the optimum minimum grain size is for common epoxies.
I think I used .01in as my minimum but I can’t remember why I had that number honestly
I meant to ask about this much earlier, but would you mind explaining more about the spindle length issue? Im not sure I fully understand the different factors involved here
For this little lathe it may not be much of an issue, depending on how it is going to be used, how much power the Moter will have and the types of materials and cutters planned to be turned and used.
However the lathe makers of old had a general practice of placing the head stock bearings at a minimum distance of 1.5 times the diameter of the total swing, now some of this may have been due to the need for more space for the widths of the flat belt pulleys allowing for multiple speed changes, later being reserved for the transmission gears as in gearhead lathes. But a lot of it was to reduce the loading on the smaller bearing at the rear of the spindle and to help in controlling runout when turning or facing. This T reduce shaft flex due to the longer length many manufactures had spindles much thicker at one end not only because the bearing at the chuck end was larger since it sees more loading but since the forces were lower at the other end not as much material was required there,
Think of how the spindles on heavy duty truck and trailer axles are designed the outer bearing is smaller than the inner bearing because the loading is much less.
Very old heavy duty lathes had a split sleeve bearing at the big end of the spindle with a large flat thrust bearing sometimes made out of leaded bronze, and sometimes a caged ball bearing on the other end or just a smaller split sleeve bearing, Latter on the practice of utilizing tapered roller bearings on the spindles became the norm as they were easier to adjust by adding an amount of preloading the better lathes took this one step further and added a angular contact caged ball bearing between the larger tapered roller bearing and the spindle nose. Some lathe manufacturers did away with the tapered roller bearings altogether in lew of using multiple pairs of radial and angular contact bearings using an angular contact bearing facing the opposite direction at the rear of the spindle, this idea worked fine for drill presses but was very lacking in holding the spindle true as the bearings wore a little. Some car manufactures used ball bearings for their front wheel bearings that were not angular contact it proved to not be a good idea and abandoned the design. it wasn't until front wheel drive cars started becoming popular that ball bearings returned only now being angular contact.
So, since I am often accused of liking to type to summarize, when building a lathe or anything else for that matter look to the manufactures of old especially those who are still around after a 100 years and see how they have done it what they are still doing and in what way have they altered their designs to achieve their success .
Never try to tell me it can't be done
When I have to paint I use KBS products
Thanks for all the info here, that’s fantastic. My spindle I think is the weakest part of my design right now, but currently I have a two tapered roller bearings, 1 each side of my spindle. I can’t remember the precision class, I think relatively low which makes me think I’ll need to replace them with something better to be able to cut closer to true cylinders.
Im hoping to use a 2hp motor and vfd, and carbide insert tooling to cut steel. I have no real idea how realistic this actually is but I’m hopeful, the inserts seem really nice in that they’re so cheap to change out vs full carbide cutters and all.
I guess I’ll see before too long what kind of tolerances I can actually hit, and how well (if at all) this lathe works haha
Taper rollers are extremely common on lathe spindles, they are probably the worst choice though. During use the spindle will heat up more than the supporting structure, usually cast iron or epoxy granite. Steel, cast iron and EG of optimal mixture all have similar expansion coefficients so the hotter spindle will expand more which loosens the bearings. That is not good. If the bearings are fitted the other way around then they will tighten. That is not good.
Frank mentioned how it should be done. At least two opposed, preloaded angular contact bearings at the business end and a simple radial load carrying bearing at the back. The back bearing should be allowed to move as the shaft expands, often this is a common deep groove ball bearing with a fit in its housing which allows sliding, sometimes this is loaded axially with a spring. I prefer to use a parallel roller for this task. You can have proper fits for the races but the inner race is free to float axially. This is a more expensive option than two taper rollers. It has been on my to-do list for a long time to convert my lathe to this arrangement. Maybe I should make a new headstock out of EG, it might be easier?
Another way, probably cheaper is to have two taper rollers back to back with some preload at the front and simple ball or roller bearing at the rear.
Have you seen the video of Dan Gelbert's granite high precision lathe
Yeah that does make sense. With this spindle completed hopefully I’ll be able to make myself a “final” spindle incorporating some more sophisticated designs.
I had thought that angular contact bearings could be used without preload, although to be fair I’ve done pretty minimal research on them, mostly just enough to learn that they probably would be too expensive to be my first spindle attempt haha.
Actually on that note, how are back to back bearings generally preloaded for a lathe? Ive seen some general examples but not a great idea of how this is done for machine tools.
I actually just watched some of Dan Gelbarts videos recently, that’s really amazing. I thought air bearings required tolerances in the range of ~.0001-.0002 so well out of my abilities, but what an incredible machine
They are always preloaded in this type of application. If you buy top quality then they will come as matched pairs with the preload built in. Being practical, good results can be obtained with lower quality bearings which are more affordable. Preload can be done with shims between inner or outer races, depending on which way round you mount them.
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