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Epoxy granite metal lathe - videos
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A nice job, but he made the same mistake the mini lathe makers do. The spindle is way too short
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Quote:
Originally Posted by
Frank S
A nice job, but he made the same mistake the mini lathe makers do. The spindle is way too short
Is your concern the distance between the bearings or is there something else on your mind?
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Quote:
Originally Posted by
tonyfoale
Is your concern the distance between the bearings or is there something else on your mind?
got it in one. Lathe manufactures of old, used to make the distance between the spindle bearings significantly greater than total swing diameter up to 1 1/2 to 2 times the distance to help control runout and reduce the loading on the rear bearing which could be smaller than the bearing behind the chuck.
Even the manufactures like Southbend, Sheldon and Atlas, who primarily made smaller lathes maintained a slightly longer distance than total swing diameter, but their spindles were slimmer with a smaller overall diameter creating a possibility for flex along its length.
Many of the mini lathes have a spindle length so short the distance between the bearings is only the swing radius, done this way because their thoughts are the lathe since the lathe is small and light weight the cut and feeds will be only a fraction of what a larger lathe is capable of preforming so runout and flex will be negligible or nonexistent in most cases, allowing them to save weight and costs of manufacture.
the Lathe this guy has made out of granite will have great stability for its size due to its mass and the rigidity of the stone the one limiting factor will be the length of the spindle and probably the diameter of the bearing next to the chuck
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Quote:
Originally Posted by
Frank S
got it in one. Lathe manufactures of old, used to make the distance between the spindle bearings significantly greater than total swing diameter up to 1 1/2 to 2 times the distance to help control runout and reduce the loading on the rear bearing which could be smaller than the bearing behind the chuck.............
...............the Lathe this guy has made out of granite will have great stability for its size due to its mass and the rigidity of the stone the one limiting factor will be the length of the spindle and probably the diameter of the bearing next to the chuck
One of the reasons that some machines have a large distance between the bearings is to have room for speed change pulleys etc. There is a compromise here behind bearing loading and shaft rigidity. You reduce both bearing loads with greater spacing but you lengthen the spindle which increases its compliance. Bearing loads can be catered for by appropriate bearing selection. In this case the diameter and wall thickness of the spindle would indicate that shaft rigidity should not be an issue. My gut feeling is that this lathe is in a good part of the compromise curve.
This lathe does not have great stability as the video shows. What would concern me about this lathe is its susceptibility to temperature change, stated to be due to the different coefficients of expansion between the bed and steel fittings. Which surprised me because epoxy granite is often stated to have a similar coefficient to steel.
This problem that the builder had with this lathe is of great concern to me because I am currently gathering bits to make a precision surface grinder based on epoxy granite. I just rechecked expansion coefficients and got this based on steel at 100%
Steel 100
Epoxy 425
Granite 60
The epoxy is very high and the granite is low, but what counts is the epoxy granite mix. I did a simple mixture calculation and got proportions for the mix of 89% granite with 11% epoxy which would have the same coefficient as steel. General guidelines for epoxy granite are 90 to 95% granite. I have seen videos of homemade tools which use a much lower percentage, I guess because it needs the right mix of aggregate sizes to fill 90% and also mixing 90% with 10% resin would not be physically easy. I suspect that this lathe has too low a granite percentage, which would explain why the rails bent so much when the temperature changed. This emphasises the importance of getting the correct aggregate sizes ratio and then epoxy ratio. Is pretty easy to test how well you have mixed aggregate sizes to fill 90%. Mix the different aggregate sizes and use water to check the void volume, only then get the appropriate amount of resin mixed.
In the case of the featured lathe the pictures of the bed show that the smaller aggregate, sand, has fallen to the bottom (I guess that was the top when cast) and the larger pieces are at the top, both areas will have too much void space which will need to be filled with excessive resin. The small grains need to be well mixed with the big stuff to fill the voids.
I had been thinking about using solid granite rather than epoxy granite but it needs that ~10% resin to match the coefficient of steel.
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Quote:
Originally Posted by
tonyfoale
One of the reasons that some machines have a large distance between the bearings is to have room for speed change pulleys etc. There is a compromise here behind bearing loading and shaft rigidity. You reduce both bearing loads with greater spacing but you lengthen the spindle which increases its compliance. Bearing loads can be catered for by appropriate bearing selection. In this case the diameter and wall thickness of the spindle would indicate that shaft rigidity should not be an issue. My gut feeling is that this lathe is in a good part of the compromise curve.
This lathe does not have great stability as the video shows. What would concern me about this lathe is its susceptibility to temperature change, stated to be due to the different coefficients of expansion between the bed and steel fittings. Which surprised me because epoxy granite is often stated to have a similar coefficient to steel.
This problem that the builder had with this lathe is of great concern to me because I am currently gathering bits to make a precision surface grinder based on epoxy granite. I just rechecked expansion coefficients and got this based on steel at 100%
Steel 100
Epoxy 425
Granite 60
The epoxy is very high and the granite is low, but what counts is the epoxy granite mix. I did a simple mixture calculation and got proportions for the mix of 89% granite with 11% epoxy which would have the same coefficient as steel. General guidelines for epoxy granite are 90 to 95% granite. I have seen videos of homemade tools which use a much lower percentage, I guess because it needs the right mix of aggregate sizes to fill 90% and also mixing 90% with 10% resin would not be physically easy. I suspect that this lathe has too low a granite percentage, which would explain why the rails bent so much when the temperature changed. This emphasises the importance of getting the correct aggregate sizes ratio and then epoxy ratio. Is pretty easy to test how well you have mixed aggregate sizes to fill 90%. Mix the different aggregate sizes and use water to check the void volume, only then get the appropriate amount of resin mixed.
In the case of the featured lathe the pictures of the bed show that the smaller aggregate, sand, has fallen to the bottom (I guess that was the top when cast) and the larger pieces are at the top, both areas will have too much void space which will need to be filled with excessive resin. The small grains need to be well mixed with the big stuff to fill the voids.
I had been thinking about using solid granite rather than epoxy granite but it needs that ~10% resin to match the coefficient of steel.
I'll be the first to admit I never watch anyone's videos to their entirety if the video is longer than half a minute. Additionally, I hardly ever put on my head [phones to listen to their dialog, consequentially I did not note anything about expansion issues and wouldn't think there could be much change within the normal range of temperature changes in most shops. where the climate could be controlled within 10 to 20 degrees one way or another, I know where there is a huge lathe made out of concrete with iron rails for the ways that is over 100 years old it is used to make 100 ft long shafts for hydroelectric turbines
I did notice in one of his videos where he used tapered roller bearings for the spindle bearings. A lot of lathes used those but nearly all of them that are of any quality also have an angular contact ball bearing next to the larger roller bearing at the chuck end of the spindle
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Quote:
Originally Posted by
Frank S
I'll be the first to admit I never watch anyone's videos to their entirety if the video is longer than half a minute. Additionally, I hardly ever put on my head [phones to listen to their dialog, consequentially I did not note anything about expansion issues and wouldn't think there could be much change within the normal range of temperature changes in most shops. where the climate could be controlled within 10 to 20 degrees one way or another, I know where there is a huge lathe made out of concrete with iron rails for the ways that is over 100 years old it is used to make 100 ft long shafts for hydroelectric turbines
I did notice in one of his videos where he used tapered roller bearings for the spindle bearings. A lot of lathes used those but nearly all of them that are of any quality also have an angular contact ball bearing next to the larger roller bearing at the chuck end of the spindle
I am normally like you regarding watching the videos posted on HMT forum but this one caught my attention because of the apparent similarity to what I plan for my surface grinder, and I watched by jumping along until there was a change in the picture. i did happen upon a section where he remeasured the alignment of the slides and there was a 5 thou vertical bend which he related to a small temperature change between that measurement and the same measurement when he aligned the slides. He then put a sample of epoxy granite in a freezer and measured it cold and room temperature and calculated the coefficient. I forget the value but it was much higher than steel, this can only occur if his resin content was excessive. It is a pity because it is one of the nicest lathe builds that I have seen here in stark contrast to a few of the recent offerings which are just plain crap.
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i just noticed that he used 11% epoxy but that was by weight, that is a big difference to 11% by volume and confirms my thoughts about it being resin rich.
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Quote:
Originally Posted by
tonyfoale
I am normally like you regarding watching the videos posted on HMT forum but this one caught my attention because of the apparent similarity to what I plan for my surface grinder, and I watched by jumping along until there was a change in the picture. i did happen upon a section where he remeasured the alignment of the slides and there was a 5 thou vertical bend which he related to a small temperature change between that measurement and the same measurement when he aligned the slides. He then put a sample of epoxy granite in a freezer and measured it cold and room temperature and calculated the coefficient. I forget the value but it was much higher than steel, this can only occur if his resin content was excessive. It is a pity because it is one of the nicest lathe builds that I have seen here in stark contrast to a few of the recent offerings which are just plain crap.
That makes me wonder if it might not have been fully cured or if it should have been aged for a while even after curing before further processing.
I'm not all that familiar the granite epoxy composite, but I have had fiberglass repairs creep due to shrinkage after they aged a while
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Hi Frank and everyone. (Hope this works first post here ever) I may be able to answer some questions about the lathe, although I’d love to hear more as it sounds like you all have quite a bit more experience than I do here.
For the epoxy granite, it’s admittedly been a few months since I’ve thought too much about it but I spent weeks reading the entire cnc zone main epoxy granite thread for the bulk of my research, and based on that I believe 11% by weight is close enough to optimal that chasing lower epoxy percents would have diminishing returns. ckelloug took stiffness measurements on many samples of eg and although around 7-8% by weight was better, 10-11% was very similar stiffness.
The look of the side of my casting looks like a total disaster lol, I absolutely see why you think it had terrible settling problems. I actually switched aggregate sources for one of my sizes of granite, the original source was black the new one white. So it looks like there was bad settling, but that was just me gradually switching to the white aggregate as I ran out of black (graded to the same size range of course).
As for thermal issues, honestly Im very concerned about that as I do not have a conditioned shop to work in, so my temps range from around 10F to 100F through the year. I think I can minimize warp with strips of steel attached to the bottom of the casting so when there’s a temp change it at least has even pressure on the top and bottom and doesn’t tend to “banana” so badly
I’ll stop typing now in case this doesn’t post or something lol, but hopefully that at least clears a few things up I forgot to mention on YouTube!
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Robert, glad that you chimed in, welcome.
Quote:
Originally Posted by
Robert Steinwandel
...... based on that I believe 11% by weight is close enough to optimal that chasing lower epoxy percents would have diminishing returns. ckelloug took stiffness measurements on many samples of eg and although around 7-8% by weight was better, 10-11% was very similar stiffness.
Stiffness is important although that can be controlled by using a steel frame cast into the EG.
Quote:
Originally Posted by
Robert Steinwandel
The look of the side of my casting looks like a total disaster lol, I absolutely see why you think it had terrible settling problems. I actually switched aggregate sources for one of my sizes of granite, the original source was black the new one white. So it looks like there was bad settling, but that was just me gradually switching to the white aggregate as I ran out of black (graded to the same size range of course).
It is good to know that settling was not a problem.
Quote:
Originally Posted by
Robert Steinwandel
As for thermal issues, honestly Im very concerned about that as I do not have a conditioned shop to work in, so my temps range from around 10F to 100F through the year. I think I can minimize warp with strips of steel attached to the bottom of the casting so when there’s a temp change it at least has even pressure on the top and bottom and doesn’t tend to “banana” so badly
Yes, that is cause for concern. I have read a few of the CNCzone posts and saw that stiffness was of more concern than thermal stability. This is a big mistake because, as you have found, thermal distortion can easily exceed any potential distortion from load. When I saw the problems that you had I was shocked because I have often seen that EG had a similar thermal expansion coefficient to steel. It was simple calculation to see that based on the coefficients of the constituents an 11% resin content by volume would match the coefficient of steel. There would be no commercial machines made if they had the same problems that you have.
As I mentioned before, your lathe design/construction is very high up on my list of best DIY lathes, pity about the thermal issues. If you do add steel underneath I would suggest that you mirror what you have on top, that would give a more balanced structure. Perhaps a better solution, although not easy to do, would be to drill one or two holes through the length of the base and fit steel rods threaded on the ends so that you can pretension them like prestressed concrete. The tension could be adjusted, after experimentation to provide the minimum distort over the temp range.
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Here's a thought, call it random brain flatulence if you like. When considering making castings out of a composite of materials would it not be feasible and indeed possibly prudent to create a wireframe steel endoskeleton out of something like small rebar maybe even as small as 1/4inch or an open woven wire mesh with a minimal mesh weave size of about 1 inch square for larger thicker castings possibly 2 or more layers, My line of thinking is the added tensile strength of the steel going in so many different directions within the cast member would serve to stiffen and possibly help to stabilize movement of flex due to climatical or temperature variances . To help insure a good cohesive bond between the mixture and the steel i think I would sand blast the steel to remove any mill scale as well as to create a rougher surface for the bonding.
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Quote:
Originally Posted by
Frank S
Here's a thought, call it random brain flatulence if you like. When considering making castings out of a composite of materials would it not be feasible and indeed possibly prudent to create a wireframe steel endoskeleton out of something like small rebar maybe even as small as 1/4inch or an open woven wire mesh with a minimal mesh weave size of about 1 inch square for larger thicker castings possibly 2 or more layers, My line of thinking is the added tensile strength of the steel going in so many different directions within the cast member would serve to stiffen and possibly help to stabilize movement of flex due to climatical or temperature variances . To help insure a good cohesive bond between the mixture and the steel i think I would sand blast the steel to remove any mill scale as well as to create a rougher surface for the bonding.
Frank, that is not a brain fart. It is not uncommon with these constructions to have a cast in frame and in fact is what I plan for my upcoming surface grinder. It is not the tensile strength that is important but the rigidity of steel. There another poster on this forum who made a large router, I think that it was with concrete not epoxy granite, in which he had a steel frame work. Search this forum for Bongo and you'll probably find it. Interesting chap who enjoys his work.
i get the idea that you are not all that familiar with the use of epoxy granite for machine tools but it is a well proven technique used commercially as well as for DIY stuff. Commercially it is used for prototypes and one offs because it is cheaper than getting one offs in cast iron, the traditional material. For DIY it is a method that can be used at home. When done properly it can result in a superior machine than cast iron because it has a lot more damping. Unfortunately a lot of DIY efforts are spoilt because the builders use aluminium instead of steel. Aluminium has 3 times the compliance of steel and a higher coefficient of thermal expansion. However, aluminium is easier to machine if you have a router or low quality mill.
Even though epoxy granite is a technique that can be used for homebuilt, that does not mean that it is always done well. Youtube is full of poor quality builds and a small number of quality builds. I intend to use a steel structure for my precision surface grinder embedded in epoxy granite but the steel will be bolted together not welded in order to eliminate the possibility of internal stresses disturbing the initial alignment. I shall be using linear rails like Robert used on his lathe.
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Tony- thanks for all the kind words
Not sure if you’ve already been testing mixes, or maybe you’ve worked with EG for a million years and I have nothing useful to say here, but let me trumpet opinions into the void and see what comes back hahaha
I found that 11% by weight was about the lowest epoxy percent I could do without starting to get more voids. Before HEAVY vibration with a vibrating table it is like damp sand and rocks (and extremely stiff to mix), it barely looks to have any epoxy on it at all. It only gets harder to mix the aggregate and epoxy together as the epoxy percentage drops, so 11% was about all my poor hands could take lol.
I was going to just epoxy the same cross section of steel onto the bottom of my lathe as it has on the top, but I think your tensioning rod idea is much better. I can’t make a 48in deep hole though, so I may cut slots for threaded rods in the bottom of the base, using a angle grinder diamond wheel.
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Frank- that sounds like a great call, and something like some rebar running the length of my base would probably have solved most of the problems I have now with temperatures.
One thing to keep in mind though, is you will need more epoxy the more surface area there is in the mold. The large aggregate doesn’t pack well against walls and other features, so you end up needing slightly more epoxy to not have voids. This is why they generally say that your largest aggregate should be at most about 1/4 the size of your smallest feature (although not bigger than 1in or so usually since bigger rocks tend to crack more easily)
So if you add like a 1/4in wire mesh inside the mold, you add a lot of surface area and small features, which would dramatically increase the epoxy you’d need in my experience. Personally I might put like rebar or threaded rod in the mold as the steel framing, but Id probably stay away from mesh screens (the surface roughness of the rods/rebar shouldn’t effect the epoxy too much more than a smooth rod and getting a mechanical grip from the epoxy is always nice)
I could absolutely be 180 degrees totally wrong here, but just my thoughts based on my experience with EG so far
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Quote:
Originally Posted by
Robert Steinwandel
I found that 11% by weight was about the lowest epoxy percent I could do without starting to get more voids. Before HEAVY vibration with a vibrating table it is like damp sand and rocks (and extremely stiff to mix), it barely looks to have any epoxy on it at all. It only gets harder to mix the aggregate and epoxy together as the epoxy percentage drops, so 11% was about all my poor hands could take lol.
In an earlier post I did say " mixing 90% with 10% resin would not be physically easy."
Quote:
Originally Posted by
Robert Steinwandel
I was going to just epoxy the same cross section of steel onto the bottom of my lathe as it has on the top, but I think your tensioning rod idea is much better. I can’t make a 48in deep hole though, so I may cut slots for threaded rods in the bottom of the base, using a angle grinder diamond wheel.
I did not think of it but cutting slots would be a very practical way of doing this. Good luck with it.
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Just as a thought I personally think if I were to consider a build such as one of these that I would first screen my aggerate to insure a more consistent size any larger rocks could be cracked or broken into smaller pieces this would reduce the possibility of having rocks in the mix with natural cracks hidden in them.
I know this is totally off base from what we are discussing but I have seen concrete structures crack in cold temperatures. The causes were determined the cracking was form moisture trapped in minute fissures of the larger rocks of the aggerate. Enough fissures with enough trapped moisture once fully cold soaked and boom the whole side of a concrete wall would look like mortar rounds went off in it
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Quote:
Originally Posted by
Robert Steinwandel
...... you will need more epoxy the more surface area there is in the mold. The large aggregate doesn’t pack well against walls and other features, so you end up needing slightly more epoxy to not have voids.
Voids on the surface are a cosmetic issue only, you should not compromise the bulk to the needs of appearance. It is easy to get a nice finish after mould release by filling the surface with a resin and glass micro-balloons mix.
Quote:
Originally Posted by
Robert Steinwandel
So if you add like a 1/4in wire mesh inside the mold, you add a lot of surface area and small features, which would dramatically increase the epoxy you’d need in my experience. Personally I might put like rebar or threaded rod in the mold as the steel framing, but Id probably stay away from mesh screens (the surface roughness of the rods/rebar shouldn’t effect the epoxy too much more than a smooth rod and getting a mechanical grip from the epoxy is always nice)
I would stay away from any mesh, that makes it way too hard to avoid voids. Rebar set in along the length of the casting is a good method but I would sand blast it first and maybe paint it with neat resin just before casting. I see that several people advocate using a fast cure hardener, I go the other way and use the slowest hardener. Apart from the time benefits of working with a slow set, the exothermic reaction will significantly heat a large casting during a fast cure. If the cure takes 24 hours then the heat has so much time to dissipate that the temperature rise is very low, which results in a more stable casting.
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Quote:
Originally Posted by
Frank S
Just as a thought I personally think if I were to consider a build such as one of these that I would first screen my aggerate to insure a more consistent size any larger rocks could be cracked or broken into smaller pieces this would reduce the possibility of having rocks in the mix with natural cracks hidden in them.
Frank, In order to a high enough granite percentage it is necessary to have a mixture of aggregate sizes. The voids between the large pieces get filled with the medium size pieces and the voids in the medium size pieces get filled with the small pieces. Other voids are hopefully filled with epoxy.
Quote:
Originally Posted by
Frank S
I know this is totally off base from what we are discussing but I have seen concrete structures crack in cold temperatures. The causes were determined the cracking was form moisture trapped in minute fissures of the larger rocks of the aggerate. Enough fissures with enough trapped moisture once fully cold soaked and boom the whole side of a concrete wall would look like mortar rounds went off in it
Very interesting. it wasn't until I came to live in NH for 3 years that I began to appreciate the effects that water/ice and very cold weather could have on everyday things. It was an education but I am glad to be back in warmer climes, with the cold being a distant memory.
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Sorry tony, I must have missed that in your earlier post, but I certainly agree lol
When I was talking about voids I was thinking of interior voids. Reading back though I phrased it very poorly. What I noticed -when there were lots of small features, the features sort of “used up” more of the epoxy content of the EG, and there were more voids throughout because there was less epoxy for the remainder of the casting. That was my impression at least, I wish I had been taking videos for YouTube back then as it was a really interesting effect.
Frank- I bought my stone from a landscaping store where it was stored outside, so everything even my sand was quite wet. I rinsed the fine dust off everything then dried it in trays in the oven which seemed very effective
I don’t know how much the fine dust effects things, but I had read that theres a minimum coating thickness of epoxy. Like no matter how small the particle, to coat it completely you need at least a layer of epoxy around it that’s a certain thickness. So very tiny particles end up not being very effective for lowering epoxy percentage, and just make the EG harder to mix. Has anyone heard anything similar to this? I think it was a paper by LeGarde or something like that (old paper like at least 50 years old I think)
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Quote:
Originally Posted by
Robert Steinwandel
.........Like no matter how small the particle, to coat it completely you need at least a layer of epoxy around it that’s a certain thickness. So very tiny particles end up not being very effective for lowering epoxy percentage,..........
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.
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I think I used .01in as my minimum but I can’t remember why I had that number honestly
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Quote:
Originally Posted by
Frank S
A nice job, but he made the same mistake the mini lathe makers do. The spindle is way too short
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
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Quote:
Originally Posted by
Robert Steinwandel
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 .
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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
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Quote:
Originally Posted by
Robert Steinwandel
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.
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 https://www.youtube.com/watch?v=sFrVdoOhu1Q&t=1s
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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
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Quote:
Originally Posted by
Robert Steinwandel
I had thought that angular contact bearings could be used without preload,
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.
