This is a supplemental article to accompany my Youtube video “Super accurate circle sanding jig”
Sometimes it is necessary to cut quite accurate circles and this can be difficult in the small work shop, using a compass and then sanding to size is easy enough but it is easy to go wrong too, you can sand over the line very quickly and end up with a slightly bumpy or oval-ish circle. Not much use if you are making something like the spindle moulder guide ring shown above where an off centre or wonky circle will give inconsistent machining.
A router with a circle cutting jig is also an option but they can be tricky to set up and it can take a few attempts to dial in to a specific diameter. They also tend to have a relative high minimum working diameter, since they work around a pivot point which has to be outside of the router base and this can leave a minimum diameter of around 200mm in some cases.
This is where the circle sanding jig comes in handy, the video above shows how to make a very accurate jig with technically no limits to size. It also shows a fairly simple but very effective dial to give super fine adjustment which allows for very accurate dialling of the final diameter. it only took a few adjustments and fine sanding to achieve exactly 115.00mm which is pretty impressive even for a metal working lathe!
The dial adjuster is simply a block of wood with an assembly made up of a fairly stiff and long spring, ball bearing and an adjusting screw. The spring simply applies tension to the screw to stop it rotating under vibration from the sander, and the ball bearing provides a smooth interface so that the rotation doesn’t feel clunky against the spring. Size, length, spring rate etc aren’t really too important, I just used random stuff I happened to have available.
I drilled a 9mm hole which accommodates the M6 threaded insert, and the depth was dictated by the spring/bolt assembly. I just sat them side by side with some over lap to ensure constant tension even if the bolt is unscrewed quite far. The spring is about 60mm long, 6mm diameter and about 0.6mm wire thickness which should give some indication of stiffness. The ball just needs to be able to fit into the hole and not fall through the spring, it could be replaced with a small washer.
Why 45 degrees
Edit 11/03/2020. The paragraph below is incorrect, see end of post for correction.
Setting the sliding mechanism on the circle sanding jig to 45 degrees is a very clever way of adding fine definition to any adjustment. It could be made even finer by reducing the angle even further, but 45 degrees is convenient for circle cutting because it unifies adjustment distance to the reduction of diameter, in other words; 2mm of adjustment at 45 degrees equals 1mm of adjustment at 90 degrees (to the sanding disc). but 1mm sanded from a circle reduces the diameter by 2mm! so 2mm of adjustment on the slide equals a 2mm adjustment to the diameter.
And since the thread pitch of the adjusting screw is 1mm; one rotation equals 1mm reduction in diameter, or 1/4 turn equals 0.25mm adjustment etc. In the video I found it very easy to quickly sand a small and tight fitting holder for the 30mm bore of the guide ring bearing, meaning I could perfectly sand the guide rings while attached to the bearing.
The beauty of this circle sanding jig is it could really be made from anything, there is no point including plans as the size will be dictated by the size of your machine and the size of the circles you want to sand. I made it approximately the size of my sander bed because I will only really sand fairly small circles but it could be made to any size.
The only real criteria is that the sliding mechanism be made of fairly smooth wood to allow it to slide nice and easy without being clunky or jerk in and out. For that reason; chipboard is probably not a good idea! I used 12mm plywood because I have plenty of off-cuts lying around. I made the strips for the sliding mechanism around 50mm wide to give plenty of support for the circles.
The pivot for mounting circles depends on what the circle is for, hardly any pressure is needed while sanding so a panel pin is probably sufficient in most circumstances. If you do not want a hole though the centre of the circle then a pin knocked through from the underside so the just the point sticks though would create a fairly non intrusive pivot.
A completely non intrusive pivot could be made using a sacrificial circle which is cut just under the size of the required circle and fixed to the slide with a pin driven flush with the surface as the pivot, this creates a small rotating turntable. Then the required circles can be fixed with double sided tape and sanded as normal.
At the end of the video I used a brass rod pivot for some wood slats as part of another project, those slats will actually pivot on the same size rod so it made sense to sand the radii using the same pivot. The spring loaded adjuster came into its own here and sanding the 40 odd slats without any unintended vibration adjustment was a breeze!
Those slats will feature in the next video which will hopefully be soon, and making these has give me some other ideas to add to the millions of jobs id like to have time for! no doubt you will see this sanding jig a lot in the future so stay tuned!
Ok so i made a mistake! Well I have been wrong before so it didn’t come as a huge shock… I got schooled on trigonometry by one of my Youtube viewers, who pointed out my incorrect logic that setting the slide to 45 degrees would result in a 1-1 ratio of distance on the slider to reduction in diameter. In fact it should have been set to 30 degrees from the sanding disk, or 60 degrees from where the slider would be on a normal circle sanding jig.
“At 45 degrees 1mm of slide travel will take 1.4142mm from the diameter. You might recognize that as the square root of 2, or 2 * cos(45). To do what you want, you need the angle to be 60 degrees, cos(60) = 0.5.
In your example a 1/4 turn of your 1mm pitch screw takes you from 116.24mm to 115.90 a difference of 0.34mm if you multiply 0.25mm by the sqrt(2) you get 0.35mm so your jig is within about 0.01mm of what it should do, that is really good. But to do what you want to do, you need to change it to 60 degrees from perpendicular to the sanding disk.”
Thanks to Chris for the above explanation.
On reflection this doesn’t mean the 45 degree jig useless, it is still a very effective and accurate jig, but the prediction of how an adjustment affects diameter is slightly off. I can and will still use the jig and can just creep up on final diameters as i did in the video, but you can at least learn by my mistake and make your jig to 60 degrees if predictable adjustments are desirable.
While I am here, I was also inundated with a complaint that I had referenced to a “full detailed plan” of the jig in the narration of the video, but then when writing this blog I decided it was pointless. Well below you will now find a 3d sketchup model of both the original 45 degree jig and a new 60 degree jig which will live up to the original claims of a 1mm adjustment of the slide to give 1mm reduction in diameter.
I even included a cut away of the adjuster mechanism to show how this works internally, just select the top piece and then right click and select “hide” from the menu. the spring in the model is in no way accurate to the original I made though and you should adjust the hole depth to suit whatever length of spring and screw you use.