Introduction: DIY Vibrating Flat Lap

>>First off: This is my first Instructable, so bear with me if something seems to be confusing. Just ask in the comments if things are unclear. I live in germany, so we only heard of inches for plumbing supplies, but for our general measurements we use the metric system. I tried to supply some units in both cm and inches but not for all things through.<<

As a rock and fossil collector (collecting as in "stumbling through the muddy field picking up rocks", not "buying finished display pieces") one quickly piles up more samples than can ever be polished in a reasonable time. At least not if family and wife have anything to say about how long you spend in the shed with your disc sander and the garden hose for water cooling... ;)

After a while I bought a lyman tumbler and added a rubber tumble pot for rock polishing. That works fine for pebbles and smaller cuts, but it also abrades the edge and works the uncut faces of geodes and slabs which is often not wanted.

I looked at commercial vibratory flat laps built by Raytech, Covington, Homberg & Brusius and the like. But those are very expensive, for what I saw was mainly a flat metal pan with three springs and a motor in a metal case. They are very good machines and for professional mineral sales people these will make their money back in no time, but for a hobbyist like me it was too I decided to build my own.

I wanted something I can dismantle and stow away nicely. I quickly arrived at the idea to use two wooden trestles that I have in the workshop anyway for the frame, and built my design up from that. If you don't have or need these for other diy projects, you can basically use any frame you like. From an old table, to a chair or whatever, it just has to be big enough for the pan to hang free. I also opted to attach the motor with magnets, so I don't have to haul the whole thing to the sink for washing steps or fiddle with screws.


- 2 wooden trestles (or other frame of choosing)

- 3 screw in hooks

- about 2 m of 3mm steel rope or 8mm steel chain

- (if using steel rope) 6 steel wire rope clamps

- 3 S-shaped-hooks for hanging the pan to the rope/chain

- 3 sturdy washing machine tub suspension springs (I took mine out of a Miele washing machine I scrapped. If you buy these, use the stronger springs for heavy load machines.)

- 2 steel cooking pots of 40cm diameter (12") and about 10cm high (try to get these on some discount if possible, mine cost me about 40€/50$ each)

- 4 iron plates about 10x20cm, usually used for joining wooden beams together or scrap iron plates you happen to have. Needs to be sturdy (2mm thick at least) and strongly magnetic.

- asynchronous vibration motor 90W. (may need a plug)

- 4 countersunk cup/pot mounting supermagnets with at least 30kg of force each (holes fitting your motor)

- 4 screws, self locking nuts and washers fitting the magnets and your motor base.

- 2 pieces of 1" garden hose, about 1,2m long each (if you use different size of pots this needs to fit around the inside)

- 30cm of 3/4" garden hose that firmly fits into the 1" hose

- industrial felt sheet 40x40cm, 5mm thick (or dimensions fitting your pan)

- silicon adhesive glue

Step 1: Building the Grinding and Polishing Pans

Grinding pan:

I first marked a nice regular pattern for drilling dimples into the pan surface.

The functions of these are:

a.) preventing the slabs pulling themselfes onto the pan surface like rubber suckers, preventing the grit to come between slab and pan.

b.) allowing for better exchange between spent and fresh grit under the slabs.

c.) sinking small pieces of rock that come detached from your specimens.

d.) easier removal of the slabs from the pan after grinding (no tight vaccum under the slab)

I added a bit of water for cooling and used a 10mm HSS drill bit to get the dimples drilled into the stainless steel pan.

After that, I welded the iron plates to the bottom of the pan, because the magnets I want to attach the motor with do stick poorly (if at all) to stainless steel. Make sure the welds are better than my shoddy work on that image, as they otherwise will come loose after some hours of vibration. You can alternatively fix these onto the pan with metal screws (use some loctite or superglue to prevent loosening) if you like and don't have access to a welder.

Next step is adding a rubber hose bumper along the rim, to prevent the rocks from touching the pan edges, where it is sloped. I just laid the 1" hose into the pan and cut it with a garden scissor. I then connected that into a ring with a 8cm piece of the 3/4" hose, as you can see in the pictures. I glued that ring into the pan with some silicon rubber adhesive and also filled the gaps between pan and hose with adhesive. Make absolutely sure there are no holes where grit could get in and hide to mix into the finer grit of later stages of grinding!

Polishing Pan:

The polishing pan is for the final high gloss shine on your slabs. It is only to be used with the final polishing compound, and thus to be lined with a nice sturdy felt. (At first I thought I could use only one pan for everything, but even the minute irregularites of the steel and rock surfaces after a 1200 grit pre-polish result in an uneven, patchy polish with the aluminumoxide. The felt lining is really needed for gapping.)

First I also attached two iron plates for the magnets to the bottom as I did with the grinding pan.

I marked the circle that would fit into the pan on the felt sheet, and cut the felt with a scissor normally used for sheet metal. It has some serration on the cutting edge that grips neatly into the felt.

Then I added silicone rubber adhesive in a spoke pattern with a bigger dollop in the center. The felt got put on and run with a rubber roller and some firm massaging with my hands from center to outside edge until I was sure the glue had spread out and mixed into the felt. I then took the head off my rubber mallet and tapped the felt where I felt it was still a bit uneven until these bulges and dents were evened out as well.

Lastly I fashioned a hose bumper like for the grinding pan, but this time I used some small diameter tubing (air hose tubing for aquariums for example) to line the gap between the 1" hose and the felt. As I am only using the one polishing agent for that pan, I do not need to worry about grid getting stuck somewhere. I glued the 1" hose only in place at a few spots on the top, so I can remove it more easily if I need to replace the felt someday.

Step 2: Hanging Your Pans

Drilling holds into the pans:

To hang the pans onto the frame, I opted to go with a 3-point system, as these are also unsed in the commercial machines. It makes leveling the pan the easiest. (You want the first hole to be just off to one side of one pot handle, so it is easier to hold the pan there when fastening and unfastening it.)

Start with measuring the diameter of where you want your holes to be positioned on the rim of the pan (mark these two points on the pan rim). For my 40cm pots, it was coming to about 42.2cm diameter. Mark that measurement on a straight scrap piece of wood, then mark the center of that length as well. Then divide one side again by half, so you get the mark at the half radius (the mark at about 31.5cm in the picture). This half-radius is where you clamp down the bottom edge of a right angle ruler and run another piece of straight scrap wood against that. When you lay this contraption down on your pan, it marks pretty accurately the three equidistant points along the pan where you need to drill holes for hanging.

(If you want to be even more precise, mark the points and measure their 3 distances of each other and average it: Add all three measurements up, and divide the sum by three to get the correct distance, then correct your points to be at this distance. Usually this does not need to be that precise, though.)

Use a center punch to make a little dimple where you want to drill, so your drill bit wont veer off.

I used a 5mm HSS drill bit, to drill through the steel rim, and then an 8mm drill bit for deburring the opposite side (these razorsharp burrs will cut you at one point or an other if you don't remove them!).

Making the steel rope / chain hangers:

(Note: Different modes of suspending your pan have some impact on the results. The springs allow for movement in all 3 dimensions and work well for the first few coarser grain grinds. The chains and steel rope channel the vibrations into the x-y plane along the pan surface as they don't have much give in the z direction. This allows for a nice smoothing during the runs with 400 grit and upwards as well as for polishing.)

Now screw the hooks in your wooden frame of choosing (in my case the trestles). One goes in the middle of one trestle and the two on the other side in the other trestle are spaced like the holes on your pan.

Make three pieces of steel rope into hangers of same length with one loop on each end using the steel wire rope clamps. If you want to use metal chains, cut three pieces of chain into the same size. The lenght depends in both cases on the amount of clearing you need to have so pan and attached motor under it can hang free in the frame.

The springs don't need to be modified to hang your pan, unless yours don't come with a hook on either side.

Leveling the pan:

This is achieved easiest by turning the screw hooks in or out if the frame. You can level using a bi-directional spirit levels as I have in the picture, or just with a normal one. Start with the single hook on one trestle. The spirit level going between this hooks connection on the pan, to halfway between those of the two other hooks. Once that direction is level, turn your single spirit level 90°, so it is sitting on the pan between the two trestles now. (If it was going East-West, it is now facing North-South.) Adjust the two hooks on the other trestle to level the pan in that direction. Congratulations, your pan is now leveled.

Step 3: Power Your Engines

The asynchronous motors available on marketplaces that look like mine usually don't come with a plug, so you have to attach your own. I opted for one with an integrated switch, so I can easily start and stop the motor without having to unplug things.

This kind of motor allows for adjustment of vibration strength. You can remove the silver casings on either side and remove some of the counterweight plates. I went with four plates each side (in the picture only two are mounted, but that was a bit on the light side for my taste). I used a bit of leftover metal tube and cut off a length to compensate for the missing plates, so the remaining ones can't move around along the axle. You can probably use a bit of rubber tubing or duct tape as well to fix them in place. Put the cover casings back before trying anything, we don't need any accidents here.

Then I screwed the supermagnets with the washers and self-locking nuts onto the motor baseplate and clonked it onto the pan bottom for the first time. (Catching my finger between pan and motor to some discomfort and swearing...did I write about accidents a minute ago? Supermagnets are nasty, especially if you combine them to a holding force of 120kg+. Rubbing my stricken fingerjoint, I was satisfied to see the motor stayed nicely put on its place on the bottom of the pan when I ran it for testing.)

I first thought I needed rubber sleeves (see picture) over the magnets to increase the grip, but those wear through quite fast and aren't really needed.

Step 4: Bumper Rings

Bumper rings are there to protect the edges of slabs and geodes from touching each other during the grind and polish.

At first I tried the aquarium tubing, but that turned out too light and easily lifted by the edges of geodes even if I filled them with sand. Maybe fine leadshot would work as filling, but I went with rubber sleeves used for joining steel pipes in plumbing instead. I ordered a few sizes and got about three out of them per sleeve. They have a straight wall, as opposed to the curved one the tube rings had and the thick rubber brings more weight as well. They hold up nicely and so far no piece has wriggled its way out under them.

Step 5: Thats It! Grind Away...

Now you can fill your grinding pan with about half a litre of water, a tablespoon of 80 grit silicon carbide and as many geodes, slabs and cut rock halves you can fit.

Running the pan for 12 hours straight, the motor gets quite warm to the touch, but not concerningly hot. I use a smaller hammer as a levered wedge to pry the motor off the pan as you can't really comfortably remove it by hand only. Still, that strong magnetism is what I need to keep the motor from dancing away. :)

I hope you enjoyed my Instructible and may your rocks turn out smooth and shiny! Let me know if you found this helpful in the comments. I am also interested in seeing your DIY flat laps, perhaps with improvements and functions I have not thought of yet. :)

Have a nice day.

- Mario.