Author Topic: Finishing problems (Read 33549 times)

Darren · « **Reply #25 on:** November 14, 2008, 06:23:23 PM »

Quote from: bogstandard on November 14, 2008, 06:12:55 PM

That is a bit better than sending them on safari.

Why do They do that? Hardly any incentive for the rest of them is it.

Or perhaps it is, pay once, pay twice.....

Darren · « **Reply #26 on:** November 16, 2008, 05:35:16 AM »

Bought some Gunk (engine cleaner) is a spray can yesterday from the "local" market stall. (and a set of 5 HSS slitting saws, I'm sure they'll come in handy at some point)

So last night I cleaned the 7x12 down a bit. Took the back guard off to get better access at the read bedway and to my surprise there was loads of paint on the underside.
Thinking that this is what I may have been measuring I set about cleaning it off. Some of it was pretty thick too, I had to carve it off carefully in layers. It didn't cover the whole width, but it might have been catching the carriageway along its travel.

Alas no improvement. Now I have better access to that area I can measure it more accurately. I'm getting up to 7thou difference from one end of the bed to the other, with about 3.5thou over the used section of the bed.

Sadly, I think this bed needs some surgery, otherwise I'm flogging a dead horse here.

John if your kind offer is still open we'll have to form a plan .......I'll pm you later if you don't mind

Regards
Darren

bogstandard · « **Reply #27 on:** November 16, 2008, 06:35:33 AM »

No Problems Darren.

John

rleete · « **Reply #28 on:** December 12, 2008, 09:12:33 AM »

Darren,

Any resolution on this? My lathe (Homier 7 x 12) is doing the same thing, and I'm going nuts trying to fix it. This happens all the time - not just when using the leadscrew, but in all turning work. I initially thought it was the headstock bearings, but I tightened them up and it didn't help.

Gibs & half nuts have been adjusted over and over. I improved backlash, and tightened it up a bit, but the "turning marks" are still visible on parts. It means a lot of extra work finishing parts, and I have to account for more material removal (sanding/polishing) and it's causing me to tear my already thinning hair out!

Anyone?

Darren · « **Reply #29 on:** December 12, 2008, 09:35:16 AM »

Hi rleete ?

Nothing done as yet I don't believe, but there is no hurry on this as I have another lathe to use. I'm not personally the one being brave with the bed work

But when the time comes it will be cataloged on these very pages. Should be sometime soon so keep an eye out.

On the head bearings. The standards are plain roller bearings, I changed mine with tapered types when it was nearly new in an attempt to solve this problem thinking it might be the cause.
It wasn't, made no difference to the finish.

I have to agree with you this problem really spoils this otherwise very capable little lathe.

BTW, welcome to the forum, please let us know what you make and interests are. I'm guessing you are in the US?

Darren

rleete · « **Reply #30 on:** December 12, 2008, 09:45:05 AM »

Yes, in the US. Rochester, NY to be exact.

I'm a mechanical designer (tooling & fixtures, mostly), and a novice machinist. I have a HF micro mill and the lathe. My current projects are in the model steam engines category. I'm also somewhat of a car nut.

I found this forum several weeks ago, and just joined H M E M, so decided to sign up here as well.

Bernd · « **Reply #31 on:** December 12, 2008, 10:13:11 AM »

Quote from: rleete on December 12, 2008, 09:45:05 AM

Yes, in the US. Rochester, NY to be exact.

Hey rleete I'm 20 miles south of you, West Bloomfield to be exact.

Bernd

rleete · « **Reply #32 on:** December 12, 2008, 10:41:56 AM »

Hi Bernd. Nice website. I see you have the same PM research engine I have. Buying that kit (eBay) is what got me started on this whole exercise.

I have a single vertical wobbler in process, as well as parts for several more. I figured I had better get some machining experience before tackling the PM kit, so as to not mess it up.

Bernd · « **Reply #33 on:** December 12, 2008, 10:46:53 AM »

Great.

I'm extending an invite if you if you want to see the shop personally.

Bernd

rleete · « **Reply #34 on:** December 12, 2008, 11:03:56 AM »

Thanks. Maybe the week after Christmas? I have a couple of weeks vacation, and I plan on spending most of it in the shop.

Bernd · « **Reply #35 on:** December 12, 2008, 12:37:38 PM »

No problem. I'm retired, wife says retarded, but what does she know

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 · « **Reply #36 on:** December 12, 2008, 12:49:57 PM »

Hi Guys

I've got a similar surface finish problem with my Chester 3 in 1. My little grey cells :

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

Have Fun

Stew

bogstandard · « **Reply #37 on:** December 12, 2008, 01:35:24 PM »

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 · « **Reply #38 on:** December 12, 2008, 01:58:12 PM »

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 · « **Reply #39 on:** December 12, 2008, 02:59:43 PM »

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.

rleete · « **Reply #40 on:** December 12, 2008, 04:50:44 PM »

Just finished tearing down the compound slide. The gibs are poorly machined, as most would expect. But, the damn things are bent! Both are warped 1/4 from the end, like they got caught in something. I straightened them both, easily. A bit of sanding on the surface trued them up quite well. I can't see how that won't improve things, and I'm very surprised I didn't see it when I disassembled for cleaning. I also retapped the screw holes, as one set screw felt like it was binding. It feels much smoother now.

The ways are flat (at least on top), from my inspection with a straight edge. Thickness appears uniform, but there is some paint on the bottom side I scraped away. Also a couple of minor burrs, which didn't look to affect anything, but I stoned them off anyway. I'll keep going and let you all know how it turns out.

Darren · « **Reply #41 on:** December 12, 2008, 04:54:42 PM »

Yes please do, tell us anything you find that seems out of place.....

I know the gibs are not brill, nothing more than a bit of steel chopped of a length of bar,
but bent

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