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What kind of Backlash do you find acceptable / expect ?
I believe that this is a valid question Jo - but I must be sure that I understand you right.You suggest that the saddle isn't moving in a straight line, if I understand you right ?How would you measure it - both play and "crabbing" ?
At the risk of teaching granny to suck eggs, have you been round all aspects of the saddle, cross slide and top slide and ensured all is tight?
P.S. 1. As you get older and have sufficient finances behind you, you may not want to continue to be one of the "Make do and Mend" generation .
I have tried to measure the rise in height off the Cross Slide, over the Prism part of the Bed, when I try to push it to opposite sides in the front and back and I see 0.06mm. on the clock and absolutely zero if I do the same with the clock over the flat rear part of the Bed.Thanks for the suggestions so far, but I have another appointment for the rest of the day and will be back in the shop tomorrow.
one way of taking out the backlash is to tie a piece of string to the cross slide and with a heavy weight and dangle it over the back of the lathe ??!!! never tried it but just an idea ??willy
Quote from: Admiral_dk on December 28, 2020, 10:43:39 PMWhat kind of Backlash do you find acceptable / expect ?Hope I don't step on toes with this....but backlash really doesn't matter!Why? There has to be some, else the thread wouldn't turn. Backlash is clearance between the thread flanks and even on a new feedscrew is going to be enough that it will mess you up if the backlash is taken up by say pushing on the crossfeed. So, sans an antibacklash arrangement, we know no matter what we will have enough BL that exactly how much there is is immaterial. 5 thou or 50, it won't much matter.Where backlash (imo) gained its status is in inspecting used machines were it can be an indication of overall wear. Lots of backlash is bad not because it makes a difference in how far the tool is advanced, but because it suggests an well worn machineWhats going on with yours? The carriage might be crabbing as suggested, or the overall fit of the dovetail might be loose enough that vibration is moving it. I was doing some internal grinding the last few weeks and with vibration of the high speed motor everything wanted to move. Tightening the gib worked for me, but you don't it always really tight or it will wear more quickly.You can check for crabbing by taking something known to be round and cylindrical, centreing it in the four jaw (centre in two planes!) to a 10th of a thou. Then with an indicator on the tool post, sweep the work in a few spots. If you get different readings, the motion of the carriage is not parallel to the lathe's axis (either crabbing or leveling). Crabbing should be identifiable to apply a force the carriage by hand and seeing how the needle moves. This little set up btw is really great for tweaking leveling on fussy work.When I'm doing really fussy work, like striving for a 10th to fit an expensive spindle bearing say, I get rid of the compound and mount an indicator on the cross slide dovetail. My indicator mount also has two cylinders with springs to press the crossfeed screw against the correctly flank of the nut. You have to hold your tongue just so, but I can usually do better than .0002" with this set up.Its not trivial getting a perfectly working machine...its why the good quality ones cost so much when new and masochist like me spend so much time reconditioning (scraping) them. So don't feel bad or get frustrated if it takes a bit of chasing. It is however very when they do perform well. If none of this helps, the next thing I'd do get it apart and start check fits with some blue. Some sort of spring arrangement loading the system against the nut might be an experiment to try - see if that fixed it. If it does, imo its further evidence the crossfeed dovetails need attention.Here's a couple shots of what I mentionedeverything is solid and scraped to fit, compound removed and replaced with a T slot plateCross feed indicator - digital is great for this, micron accuracy and easy to switch between imperial and the french system (haha couldn't resist), err metricHere the sweep tool. Its essentially a spindle with very accurate AC bearings. I mostly use it in conjunction with reconditioning (second photo) but its easily mounted in the tool post for this sort of checking. btw, in the second picture is a Holbrook B8, marvelous lathe and a current project
Nice set up! Are you documenting this rebuild anywhere?....Love to follow along!....Seems we share a similar sickness....LOLDave
I do know about having the compound @ 1 degree from the Z-Axis - so that increasing it 1mm. results in a much finer cut - how much depends on the rest of dimension off the Lathe + Tools Peter who told me this, used this technique to adjust the depth cut to 0.001mm. accuracy on the big lathe at his work, when needed.
Mikes tip about the Tool MUST be just as sharp as a wood chisel or other good cutting tools and that it should be able to easily cut your fingernails and produce a smooth shave / cut from said nail - has also told me that most of my old HSS were not good enough (sharp yes, but not enough).