Having side-tracked from straight-out woodturning into doing Intarsia pictures I had the need for a flap sander for final sanding of the irregular shaped pieces. None being commercially available I had to make my own.
The
sander is designed for use in a lathe held between a Jacobs chuck and live centre
in the tailstock
The sander is basically a 12 segment cylinder held together
by tapered end caps, which when tightened, force the segments closer together
thus holding the abrasive flaps firmly.
The segments are made by rip-sawing a 25 mm thick strip of timber (I used radiata pine) to make 12 off segments 22 mm wide on the base and the saw blade set to 15 degrees from the vertical, so that when both sides of the segment are cut the included angle is 30 degrees. ACCURACY is paramount.
PUT YOUR THINKING -CAP on now, and visualise the drum revolving towards
you so that the abrasive flaps are facing you with the rubber support behind
it. While the strip is in one piece (or in multiples of 3 or more segments)
run the correct face across a router bit or a saw blade to create a recess 15mm
wide for the rubber insertion. The rubber should be about half a millimetre
proud of the surface when fitted so that it holds the abrasive flap and prevents
slipping.
When this has been done, the strip(s) can be cut into the 12 segments and held together with a worm-drive clip placed about 20 - 25mm from each end. A plug is inserted in each end and the cylinder mounted on the lathe between centres and each end turned down to 53 mm diameter and 8mm long to the shoulder. It is not necessary to turn the outside or the inside of the cylinder.
Using a hardwood (I used Jarrah; because it mustn't split when being tightened) turn the end caps, being accurate with the centre holes which should be a GOOD fit on the shaft. Slop here could mean wobbling and vibration in use. Turn the wooden core (any timber will do) to a diameter of 33mm and drill a 13mm hole accurately through the centre for the full length. This core centralises the segments during final assembly and should have a 1 - 2mm clearance inside the cylinder when finally assembled.
Make the shaft using all threaded rod or plain rod threaded just the
required amount. One end should be drilled with a centre-drill so that a live
centre in the tailstock can sit in it; and the other end turned down to suit
your lathe chuck or a Jacobs chuck. (10mm suits my Jacobs chuck) In any case
the thread must be removed concentrically, otherwise there will be excessive
vibration.
Before final assembly, I screwed a nut on the tailstock end and using a centre punch I bruised the threads of the nut and shaft to lock the nut in place. Two nuts are used at the chuck end, as it is imperative that the nuts do not slacken-off when in use. (I use my sander at 1200 RPM). If you have faith in nylon-ring locking nuts, by all means use them.
Cut 12 pieces of 2mm thick, canvas reinforced, insertion rubber (car tube rubber is not rigid enough) 100mm X 50mm and fasten one with contact adhesive into the recess of each segment. Do not skimp the glue, because you don't want the rubber to be displaced when doing the final assembly. You need to be an octopus as it is, so don't be lousy with the glue.
I use 240 grit, 100mm wide, cloth-backed abrasive strip and cut 12 pieces 85mm long. I have made a jig that accepts the 100mm width with a hinged flap that is 50mm wide across one end of the jig. The piece of abrasive is inserted in the jig; the flap is pressed down to hold the abrasive and with a trimming knife I cut from the flap to the end of the piece in slots in the base of the jig, which are spaced to give me 7 fingers 14mm wide.
FINAL ASSEMBLY is accomplished by setting your teeth and your
protruding tongue at the correct angle and plenty of patience. I have turned
up a base 180mm diameter that is recessed to depth of 35mm to accept the end
cap and a further recess to accept the nut and protruding shaft (tailstock end),
so that the cylinder will stand vertically in it. The shoulder of the segments
should sit on the face of the jig.
I use a worm-drive clip that opens out
to 155mm diameter (or alternatively you can cut a ring from 3mm MDF with the
same size hole) The clip or ring sits on 3 or 4 narrow blocks that positions
it at or just above centre height of the cylinder.
I AGAIN REFER you to the paragraph about putting your thinking cap on! With the wooden core in place I insert 11 of the 12 segments in place facing in the correct direction( tip: with a felt tipped pen draw arrows pointing in the correct direction on the shoulder ends of a few segments).
Go round again inserting each sanding flap in front of the rubber, pushing it in to the core, top and bottom. Then winkle the last segment into place; and using a thin paint scraper insert the last flap. Cover with the end cap and screw down finger tight. Breathe a sigh of relief and relax.
The whole assembly can then be placed on the lathe and straighten all the abrasive flaps so that they are all in line. Tension up the nut and locking nut, so all is locked up firmly, and make sure everything is clear. (remember that the ends of the flaps now make up a 200mm cylinder)
In the warm weather at 1200 RPM the sander creates a lovely, cooling breeze, and in the cold weather it does the same thing, but not so lovely. This sander works like a breeze (pun intended), not only for Intarsia pieces, but also for other applications, such as softening sharp corners on objects when objectionable.
ENJOY!
