Correct stone for scratches on 932 saddle?

[EVMiller]

I have a few superficial scratches on the top saddle. You can't feel them but I figured I should hit them with a stone before reassembling everything. Should I bother doing it? If so, what type of stone is used for this?

 

[benmychree]

India stone is good for such stuff.

 

[davidpbest]

Get yourself a pair of precision ground bench stones. I use them all the time on vise jaws, table surfaces, 123 blocks. Watch this video to understand how they work.

You can buy them here.

 

[EVMiller]

Thanks!

 

[graham-xrf]

I have a few superficial scratches on the top saddle. You can't feel them but I figured I should hit them with a stone before reassembling everything. Should I bother doing it? If so, what type of stone is used for this?
View attachment 362168View attachment 362167

It depends on how deep are the scratches, and they do not look very deep at all!

The general value of precision ground stones as mentioned by @davidpbest is unquestionable, but it misses Renzetti's point about what precision ground flat stones do. They are flat such that the pressure beneath them is so low they do not remove much material, if any.
You can rub a flat stone even on a pristine gauge block, and not harm it, nor change it's dimension at all. The stone will knock down burrs and micron irregularities, and effect a "flat polishing" action that may leave all or nearly all the metal still there, but flatter, and more mirror-like. This action happens, even if the stone is as coarse as #300.

The scratches in your surface are not deep, and would rub away with abrasive paper if done in a series of grit sizes. You will not get this effect with precision ground flat stones. If you succeeded then then at least one of the (expensive) flat stones would end up no longer flat, but the nature of the flat stones is that even if you tried, you might wear yourself out only putting a better polish there without removing the scratches.

So - if nothing has to slide over the scratches, nor take a position that depends on that scratched surface, so that we are talking about visual appearance only, then start with #180 on a wood block, and take them out, blending the action over a larger area than the scratches. When they have just about disappeared, move to #240. If they have not gone at #320, back up to #240. Move to the next grade when you have ground out the marks of the previous grade. To match the final surface of the rest of the metal, it looks to be about #400 or #600 but you can choose where to stop.

The fact you will have "lowered the surface" to take out the scratches will not be visible just to look at, because you blended it over a larger area, and the actual depth change is miniscule. A scratch even only a tenth or two deep is still so very visible, and to carefully grind it away is not going to affect anything. A deep scratch like say 0.002" in a critical way surface is a very different matter.

My point is, you can fix this, but precision ground flat stones, though lovely to have, just won't work to remove scratches without ruining the stones, and I think, is not the way to go.

 

[davidpbest]

It depends on how deep are the scratches, and they do not look very deep at all!

The general value of precision ground stones as mentioned by @davidpbest is unquestionable, but it misses Renzetti's point about what precision ground flat stones do. They are flat such that the pressure beneath them is so low they do not remove much material, if any.
You can rub a flat stone even on a pristine gauge block, and not harm it, nor change it's dimension at all. The stone will knock down burrs and micron irregularities, and effect a "flat polishing" action that may leave all or nearly all the metal still there, but flatter, and more mirror-like. This action happens, even if the stone is as coarse as #300.

The scratches in your surface are not deep, and would rub away with abrasive paper if done in a series of grit sizes. You will not get this effect with precision ground flat stones. If you succeeded then then at least one of the (expensive) flat stones would end up no longer flat, but the nature of the flat stones is that even if you tried, you might wear yourself out only putting a better polish there without removing the scratches.

So - if nothing has to slide over the scratches, nor take a position that depends on that scratched surface, so that we are talking about visual appearance only, then start with #180 on a wood block, and take them out, blending the action over a larger area than the scratches. When they have just about disappeared, move to #240. If they have not gone at #320, back up to #240. Move to the next grade when you have ground out the marks of the previous grade. To match the final surface of the rest of the metal, it looks to be about #400 or #600 but you can choose where to stop.

The fact you will have "lowered the surface" to take out the scratches will not be visible just to look at, because you blended it over a larger area, and the actual depth change is miniscule. A scratch even only a tenth or two deep is still so very visible, and to carefully grind it away is not going to affect anything. A deep scratch like say 0.002" in a critical way surface is a very different matter.

My point is, you can fix this, but precision ground flat stones, though lovely to have, just won't work to remove scratches without ruining the stones, and I think, is not the way to go.

Disagree completely. If these are precision reference way surfaces, you either want to:

1. level off any burrs at the edges of the scratches using precision ground stones and removing as little metal as possible, or
2. have the entire reference surfaces precision ground and/or scraped to new dead-flat reference surfaces again.

Removing cosmetic scratches with abrasive paper serves no functional purpose and will likely lead to creating uneven surfaces, which is exactly what you don’t want.

 

[graham-xrf]

Disagree completely. If these are precision reference way surfaces, you either want to:

1. level off any burrs at the edges of the scratches using precision ground stones and removing as little metal as possible, or
2. have the entire reference surfaces precision ground and/or scraped to new dead-flat reference surfaces again.

Removing cosmetic scratches with abrasive paper serves no functional purpose and will likely lead to creating uneven surfaces, which is exactly what you don’t want.

Perhaps I misunderstand.
I took it that the surface was not a bearing surface. The scraped surface in the picture was elsewhere.
I qualified what I said in the fourth paragraph ..
"So - if nothing has to slide over the scratches, nor take a position that depends on that scratched surface.. etc"
If the surface is a bearing surface, then you are correct. Other than stoning to remove burrs, there is not much one can do.

There were not enough words in your posting to make clear the context where I took it that @EVMiller was intending to use stones to remove the scratches, as opposed to using precision ground stones to simply ensure, if a bearing surface, that there were no burrs, and it could function normally.

I believe I made it abundantly clear that using abrasives in the way I describe is for surfaces that do not have anything slide over them.
Our understanding of how precision ground flat stones work is apparently the same. We appear to be in agreement, except perhaps in the detail on how much metal is removed by them. In my experience, almost zero! They effect a smoothing, or polishing action. I did not succeed in "grinding away" anything, even though I pushed hard (on surface plate).

 

[EVMiller]

There were not enough words in your posting to make clear the context where I took it that @EVMiller was intending to use stones to remove the scratches, as opposed to using precision ground stones to simply ensure, if a bearing surface, that there were no burrs, and it could function normally.

I am planning on using a precision ground stone. I should have included more info in my initial post.

Excellent information on what to do as well as what not to do. Thanks @davidpbest and @graham-xrf

 

[RJSakowski]

Scratches are like battle scars. They are sign of use. The first scratch or ding is always the most disturbing. I have long thought that it would be best to intentionally put a scratch or ding into a new machine to eliminate the trauma when that first unintentional one appears.

Scratches of themselves aren't detrimental; to the operation of a machine. Bearing surfaces are often purposely scratched to provide a micro channel for lubrication. The burrs that occur as a result of a scratch or ding are but they can be removed fairly easily by stoning.

An interesting exercise. is to measure the thickness of a piece of stock and then manually stone it for a while and remeasure. It takes an awful lot of effort to remove a measurable amount of material. Even more so on hardened stock.

 

[graham-xrf]

@RJSakowski :
There are two points of emotional trauma.
The first is at the first ding, more or less as you describe, although I would distinguish between a bit of a scratch, and a more catastrophic "ding".
The other is the time when you drill your first hole, or make your first cut (into the machine body), for whatever mod you are doing. It happened to me when I started with the hacksaw cutting away the business end of my compound's ACME nut!