The Shop > Tools
Finishing problems
Bernd:
No problem. I'm retired, wife says retarded, but what does she know :D so any time of day would be fine by me. PM me with a time of your convenice and I'll give you directions on how to get here.
Bernd
sbwhart:
Hi Guys
I've got a similar surface finish problem with my Chester 3 in 1. My little grey cells : :scratch: have been working on the problem on and off for the last couple of years. I was wundering if anyone has checked their jib strips out for straightness ?, the guy who runs the mini lathe and mini mill site recomends lapping them flat, as anyone tried this ?. I've got a job in machine at the moment as soon as its finished I'm going to check it out, Johns got a surface grinder and lives a short walk from my home so hopefully he'll let me use it on my jib strips :thumbup:
Have Fun
Stew
bogstandard:
I will be starting Darrens lathe soon, and I will do a write up in this post. I can't guarantee it will solve the problem, but it definitely won't harm the lathe, and it will also show if this is the root of the problem for Darren.
Basically I will be skimming the underside of each side of the bed rails to get them parallel (there is a rather large sticky out bit underneath each one that is doing the lathe no favours, set the jibs to the tight spot, and anywhere else the saddle wobbles about), and then fitting tapered jib strips in brass or bronze (depends on what I can get my hands on).
All will be revealed.
John
rleete:
I talked to the diamond machinists at work today about this. Recommendations were that it was the tool and the ways. Oh, and the operator. Something about the nut at the wheels being off... :)
I was told that the finish cut should be at a faster machine speed, and that the tool should have a large radius. We manufacture tooling for lenses, and some of the ones we do are plain mirrors - called "planos" in the biz. The tools they use are large radius diamonds (large for them being in the .25"-.31" range) on hard copper. When the part is done, you can't tell it's been turned, but is a circular mirror of up to 24" diameter. Plus or minus .0005 is wide open tolerances for these guys.
As to the ways, they said they are too lose, and when I'm taking larger cuts to get to the finished diameter, the wavyness is the result of uneven pressure on the tool against the work. It sort of work hardens the surface, so that the finihed cut can't remove all of it evenly. So lighter cuts on the turning if I can't tighten up the ways more. Lapping the gibs should help, but I've been avoiding it. It was also recommended that I back off the leadscrew nuts before tightening the ways, as that may be influencing the way the slide fits on the bed.
So, I'll try tearing down the machine and lapping the gibs. Then reassemble, paying particular attention to get the best adjustment at the point where I do the most turning (near the headstock), and then worry about backlash. I'll post results when I get it dialed in.
Darren:
Ok,
I'm not disputing what you say above but the first thing I would advise is to accurately measure the bed ways for even thickness and parallelism.
My lathe that show the same symptoms as yours was way out. The bed was thicker in the middle than at either end. This meant that the gibs could not be adjusted properly.
It certainly needed a little more than hand lapping, I think lapping is for ways that are almost correct and just need a bit of fine tuning to finish them off.
I also tried finishing at different speeds, different tooling etc, non of it made any difference. Besides, you can't really up the speed for larger diameter work so better to find the cause to see if you can cure than to try and work around it.
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