What is the end result of scraping in a machine?

[Wobbles42]

This may seem obvious, but I haven't seen it clearly explained. Aside from being a good learning opportunity, are there real performance gains to be had by scraping in a lower end machine?

It seems many people have undertaken this project. I've found a lot of info on *how* to do this, and I understand the advantages of scraping as a process, and most people indicate that it makes a difference, but it isn't clear what that difference is.

I am curious what kind of changes people see in the machines themselves after improving the ways geometry. What sort of improvement is observed?

A little Sieg X2 is what I have access to, and I am curious what potential exists. People joke that these machines can be "casting kits" more than finished machines -- with a little love can they be turned into a more refined tool?

 

[markba633csi]

On older machines it's done to compensate for wear and return the machine to factory specifications. On new inexpensive machines it's sometimes done to correct manufacturing defects as received from the factory, or get closer to an ideal, with the end user performing the dual role of quality control and rework department, in most cases for zero money LOL
-Mark

 

[benmychree]

In a word, ACCURACY, where is did not exist before.

 

[Ulma Doctor]

increased bearing surface
when properly scraped and flaked, the wear surfaces use the film of oil, to float the assemblies they carry- with little friction due to large surface area to support the load
wear surfaces that don't share the capacity to trap oil, wear away quickly due to increased friction, coupled with the very few areas of contact supporting the entire load, lends to rapid wear

 

[C-Bag]

All the above are very true. In my case with the compound on my 9x20 it reduced to zero stiction while fixing the inaccuracy of the way the dovetaiI was machined. I was not aware of this stiction until I read the Connelly book although I’d felt it in mechanisms before. It seems counterintuitive but if you get two smooth surfaces together with a fluid between them like two pieces of glass, they will stick. You can overcome it, but it makes mechanisms “sticky”. So not only is the goal to increase the precision of the fit it also reduces stiction which like in the case of my lathe carriage (which needs to be scraped) it would smooth the feed and make for more consistent finish.

 

[Wobbles42]

Thanks everyone! That's exactly the info I was looking for. I just didn't know what to look for. I think I am going to try the project.

I have an extra saddle assembly for the mill, so it will be interesting to compare (and I have one chance to get it wrong if it comes to that).

 

[Hellkell]

It seems counterintuitive that you’d scrape something flat then flake it to allow oil to get in. Doesn’t that reduce the flatness again? negating the scraping.
Wouldn’t a less than flat surface (non-scraped) do the same thing (allowing oil to be trapped)?

 

[markba633csi]

The average flatness would remain the same after flaking if done evenly over the whole surface- the overall accuracy would remain. However, the fit between two parts may be way off requiring re-milling and scraping
-M

 

[Ulma Doctor]

It seems counterintuitive that you’d scrape something flat then flake it to allow oil to get in. Doesn’t that reduce the flatness again? negating the scraping.
Wouldn’t a less than flat surface (non-scraped) do the same thing (allowing oil to be trapped)?

The flaking is done on the bottom surface of the top sliding member. The voids created by the flaking keeps the oil trapped rather than allowing it to be squeezed and run out, creating a bearing surface. The inability to compress a trapped liquid is the very same principle that floats the assemblies, rather than have them grind together.

 

[C-Bag]

My impression from the Connolly book was flaking was more ornamental than functional as scraping is. I’ve seen more and more especially on these crazy priced eBay machines where there is flaking with no underlying scraping. And truly effective scraping would not be on an up facing surface, like my ways, it would be on the bottom of the carriage. This where you would want the cushion of oil trapped, not necessarily on the ways where it would also attract dirt and grit.