Thread: Al250g update

I was going to ask that. but yesterday you said you had done a few passes before there was an issue.

You havent got powerfeed engaged when you close the halfnuts?(on most* lathes they are interlocked so you can't... not sure about your lathe)

Stuart

*most maybe an over statement.......not like I have used that many lathes............... so "all lathes I've had anything to do with"

Thanks for your response Stewart.
I disengage the power feed (into the neutral position) before I try and engage the half nuts today.

You're right i did manage to do a couple of passes with the half nuts engaged initially--just enough to make a hairline on the job, but as soon as I tried to take a 'tiny' cut then everything stopped.

I got a mirror to check what happens when the half nut is engaged and on the 'full engagement' the half nut lifts the lead screw about 2mm. ie it pushes up against the lead screw and bends it.
I wonder if thats supposed to happen?

regards
Brian

Originally Posted by Briangoldcoast

Thanks for your response Stewart.
I disengage the power feed (into the neutral position) before I try and engage the half nuts today.

You're right i did manage to do a couple of passes with the half nuts engaged initially--just enough to make a hairline on the job, but as soon as I tried to take a 'tiny' cut then everything stopped.

I got a mirror to check what happens when the half nut is engaged and on the 'full engagement' the half nut lifts the lead screw about 2mm. ie it pushes up against the lead screw and bends it.
I wonder if thats supposed to happen?

regards
Brian

Brian, no the leadscrew is supposed to be parallel to the bedways in two planes - when specified in the inspection record then usually to better than +/- 0.2mm. At the same time, the leadscrew is also supposed to be at the center of the half nuts vertically and horizontally. That is best checked by closing slowly the halfnuts anc checking that the leascrew is not getting deflected, do this with the apron fully left and fully right. Your eye can perfectly see any deflection of more than 0.1mm, you can also feel this with your finger - no measurement required to check this.

Vertical adjustment of the leadscrew is usually possible at the left by moving the norton gearebox up/down, and at the right by moving the leadscrew support bracket up/down. Sometimes the mounting screw holes need be enlarged to get some adjustment travel. It is usually a bad idea to shim or mill the apron vertically, as this would cause binding or slop with the fixed rack.

Horizontal (Y-axis) adjustment of the leadscrew is not normally necessary, and an adjustment is not provided. But in some cases where components of two lathes are mixed this adjustment can be off. Say a lathe was fitted with the apron of another lathe, or with the Norton geaqrbox of another lathe. Adjustment depends on how the lathe is made. Sometimes shims can be added between Norton gearbox and the bed, or between the right leadscrew bracket and the bed. Sometimes either the Norton gearbox housing or the right LS bracket need to milled a little to get the Leadscrew to line up with the apron or worm drive or/ and the half nuts.

Thanks cba--I appreciate your input, and it's a good datum line for me to follow this through on the weekend.

HOWEVER!!--- and Stewart please note----
I 'discovered' the lever that locks the half nuts onto the lead screw has THREE positions.
Position 1--- neutral at 45 Degrees to the horizontal up
Position 2 ---Horizontal
Position 3--- 45 degrees to the horizontal down.

If I do what I had been doing--slide the lever down to position 3 with as little pause as I could, the carriage locked--everything locked.
If I pause the lever at the horizontal momentarily then ease into position 3 then the carriage moves along the ways ok.

I may be kidding myself-----but I wonder if any of you 'older' machinists might tell me--" of course that's right you B----y goose".
Maybe the problem has been operator error??
I will appreciate any observations.

Are the half nuts full of crud?

Originally Posted by Briangoldcoast

Thanks cba--I appreciate your input, and it's a good datum line for me to follow this through on the weekend.

HOWEVER!!--- and Stewart please note----
I 'discovered' the lever that locks the half nuts onto the lead screw has THREE positions.
Position 1--- neutral at 45 Degrees to the horizontal up
Position 2 ---Horizontal
Position 3--- 45 degrees to the horizontal down.

If I do what I had been doing--slide the lever down to position 3 with as little pause as I could, the carriage locked--everything locked.
If I pause the lever at the horizontal momentarily then ease into position 3 then the carriage moves along the ways ok.

I may be kidding myself-----but I wonder if any of you 'older' machinists might tell me--" of course that's right you B----y goose".
Maybe the problem has been operator error??
I will appreciate any observations.

I suspect that you re closing the half nuts on the crest of the leadscrew thread because this would probably give you the problem you're experiencing. You need to ease the lever shut about on the mark (a little before) and the half nuts should drop into engagement. If you close rapidly and force them when you're closing on a crest - I dunno what would happen. You can't do it on my Chipmaster as the leadscrew is way too stout to move out of line, but a light lathe with flexible screw - maybe.

Slowest spindle speed, fine thread, just practice closing the half nuts on the dial marks without cutting anything - see how you get on. Most certainly the LS should not be forced out of alignment.

PDW

Originally Posted by PDW

I suspect that you re closing the half nuts on the crest of the leadscrew thread because this would probably give you the problem you're experiencing.

When I do that the lever just wont move all the way.....
Even the bending of the leadscrew... while ummm "not good", could it really lock the leadscrew?
I've been wondering if its something to do with the keyway....

If the gearbox ratio is tall enough turning the leadscrew can be difficult anyway, so the leadscrew being hard to turn may not necessarily be the problem.

There are linkage adjustments for half nuts and auto feed on most Asian lathes, when adjusted properly you should only be able to engage half nut or auto feed. However when not adjusted properly you can actually engage both, partially or fully. The only way to check is to pull the apron off and check the linkage adjustments are correct, it may not be evident by just flicking levers back and forth, you have to physically look.

Asian bench lathes are made to a price specification, not a quality specification. For that reason it helps to familiarize yourself with the design and quality parameters and how everything fits and works together before use. If nothing else it will give you a good indication of what is wrong when trying to diagnose faults.

When a lathe violently jumps out of gear like that you can bet your bottom dollar it's going to do some damage or cause some undue wear somewhere else along the drive train which will cause other problems later on.

Originally Posted by Stustoys

.....
Even the bending of the leadscrew... while ummm "not good", could it really lock the leadscrew?
....

Stu, it depends on the apron design. On a lathe with two separate shafts for threading and automatic feed, I do not think that misalignment of any of the two shafts could ever cause it to lock.

On a lathe with one single, slotted, leadscrew (used for both threading and automatic feed) it very much depends on how the apron is designed. If closing the half nuts deflects the LS, then either the LS is misaligned, or the hollow worm for automatic feed is not aligned with the half nuts. If its this hollow worm wheel that forces the LS off center, then some fairly large forces can be involved, and yes I can imagine this could lead to a lockup and even a broken gear tooth. The only way to find out may be to take the apron apart, clean and inspect for wear or obvious problems, then reassemble and check alignments.

Only slight misalignment of the worm usually manifests itself in a jerky automatic feed that becomes visible on surface finish, whereas gross misalignment of the worm does cause very rapid wear (signs of metal powder under the apron) and broken teeth.

One particular Chinese lathe is known for the slot in the LS being that badly finished and aligned that it can chew up the half nuts in less than one hour total threading time. The half nuts end up stripped blank of any thread.

Sounds like there is something seriously wrong in the quick change gearbox.

Two gear ratios meshing at once would lock it up.

I suggest you look in there for an answer.

Originally Posted by Briangoldcoast

I may be kidding myself-----but I wonder if any of you 'older' machinists might tell me--" of course that's right you B----y goose".
Maybe the problem has been operator error??

Maybe, maybe not, but I'd never call someone a B----y goose because their lathe worked differently from mine.
Every machine tool has it's quirks and I don't think there would be anyone here who could honestly say that in the absence of instructions they have never had to puzzle out how something complex worked.
As long as you are learning as you go then you are coming out ahead.

Michael

Originally Posted by Stustoys

When I do that the lever just wont move all the way.....
Even the bending of the leadscrew... while ummm "not good", could it really lock the leadscrew?
I've been wondering if its something to do with the keyway....

Like I said, is the half nuts full of crud... Close them and acts like a brake..

RC--
I took some time this morning and took your advice and crawled under with some paper and cotton buds and wiped out the half nuts. Fortunately the only thing in them was a film of oil.
Thank you for your thoughtful idea.

Michael G-- Thank you for your reassuring words. You're right--there was no operational manual came with the lathe, just a folder of exploded views and some simple warnings--I'm sure everyone has seen the extent of Chinese /English instructions.
Up till now I have either worked things out or asked the help of a few mates or more importantly the helpful guys here on the forum. I'm always grateful for all the ideas and help that comes my way here.

sacc51-- I agree that pulling the apron off and adjusting the half nuts might be the next step.. I had been trying to avoid getting into new territory there but time I guess will tell.. Thanks for your suggestion.

cba---You're points are very valid I think, and I amy need to explore this further. However the lead screw and finish generaly is pretty fair for this unit--I think.


PDW.- I have been following your advice and while everything is not totally foolproof, I have been at least getting better results and operation,. I suspect there's some adjustment needed and now I know where to focus I will take the time and explore all this further.

Thanks everyone for your thoughtful suggestions

If the half nuts are now, clean, could they be clamping too tight and acting like a brake still?

Close them and move the carriage with the handwheel back and forward and is there a slight bit of movement?

3/4 close them and is there binding?, does the binding get worse as they close more and more?