Super 7 not facing flat

[RoParticipant @ro]

Hi, I'm trying to face off a 5 inch steel disc on the faceplate of my Super 7, but the facing cut is producing a convex surface.

As you can hopefully see in the picture above, if I hold a parallel across the face, flush to one side I get a gap on the other side. Using feeler gauges I have measured this gap to be approx. 0.4mm.

I have checked this with rulers and other parallels, it is not he parallel at fault.

Holding a parallel in the 3 jaw chuck against the face of the chuck and running a DTI against it as I move the cross slide shows that as the tool moves from the edge of the chuck the the centre, the tool moves out by 0.2mm. This feels consistent with the facing results above. In case the face of the 3 jaw was not flat, I span the chuck round by 180 degrees and took the measurement again – exactly the same result.

I have stripped down the cross slide and saddle, cleaned out everything and reassembled it no no avail.

I thought it might me the head out of alignment with the bed, so I turned up a test. bar in the 3 jaw to see if it produced a taper. It was out by 1.5 thou over 6 inches, so I'm pretty sure the headstock is aligned.

Please help! I'm running out of ideas as to what could be wrong!

 

thanks

 

ro

Edited By Ro on 18/07/2020 10:24:45

[RoParticipant @ro]

[Nick HughesParticipant @nickhughes97026]

Either a worn or badly adjusted saddle. as when new, they are set to face slightly Concave.

Discussed before an here :-**LINK**

[not done it yetParticipant @notdoneityet]

Nearly all lathes cut concave to an extent. Think how things would stand upright on the face if convex. Yours will clearly wobble a tiny bit on a true and flat surface unless you dish the centre.🙂

A very slight headstock mis-alignment would still cut a parallel bar, if held between centres (a thou and a half, over ~6”? Are you locking down the saddle while facing? Have you tried a second cut, at the same setting, but at much higher rotational speed? Is your cutter truly sharp?

[RoParticipant @ro]

Posted by Nick Hughes on 18/07/2020 10:38:40:

Either a worn or badly adjusted saddle. as when new, they are set to face slightly Concave.

Discussed before an here :-**LINK**

Thanks Nick. that's an interesting read. you say it could be a badly adjusted saddle, am I right in thinking that the only adjustment I have is the gib strip? I have tried the cuts with the gib at "normal" settings, and with all the screws locking it tight, but still have the same issues.

Posted by not done it yet on 18/07/2020 10:47:18:

A very slight headstock mis-alignment would still cut a parallel bar, if held between centres (a thou and a half, over ~6”? Are you locking down the saddle while facing? Have you tried a second cut, at the same setting, but at much higher rotational speed? Is your cutter truly sharp?

Hi NDIY, I turned the test bar just held in the 3 jaw, no tail support (just very light cuts ).

Saddle is locked (and, as above I've tried locking it using the gib adjustments too), and I've tried it with a freshly honed HSS tool and a brand new CCGT insert, at cut depths ranging from 5 thou to 0.5 though – same results every time.

Thank you both for your suggestions!

ro

[HopperParticipant @hopper]

Most likely a worn saddle, on the surface that bears on the vertical shear surface. Is it pre-1972 with the "narrow guide" where the saddle bears on the vertical surface of the front bed shear? If so, a wide guide conversion to bear on the back of the rear shear will most cure the problem. (Done by attaching a strip of flat ground stock about 1/16" thick to the saddle surface at the back.) It could also be bed wear on the vertical surface right there.

When you stripped the saddle, did you put a straight edge over the vertical guiding surface? If there is wear there, this surface will be worn convex and allow the saddle to rock instead of holding it dead square to the lathe axis. (or close enough). Cure for that, other than the wide guide conversion, is to remachine that surface back flat in a milling machine after setting up the cross slide ways dead square to the mill table travel axis.

Bodges: 1. Use the gib screws to pivot the saddle around to correct angle by tightening the screws at one end more than the the other. Use your parallel in the chuck and a dial indicator to check when you get it set square and then nip it up. (Check your parallel is sitting square by a dial indicator on the carriage bearing on one end, set it to zero, then rotate chuck 180 degrees and the dial indicator should read zero again.)

2. Swing the topslide around and set it to face flat by the same method. To advance the tool to take a cut, you will have to slacken the tool clamping screws slightly and tap the tool bit forward a few thou at a time. Bit hit and miss but if you just want to face flat without achieving a specific dimension it works. Common practice on large, clapped out lathes in ancient machine shops in industry (mostly in remote locations where you can't just send out for a new lathe.)

Edited By Hopper on 18/07/2020 11:37:10

[RoParticipant @ro]

Hi Hopper,

Thanks for the info – you were absolutely right! The guiding surface was warn convex, I can't believe I didn't check it when I had it apart before.

I have done the cuts that I needed to do with a 0.1mm shim in between the rear of the guiding surface and the bed, and the gibs tightened up. This has given be a perfectly flat cut.

Luckily my lathe is pre-1972 (albeit with a 1990's bed) so it does have the "narrow guide". I'll order up some 1/16" GFS and see if I can do a wide guide conversion.

Any ideas on the best way to attach the GFS to the rear shear?

Thanks again – I can't believe you were able to diagnose the issue without seeing it!

ro

[HopperParticipant @hopper]

LOL. Been there. Done that.

There was an article in MEW Issue 287 last year on how to do the wide guide conversion without needing to use a milling machine.

I used Loctite to hold my strip in place, plus two small silver steel dowel pins about 3/16" diameter just for good measure. I clamped the strip in place while the Loctite dried by putting it on the bed and tightening up the gib screws.

Also you will probably need to drill out the apron mounting holes and possibly machine down the surfaces on the two lead screw mounting brackets to get the leadscrew to half nut alignment back to normal.

And probably scrape the horizontal surface down flat so it does not ride up on the unworn strip at the very front etc. Jiust blue up the least worn area of the bed and scrape it to match that.

Its all outlined in the article in detail.

[KWILParticipant @kwil]

Last but by no means least, be careful with the right hand gib screw setting, over tightening here is the dominant reason for causing excessive wear.

[RoParticipant @ro]

Cool, thanks for the advice all. I’ll try to remember not to over tighten that screw!

For now, I’ll just keep a 0.1mm shim close by until I get round to either the wide guide conversion or trying to scrape in the current guiding surface.

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