Lathe rebuild questions....

[dobbslc]

I need a bit of advice as I'm rebuilding part of my lathe.
It's a Chester Centurion combo machine.
The cross slide was very unpredictable in its action so I started stripping things down and found a few worn bushes etc. I've emailed Chester about spares and I'm waiting for a reply.
Forgive me if I call things the wrong name as I'm fairly new to the machining world.
I've been getting very poor finishes while facing and turning, one minute all is fine then it all goes pear shaped! When facing I can get a very uneven finish that's visible to the eye concave convex all over the place. Would a worn cross slide nut and worn bushes in the handle shafts cause this?
Also how do I set the gib strip? Is anyone familiar with this machine or one like it?
Thanks
Dobbs

 

[ford34/2]

this one ??
.

Images are for illustration purposes only

Centurion 3 in 1 Machine
Our Price: £1,704.00 inc VAT

Description
The ultimate in combination machines the Centurion has the capacity to cover all the requirements of the home engineer. Taper roller bearings on both mill and lathe spindles, along with conventional belt and pulley drive systems provide for longevity and accuracy. Large 28mm spindle bore and a 420mm diameter lathe capability are features of this machine.


Centre Height 210mm
Swing over Bed 420mm
Swing over Crosslide 270mm
Distance between Centres 500mm
Width of Bed 139mm
Longitudinal Travel 440mm
Spindle Bore 28mm
Spindle Taper MT4
Number of Spindle Speeds 7
Range of Speeds - Lathe 160-1360rpm
Metric Threads steps 0.2-6mm in 22 steps
Imperial Threads 4-120 t.p.i.in 38 steps
Automatic Feeds 0.05-0.35 in 12 steps
Tailstock Taper MT3
Tailstock Quill Travel 80mm
Compound Slide Travel 80mm
Crosslide Travel 200mm
Millhead Spindle Taper MT3
Millhead Quill Travel 110mm
Distance between Spindle and Table 306mm
Millhead Spindle Speeds 16
Range of Speeds - Mill 120-3000rpm
Milling Table Size 475 x 160mm
T Slot Size 12mm
Drilling Capacity 25mm
End Mill Capacity 28mm
Fly Cutter Capacity 80mm
Lathe Motor 3/4 hp
Milling Power 3/4 hp
Power Supply 240volt
Net Weight 250kgs
Dimensions (LxWxH) 1570 x 480 x 955mm
Max Distance Spindle to Column 285mm

FEATURES:

• Powered Crossfeed
• Separate Motor for Lathe and Mill
• Large Milling Table
• Leadscrew Guard
• 28mm Spindle Bore
• Heavy Duty Cast Iron Construction
• Capacitor Start Motors for High Torque Starting

STANDARD ACCESSORIES:

• 125mm 3-Jaw Chuck
• 1-13mm Drill Chuck
• MT3 Drill Chuck Arbor
• MT4 & MT3 Steel Centres
• Manual & Parts List
• Milling Drawbar
• 4-Way Toolpost
• Combined Vice /Compound Slide
• Change Gears
• Lathe & Drill Chuck Guards
• 10mm Lathe Tools

Please Note: These machines are not rated for continuous use in an industrial working environment.

 

[CRB]

Also how do I set the gib strip? Is anyone familiar with this machine or one like it?
Thanks
Dobbs

Gib strip maladjustment would allow the carriage, to rock around giving the varied results you have experienced.

I had always adjusted by screw and locknut, located on one side of the Vway. I read recently, that removing the hand-wheel and screw, and then adjusting gives a better setting.

Either way, undo the locknuts and tighten the screws until there is no slop but movement is smooth without being easy. Much easier to feel than describe.

 

[Digger6135]

To be honest I would not want one of those combination machines as a gift, they lack ridgity and capacity when used as a milling machine.

Digger6135

 

[dobbslc]

Cheers CRB
The gib has two large headed screws holding it in, one at the back that sits over the end of the gib and one at the front where the head fits into a slot in one edge of the gib, I've found that gib has popped out over the screw head a couple of times and that either binds the cross slide or let's it run too loose.
There is quite a large amount of adjustment on it that seems to make little difference, maybe if I can disconnect the hand wheel and screw it will allow more feel of the adjustment.

 

[DanielW]

Two screws on the gibs? I'm sure theres three grub screws on the crosslide.

Change your gibs for brass type and try to fix the backlash on the leadscrew.

Join a modellers forum as most people who use these mini machines have either the lathe or mill or combo.

http://www.model-engineer.co.uk/

 

[graffian]

A screw at the front and back of the slide and the head of
the front one sit's in a slot in the jib strip?

 

[Wedg1e]

Cheers CRB
The gib has two large headed screws holding it in, one at the back that sits over the end of the gib and one at the front where the head fits into a slot in one edge of the gib, I've found that gib has popped out over the screw head a couple of times and that either binds the cross slide or let's it run too loose.
There is quite a large amount of adjustment on it that seems to make little difference, maybe if I can disconnect the hand wheel and screw it will allow more feel of the adjustment.

This sounds like it's a tapered gib strip and the screws are supposed to lock it in place at whatever 'depth' gives the correct sliding feel. I imagine you're supposed to slacken the rear one, adjust the front one to slide the gib back and forth and then wind the rear screw back in to keep it at the setting.
Of course if the front screw's head is a sloppy fit in the gib's slot then it won't stay put, much as you say... is there scope for making a better-fitting screw?

 

[DanielW]

You would have thought their website forum technical support would help you out,thats a poor run thing when they cannot backthemselves up with technical support to the machines they sell. what a bummer.

 

[DanielW]

Show us a pic of what sort of a bad job its doing? is creating ridges on the metal piece job?

 

[Pete.]

Yep tapered gib. If the adjuster is riding over the slot in the gib then the gib will move causing the cross-slide to bind one way and come loose the other. it needs to be held very securely between the adjusters. The one in the slot tightens the gib, the other one locks it in place once adjusted.

 

[dobbslc]

Thanks Pete that's what I thought, when /if I get the spares I'll spend some time checking and setting the gib. The slot in the gib is about 2mm wider than the adjuster screw head so it has some play, I might have to machine a new one up.
I know some people don't like these machines as they are seen as hobby lathes but for what I'm doing it's fine when it works....
I'm not skimming bus brake drums only making spacers etc for my bike so it's mainly alloy.
I'll try to get some pics up when I can.
Rob

 

[graffian]

the adjuster screw head so it has some play, I might have to machine a new one up.

The screw on the other end of the jibstrip will push the screw head against the end of the slot, so there is no backlash in the strip..
My lathe has the same setup, I had to rebuild the end of the strip as it was broken off, I brazed a bit of steel to it and scraped it flat, it was
ten years ago and I haven't adjusted it since.
I adjusted it with the cross slide nut left out, so I could push the slide back and fore, I set the strip so it was
pretty hard to push it across the unworn bit[the lathe is 60 years old].

 

[dobbslc]

Two screws on the gibs? I'm sure theres three grub screws on the crosslide.

Change your gibs for brass type and try to fix the backlash on the leadscrew.

Join a modellers forum as most people who use these mini machines have either the lathe or mill or combo.

http://www.model-engineer.co.uk/

Thanks chaps. I'm going to order a new lead screw nut? As mines quite worn with about 1/8" slop on it.
This machine isn't a mini one though, it's like the one in the link above but the long bed version with 800 mm between centres.
I'll post some photos I've taken when I get a chance on my phone ( the Kindle won't do it)

 

[dobbslc]

A screw at the front and back of the slide and the head of
the front one sit's in a slot in the jib strip?

That's the one.

 

[dobbslc]

Here's a few pics.

Gib slot with one of the screws



Some of the pieces.

that gear is very worn.


The slideways, they look worse in the photos, no grooves just light markings.

 

[MrFluffy]

You can make a poor mans anti backlash nut by sawing 80% through the nut then drilling and tapping a bolt axially with it to close up the gap a bit. Google for diy anti backlash nut and you will see lots of ideas.
The only real fly in the ointment is when the leadscrew is also worn, you have to set it up to have zero backlash on the unworn section or it binds up on that bit if set for the worn bit same as the gibbs.

 

[dobbslc]

The nut has been modified already, they aren't meant to be slotted like mine is.
I've just ordered the bits in the photos plus a bit more to rebuild it, wasn't as expensive as I thought it would be either.
I've assemble it without the nuts etc and fitted the gibs, it has a slightly loose spot near the chuck so will need a bit of setting up when I fit the spares.
I'm also going to polish up the gib on my surface plate with a bit of fine wet and dry, hopefully that will smooth things out a bit too.

 

[MrFluffy]

Slotting the not axial like in your photo lets it bite down to a smaller thread but the anti backlash nuts work by making the cut across the nut and bringing a couple of the thread roots together changing the pitch slightly in that area so their flanks take up the play.
If you have a new nut and it still feels pants its always a get out of jail trick to have.

 

[CRB]

It has a slightly loose spot near the chuck so will need a bit of setting up when I fit the spares.

That will happen any way, unless you can use the whole bed evenly over its' lifetime.....Never happen.