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Atlas 618 not facing square

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WCraig

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Hi:

When I face stock with my Atlas 618, the resulting surface is slightly convex. For example, I just faced some 1.25” aluminum and when I hold a straightedge across the newly cut face, it rocks every so slightly. I can get the feeler gauges out to measure it but I would guess a couple of thous.

I think my technique is good. I’m using the cross slide with the carriage locked to the ways. Don't think the compound is moving since this happens repeatedly. I also drilled a centre hole in the stock so it is not a nub left by being a touch off centre.

Does this mean that there is uneven wear in the cross slide dovetails? The gib does have obvious rub marks. How would one compensate or fix this?

In one of Keith Rucker’s videos, he said that a properly set up lathe should leave a an exceeding slightly concave surface. On the order of 1/2 thou over the working radius (8 inches in his case). This is so that faced parts will stand straight on a flat surface. Mine would wobble somewhat.

Craig
 

tq60

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Atlas lathe is not that rigid.

Could it be that the further out in diameter the cutting speed applies more force to the cutter resulting in movement that allows the cutter rum to change position a bit.

Make everything as short as possible with minimum stick out and make tinny cut to test.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 

Superburban

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Has the lathe been taken apart? I could see a headstock out of alignment doing that. Have you tested it with different lengths from the chuck? I would think a worn way, would give different results as you get away from the headstock. I can't imagine a worn saddle, without worn bed ways, so I would think it would also give differing results at different distances.

Worn cross slide dovetails? Generally if you adjust the gibs, then move the carriage, it will usually get tighter, or looser, as the two dove tails will not be parallel to each other.
 

RJSakowski

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Some lathes have adjustable headstocks but that is not the case for the 618 (at least, mine) Because it uses flat ways, it would be possible to correct the non square condition by honing or scraping the carriage where it comes in contact with the front side of the front way.
 

markba633csi

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I have a Mk2 Atlas that does that. It's the headstock to bed fitup that's just slightly off, since I swapped heads with another machine. The spindle is just a hair counterclockwise if you were to be looking straight down at it. It also turns larger diameter by a tiny bit as you move further from the chuck.
I am migrating to another lathe so I doubt I'll bother trying to fix it, I just do short parts or turn between centers.
mark
 

WCraig

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Wow, multiple replies in the space of a few minutes!

Has the lathe been taken apart? I could see a headstock out of alignment doing that. Have you tested it with different lengths from the chuck? I would think a worn way, would give different results as you get away from the headstock. I can't imagine a worn saddle, without worn bed ways, so I would think it would also give differing results at different distances.

Worn cross slide dovetails? Generally if you adjust the gibs, then move the carriage, it will usually get tighter, or looser, as the two dove tails will not be parallel to each other.
I've used 'Rollie's Dad's Method' to align the axis of rotation with the bed. I got it down so that the vertical was zero and the horizontal was out by 4 thous over about 10 inches. That remaining difference might account for some of the problem but it couldn't be the entire reason.

Some lathes have adjustable headstocks but that is not the case for the 618 (at least, mine) Because it uses flat ways, it would be possible to correct the non square condition by honing or scraping the carriage where it comes in contact with the front side of the front way.
I should have thought of that. I think I'll check Rucker's video. I think he used a massive right-angle granite block to test the carriage and cross slide.

I have a Mk2 Atlas that does that. It's the headstock to bed fitup that's just slightly off, since I swapped heads with another machine. The spindle is just a hair counterclockwise if you were to be looking straight down at it. It also turns larger diameter by a tiny bit as you move further from the chuck.
I am migrating to another lathe so I doubt I'll bother trying to fix it, I just do short parts or turn between centers.
mark
I did have the headstock off the ways when I was rebuilding the machine. As noted above, however, I did some work to ensure the axis of rotation is aligned as closely as possible with the bed. It would be...unwelcome...to have to take things apart enough to try to shift the headstock.

Thanks everybody.

Craig
 

markba633csi

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We know a thing or two cuz we've seen a thing or two LOL
 

WCraig

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Argh, it seems that the problem was mostly operator error! The workpiece wasn't tight enough in the chuck jaws. In my defence, the chuck key that came with my brand new 4-jaw chuck is much smaller than the one for the old 3-jaw. I think I was applying the same pressure but had far less leverage and therefore the jaws weren't nearly as tight as they should have been. Anyway, the work must have been squirming in the chuck somewhat with more being cut from the outer diameter than when it got to the center.

After really (no, I mean REALLY) torquing the jaws tight, I re-faced the work and got a much better surface. Weirdly, I had more problems with chatter on these cuts?

I think I may try to find (or make) a chuck key with a bigger T-handle and use that for the final snug-down on the work.

Sorry for the wild goose chase, everyone.

Craig
Still plan to re-check alignment with Rollie's Dad's Method ("RDM"?).
 

wa5cab

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Atlas lathe is not that rigid.

Could it be that the further out in diameter the cutting speed applies more force to the cutter resulting in movement that allows the cutter rum to change position a bit.

Make everything as short as possible with minimum stick out and make tinny cut to test.

Sent from my SAMSUNG-SM-G930A using Tapatalk
No, if that were happening, it would result in the opposite thing happening. The cutter would cut less near the OD than near the ID.
 

wa5cab

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One other thing that I will mention is that the reaction of the carriage during turning and during facing from OD to center is to move toward the operator. And in most cases, there is a torque component applied through the compound and cross slide to the carriage in the CCW direction as seen from above. Consequently, most wear will occur on the carriage gib and on the rear of the rear way. The front of the front way and front of the carriage gets relatively little wear.
 
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