Thoughts on a lathe gib

[alanganes]

Been away for work a bit lately and that has kept me out of the shop. Now that I'm home for the time being, I was doing a bit of tweaking and cleaning on one of my lathes an, old Jet 10x24.

Looking at the gib on the cross slide, it is of the pretty typical parallelogram profile with a notch on one end that engages the adjusting screw. Nothing unusual. What I did note was that one side was clearly hand scraped at many points along its length.

So why would the manufacturer have done this? It is a fairly long and somewhat thin piece, and it gets sandwiched between the slide and the carriage dovetail. Why would it need to be scraped? I can see the need of it to be pretty flat to assure lots of contact area and that the thickness is even. Were I to make another one of these (theoretical question...) would simply doing a decent once-over with a surface grinder be "close enough" for it to work properly?

Mostly just curious, but the answers have obvious applications for any future projects.

Interested in the group wisdom...

Thanks!

-Al

[DaveH]

The "scraping hollows" hold the oil
 
DaveH

[Highpower]

As far as I know it is the same reason you find scraped 'ways' on a mill or on a lathe bed. Hand scraping remove high spots  to make the surface flat but also leave slight pockets to hold an oil film to keep things moving smoothly. Now think about a set of precision gauge blocks that are ground perfectly smooth and flat. When you 'wring' them together, they don't let go! There is no room for air or an oil film between them to slide against each other so they get stuck together. Not what you want to have happen on a machine.

Another procedure, 'flaking' takes it a step farther to add even more areas to hold oil.

[alanganes]

The fact that a scraped surface holds oil makes sense, though it seems funny to me that it would be done on a machine like this for just that purpose. The ways were clearly not scraped, and unless the gib were ground to a highly polished state (like a gage block) there would still be some of that I'd think. The marks on this do not have the regular patterned look of hand flaking.

I can see it being done to take off high spots, but if that was the primary reason it seems a pretty slow way to achieve something that could be done by surface grinding it. Particularly since while this is a fairly decent machine for it's era and what it is, it was clearly never in the "cost is no object" class of machine like a Hardinge or something.

I'm not discounting those thoughts, just an interesting thing to chew on. I've taken a few old and not so old machine tools of various US and Asian manufacturers apart along the way (no pun...), and have never seen a scraped gib before. It sort of jumped out at me as unusual. I'm sure many here have seen their share of disassembled machine tools, have you guys seen scraped gibs on your machines? It's not impossible that I am just grossly unobservant and this is just donning on me...

Thanks for chiming it!

[Highpower]

I'm certainly no expert at anything machining related. Maybe I'm just looking at it from the other side of the fence. I have Asian machines and both my lathe and mill have 'scraped' tapered gibs like you describe. A rather crudely done job of it of course - but scraped they are. I just assumed it was done that way 'over there' because hand scrapers are cheaper than surface grinders, and you can put scrapers into the hands of a lot of workers in a factory vs. putting each worker on a surface grinder.

I would venture a guess though that there are quite a few more careful hours invested in scraping a Hardinge compared to import machines though, with a much higher spot-per-inch count.    

I've been telling myself I'm going to pull the gib out of the knee on my mill and re-scrape it, since day one. In between too tight, and too loose, lies shudder and stick and slip. I don't think it's what you would call 'flat'. Maybe in the next 5 years or so.....