Epoxy granite metal lathe - videos

[Frank S]

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

[tonyfoale]

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.

[tonyfoale]

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.

[Frank S]

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

[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.
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

[tonyfoale]

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.

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.