Expandable drums and flexible belts

The humble expandable rubber drum and smaller, slotted expanding drums are important tools for carvers.  The smaller sizes give us the opportunity to grind surfaces which are depressed or concave (curved inwards along one or two axes) with accuracy where otherwise a large dental burr might be needed.  I use 6, 12 & 25 mm diameter models on a regular basis.  The large drums allow us to grind and polish surfaces which may be curved along one axis (a ridge) or two axes (convex) and I use a 150 mm diameter x 38 mm wide drum.

Picture 1 on left – a 150 mm diameter expandable drum on its mandrel

The tools themselves are usually mounted on a mandrel which fits into a chuck.  Make sure you mount the drum on the mandrel the correct way around so it is thrown open by the direction of rotation, and not closed.  The point of the expansion is so a belt can easily be slid onto the drum when it is at rest and, when rotated, the diameter of the drum expands and locks the belt in place through centrifugal force.  The smallest drums expand due to tightening either a screw or nut located at one end.
 
I’ll discuss the pneumatic sanding drums in a later blog as they are quite a different animal.

Picture 2 on left – the 150 mm diameter drum rotates anticlockwise so it expands in use.  If mounted the wrong way around it would close instead and the belt would slip off

Drums begin at 6 mm diameter x 15 mm long (the small belts which fit them are sometimes called spiral bands) all the way up to 200 mm diameter x 75 mm wide.

Picture 3 on left – the 6, 12 & 25 mm diameter drums, with spiral bands

Belts can be covered in grits ranging from 60 (very coarse) – 50,000 (polishing) in either silicon carbide or diamond.  I prefer the latter, and they are not expensive for the work they do.  I have a number of well-used (they become less aggressive as they wear) and newer, “sharper” ones in the following sizes: 60, 120, 240, 400, 600 & 1500.  I hardly use the last one as I prefer other methods to polish my work.

Picture 4 on left – showing the diamond patterns on four belts, from left to right they are 120, 600, 400 & 1500 grit

Flexible belts can have the diamonds attached by either electroplating or resin bonding.  One method is basically as good as the other.  Some belts have a regular pattern of distinct “dots” over the surface of the belt whilst others have tighter coverage of the abrasive.  The coarser dots can fly off in use, so wear eye protection at all times.  I picked up a couple of beautiful coarse belts from a UK company recently which have a tight overlapping pattern of abrasive – they are very aggressive (the belt on the left in Picture 4 above) and good for cutting back on the rough!
 
A good supplier for the larger drums is Rayspan, and AbTec (UK) is good for the small slotted expanding drums.  Crystalite/Abrasive Technology (USA) make a good range of belts and spiral bands, but there are many more excellent producers out there.
 
Important points to remember:

• some belts are designed to only run one-way whilst others are bi-directional – look for the arrow on the inside of the bands that indicate the uni-directional ones

Picture 5 on left – showing four belts, three of which are uni-directional

• most belts are designed to be used wet – I use a water-fed flap of cloth to wet the entire width of the belt when in use (please see Picture 3 in the “Splash boxes” blog below)
  
• use the whole width of the belt to ensure even wear
• don’t press too hard on the belts – you distort it, causing it to wear more quickly.  Some belts will slip along their drums if you press too hard so you then need to stop and reposition them
• it helps to grind each area at least twice, at right-angles, to remove any deep scratches from previous coarser grades of grit
• hold the workpiece lightly but firmly with both hands and rotate your wrists to bring different areas of the work into contact with the abrasive
• watch your fingernails and knuckles – you can cut yourself if not careful, especially with the coarser grits
• I run the small slotted drums at around 750 rpm, and my large expandable drum at no more than 1500 rpm (though individual manufacturers recommend maximum rpm of between 1750 and 2400 from a quick search of the internet).  Some specialist, non-lapidary drums have higher maximum rpm’s but they don’t form a part of this discussion
• never run the expandable drums without a belt or there will be pieces of rubber flying past your ears
• don’t press your work onto the belt edge because you will damage the belt and the rubber drum underneath

 
I hope the notes above de-mystify these useful tools for you?