Colchester student cross slide backlash

  1. Mashmoreuk New Member

    Messages:
    6
    Location:
    Berkshire
    Hello! First post on here but been browsing for a while.

    I've got a good 2mm of play if I push and pull on the tool post, and obviously this effects the cut.

    I've tried changing the bronze nut on the slide with no joy... so I'm thinking this could be the screw...

    Or am I better of trying to use the later style nut which has the adjustment so you can preload the thread almost?

    Mine currently is just a nut no fancy adjustment. But its the only thing stopping me enjoying this lathe
     
  2. grim_d

    grim_d Unlikeable idiot.

    Messages:
    2,866
    Location:
    Scotland - Ayrshire
    If the play is in the cross slide it is likely wear in the screw or nut as you suggest however it shouldn't affect your cut as when you advance the tool into the cut the backlash is taken up regardless of the wear.

    Before changing the nut on my Harrison I had more than 2mm play, sure the backlash was annoying but did not affect the cut.

    Are all your gibs adjusted correctly? I believe students have taper gibs on the cross slide.
     
  3. Mashmoreuk New Member

    Messages:
    6
    Location:
    Berkshire

    Yeah I suppose the constant pressure keeps the tool post "tight"
    I just cannot get a good finish, there's really fine chatter marks,

    Aswell as anything below 10mm OD becomes impossible to get any kinda finish on.

    It's strange
     
    Brad93 likes this.
  4. grim_d

    grim_d Unlikeable idiot.

    Messages:
    2,866
    Location:
    Scotland - Ayrshire
    Have you check all the usual suspects? Gibs are adjusted correctly nothing else loose or misadjusted, tool on centre height, tool geometry, speeds and feeds etc.

    If the spindle bearings are adjustable do they need tightened up?
     
  5. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    That's logical, but what I've found on my Drummond is that the tool can get drawn into the workpiece when it starts to cut, especially when parting off. I checked this with a DTI. I don't know anything about the Colchester, but on the Drummond I locked the screw to the body of the slide so it wouldn't turn, but could still move the toolpost in a parallel direction to the screw, so it had to be wear on the screw and/or nut.

    If you can move the slide from side to side or twist it I'd be looking at the gibs.
     
  6. Richard.

    Richard. Forum Supporter

    Messages:
    18,350
    Location:
    Cambridgeshire
    You shouldn’t get chatter from back lash alone. Every nut and thread in the world will have some or the principle simply doesn’t work. If you disengage far enough then re engage to take your cut all that back lash is taken up on the other side of the thread. If your taking an especially deep cut or finding it is pulling in when parting off just lock the slide before taking the cut. Chatter is more likely to be something else. Flex in the work piece or tooling. Tool off centre height, wrong feed speed to rpm ratio etc. For the perfect finish cut everything needs to be spot on.
     
    Pete. and fizzy like this.
  7. MBB Forum Supporter

    Messages:
    1,045
    Location:
    northumberland
    Make sure the cross slide screw has no back and forth movement in its mounting, adjust using the nut holding the hand wheel on.
     
    rtbcomp likes this.
  8. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    Parting:- when you pull away from the work the thrust is taken on the front of the screw thread and the back of the nut thread, which means the tool can't be drawn in. However to re-engage with the work the screw has to push the tool towards the work, which means the thrust is now taken on the back of the screw thread, and there is clearance between the front of the screw thread and the back of the nut thread, thus the tool can be drawn in.

    You can't lock the slide when parting or facing, I do lock the saddle to the bed though.

    If you're machining along the length of a bar then moving the slide towards the centre and then moving it back out to the required position (off the end of the bar) before engaging will compensate for backlash as Richard says. the slide can also be locked.
     
  9. slim_boy_fat

    slim_boy_fat Forum Supporter

    I've 'temporarily' cured the backlash on my ML7 cross-slide. Replaced the brass[?] washer behind the dial with a spring one - it's better and will do until I figure out a 'proper' way. :ashamed:
     
    Gwil likes this.
  10. R-D-R Forum Supporter

    Messages:
    1,135
    Location:
    Derbyshire - England
    I'm still learning on the lathe, its amazing how little things impact the Finnish. Are you able to lock the compound slide and cross slide off to hold them tight and see if your Finnish is impacted? I found tool stick out was important, I had it too far out to start with but now I have it as close to the holder as possible and I can get nice shiny finish. Does your cut improve or change as you move along toward the chuck? it could be the work piece needs better support.

    Speed and Feed is a bit like Amps and Volts on the welder its a fine balance to get it just nice.
     
  11. waddycall

    waddycall Member

    Messages:
    1,143
    United Kingdom
    I have a shonky old lathe with loads of backlash everywhere. I can get a mirror finish setting the tool so it just rubs slightly after taking the cut. LH Sparey describes how in “the amateurs lathe”.
     
    slim_boy_fat likes this.
  12. I think severe winters where the reindeer struggle and a shortage of Baltic herring have a bigger impact on the Finnish more than your ability to use a lathe!
    :scared:
     
    Gwil, Seadog, waddycall and 2 others like this.
  13. Screwdriver

    Screwdriver Member

    Messages:
    6,403
    UK London

    Well that is the complete opposite of what I have been led to believe. Are you sure this is correct?

    What you appear to be suggesting is to somehow set the tool to resist "being pulled into" the cut by winding OUT prior to making a cut. Well then how do you advance the tool from that position? Surely at some stage you would have to wind it in? Secondly if you are referring to say the carriage cross slide (and not the compound slide) either the backlash is going to resist tool pressure OR resist being drawn in. It cannot resist both! If the cross slide is pulled back and the compound slide pushed forwards to make the cut (or v,v,), you have the worst of both worlds with backlash in both directions!

    As for winding the carriage to the centre then back before making a cut, when you engage drive to take the cut, the carriage will sit there stationary until all of the backlash is taken up by the lead screw.

    I have always understood one removes backlash by advancing the tool, thus the tool is set to resist tool pressure. A very similar scenario is of course during milling operations where climb cutting should be avoided for significant material removal.
     
  14. MBB Forum Supporter

    Messages:
    1,045
    Location:
    northumberland
    If you have 2mm play that is nearly more than the pitch of your screw. Something seriously wrong with cross slide nut not secured to the cross slide or movement on the screw being able to move back and forward against the cross slide thrust bearings.
     
  15. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    That's the point I made:


    Exactly.

    I said slide, not carriage. That doesn't matter because the tendency to draw the tool in is perpendicular to the lead screw, and with slide locked it can't move.

    I should have made it clear I meant moving the tool towards the centre of the workpiece along its radius. [/QUOTE]

    I'd agree with that, the backlash has to be taken up this way or you'd never get the tool to engage with the workpiece, but it still leaves the tool open to being drawn in.

    Basically you can't have it both ways. I'm sure if the slide screw and nut are in good condition the backlash will be insignificant, but problems arise when either or both are worn.
     
  16. spencer 427 Member

    Messages:
    6,925
    Location:
    uk colchester
    It could be that the cross slide ways are worn and being a Colchester student most probably is. This would definitely contribute towards chatter
     
    Pete. likes this.
  17. Screwdriver

    Screwdriver Member

    Messages:
    6,403
    UK London
    Oh ok, still not sure about setting the slide by winding out before a turning operation. I would expect tool pressure to remove any backlash as soon as you start the cut. Locking up the slide is really the best option.

    The only time I have a problem with the tool being pulled into the workpiece is while parting off and even then, right at the end of the cut.

    I hate parting off with a passion. :mad:
     
    slim_boy_fat likes this.
  18. Pete.

    Pete. Member

    Messages:
    9,427
    Location:
    Kent, UK
    Plenty of production lathes had lever or rack and pinion feeds for quick and repeatable actions, they didn't rely on screws yet they produced good consistent finished parts. If your parting tool is digging in it's the slides, not the screw. If the small diameters are suffering bad finishes it's either the slides or the spindle bearings. Improving the screw and nut might improve the finish but only because the tighter fitting parts mask the real problem by restricting the unwanted movement a bit.

    No1 - check the spindle movement. It's unlikely to be that but it's the easiest thing to check. Stick a dial gauge on the spindle at the back of the chuck. Put a bar in the chuck and lever on it up and down, see how much the dial moves, it should not be a lot.
     
    Richard. likes this.
  19. slim_boy_fat

    slim_boy_fat Forum Supporter

    That's the problem I get, and I part at the slowest speed and feed the tool in very gradually. I use cutting oil and unwind on the cut a few times too, but still..... I even managed to break the one end of the indexable insert yesterday as I was test setting it up. :ashamed:
     
  20. rtbcomp

    rtbcomp Moderator Staff Member

    Messages:
    18,615
    Location:
    Sheffield UK
    This is interesting

     
    slim_boy_fat likes this.
Advertisements