Chester 920 traverse issue

[flintandsteel]

Had this lathe about10 years now. Sorted, renewed and fixed most things and it does what it should.... till this morning.
As the table moves along the travers screw (not sure of technical name) it reaches a point where something must jam up as it gets closer to the chuck- sufficient to kick it out of gear.
Checked the under table box and all fine in there.
Only thing I did this morning was to change up on the gear box, try the speed then back to where it was.
Reset the drive gears so it can power the traverse screw again (table not engaged) and runs fine.
Fingers crossed engaged the table and it travelled the full length of the bed till about 6" off the chuck then it kicked out of gear again.
Thinking gear box issue. Just tried it in a different gear - same problem
Thoughts?

 

[flintandsteel]

Had a look in the gear/speeds box and all looks fine.
No bits of metal or teeth lying around
Worn transverse feed bar?

 

[ChaiLatte]

When you traverse the carriage along the bed manually, using the handwheel, does it feel any harder at the same point as the power-traverse struggles?

Have a look at the rack in which the manual handwheel pinion engages to make sure it is clean and sound at the problem point.

I think what you call the traverse screw would normally be called the leadscrew. Have you checked that is not burred or bunged up at the problem point? You can clean the leadscrew by setting it moving using the gearbox, making sure the carriage is not engaged to it and passing a piece of string round it in a U-shape. Holding both ends of the string, it will clean out the groove of the thread.

 

[Amplidyne]

As ChaiLatte says, it sounds more like either a problem with muck on the leadscrew itself, or with the saddle movement sticking somewhere rather than an actual gearbox problem. Sounds as if it's loading up the leadscrew / drive at some point, and kicking it out of gear somehow.

 

[flintandsteel]

Thank you all so much.
Last night had a quick look for on line (thesaleroom.com and Easylive etc) just in case. Some interesting old classic beasties on those sites and more modern "don't bother" types as well..
Anyways, it took a while but finally got it after several pieces of string and caressing it with a wire brush.
Noted some dark dust (just a little) in places in the groove on the leadscrew. Maybe a carriage strip down is in order if it reoccurs.

 

[ChaiLatte]

Noted some dark dust (just a little) in places in the groove on the leadscrew. Maybe a carriage strip down is in order if it reoccurs.

Any gunk on the leadscrew will not come from the carriage. It will be swarf falling onto the leadscrew from above where you are cutting.

Modern lathes have telescopic leadscrew covers, as much for safety so your long hair does not get dragged into it when machining in flip flops, baggy gloves and tight swimming trunks (tight gloves and baggy trunks are a lower risk), as for swarf protection.

A search for 'lathe leadscrew shield/guard/cover' would give you some ideas of what is possible and whether the hassle of making one is less than the string trick once a year.

 

[Amplidyne]

Those "flat spring" type shields are available as an aftermarket thing for most lathes I think.
Just brushing the leadscrew down after each use is a cheap substitute though. I keep an oily old paint brush by the machine.

 

[Fergie 307]

Old toothbrush is good especially if it has quite a fine pitch leadscrew.

 

[flintandsteel]

Thought I had it but no. Kicked out again.
The lead screw looks to be more worn at the chuck end which I guess is to be expected.
Kick out at roughly the same location, so wondered if it was a combination of screw and saddle gear failing meshing at that point.
Part stripped the carriage and freed the lead screw at the right hand side. However it all need so come out. To do that the left side needs to be freed. Now is that by removing the circlip in the drive compartment or is there a knock out pin in the linkage rhs of gear box? Don't want to go hitting it till I know.
Manually moving the carriage about, a large worn out washer dropped into the tray. Think this might be the issue. Washer about 2.7mm thick Worn thin and 1 edge missing) same internal dia a lead screw dia, and about 30mm o/d but with a flat on one side.
Think it's been floating and dropping into the threads while traversing.
So where / between what does it sit? Standard and replaceable or of special steel?

 

[Fergie 307]

If it is a hardened ground washer then these can be obtained in various sizes from simply bearings or other bearing suppliers.
Hopefully a search of Google might get you a manual for the machine with an exploded drawing to help with where it goes.
Sounds to me as though you might need to replace the screw, but certainly worth getting whatever the issue is with the washer sorted first. With luck that might get you going again.
Pretty generic Chinese machine so if Chester can't help or don't provide back up might be worth having a look at Warco, Amadeal etc to find someone who sells the same machine and might be able to help with spares.

 

[flintandsteel]

Finally figured out the issue. Pic later if I can get the computer to talk to the camera.
In the drive box there's a worm gear sits in a cast housing. Well it did but the rhs of the housing is gone. Not in my ownership. The washer was the remains of the housing hanging on.!!!
So now the worm gear moves forward and jams in the housing!!!!
Could maybe sort it or get it done unless someone has a scrap Chester. Failing that it will work on the threading setting (I think) but is that a realistic get around?
Suppose I could turn an insert then fix it somehow?

 

[Fergie 307]

Once you can get some pictures up of the broken part I am sure there will be some suggestions for a repair.

 

[ChaiLatte]

...unless someone has a scrap Chester...

This is a long shot, but if you have any contacts in America, the same machine is likely sold under the Grizzly brand (it is a generic 9 x 20 lathe). They offer a reasonable spares backup.

It would be helpful to link to a good copy of a manual for the machine (Chester's own one does not show the half nuts, so is not the best. The Grizzly one may be better). Then you can describe parts using the same names as the manual, give part number and page references and everyone interested will understand.

 

[flintandsteel]

Appreciate that, but still have computer and camera not talking to each other.
From the red cover Chester 920 manual it's part No.4034 in the apron assembly.
I think the newer machines differ.
I have a Grizzly manual but no parts list with it.
If not available I can see a new side could be turned and brazed in place but that's beyond my skill set or equipment.

 

[flintandsteel]

Finally got it
It's part No.2 (4032 in the manual) which houses worm gear No.3
The left side of mine has gone.
Were the broken left side cut down I'm sure a new piece could be made up and fixed in place. Beyond my skill and other equipment.
On newer machines I think they've changed this part.
Mine is old, with me 10 years, previous owner maybe 20 and he got it from a Tec College. Could easily be 30 years +. Also I think early ones were British made, not nasty Chinese stuff

Can assistance or recommendation be requested here for someone who could do the repair?
View attachment 197702

 

[ChaiLatte]

I have found two different copies of the manual online:

https://www.chesterhobbystore.com/wp-content/uploads/2022/12/920-Manual.pdf


The part No. 2 seems to tally with what you say, but the 4032 is not the same. Hence, please link to the manual you have or refer to one that is publically available.

It looks like a U-shaped part with two large holes and three (maybe threaded) holes. It would be better to make the whole part anew from steel, possibly incorporating a bronze bush in the large holes, rather than trying to repair the old one.

If you own a hacksaw, a drill press and a file, you have the necessary tools to make a new one. It would be wise to put as large a fillet radius on the internal corners of the U as space would permit.

The first step in procuring a replacement will be to make a dimensioned drawing of the part. If you have woodworking skills, make one from wood as then you can determine fillet radii and bush possiblity on the machine.

 

[Fergie 307]

I would agree with ChaiLatte, if the drawing he has posted is the correct part, or very similar, it would be pretty easy to make.
I can't open the attachment you posted.

 

[flintandsteel]

View attachment Cester Apron.jpg

Only the lower half of the lhs left and worm jammed forward in the bracket.
Later models don't have the same setup.

 

[ChaiLatte]

This is food for thought or discussion. I think it is correct to say that the end that has worn away is the end facing the headstock. That would make sense as the lathe would spend 90% of its time feeding in that direction.

The original part would be cast iron as it is cheap when you are making 1000 machines and it also provides a good bearing for the leadscrew to run in. However, being soft, it has worn away...

Can you rub the worm with a file and see if it is hardened.

With any new one you make, consider the possiblity of using a thrust bearing there - usually a three piece item with two hardened washers and a ball or needle race between them. If that won't work, a single hardened washer against the new part, a sintered bronze (Oilite) washer and then the worm might prolong the life of the assembly.

When you reassemble, the worm might be able to be turned end for end to give it a new life.

 

[Fergie 307]

You can actually use the lathe to do most if not all of the machining.
This assumes you have a four jaw independent chuck?
If you start with a suitable, slightly oversize block of steel then you can machine the outer 6 surfaces to get an accurately sized and squared block.
You can then machine the central bore for the shaft.
I would then probably look to remove the bulk of the material from the centre by boring that out.
If you can then mount the part on the top of the saddle, in place of the toolpost, then you can potentially take out the remaining material from the centre in the lathe as well, by mounting a milling cutter in the chuck.
If you can't do that then you could remove the last of it using a cutting disc in a Dremel or similar and or some work with a file.
If the rounded shape of the sides is important, for clearance or whatever, then you can simply grind them to shape afterwards.
ChaiLatte's observation about incorporating thrust washers is a good idea, if there is room. Ideally you might make the overall length slightly greater to allow for this, assuming the overall length is not critical for it to fit with the surrounding components.
If in doubt make a suitable wooden block to see how much room you have to play with.