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| plastic intermediate gears or metal for mini-lathe? |
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| lordedmond:
cricky that must be a small machine 6004-RS2 has a 20mm bore ? Stuart |
| drmico60:
Hi Woodsman22, The plastic change gears may vary in dimensions due to temperature changes but simple size change will not alter the accuracy of thread cutting. If you have a 25 tooth gear driving a 50 tooth gear then the ratio will always be 1:2 irrespective of the size of the gears. The only thing that could affect the accuracy would be if the tooth spacings varied on the gears. This is not very likely since they will have been moulded in a precision master mould. The same will be true for the main spindle gears and the intermediate shaft gears. The main reason for changing to metal gears is that some users have broken the plastic gears. This usually happens as a result of misuse. In the early days of my lathe I broke my 80 tooth change gear by driving the carriage into the headstock. If that had not happened something else somewhere would have broken or failed (perhaps the electronic speed controller due to overload). The gear was cheap to replace whereas other damage may have been much more expensive (new controller board is circa £80, new gear circa £5). For this reason I do not believe it is a good idea to have metal change gears. There may be some justification for metal gears on the spindle since I have heard of these breaking under load but even here I would be tempted to put the metal gear on the spindle but keep the layshaft one in plastic in order to provide a weak link to break in the event of a serious jam up. I would keep the layshaft in plastic since it is less hassle to replace than the spindle gear if it breaks. I actually planned to do this on my lathe and ordered the metal gear for the spindle. I did not fit it immediately on arrival and some months later when I went to fit it I noticed that the metal gear was cracked. There is not much metal between the top of the keyway and the root of the gear teeth. So much for greater robustness of metal gears. Another reason for keeping with plastic gears is that they are much less noisy than metal gears and they require little lubrication. If you decide to fit metal spindle gears then also think how you are going to get oil or grease to them. Perhaps drill some holes in the headstock close to the gears to permit access for spray grease. Thread the holes and block them with screws to prevent access of dirt during normal use. Why do you need super accurate threading? The geometric relationship between the spindle rotation and the carriage movement through the gears ensures sufficient accuracy for most purposes. Mike |
| awemawson:
Bigger professional lathes usually have a 'shear pin' in the drive chain to the lead and feed screws. Usually made of soft brass and just comprising a collar pegged to a shaft with the shear pin. Later Colchesters had a spring loaded detent mechanism for the same purpose. Andrew |
| vtsteam:
My guess is that Woodsman is already decided on metal gears for threading, has purchased and installed them, has already read the pros and cons elsewhere, is asking about longevity of a plastic spindle drive gear and actually wants practical advice on installing different spindle bearings after purchasing them and removing the headstock for that purpose. If so, Woodsman, it would be helpful if you gave specific details of your bearings, and state of disassembly of your headstock, photos would be very helpful, and a description of the specific problem you think you are facing in installing those bearings, if any. Then you will get practical advice. This is an extremely helpful forum. |
| lordedmond:
Hi I did have a 7 X 14 lathe a good few years ago ( is it that long how time fly's ) IMHO replace the change gears and the bearings but retain the plastic speed shift gear in the head stock if only for your ears the gear that you can buy are poor and are soft and poorly formed , with these in you have a safety for a crash ( its not if its when ) they will break and are cheap to replace with metal ones in the next weakes bit will fail and may not be so cheap. as you have stated you are new to ME but are you new to lathe work ? as for info try here I am swear that it may not be you lathe but they are pretty generic http://www.arceurotrade.co.uk/machineguides/Mini-Lathe-Angular-Contact-Bearing-Change-Guide.pdf also plenty of other info on the site no link with the from only a customer Stuart |
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