Motorized Rim Set Sander:
By: Tim McKnight, 1/19/2004
They, those three guys in Indiana, have said "necessity is the mother of all invention". I guess I have partially used that axiom as an approach to my present dished rim set sander. As I have been building acoustic guitars, a dozen plus years, my methodology and building prowess and tools have evolved. My first tool to sand the proper arch on the rim set was a flat board with a tapered wedge attached to it. This tool served its purpose but required lots of trial and error to provide a seamless fit of my tops and backs to the rim sets.
My next tools were a couple of concave dishes made by gluing two pieces of 3/4" MDF and routing a concave dish for the 15 and 25 foot back and top radius' that I currently use. By applying 3M spray adhesive, I attached 3" wide strips of 80 grit sandpaper to these dishes. Then I bolted a 1/2" black pipe flange base and an 8" pipe nipple to my work bench which enabled me to manually pivot the sanding dish back and forth, around the pipe, sanding the proper arch on the rim set. The rim set was secured to my work bench by clamping the outside form which prevented the rim set from moving as I sanded the arch onto the lining and sides. This worked well but as my batch sizes increased this required more elbow grease which quickly got me wondering how could I motorize the process?
One evening, while surfing Frank Ford's site at www.frets.com, I came across a field trip article in which he snapped some pictures of a luthier sanding rim sets with a modified potter's wheel. Well, wouldn't you know it? I had a potter's wheel setting in the corner of my shop not being used. I installed the largest pulley possible (6" diameter) on the potter's wheel and coupled it to an 1140 RPM 1/3 HP motor and then I had a motorized sander. The only problem was that it turned WAY to fast at approximately 380 RPM. It worked for the current batch of 6 guitars that I was building at that time but I filed a mental note that before the next batch I would need to slow it down. High speed sanding is not a good thing when sanding rim sets. It generates a huge cloud of sanding dust, requires a respirator, greatly increases the possibility of an air borne rim set [ask me how I know this] and makes a real mess in the shop.
A few months later, Mario Proulx, posted a picture on the www.MIMF.com forum, of his new motorized sander, which he built beneath his work bench. Mario incorporated a unique quick disconnect drive chuck that allowed the dish to be attached and removed quickly with no tools. He also employed the use of a jack shaft or intermediate speed reduction system, to slow his sander down. Remembering that I needed to slow my sander down, I borrowed these two ideas from Mario and set off to build mine. However, I need to share the credit with him.
Not wanting to incorporate my sander design into my work bench, I decided to build my sander as a free standing unit. The case is made of materials that I already had on hand, 3/4" MDF.
The sanding dish drive shaft was intentionally set 6" forward of center so the sanding dish would hang over the front edge. This was done to minimize strain on my back while holding the rim set and form on the rotating sanding wheel. My dished sanding wheels measure 22 1/2" in diameter x 1 1/2" thick.
This picture shows the internals of the sander and the lower storage area for additional discs. Some folks on the www.MIMF.com forum suggested making the drive area smaller which would provide more storage area below. Their ideas were considered but, having worked in small cramped spaces, I opted to make the drive section large enough to provide ample maintenance room should the need arise.
This close up is of the drive section and the jackshaft belt reduction. The jackshaft sets in two 1/4" x 1" x 2" flat cold rolled steel straps. Steel was used so the shaft would bear against steel as opposed to bearing against the much softer MDF. The two pulleys on the jackshaft are joined together as one piece and they rotate on 1/2" ID bronze bushings on a fixed stationary shaft. The front shaft rotates on two 1/2" flange block bearings. I used several 1/2" ID shaft collars, with sets screws, to set the heights of the shafts and pulleys. Using hindsight I would suggest anyone wishing to build a sander like this to consider incorporating a belt tensioner between the jackshaft and dish drive shaft. My belt isn't slipping [yet] but, I suspect I will be adding a tensioner down the road when the belt stretches. My final disc speed is just over 100 RPM which is almost perfect. It is slow enough that material isn't removed too fast and the dust falls gently to the floor without generating a huge airborne cloud.
This picture shows how the motor is attached to the cabinet via a 4" door hinge. A 5/16" x 4" carriage bolt on the opposite side of the motor base provides the proper belt tension. All motor wiring is secured to the cabinet using 1/2" Romex Staples. A quickie MDF belt guard was installed to keep any dust or critters from falling onto the belt or motor.
This view shows the power switch is easily accessible. A cord hanger is installed to keep the coiled cord out of harms way when not in use. Again, all wiring is secured to the cabinet using Romex Staples. The wooden block below the switch is for the "barn door" type dead bolt latch that secures the door.
The 12" diameter Lazy Susan provides the low friction rotating surface for the dish to ride on. There is a 1/2" shim installed below my Lazy Susan so the drive chuck would be below the top of the Lazy Susan. If the drive chuck is too high the dish will wobble from side to side as it rotates. It is important that the sanding disc lay perfectly flat on the Lazy Susan. I used Royal Purple spray lubricant to lubricate the open ball bearings on the Lazy Susan. You should use some type of dry type of lubricant so the sanding dust will not stick to the ball bearings. There are several brands on the market which go on wet and then the solvents evaporate, after a few minutes, leaving a dry film lubricant. Molybdenumdisulphide is another good dry lube alternative.
A close up of the sander drive chuck which is made from 1/4" x 2" x 5" cold rolled steel. The center post is 1" diameter x 1" long cold rolled steel with a 1/2" inside diameter hole bored through it for the Thompson drive shaft. There are also two holes drilled and tapped for set screws which lock the drive chuck to the shaft. A third hole was drilled to use a roll pin through the shaft should the sets screws fail. This has not been needed to date. The two outer posts are 1/2" x 2 1/2" Shoulder Bolts with the threads cut off. All three posts are braze welded to the 1/4" x 2" x 5" plate. I used Thompson precision ground shafting, which is very hard and the set screws will not "dig" into it. Flats were ground wherever set screws would line up on the shaft. You could also use mild steel shafting in place of Thompson shafting and this would eliminate the need to grind flats on the shaft.
This shows three female holes on the bottom of the sanding dish in which the drive chuck male posts fit into.
The final picture shows a rim set setting on top of the sander. It should be mentioned that I use internal stiffeners to hold the rim set tight against the form. It is difficult to sand the rim set unsupported as it tends to flex and vibrate too much. Gloves are also recommended to be worn while sanding unless a close manicure is desired.
I had used a lot of material I already had on hand to construct this sander. My only purchases were for shafts, bearings, pulleys and locking collars from Motion Industires. I already had a used motor but other motor RPM's and HP could be used as long as you plan for a final disc speed of around 100 RPM. My total out of pocket costs for this project was approximately $40.00. If you need more information or have any questions feel free to email Tim at mcknightguitars@yahoo.com
Bill of
Materials:
Cabinet: 24 3/4" wide 3/4" MDF
24" deep " "
32" high " "
Top, shelf & bottom:
24" wide " "
23 1/4" deep " "
Door: 24 3/4" wide x 32" high x 3/8 thick plywood
Motor: 120VAC
1/3 HP
1140 RPM
Pulleys: Purchased from Motion Industries
Motor 3” diameter
Jack shaft 11" diameter & 2" diameter
Dish shaft 4" diameter
Belts: Motion Industries
Motor to jackshaft
Jackshaft to dish
Bearings: Motion Industries
Jackshaft 1/2" x 3/4" long bronze oil impregnated bushings
(2) 1/2" Two bolt flange block bearing
(2) 1/2" ID x 1" OD bronze washers [above and below jackshaft pulleys]
Shaft: Motion Industries
(2) 1/2" diameter x 18" long Thompson shafting
(8) 1/2" ID Shaft collars with set screws
Chuck:
(1) 1/4" x 2" x 5" Cold rolled steel
(2) 1/2" x 2 1/2" Shoulder bolts (with threads cut off)
(1) 1" x 1" Round cold rolled steel. (Bore ID to 1/2", drill & tap for 2 set screws)
Electrical:
12' 12-3 Rubber covered cord
(1) 120VAC Male plug
(1) 2" x 4" 1/2" knock out steel work box
(3) 1/2" Romex clamps
(1) 15 Amp light switch
(1) 2" x 4" light switch cover plate
Hardware:
1 lb 1 5/8" Drywall screws
(1) Titebond glue
(1) 4" Heavy duty door hinge (to pivot motor as a belt tensioner)
(1) 1/4" Barn type bolt door latch
(1) 5/16" x 4" Carriage bolt for motor tensioner
(2) 2" door hinges
(1) 12" Diameter HD Lazy Susan (Lowes)