Lucky accident

[sshire]

Lucky Accident
Somewhere, on virtually every forum having anything to do with machining and small lathes or mills, you'll find something like this.
"Chinese machines are a kit and will require some level of tuning, adjusting, remaking parts, etc. to get them to a workable state."
I've seen that and so have you. So what do we do?
Clean off the shipping gunk. Lubricate them. Replace 4 way tool posts with the quick change variety. Add a DRO. Better live centers. Better chucks, etc. I've done that. 
Finding endless belt changing a PITA (turn and face at one speed, change the belts to different pulleys for parting, change back, change to yet a different speed for center drilling and reaming, change back), I replaced the 120 V single speed motor for a 220 V Leeson motor and Teco VFD to give me variable speed with lots of low end torque. Spectacular.
So all of the above made my lathe much better than it was out of the crate. But...
I was never happy with the lack of smoothness of the cross slide and compound slide. It seems as if I was forever adjusting the gibs. A bit tight but no wiggle in the slides. A bit loose, easier to turn the handwheels but movement in the slides. Sometimes I got lucky and hit a sweet spot but after a few days that was gone.
I had read a number of articles about lapping the slide dovetails and figured why not? The "professional" machining forums screamed, "NO!! Never lap the gib and dovetails!" Most of the "amateurs" who had done it said "big improvement". What to do. 
Removed toolpost, compound, crossslide screw and nut and applied Timesaver lapping compound. This starts out at the stated grit and progressively becomes less effective until it won't cut at all. Put fresh Timesaver on, rinse and repeat. Move the compound by hand back and forth for about 45 minutes replacing Timesaver when it feels like nothing is happening. As the slide movement gets easier, tighten the gibs a bit and continue.
When I felt that the movement was pretty smooth (and my arms felt like they would drop off), I cleaned everything very well and started to reassemble everything.
I noticed that the gib strip had a curve in it. I really hadn't paid this any close attention but everything I had read indicated that this wasn't right.
Maybe it's supposed to have a slight spring but this seemed to spring in the wrong direction. Very little. Maybe 50 thou off the surface plate at the center.
I can bend that straight. Take that gib strip!  I laid it across two parallels at either end, and very gently lowered the handle of the press. It moved (honest) maybe 20 thou and SNAP. Now I have two gibs, evenly divided at the locating slot. NEW PROJECT.
.25 brass flat bar. Cut, faced, 30 degree bevels on each side, new locating slot milled. Done. (Uh, 2 1/2 hours later).
So how did this all work out.
Gib adjustment is no longer a PITA. Much easier to set and much less fiddling. And the cross slide movement. No wiggle and smooth as glass. It's like a new lathe. Can't wait to do the same for the compound slide. Timesaver and brass gib strip for sure.

Best
Stan

[steamer]

Glad it's working out for you Stan.   What surfaces had metal removed?
It would seem that using lapping compound would remove material from both surfaces,,,ie they may fit, but may not be straight....
I suppose if it's working for you..

Dave
 

[sshire]

Dave
The undercut of the dovetails.  I doubt that there was much metal removed. It appears that there were a few rough spots that the 400 grit smoothed out. It seemed to me that the surfaces between the gib and the dovetail had the most work and since the gib is adjustable I doubt that the "straightness" was affected. I will run the cross slide with a DTI to check, but I have no "before" numbers to compare. Since you've done such an amazing job on the SB and know a whole bunch more about this than I do, what would be an acceptable tolerance for the cross slide movement (assuming that 90 degrees to the ways is perfect)?

Best
Stan

[John Rudd]

Somewhere, on virtually every forum having anything to do with machining and small lathes or mills, you'll find something like this.
"Chinese machines are a kit and will require some level of tuning, adjusting, remaking parts, etc. to get them to a workable state."
I noticed that the gib strip had a curve in it. I can bend that straight. Take that gib strip!  I laid it across two parallels at either end, and very gently lowered the handle of the press. It moved (honest) maybe 20 thou and SNAP. Now I have two gibs, evenly divided at the locating slot.

Lol made me laugh...I know I shouldnt...but been there done that and got a Tee shirt..

But the consoling thing is you got out of it...and have a working machine...Well done that man!!

[Stilldrillin]

A good (eventual) outcome,Stan. 

I've heard, the gibs will break! 


I've set up one lathe, and two mills........ All gibs were bent, but made of steel. 

So, I clonked 'em straight with a lead tup. Then finished on a "surface plate", covered with emery cloth.

I always file the gib screws to a point, better to locate in the gib strip dimple.

Never had gib trouble since....... 

David D

[steamer]

Hi Stan,
Thanks for the response.
I think your asking me how square the cross slide should be to the axis of the bed ways?

According to Connelly....

As an assembly on the bench, the cross slide should be square to the V ways to .0005" in 6 inches maximum.  The direction of the error is to the left, toward the HS, as the Cross slide is advanced  This may be refined at final alignment with the headstock.

Parallelism between the angled ways of the dovetail is much tighter to say nothing of straightness of the guiding surface of the dovetail....the one closest to the headstock.   

But please don't take any of this as a critizism.  There are many ways to solve problems....and if the dovetail doesn't have enough clearance, a little compound will create some at the root.  My concern with lapping compounds in this application is the lack of control over exactly what is being removed...and where?  It's kinda a blind operation.
    

Dave

[steamer]

Hey Dave,

Right!   but your guiding the abrasive on your surface plate....so the plate geometry doesn't change, just the gib.   

Dave