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Improvements to the Mini-Lathe Half Nut Mechanism?

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Scrim	19/02/2011 17:42:45
19 forum posts	Whilst adding a plastic chip guard behind my SC3 mini lathe apron I decided to strip and improve the half nut mechanism. In the case my 'Super C3' (SC3) it's more a case of a 'half of a nut' mechanism in fact as only the lower half of the usual pair of half-nuts is present because the new leadscrew guard doesn't leave any room for the upper one. It still seems to engage and drive the saddle ok, but I found the engaging lever would often stick so I wanted to improve it. I also noted the usual mini-lathe problem with the dovetails and gibs making poor contact so I expected to make some marked improvements by lapping them to a better fit. However, the more I study the mechanism the more I think the manufacturers need to redesign it from scratch. It's quite a surprise it works at all as far as I can tell. Virtually all the dovetail in contact with the half nut is missing on one side where a circular recess has been cut for the cam type (?) mechanism that the handle turns, and on the other side the gib is held in place by two screws+washers. It looks like a total bodge and I suspect any minor attempts to improve it may stop it working by what ever mysterious means it currently functions. Are there any articles out there about this issue and ways to resolve it, because I can't find anything beyond a mention that the mechanism includes a gib? Scrim
Scrim	20/02/2011 22:20:41
19 forum posts	Looks like I might be on my own with this one then! Scrim
john swift 1	21/02/2011 00:02:55
318 forum posts 183 photos	Hi Scrim , I expect its just the same as the standard C2 and C3 lathes so your not alone , but could be the first to find the solution ! while there is lots of details of mods to the C2/C3 lathes on the web a half nut mod isn't to be found the nearest for a C6 lathe can be found here --- http://www.toolsandmods.com/apron.html While I've made some mods to a Machinemart CL300m lathe eg. replace the plastic back gears with metal ones , replace the spindle ball races with taper roller bearings and scrap metal gibs with brass gibs the half nut mechanism is some way down the to do list John
Scrim	21/02/2011 14:53:07
19 forum posts	Thanks John, It's a good point about the half nut mechanism being well down the to do list! I wouldn't have bothered looking at it if I hadn't already been working on the apron. Also, it does seem my mechanism is a little crankier than some as a friend says they don't get the sticking problem I experienced. I'm tempted to just try to get it working as before (not necessarily a trivial exercise!) and get back to the both more important and more easily fixed issues for now. Scrim
AES	21/02/2011 15:19:48
85 forum posts 1 photos	Scrim, QUOTE: I'm tempted to just try to get it working as before UNQUOTE: Please don't do that - what I need (and I suspect many others too) is for you to spend many hours working out a real good mod, then more hours photographing and writing it up, then get David Clark to publish it in MEW. We'll all benefit that way! (Grin). Seriously, on my lathe (mine is actually a "Real Bull", not a Sieg) I did find it took me quite a time to set that up nicely at the start (after cleaning n degreasing) so that the self act engaged/disengaged positively each time. But so far that seems to be more or less OK still. But I would like a better system for that system though - as you say, it's well down the list still! Where I do have a much bigger problem is with lowering the self act rack on the bed casting and/or adjusting the carriage handwheel to take out the LOADS of backlash I've got - I've had some success with that but if I want a decent finish I still use the self act only (not the handwheel) 'cos the handwheel has so much backlash n general "slop" that I just CANNOT get a decent finish using the handwheel - with the carriage adjusted decently (no "slop" to the bed) it's pretty tight to turn the handwheel and it tends to "jerk" - PERFECT for a good finish! So while you're at it, do a decent mod for that too please (grin again) - and YES, I have seen a mod on one of the US fora re machining up an eccentric bust for the handwheel bearing. Krgds AES
Scrim	21/02/2011 17:25:20
19 forum posts	Fear not - if I come up with anything I'll pass it on one way or another! Scrim
Stub Mandrel	25/02/2011 21:29:48
4318 forum posts 291 photos 1 articles	Hard to comment without seeing a photo. The original mini-lathe arrangement is quite good IMHO. It even has an adjuster to set how closely the half-nuts close on the leadscrew - obviously pointless with an 'half-nut'. Neil
Scrim	26/02/2011 19:50:36
19 forum posts	Thanks folks. For now I've made do with the following which has improved things a lot: I fitted a large diameter thin spring washer between the apron and the 'cam' type disk that's rotated by the engaging handle. I cleaned up the face of the handle which bears on the apron by facing in the lathe (with no apron you have to hold the saddle against a bed stop and use the top slide to feed the tool to the left). I replaced the two soft washers which hold the gib in place with two strong steel 'washers'. I used the ends of the two feeler gauge strips that already had holes bored in them so all I had to do was cut off with a Dremel and clean the edges up. With adjustment of the gib it now has a smooth and precise feeling action. It doesn't appear that using a single half nut detracts from the mechanisms function, so I won't worry about that as I like having the guard to keep swarf off the lead screw. ARC actually offered to send me the upper half nut when I bought the Super C3, but I decided not to take them up on it for some reason. Scrim

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