I own an HBM 450 which is a 250x450mm benchtop lathe (chinese obviously). I feel that the weakest part of the whole machine is the cross slide carriage. No matter how I adjust the gibs, I can not achieve a precise, smooth movement, at least not the precise I want it to be.
Because of that I am considering replacing the dovetail guides with HIWIN HGH 20HA sliding rails. It will require some machining in the existing components, but nothing too hard to do. My main question is....Have anyone walked this road before? How the final rigidity of the entire setup will be affected?..Better (I hope) or worse?
what i did with my southbend lathe was to tighten the gib until the cross slide stuck a bit on both ends of the travel, and then moved it back and forth with plenty of oil. (take the leadscrew and the nut out!)
Where it made metal on metal contact on at the ends of the travel, the cast iron gets polished. take a file, grind a radius on the end of it at a 5 degree angle (you will need to grind the end of it about 45 degrees to get up close to the inside of the dovetail, but the edge for scraping should be about 85 degrees), and use it to scrape down the shiny spots on the carriage flat way. do not take the cross slide assembly off and scrape it. (if you have a surface plate you can first scrape the flat ways of the cross slide assembly)
as you scrape down the shiny spots it will be a more uniform clearance across the entire motion. don't scrape the dovetail, just the flat surfaces. the dovetail is what sets the straightness and 90 degree angle relative to the carriage.
You will need to take a dremel with a half inch diameter grinding wheel, or 1 inch diameter cut off blade, and grind about 10 divots per square inch of the flat portion of the dovetail. These are needed to give the oil somewhere to go. do not grind divots on the exposed dovetail of the lathe carriage, only the cross slide assembly. without these divots you will need to use very thin oil (i was using heating oil) for the above lapping/scraping procedure i just described, and it still won't work right. use lots of oil to flush the metal particles out as you scrape the carriage.
within a few hours of work you should have the clearance down to a uniform 2 tenth across the entire range of travel. (in my case, i had to remove about .002" of metal at both ends of the flat ways of the carriage assembly, because my lathe is from 1943 and the cross slide was probably at least .003" low in the middle of the travel). i basically averaged the wear out across the entire length of travel, rather than trying to get it perfectly flat and straight. since your lathe is relatively new, you should not have to remove more than .0005" of metal)
your lathe has probably been ground, but the parts were never scraped to fit each other, or the price would be at least double. you might have less than 10% contact between the two parts as it is right now.
anyhow I do not believe the time and expense of adding linear ways to your lathe is worth it, unless you want to make a lightweight cnc machine out of it and leave it running week after week making parts..
If you have a micrometer and some pins of uniform diameter, measure the distance across the dovetail on both ends. if its more than .001" out of spec (which is a lot), then you will need to selectively scrape the flat ways only at the narrow end so that the dovetail widens only at one end.. doing so will selectively average out the error.. but you still won't know if its straight.