Polishing - basic knowledge

 

In the following I would like to give you a little basic knowledge about polish and polished surfaces on different materials.

 

First important insight : Even a perfectly polished surface is only a scratched surface!

 

Every surface processed by grinding or polishing shows scratches. Depth and width of these scratches determine the optical impression.
In ornamental cuts, for example on watches or jewelry, the optical impression is influenced by grain and stroke depth. 
The intended strokes (scratches) usually run strictly parallel here to create an optically appealing matte surface appearance.

However, a surface of a wristwatch or piece of jewelry that is perfectly mirror-polished to the eye is not free of scratches. 
They are just far too small in depth and width to be visible to the naked eye.

 

With a good polish, we therefore aim for an "invisible scratch pattern" on the surface - much too fine to be perceived by the human eye or standard lens magnification.

Sounds simple in theory, but in practice there are many factors that stand in the way of a perfect result.
I would like to discuss some of these factors in more detail below. 

 

1. material hardness

 

The most important variable/factor in polish removal of different metals is their specific hardness.
The softer the metal, the easier and faster the polishing process. 
Gold, for example, is a relatively soft metal and can therefore be polished quickly and easily. 
The same applies to brass and copper, for example - their specific hardness is similar to that of gold. 

Stainless steel or titanium are significantly harder than the metals just mentioned. Compared to gold, the polishing removal rate under the same conditions is therefore many times lower. 

With the right polishing tools and polishing compound, however, an excellent result can be achieved quickly.

 

In the case of the metals mentioned so far, the heat generated during polishing does not play a role, since their structure is not changed by the comparatively low heat during the polishing process.

Plastics behave quite differently. They are comparatively soft and can therefore actually be polished quickly and easily.
However, plastics such as Plexiglas, but also paint surfaces or amber (resin) can melt and smear if they are excessively heated by the friction during the polishing process. 
This causes irreparable structural damage.
In this case, it is essential to dose the speed and polishing duration ("cooling pauses") accordingly. 

Hardened metals, DLC (Diamond-like Carbon) coated surfaces or glass can hardly or not at all be polished with normal methods due to their hardness.
Special polishing agents (with as high a proportion of fine diamond dust particles as possible) are required here to achieve removal.

 

2. the depth of scratches or damage on the surface to be polished.

 

The depth of scratches of light wear marks, for example on a wristwatch or a piece of jewelry, are in the 100th of a millimeter range. This damage can be completely removed by professional polishing. Medium to heavy scratches or light damage, on the other hand, are in the 10th of a millimeter range and cannot simply be removed by polishing. 
The amount of material removed during the polishing process is too small - even if you polish for a very long time, the medium to heavy scratches and light damage are still visible when you look closely. The edges of the scratch or damage are rounded off considerably, but not completely removed to the depth.  

In order to achieve a perfect restoration result with medium to severe surface damage, the entire surface must first be ground flat with a hard medium ( lapping process ) or material must be welded on beforehand.

This is an extensive topic, which I will cover in detail separately elsewhere.

 

What can be achieved with a professional polish and where are the limits?

A practical example:

If you look at a mirror-polished front surface (side surface of a watch case) of a watch that has been worn carefully but regularly for many years, you will typically find 95 percent of light scratches
in the 100th depth range. However, there will also be some medium depth scratches and perhaps even a deeper bump that resulted from carelessness.Hundreds of the light scratches lie like a matte haze on the surface, greatly reducing the gloss level compared to when new.

When polishing without pre-sanding, all light scratches can be completely removed with good equipment. The optical impression improves considerably - the surface sparkles and reflects, just like on the first day.
On closer inspection, however, the few deeper scratches and the impact damage mentioned as an example can still be guessed at. However, these damages aren't that noticeable anymore, the sharp edges of the deeper scratches were rounded off (broken) during the polishing process.

The overall optical impression is improved enormously by the polishing process alone.

 

3 . The horror for many watch lovers - polishing damage!

Now I would like to go into more detail about the dangers of polishing. Polishing dents, rounding of edges to the loss of contours, ... the horror for watch lovers.
Unfortunately, even in the professional field, polishing is very often not done professionally. Many watch lovers therefore explicitly refrain from polishing, e.g. as part of a revision at the watchmaker.

Incidentally, a simple rule applies to all polishing damage: the softer and more polishable the material, the faster and more severe the unintentional damage.

But now the good news - with a little basic knowledge and the right polishing tools, this damage can be completely avoided. 

Polishing dents are usually caused by the typical small polishing attachments that are offered in different shapes as accessories to the well-known rotary tools in every hardware store.  If you work intensively and specifically on the area of a deeper scratch with a small hard felt attachment, it does not take long for the polishing attachment to become saturated with polish abrasion and thus harden. As a result, excessive material is removed from the small area. The narrow scratch is then just swapped for a wide dent, so to speak. A really bad deal in my eyes!
Such a dent can later only be repaired professionally by applying material via laser welding. 
The second possibility to fix it would be to level the affected area in its entirety by grinding (surface lapping). This, however, entails a high loss of material of the entire surface.

In addition to the dangerous small polishing attachments, there are also the large polishing wheels - the "professional equipment", so to speak. A large wheel polishes much more flat, the danger of dents is reduced. 

BUT .....

These polishing wheels, which can be found in the workshop of any watchmaker, jeweler or goldsmith, are usually soft.
To create the necessary friction and polishing heat, the surface to be polished is pressed deep into the face of the polishing wheel. In doing so, one has very little control over the polishing process! 
The highest abrasion occurs at the edges of the surface to be polished, which changes the shape of edges / contours to soft or even rounded, depending on the polishing time. 

Even adjacent surfaces are unintentionally polished beyond the edge. Typical examples are matte surfaces on the lugs of a wristwatch ( lugs are the connecting elements for bracelets on watch cases), which are also attacked during the polishing of the front sides of the case and thus lose their matte finish / ornamental polish.
Of course, you can mask these adjacent surfaces before the polishing process, e.g. with protective lacquer. However, it is much better to use a polishing tool that is functionally gentle on the edges and does not touch adjacent angled surfaces unintentionally.  

This criterion, the protection of the edges, was one of the main objectives in the development of the Tomtools polishing attachment.

 

4. holograms:

Holograms are scratches caused by the polishing process itself.
Ideally, material is removed so finely from the surface during polishing that the resulting new scratches are no longer visible to the human eye or with a magnifying glass.
If a coarser polish is used, for example for a necessary pre-polish, visible holograms are quite normal. These should be completely removed in the next step, fine polishing.

Unfortunately, holograms often remain clearly visible even when the finest polishing compound is used.

There are several reasons for this

- the difference in grit sizes in the polishing compound from pre-polish to fine polish is too great. 

- the accidental mixing of polishing compound (e.g. due to polishing compound residues of the pre-polish on the workpiece or on the polishing medium, it mixes with the fine polish) 

- the polishing medium is saturated with abrasion and begins to harden. Hardened, dirty polishing media, whether small felt attachments, cotton discs or large polishing wheels create visible new scratches.

 

Solution:

I offer coordinated polishing compound of pre-polish and fine polish to prevent holograms due to high grit differences. 
While the large polishing wheels can only be roughly cleaned in use, the inexpensive Tomtools exchangeable media with low material consumption can be changed in seconds.

A fresh polishing medium, is in my eyes one of the most important prerequisites for a perfect fine polish free of holograms. 

All the problems mentioned in this paragraph can be conveniently avoided with the Tomtools polishing attachment set.