lathe cross slide 'dishing'

[Miner]

Hi Andy,
Others may disagree, but if it were me? I'd make a new gib. Sooner or later any slack thruout the whole system will rear it's ugly head with chatter at the tool tip. I've been more than suprised a few times by thinking one thing, yet the tool itself doesn't cooperate with logic.

Do you have the George Thomas book The Model Engineers Workshop Manual? While it's about Myford lathes, he shows why the gib strip should be pinned in place to prevent even very minor movements of the gib in the direction the slide is moving, yet it still allows adjustment. If you don't have the book, just PM me your email adress and I'll scan the correct pages for you. There's some real good views about why the standard vee shaped points on the gib adjustment screws don't work as they should.

Pete

[andyf]

Thanks, Pete. PM sent.

I ran into a gib strip snag a while back, when replacing the solid nut on my little Perris's leadscrew with a half nut, running in a dovetail with a gib.  Couldn't get it to work smoothly until I realised that the adjustment screws through threaded holes in the frame were pushing the gib down the slope of the dovetail until they fouled the half nut. Easily fixed, but a lesson learned.

Pins would help prevent that happening, too.

I must shop around for some gauge plate, not that I would want to harden it, but for the sake of its ground surface.

Regards,
Andy

[Jonny]

Will be ok Andy, the screws will take that clearance up, dead easy to set up.
Just make sure the dimpling is deep enough in gibs.
Whilst at it i would drill and tap another hole to act as a lock.

A new gib needed if it was a tapered one.

[andyf]

Tapered goibs? On a cheap Chinese lathe?? 

I redid the dimples with a slot drill, as mentioned earlier, so they had flat bottoms and lined up properly with the screws. Being on a 60 deg slope, the top side of each dimple is deeper than the lower side, but I made sure thee was a bit of depth all round.

I'm currently awaiting GHT's thoughts on gibs from Pete,  but in the meantime it occurs to me that there's another way to fill the gap behind mine. Make up a thin strip, like a gib, JB Weld it to the female side of the dovetail, put pins through it and into the gib proper as recommended by Pete and GHT for location, and continue the existing tapped holes for the adjuster screws through it.

The lathe did come with a central gib screw, with no lock nut, to immobilise the slide when needed. Wish I could fit one with a little handle, to save hunting out the right Allen key, but there's no room.

Andy

[andyf]

Hi Andy,
Others may disagree, but if it were me? I'd make a new gib. Sooner or later any slack thruout the whole system will rear it's ugly head with chatter at the tool tip. I've been more than suprised a few times by thinking one thing, yet the tool itself doesn't cooperate with logic.

Do you have the George Thomas book The Model Engineers Workshop Manual? While it's about Myford lathes, he shows why the gib strip should be pinned in place to prevent even very minor movements of the gib in the direction the slide is moving, yet it still allows adjustment. If you don't have the book, just PM me your email adress and I'll scan the correct pages for you. There's some real good views about why the standard vee shaped points on the gib adjustment screws don't work as they should.

Pete

Pete, I took your advice and made a new gib, which almost fills up the enlarged gap left after I machined the dovetails to get them squarer to the spindle. As you say, these things don't always follow logic, and one end of the gib wanted to dip down so its underside fouled the slide base. A pin at that end fixed the problem, and for the sake of completeness I added one at the other end, too.

Thanks for the advice, and for recommending the GHT book. As you say, it's written around the Myford, but there's still a lot of useful stuff in it.

Andy