Help - Compound lathe gears coming loose
My Atlas 12 x 36 is exhibiting some strange behaviour lately. The compound gears that engage the QC box and the idler gear are working themselves loose when I engage the halfnuts for turning. Basically, I can make about 3 passes before the nut loosesns and the compound gears move out of engagement and the work piece gets a nice shiney spot where the bit just stay engaged. If anyone has any idea of what would cause this, I would appreciate any advise. I think the design is awful with the gears directly riding against a washer that directly rides against the nut, but if others with Atlas lathes don't have an issue, then something is just funky with mine.
Thanks, Derek G
The change gears on my SB have keyways to lock them on the shafts where this is needed (on the lead screw). The nuts are only there to hold them in place, not to provide friction for driving the shaft (lead screw). Are you perhaps missing a key?
Make it fit.
if you lathe is built anything like mine, the gears on the input/output shaft should be keyed and unable to 'loosen' the nut any
the idler gear should have a bearing or steped bushing or some sorta arrangement where you can fully tighten the bolt that secures it to the banjo, without any friction on the gear rotation (ie its unable to put any torque on the bolt)
You may be missing a key, or a washer in one of the gear assembleys.
Originally Posted by derekg
If I've got the message right, the gears are staying on their respective shafts OK but are coming out of mesh under load.
The problem is usually that either the "banjo" under the gear that drives the QCTB "swings" under load or else the intermediate gears mounted on the slot/s in the banjo or mounted to the back of the head-stock gear-box have been "pushed apart" - because of the load on them.
All that may be needed is a general "tighten-up" to stop the movement/s under load.
Because of the "pressure angle" of the gears (pretty well ANY gears) the gears are not only pushed "around" but are "pushed apart" (ie out of mesh).
The greater the load you put on your gear-trains, the greater will be the tendency to push them apart.
I deliberately set my tightening of the "banjo" nuts/screws to medium/light so that in the event of a serious "crash" the "banjo" acts as a "weak link" and moves the gears out of mesh.
Sure, I've got a few threads cocked up but I still have an entire set of gears all the way from my head-stock spindle to my lead-screw and my half-nuts.
I've done it for years. Its just a matter of getting to know your machine.
oldtiffie: Nice trick! I wonder if thats what is supposed to give in my geartrain since I don't seem to have any sacrafical gears or clutchs in the geartrain (I could have a sheer pin I guess, but I havent seen one, Clone of HF12x36)
My guess runs along the same lines as Tiffie's. Added to his comments, when setting up the gear train, use a piece of paper to set the gear mesh. If the gears are too loose, they are already on their way toward slipping out of engagement.
The other possibility is that you might not have the proper setup for the gears. The gear has a double key, and a bushing with keys for the driven gears. The bushing runs on a sleeve which should be long enough that the assembly has some float to prevent loosening the nut and washer. If you do not have this float, something is wrong and that is what needs to be addressed. Sleeves are quite simple to machine and possibly a slightly longer one will solve the problem.
Thank you for the responses. Looking at the keyed bush, there is a brass bushing in the middle that appears to be worn down. I'll see if I can make a new one a bit longer to correct the problem. Many thanks for the great feedback.
It sounds like someone might have replaced the sleeve with a bushing.
Here is a photo from the Atlas parts list showing the parts involved;
I don't have any to measure, but the straight part of the sleeve should be around 1/32" to1/16" longer than the bushing. The shoulder on the bottom is around 1/4" as I recall and larger than the bushing measured across the keys.
No particular precision needed other than a nice fit & finish on the OD of the sleeve where the bushing runs.