Backlash Or Just Poor Technique?

Spring is just as it sounds. It is an amount of force that will flex what ever your referring to & it will take its memory back when rested. Once you push something to hard it bends & can only spring back that certain amount that it has the ability to. Even holding a pencil anywhere along its shaft there is a certain amount of spring introduced to it from gravity.

Clear as mud?
 
great white said:
This is going to sound "noob", but could someone define " spring" in the context of this thread?
Click to expand...

You can put a dial indicator on the tool post and push it around a bit , some of that will be bendyness of the tool holder and cross slide and some could be slightly lose gibs (or not quite adjusted righht if you have the ones with seval scres with locknuts).

You could then try to push just the sadle about and see if that shows up.

If the top slide (compound slide) is over extedned that can add to bendyness quite a lot.

I realised i havnt explained putting a dial indicator on the tool post very well , i will try to get a photo.

Stuart
 
When moving the tool below (or above) center, it is not the distance from center that matters, it is the angle above or below the cross slide axis. The actual angle change per unit height differs with the diameter of the work. I gently pinch my pocket rule to the work with the tool. If it is vertical, the tool is on center. If the top of the rule is tipped away from me, it is above center, if tipped toward me it is below center. Eyeballing the angle is plenty good enough for what we are trying to achieve here. Try to remember the angles that work best for various tools, materials, and operations.
 
There are lots of variables with different machines , even just reversing the direction will have a movement of play just in the gearing and the wear on or in the half nuts besides the deflection issue. When I was taught in voca school we were always to back out the tool then reset and depending on fit we either skimmed or clean cut to finish. But I guess things changed from forty plus years ago.
 
My atlas lathe does that too, I have the 12" commercial. I read early on where RJ says that it's worse with deeper cuts, I have found personally that when I take a deep cut its actually less, it might be all in the tool and inserts tho, they were made for a much bigger lathe, and a deeper cut than I could ever think about trying. Iscar QNMG 431 1" holder
Have you ever heard that a good machinist can do just about anything on an old wore out lathe? For you, yes I would back the tool away every pass, or at least the one before the finish pass, and be careful with your spring passes, it's possible they could cut undersized if you don't know your machine. Most of the time a spring pass can take off another couple thou on my flexy lathe, maybe more. I always keep my compound at about 6deg, it allows you to really sneak up on the finish size
 
I have the same thing on my lathes ranging from a 400 lb. Rockwell to a 1400 lb. 14 x 40. I usually back off the cross feed on the return to the tailstock. Have done RJ method too, works well.

Bruce
 
What Chevydyl said is exactly why I take a spring pass before I take my final pass. I also ideally take no more then 70% of the cut on the last few passes.
 
great white said:
This is going to sound "noob", but could someone define " spring" in the context of this thread?
Click to expand...

Cuts the part at finished diameter with minimal load on part and machine. This can be a two edged sword, often the light depth of cut during a spring pass will bugger the finish or dimensions, be wary.

Most CNC lathes have a "number of spring passes" in the dialog screen in a threading cycle, G 67 I believe. It will make all the cutting passes in order then as many spring passes as programmed at finished diameter.

Several weeks ago I ran eighty 1 1/2-4 internal acme threaded parts using 2 spring passes, tried 3 in order to get a better finish but it was worse due to the low depth of cut.
 
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