Honey Mesquite bowl with crushed
turquoise inlay ready to be sanded
Diamond Abrasives Overview: I've always felt that there was a natural harmony between stone and wood, so it's no wonder that I was drawn to crushed stone inlays early in my woodturning career. Although I enjoy inlaying solid stones as well, crushed stone inlays have always fascinated me. With solid stones, it's very difficult to add banded rings, or fill voids. With crushed stones, it's quite easy to inlay bands, fill voids and even inlay pictures or scenes made from the stone.
Working with crushed stone is child's play compared to cutting and fitting solid stones. With solid stones, you need some type of diamond blade (dry or wet cutting) and a way to precisely cut the specific shapes you need for your inlay. In the last few years several new diamond wet saws have become available that make this job easier.
When inlaying crushed stone, you have to sand the inlay so it's flush with the surface of your woodturning. Even soft stones require quite a bit of sanding to get the rough surface close to the wood. When I first started inlaying crushed stone many years ago, I used regular woodworking abrasives like Aluminum Oxide, which was harder than the inlay stones I was using. While this worked to a degree, sanding the rough inlay surface was always somewhat of a chore.
Sanding a large crushed turquoise
inlay with 3M diamond abrasives
It was not uncommon for me to use several course 80-grit Aluminum Oxide abrasive disks just to get the bumps knocked down. Still more abrasives were needed to remove the bulk of the waste stone and even more to get near the surface of the wood. Once you are near the surface of the wood it's easy to finish sand your inlay, but getting the waste stone removed efficiently was never an easy or enjoyable task, especially when working with harder stones.
The Mohs Scale of Hardness
In the early 1800's, Friedrick Mohs introduced an arbitrary (non-linear) scale to measure the hardness of various minerals. He selected ten minerals for his scale including Talc which is very soft (value of 1), to diamond the hardest material known (value of 10). Any mineral listed on the scale can scratch any other mineral listed on the scale with a lower number than itself.
Turquoise is popular for use in crushed stone inlays
As a woodturner, it's important for you to know the Mohs hardness number of your abrasives, especially when you're sanding materials other than wood such as crushed stone, metal or glass. By knowing the Mohs hardness number of your abrasive and the crushed stones you will be working with, you can effectively sand your inlay.
Mohs Hardness Scale
Talc – 1
Calcite – 3
Fluorite – 4
Apatite Feldspar – 5
Orthoclase – 6
Quartz – 7
Topaz – 8
Corundum – 9
Diamond – 10
Popular Abrasives - Mohs Hardness Ratings
Flint (Quartz Sand) – (~7.0 Mohs)
Emery – (~7.5 – 8.5 Mohs)
Garnet – (~7.5 – 8.5 Mohs)
Aluminum Oxide – (~9.0 Mohs)
Silicon Carbide – (~9.25 Mohs)
Diamond – (10 Mohs)
All of these rough stones can be sanded
with traditional aluminum oxide abrasives
Diamonds to the Rescue
Through the years, I've experimented with many different types of abrasives for sanding crushed stone inlays, including the newer ceramic abrasives. While even standard Aluminum Oxide abrasives worked acceptably for some stones, none of the abrasives I used gave me the performance I really wanted. A few years ago I began experimenting with diamond based abrasives used by Granite countertop installers for sanding my crushed stone inlays.
Diamonds have long been used when you need the hardest and longest lasting abrasives. They have been used in various industries for many years for the most demanding abrasive work, but you rarely if ever heard of them being used in a woodturner's studio. The biggest problem with using diamond abrasives made for stone countertops is their size. Most of the disks were five or six inches in diameter, that's really too large for many woodturning projects.
When sanding large crushed stone inlays like this,
diamond abrasives are a real time saver
A few years ago 3M began producing a line of diamond based abrasives in
two and three inch diameters. I've since abandoned my larger disks and
use the small disks from 3M now, since they are much easier to work with
on smaller inlays. 3M Diamond Abrasives are available from Bruce Hoover at The Sanding Glove.
Crushed Stone Sanding Protocol
Step 1- Course (250 Micron, 70–Grit Equivalent) Diamond Disks
These disks are a micron graded, nickel bonded diamonds bonded to a strong flexible backing that will work with standard Velcro faced sanding mandrels. The diamonds are arranged on the disk in an open dot type pattern, which allows the disk to cut very aggressively. They make short work of sanding the uneven crushed stone surface, so you can quickly move on to using your finer abrasives.
3M's 3" diamond abrasive disk excels
at sanding crushed stone inlays
Although they can be used dry, they will last longer if you keep them slightly wet, or damp during use. A small pump spray bottle of water is a great way to spritz water onto the disk occasionally when sanding. Spraying the disk also helps to cool the disk and remove some of the dust residue. 2" pads cost $18.00 each, 3" pads cost $38.00 each. Pads are very durable and easily cleaned with a shot of compressed air, or water and a small soft bristle brush.
Step 2 - Fine (125, 74 and 45 Micron, 120, 220 and 320–Grit Equivalent) Diamond Disks
After the bulk of the rough stone surface has been sanded, I switch to these finer film backed diamond disks and sand the inlay with 125-micron (orange), 74-micron (teal) and 45-micron (beige) disks until the surface is flush to the turned surface. The 45-micron disks leave the surface at approximately 320-grit. These disks feature micron-graded diamonds that are resin bonded onto a 5-mil polyester film backing. They leave a uniform and consistent scratch pattern and can be used wet or dry.
These film-backed diamond abrasives are
used to sand the stone inlay up to 320 grit
These abrasives come three to a pack and are available in 2" or 3" disks. Costs range from $6.00 per pack for the 2" size, to $7.00 per pack for the 3" size disks. Disks are very durable and long lasting and are easily cleaned with water, or a soft bristle brush. Step 3 – Traditional Abrasives for Finish Sanding
Once the inlay is sanded to 320 grit, traditional
abrasives can be used to complete the sanding
You can use traditional abrasives like heat-treated Aluminum Oxide to complete the sanding of the crushed stone inlay and the project from 320-grit to the desired level. Diamond abrasives take most of the hard work out of sanding crushed stone inlays. Once the inlay is flush to the surface of the wood, it's easy to continue finish sanding with your regular abrasives (as long as the abrasives are harder than your stone inlay). In addition to Aluminum Oxide abrasives, I occasionally use Silicon Carbide, or some of the newer ceramic abrasives to finish sand, depending on the specific project.
Additional Thoughts on Diamond Abrasives
There's no debate that diamond abrasives are the most efficient abrasive for sanding various crushed stone inlays. The only real downside to using them is the cost. While they are expensive compared to other abrasives, their durability and efficiency more than make up for their added cost in my opinion. If you are working with lots of crushed stone inlays now, you'll appreciate the difference diamond abrasives will make when sanding your next inlay.
If you're only doing the occasional inlay, or you just want to experiment with using crushed stone inlays, I would use a traditional abrasive for your rough and finish sanding. It will take longer, but it will be less expensive in the long run. If you get hooked on working with crushed stone inlays in your woodturnings, take a serious look at diamond abrasive disks. You'll be glad you did!
Safety Note: You need to wear a proper respirator when working with, or sanding crushed stones. The dust from some of these stones is toxic; so make sure you wear proper breathing protection. Good quality half mask respirators with P100 dust filters and an organic vapor cartridge can be purchased for approximately $75.00. That's money well spent!
3M's half-mask respirator with organic
filter cartridges and P-100 dust filters
Safety Note: Always follow all manufacturers safety instructions before working with your lathe, or any of the tools or products you may use. If you are unsure about any operation, obtain competent professional instruction before proceeding. Use and wear all necessary safety devices during turning and observe safe woodturning practices to prevent accident or injury.
Steven D. Russell is a professional studio woodturner, teacher and writer. He has written numerous articles for international woodturning magazines, which have been published in more than 78 countries around the world. Steve has demonstrated in numerous cities across the United States. His studio, Eurowood Werks, specializes in bowls, platters and hollow forms with unique visual and tactile treatments.
Steve is also the current and founding President of the Lone Star Woodturners Association, Inc., an AAW member chapter. The LSWA is a 501(c)3 non-profit educational organization dedicated to teaching and demonstrating the art and craft of woodturning.
Steve is also a featured writer for the Guild of Master Craftsman's "Woodturning" magazine, published in London England. Woodturning magazine is the world's leading magazine for woodturners. Look for his articles covering technical topics, or project based articles in an upcoming issue.