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Poor surface finish on engine lathe

ewlsey

Diamond
Joined
Jul 14, 2009
Location
Peoria, IL
I'm chasing down the cause of poor surface finish on my engine lathe. It's a 16x40 geared head lathe, made in China. It's a stout commercial grade machine. Not a hobby home shop toy.

1045 material, 1.7" diameter, feed .014/rev, 700 RPM, .08 DOC on diameter.

004.jpg

Did some hunting. Cross slide is badly worn in the middle of travel, like .01" slop at the location where I was making this cut. So, obviously that's a huge issue.

I checked the spindle bearings at both ends with a pry bar and indicator. Seems good and tight both radial and axial. Runout on the spindle nose is basically 0.

I wanted to isolate the issue to the wear in the cross slide, so I put some bar clamps on the cross slide to hold the dovetail over and down. Then I took some more test cuts that way.

002.jpg

It's better, but certainly not good. If I use these same parameters in my CNC lathe, the finish is flawless. Am I asking too much? I'm OK with fixing the wear in the cross slide, but I'd really like to rule out other sources of the poor finish before I dive in.

Any thoughts?
 
Sometimes bearings have no play but 'noise' under load.
I used to rebuild heald boring heads. There would be no play on the indicator but required finish couldnt be attained until rebuilt with new bearings.
Another time a lathe had bad finish and it was bad drive belts sending frequencies through the drive
 
I tried several gears to rule out vibration in the gear train. Belts are good and tight. No balance issues I know of. Machine is leveled and sitting on steel pads.
 
Just dealt with a similar issue on an 17" Clausing.

Funky finish, would not feed in X, hard to turn.

I assumed that coolant and a shitty oiling system on the slide was to blame so I pulled it apart to clean.

Discovered that this lathe has been rebuilt due to the 1/8 turcite on the slide.

The turcite had came unbonded, only reason it stayed in place is cause the turcite had been pinned.
Who ever had it before just tightened the shit out of the gib.

I was lucky, the saddle and slide ways where still parallel to .002 according to measuring over pins.

Ordered new turcite and cemented it in.
Using high spot blue I rough scraped the turcite in.
Then I scarped the gib in.
After gib was scraped in close I went back and finish scraping in the turcite then finish scraped the gib.
Slide is now nice and smooth and finish is OK. It will also power feed in X now, sure sucked ass before having to hand crank across a wide face.

ewlsey, since you said it had .010 wear in the middle its sure sounding like your going to have to machine the saddle and slide with a dovetail cutter then replace what you removed with turcite and scrape it all back in.

Searched online for the turcite...
Found a place that cut to order.
1" x 26" x 1/8 thick plus cleaner and cement and shipping....$135

Took me about 6 hours to bond and scrape it all in....was worth every penny.
 
Fixing the cross slide is not an issue. I just don't want to pour time and money into the machine if there is another, bigger, issue.

I think I need to try another test cut with the tailstock.
 
I had the same surface finish problems with a chinese lathe at school, when I took a .100 cut it stopped for some reason. My pacemaker I had would get an amazing surface finish no matter what. Maybe saddle is clapped out or was not properly made.
 
Fixing the cross slide is not an issue. I just don't want to pour time and money into the machine if there is another, bigger, issue.

I think I need to try another test cut with the tailstock.

I completely understand.
The thing is though, the amount of wear you stated is a big issue just by itself.
Another issue that may be a problem is wear in the compound slide as well, compounding the issue.
Have ran lathes in the past that needed a C clamp on the compound.
 
The compound has a locking screw, and it's pretty tight.

I don't know if the clamps I put on the cross slide completely tightened it up.
 
1045 material, 1.7" diameter, feed .014/rev, 700 RPM, .08 DOC on diameter.
.
.
If I use these same parameters in my CNC lathe, the finish is flawless.

Presume same insert, same holder, but:

- is the TP the same, and with the same overhang and same 'quality' of support under it?

- is the workholding the same (chuck?, collet?, how far out from the bearings?)

What happens if you go up or down on the RPM on the manual lathe?
 
Roughly the same. CNC tool is 1" shank. Engine lathe is 3/4". Running an Aloris CXA tool post.

"Roughly" .. good one..

:)

CXA on both, can you adjust the engine lathe by enough to try the 1" holder off the CNC on it? Or the reverse, to see if the CNC becomes 'less flawless' with the 3/4" holder?

Or get a small jackscrew under the 3/4" one on the manual for at least a test cut?

There's an unwanted, but very 'regular' resonance in there somewhere. Ear should notice it, fingertips or mechanic's stethoscope localize it.

Short run of cut, clapped-out "company" lathe, one first hung a bit of body-mass as haul-back and "meat damping" on the TP, chased an actual fix later. Clamps, often, different shape and approach angles on a Rex 95, even more often.

Or not at all. Given some of the lathes had served War One, War Two, Korean War, one just went and used a different lathe.
 
Hard to see any regular pattern from the image. I’d dial back to .008” feed and see it substantially looked similar. The .01” slop means you’re gonna have the cross slide off it anyway...

Old timey trick with machines that were worn badly from always making small parts was to use a boring bar on the aloris to get the cross slide back to where it wasn’t used so much.

Good luck
Matt
 
Hard to see any regular pattern from the image. I’d dial back to .008” feed and see it substantially looked similar. The .01” slop means you’re gonna have the cross slide off it anyway...

Old timey trick with machines that were worn badly from always making small parts was to use a boring bar on the aloris to get the cross slide back to where it wasn’t used so much.

Good luck
Matt

Cheaper, faster, easier here to seek a less problematic insert, alter the RPM, feed, DOC to fit the "wear personaliy" of this particular machine until time - and money - is found to correct it.

Or replace it. Vanilla/White-bread commodity lathe, after all, not some hard-to-find specialty item.

Or just go and use the CNC machine that doesn't have the problem to begin with, this same-same tasking.
 
Fixing the cross slide is not an issue. I just don't want to pour time and money into the machine if there is another, bigger, issue.
It kind of looks like bearings, to tell the truth. Even if the cross-slide is crap, if you lock it down and clamp it, it shouldn't do that.

I think I need to try another test cut with the tailstock.
Yeah, good idea. And in a different place on the z travel, too.

But I dunno, have run some pretty clapped-out lathes and if the bearings are good, the finish is usually okay. They won't repeat and you can't trust the dials on size and they don't cut straight but load the cutter in one direction and it pretty much stays that way.

I would fear you have a headstock bearing problem ... or the chuck is not tight, the jaws are loose in their slots, something holding the part is not doing its job, something like that. But I am pretty sure you already checked all that :)
 
I did some more testing. Tried a different insert at a slightly higher RPM with the clamps installed and finish was pretty good.

I need to install the 4 jaw and test again. I think my 3 jaw is part of the problem. It's never been quite right.
 








 
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