If your happy with just a ground finish on the slideways then fine. But a proper machine is hand scraped to achieve the finish on the ways. A ground finish no matter how fine is microscopically a series of ridges and valleys, some of the ridges eventually get worn away unevenly to reduce accuracy. The worn metal from the ridges contributing to even further wear. A ground finish also has very little affinity for lubricating oil.
A hand scraped finish has lots of little hollows, these hollows can be likened to a shot peened finish, toughening the surface. The also serve to maintain a film of oil between the mating surfaces reducing friction and wear. The high spots on a scraped finish are less pronounced and less likely to wear into a grinding paste.
A hand scraped finish on a lathe would cost in the order of £2000 for a short bed machine. A grind can be done much cheaper.
Although you are absolutely right in many respects it's a very sweeping statement. In a way its stating that world class machinery are not made correctly. Very few scrape machines these days. It's one way yes i agree, but its not the only way to get superb results and with longevity.
Lets take two examples the Hardinge tool room lathe and the smart and brown tool room lathe. The beds are not scraped, in fact in my years i have never seen a centre lathe bed scraped. I have used some of the best toolroom lathes ever built and still no scraped beds! Look at the very popular colchester no scraping there either. I have the fortune to own one of the above said lathes and that compared to a scraped machine i have of similar age and work load is worse for accuracy compared to my smart and brown. To give you an idea those smart and browns were regarded as one of the elite machines of the generation, they were made with absolutely no expense spared. I have the original bill for the machine somewhere, and it was a hugely expensive machine which would equate to something like 70k now for a 2 foot centres lathe! But still no scraped bed!
It's a swings and roundabouts situation, over years there has been debates, which is better grinding or scraping. They both have pros and cons one of the big cons of scraping is in fact lack of contact area vs grinding, they are all peaks and valleys in both methods. Very often with ground ways they use a lost lubrication system which ensures constant oil between the surfaces. When you look at it in real terms for a lathe bed do you want it scraped? That can gives swarf and muck the opportunity to sit in the valleys, pick up and actually aid wear and not reduce it, I have yet to see a ground slide way pick up. So which is best ground or scrapped?
I'm not sure where you get the conclusion that scraping is like shot peening a surface and toughens it, as that is not correct. As you know scraping is the same as any process to remove metal which is a shearing action. It's not impacting the surface to compact the grain structure therefore will have no effect at all. Work hardening is either a mechanical means of impacting the surface also know as surface conditioning which something like shot peening will achieve.
Shox as long as the guys do the work do it right, you will have a nice lathe at the end of it. Nothing wrong with a ground bed or slide way anywhere, If a ground finish is no good then the world leading machine tool builders like Mazak and all the rest of them have been doing their job wrong for years.
Scott (ward eng)