I bought a set of change gears today, before i fitted them or checked them in any way i though i would try cutting a thread and leaving the nut engaged.
er, no need you can just change leadscrew direction, yes the CVA is a nice bit of kit.... I hadn't tried this before.. Infact leaving the nut engaged is how i will do all my threading untill i flog the lathe, whats the bet whatever I replace it with won't have this facility, or is this common.
Most lathes have some form of leadscrew reverse. From the simple tumbler gear on the Myford Boxford types to the inbuilt levers on the screwcutting boxes.
It's not advisable to use this method for threading return as very often when swapping over from forward to reverse you can mesh a tooth out.
-- Regards,
John Stevenson Nottingham, England.
Having a Holbrook 8B I am very interested in this remark. The guy who sold it to me extolled its high speed screw cutting facility.
The carriage has a micro-adjustable arrangement that (when set-up) nudges the carriage drive gear into neutral (either when using fine feed or when screw cutting). If screw cutting I then withdraw the tool slightly before pushing the lead screw into reverse. This can all done with the lathe running - at least when at slow/medium speeds. I have always assumed that it is meant to work that way.
I can understand a tumbler reverser gear dropping a tooth but had always assumed that my machine had a dog clutch and would always pick up at the same place. On thinking about it perhaps it can also pick up 180 deg. out of phase. Presumably of no consequence if cutting an even number of tpi but what about an odd tpi?
Should I be worried?
like the tumbler on my cincinatti, can't really see me leaning around the back and loseningthe big bolt locking it and flipping it over- besides my cincintti lost some gears to my beadroller. I am guessing the CVA has a one position dog on the drive gear it must use to slide between the two direction gears, if thats how it's done. I do have a manual and will have a look later
I cut a 27 tpi thread and made about ten passes down it to see that the tool cleared every time, i think it very unlikely i would have managed that if it was possible to get it meshing wrong.
I would have expected you to say of course the CVA has such and such... Just had a look at the manual.
I am quite suprised i didn't remember this. so how many more lathes have this facility..
I fooled around with the gears I got yesterday and putting the 127 tooth gear in the chain is a git, it really is going to have to be used with the cover off and the gear spaced away from the casting, not very elegant. I have considered belt driving to the box, as the gear[that pasky
127t] can be used in a certain position in the headstock casting but there won't be enough movement to mesh it. all this would be solved if i had bought a colchester..suppose i should see about using a 63 tooth, with my present gears. so metric threading is a piece of piss;-) with a CVA...
-- richard
Well, well, you learn something every day. I have had a CVA in daily use now for about 15 years and I never realised this. It probably won't make the slightest difference to me as I still use the screwcutting dial. I can see where this comes in for metric but not for english. I do have all the change wheels for mine but I must admit other than swapping the top gears between high and low I have never fitted the other set for metric on this machine as I have another all metric machine I keep for metric threads.
One mod I would like to do shortly is to change the start control from the headstock mounted lever which is far too close to the chuck to a traveling lever on the apron to match my other machines. Just have this as a forward / stop / reverse switch wired to an invertor so I can get fast reversing when tapping etc as the original clutch does not like being banged straight into reverse.
-- Regards,
John Stevenson Nottingham, England.
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