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25th October 2014, 09:45 AM #16Cba
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Simon, it is not only when making "something long and parallel" that bed twist matters. It is more often when you make something not that long, but need it to be accurate. Imagine yo want to turn something 20mm diameter and 50mm long that needs to be a good fit. And the micrometer shows it to be 0.02mm thicker at one end. Because the 100mm thick concrete floor has moved. Because the soil under it is reactive clay that swells and shrinks with the seasons changing moisture content. What can you do, short of changing home or the concrete floor?
- keep correcting parallelism by adjusting always the same machine foot. This works very well, but requires you to spend time making one or two test cuts to measure taper error whenever you need an accurate part. Harold Hall describes how he does this with a Myford, in "Lathework a complete course" on page 36.
- if it is a smaller bench type lathe, and the bed does not have an inherent twist, do not mount it rigidly. Let instead the bed find its own stress- and twist free position. This is done by a) putting a hard rubber interface between the lathe bed and the table, and b) only bolting down the headstock side, the bolts at the tailstock side are inserted but left loose. Now your lathe is independent from movements in the floor and bench. The downside is, the bench/cabinet is not contributing anymore to the overall stiffness/rigidity of the machine.
With regard to rubber interfaces (puck) on larger machines, the only reason I am aware of these are used, is to reduce vibrations and noise from the machine onto the building floor and from there to other machines that may not like these vibrations.
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25th October 2014 09:45 AM # ADSGoogle Adsense Advertisement
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25th October 2014, 10:37 AM #17
Hi Simon,
Don't be so sure the Elliot's base wont twist. I can read the Blohms table twisting with a .02mm/m level. Only just before it lifts one of its feet but it is measurable. Having said that the blohm weights more than twice the Elliot bit still only has a 1/2" thick CI base.
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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25th October 2014, 01:10 PM #18GOLD MEMBER
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Thanks CBA and Ueee,
The world is certainly an interesting place when you have the ability to measure such small deviations. I would never have thought that the base would enough to be measurable. That's worth noting. Well, a very smart person once said to me that structural elements should be considered springs.
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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