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Cebtec 2A Backlash Adjustment

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Steve Taylor 216/02/2011 19:24:05
35 forum posts
6 photos
How do you adjust the backlash on the Centec 2A, I think its the X axis (east-west).
I can see a bronze collar that the lead screw runs in and a bronze coloured nut that stands proud which is very tight, I dont want to force it
Steve Taylor 201/03/2011 15:17:25
35 forum posts
6 photos
OK I understand now that its a half nut and there should be a lever to disconnect it when using the lever to move the bed.
Its no longer lever operated.
 
Having adjusted the half nut as tight as possible there is less backlash but still around 15 out of 100 on the graduated dial.
I dont have any screw cutting facilities to make a new one so will need to find one.

Any ideas anyone on where to get one please

Bryan Rozier01/03/2011 15:29:40
13 forum posts
Hi Steve,
 
You can make one out of Delrin (Plastic) see this link :-
 
 
Best regards
Bryan
 
 

Steve Taylor 201/03/2011 17:01:54
35 forum posts
6 photos
Thats a great link, thank you.
I'm not sure a half nut would have the required strength
Bryan Rozier01/03/2011 17:06:56
13 forum posts
Hi Steve,
 
Did you read the whole thread? Delrin is extremely strong in this application. A number of people have used Delrin in a milling application with no problems.
 
Best regards
Bryan
 

Steve Taylor 201/03/2011 19:49:11
35 forum posts
6 photos
I'm not sure how it would mount Bryan.
Looking at the photo below thats the entire nut, there isnt a top half as its pushed up against the leadscrew, so I cant see how it would insert.
The only ways I can see it working is if I made a new carrier for it or filed out the existing one, which leaves me with no way back
 

Richard Parsons02/03/2011 08:55:56
avatar
645 forum posts
33 photos

Some 65 years ago the ‘Old Man’ bought a South Bend type lathe (it was actually a Sheldon). Every year he (and later on me) did a major service on the machine. One of the things we did was to renew all the main feed nuts. These were the cross slide nut, the compound slide nut and the split nut on the lead screw.


He had made a little jig for each nut. These held the relevant screw dead central with the nut it was to fit. The nuts were heated and the ‘Babbitt/Hoyt metal’ poured out and recovered. The screws were covered in lamp black from the acetylene torch placed in the Jigs. The nuts were re-tinned and put into the jigs and packed round with fire clay to stop the hot metal from leaking out. After a little time (and a large mug of tea and a lump of cake –none of your teatime sponge- the Babbitt/Hoyt metal with fresh added to make up the losses was melted and poured in. When it was cool enough to handle the nuts were cleaned up with a file and were re-fitted and were ready for another year’s hard work.


It is an alternative to the Hot Plastic method. Both Babbitt and Hoyt Metals are designed as a bearing metal and as they are moulded onto the thread they have little or no backlash. On the Old Man’s Sheldon they would have lasted some 5 to 7 years. With our usage they would probably last 15 to 20 years.
Hope it helps

RP
Bryan Rozier02/03/2011 10:02:38
13 forum posts
Hi Steve,
 
Do you need to dis-engage the half nut? I.e. use the rapid traverse?
 
How about buying a tap from tracey tools (around £15) and threading a cube of bronze and then making up a mounting bracket from something cheaper like mild steel angle. Leave all your mounting holes a little over size to give you some adjustment.
 
You wouldn't need the slot so shouldn't be too difficult.
 
 
Although a cube of MS with Delrin inside might also work.
 
Cheers
Bryan
 

ady02/03/2011 10:35:01
612 forum posts
50 photos
If you've got a unit which has seen a fair few years of use then you may well find it impossible to have a zero backlash situation without renewing the leadscrew itself.
 
I can adjust an old lathe I have with "zero backlash nuts" but the leadscrew nut gets pretty tight at each end because there's always more wear in the middle of an older leadscrew.
It's only a few hundredths of a millimeter but it's there, and it's easy to notice on a decent bit of kit, the nut jams up at each end.
 
Worth bearing in mind before you make a big effort, a perfect nut will not fix an imperfect leadscrew.
 
I've had zero backlash before on a renewed cross slide nut and it doesn't last long in regular use before it needs readjusted, a month or two max.
It can be readjusted to bear on one surface but a heavy cut in the wrong direction will expose any slop in the mating of the nut/screw until the surfaces rebear on one another.
 
So if you're really serious about achieving zero backlash on a regular basis, get a new leadscrew and two new adjustable nuts.
This means the leadscrew thread can bear fully on one nut in one direction, and bears fully on it's counterpart nut when it travels in the opposite direction.
It goes without saying that you will need a perfect leadscrew to achieve this nirvana over the full length of travel.

Edited By ady on 02/03/2011 11:04:55

ady03/03/2011 00:45:48
612 forum posts
50 photos
I've uploaded a couple of piccies of the nuts used on the old pultra to give you an idea of what's required in.

Pultra zero backlash nut

in my albums.
It's simple and clever in theory.
in practice however everything needs to be completely spot on to achieve a perfect result.
Steve Taylor 203/03/2011 18:31:22
35 forum posts
6 photos
The Y axis has 20 thou backlash in the middle and 8 thou at each end, where its also tight.
The Y axis nut is bronze and could be replaced using the delron method but its not worth the effort unless the leadscrew is changed at the same time.
 
The X axis is 15 thou consistently along its travel, but there isnt enough space for anything other than a half nut so it needs a authentic half nut replacement. or a new one making, tapping and cutting (in half)
 
Thanks for everyones advice, I've decided to leave it as it is, I can cope with the backlash and I know how much it is. It dosnt seem worth spending a small fortune on leadscrews and so on
ady15/07/2011 11:46:23
612 forum posts
50 photos
I've bumped this back up because you might find it worthwhile trying out a digital readout system instead.
 
A DRO system takes the backlash into account because it relies on "real" and not perceived movement and can be accurate to around a hundredth of a millimeter.
 
With a bit of thought you can achieve a very high level of accuracy on a job, graduated dials and leadscrew types become "irrelevant" because you can zero a DRO anywhere you like and also choose to use either Metric or Imperial readouts for your cuts.

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