Improvements to the Mini-Lathe Half Nut Mechanism?

[ScrimParticipant @scrim]

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

 

 

[ScrimParticipant @scrim]

[ScrimParticipant @scrim]

Looks like I might be on my own with this one then!

 

Scrim

[john swift 1Participant @johnswift1]

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

 

 

 

 

 

[ScrimParticipant @scrim]

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

[AESParticipant @aes]

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

[ScrimParticipant @scrim]

Fear not – if I come up with anything I’ll pass it on one way or another!

 

Scrim

[Sub MandrelParticipant @submandrel]

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

[ScrimParticipant @scrim]

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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