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[CHILELAP]

Greetings to everyone: We have a 6 wheeled genie, a 10" and a 6" saw and a regular 8" flat lap (not vibe).  We need to import most equipment and supplies as we are in Santiago Chile so decisions are important as importing is quite expensive and also returns and guarantees from this far away are not doable. Anyway, looking to buy an 8" expandible drum and set it up sort of DIY with an extra 1 hp motor at 1700 rpm and we`re looking for advise about following:


1.) Obviously we`ll set up the drum on a metal shaft and lock in place with a flange type setup.  But aside from this, is a full metal rim needed for the 8" expandible drum or not?  We are looking at the Covington drum: covington-engineering.com/supplies/expandable-drums/ 

As a reference, here is a similar drum with a full metal rim: www.ebay.com/itm/224049260332

2.) Our second question is about IF it is worthwhile to buy diamond belts for these 8 inch drums, as opposed to the normal silicon carbide belts (about $65.000 for a diamond belt versus $1.75 for silicon carbide).  We are looking to use the 8" drum most with 400 to 600 grit belts for a medium step for quartz crystals, and well as some free form type figures in different jaspers.

Thanks in advance.

[lithicbeads]

A third or half horse motor is plenty large for a expando and a solid metal grinding wheel on the other side. Yes for safety sake a metal interior is best. Look at the kingsley north catalog as they have top quality tools for picking a brand. I used them a lot  a long time ago and there is a danger that the wheel will explode , delaminate , if the belt breaks which happens with  old dried out sic belts . A few coarse sic belts are nice to keep some of the wear off your coarse diamond belt but diamond is vastly superior to sic, no comparison in terms of economy of work done by the belt for the price or the finish. Sic is very prone to developing dry spots on the belt under use despite copious water flow and quartz crystal reacts by spalling , a very bad outcome A quality expando should not be a risk for delaminating for many years of use if a belt breaks. I've have had it happen twice in 40 years  when sic belts came apart at the seam and once it smashed my hand and broke it. The 600 and 1200 diamond belts are crucial for a quality pre polish Good luck.

[CHILELAP]

Thnaks sincerely for the detailed reply.

[55fossil]

    I have a Covington Dual expando drum machine which I love. I also have a homemade arbor with two, 8 inch expando drums. Both machines are wet units. I would never run a dry sanding machine again. Water is better for the rocks and your lungs. I polish moistly Jasper and plume agates as well as a lot of Nephrite Jade. I do not use Diamond belts because I am pleased with the Silicon belts.
   Biggest Problem:  You have to get the correct size belt for your drums!  Too tight they will not go on, too loose and they slide around the drum and wander off the edge. There are two very similar size 8 inch belts. 
 Dry Spots on belt:  I am careful to keep my belts wet and move the rock over the entire face of the belt. I get dry spots when the belt gets dry or I work it excessively hard. I cannot comment too much on whether Diamond belts are more economical but I like the Sic belts with water.
good luck.
PS:   Covington builds good machines but like everyone there are issues with spare parts these days....