agatecollector
off to a rocking start
Member since November 2019
Posts: 3
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Post by agatecollector on Nov 29, 2019 8:20:26 GMT -5
I am trying to build a flat lap that uses 12" magnetic pads. I want to use a brushless DC motor. Commercial units use a 1/3hp motor, but bldc motors are rated with its torque value rather than power. Someone told a 5Nm bldc motor should be able to do the work. Is this value ok? too much? not enough? found a few bldc motors with 5Nm torque and 3000 rpm. Speed control is easy in bldc motors, can easily lower it to 1000 rpm. I need help with chooing a suitable size motor.
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Post by Rockoonz on Nov 29, 2019 22:33:02 GMT -5
The motor bearings in most motors would not support the weight of a 12"d plate of steel for long, especially with the intermittent side loading. You really should consider a belt drive, plus speed reduction will multiply torque so a smaller motor will work fine. As a bonus the motor can be moved further from the water. Also why not a stepper or frequency drive?
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nik
spending too much on rocks
Member since May 2019
Posts: 315
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Post by nik on Nov 29, 2019 23:40:11 GMT -5
An industrial sewing machine motor might be a good choice. I'm using one on a 6 wheel grinder, and love it. However, I fully agree with the idea of belt drive and speed reduction. 3000 rpm is scary fast for a 12" lap. Also it would let you run the motor closer to rated power and speed, which equates to less heat produced and more cooling air over the coils.
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agatecollector
off to a rocking start
Member since November 2019
Posts: 3
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Post by agatecollector on Nov 30, 2019 5:10:19 GMT -5
The motor bearings in most motors would not support the weight of a 12"d plate of steel for long, especially with the intermittent side loading. You really should consider a belt drive, plus speed reduction will multiply torque so a smaller motor will work fine. As a bonus the motor can be moved further from the water. Also why not a stepper or frequency drive? Thank you for sharing your expertise. The usual way is AC motor with belt but I like doing things the hard way The reason I want a brushless DC motor is that its quiet, less heat and direct coupling to load so a more compact unit can be built. Will try and see if it can withstand the loading
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agatecollector
off to a rocking start
Member since November 2019
Posts: 3
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Post by agatecollector on Nov 30, 2019 5:28:32 GMT -5
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jamesp
Cave Dweller
Member since October 2012
Posts: 36,154
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Post by jamesp on Nov 30, 2019 6:41:54 GMT -5
Maybe this will help with your needed power estimates agatecollector. I really like this machine. Home built 8" lap uses a 1/4 hp AC 1725 geared down to 900 rpm at disc. When using 80-120-220 discs an 8 sq in rock face with moderate pressure does slow it down. Plenty of power for coarse shaping large cabs. Curious about the cost of 12 inch rubber diamond pads ? They are sure nice for 200-400-800-1500-3000-polish steps. At a good max speed of 3000 rpm for 4 inch pads it looks like 1000 rpm is the max speed for 12 inch pads. You are aware that the rubber starts to melt above these speeds ? I never found a need to run it slower or vary the speed. If anything faster. The build uses a 2:1 14" diameter 10 gauge tank head. The rugged tank head allows welding directly to it which made fabrication much easier. Fabrication photos: www.flickr.com/photos/67205364@N06/sets/72157696891131845
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jamesp
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Member since October 2012
Posts: 36,154
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Post by jamesp on Nov 30, 2019 6:50:39 GMT -5
The motor bearings in most motors would not support the weight of a 12"d plate of steel for long, especially with the intermittent side loading. You really should consider a belt drive, plus speed reduction will multiply torque so a smaller motor will work fine. As a bonus the motor can be moved further from the water. Also why not a stepper or frequency drive? I agree with Lee here. 12" disc direct coupled is demanding for motor bearings. And the water issue, the motor is on the drip line. Consider turning on and off, even with a shaft slinger diamond contaminated water is probably going to find that motor. Little diamonds love the taste of motor seals and bearings. Motors at $80, pillar block bearings at $10.
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nik
spending too much on rocks
Member since May 2019
Posts: 315
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Post by nik on Nov 30, 2019 6:56:20 GMT -5
The linked motor would work, but I'd try to find one that runs at lower rpm (check eBay) to avoid running a large, expensive, driven pulley. Perhaps closer to 3500. The one down side to the motor that I am running is a non standard (at least to lapidary) shaft diameter and belt width, so that might be worth thinking about. However the narrower pulleys seem to do well with a 3/8 belt. I wasn't able to find an appropriately sized driven pulley for my grinder. So I went with a standard 1/2 width. The belt just rides a bit lower in the groove. It hasn't given me any problems in the few months I've been running it with roughly 70 pounds of rotating mass.
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jamesp
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Member since October 2012
Posts: 36,154
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Post by jamesp on Nov 30, 2019 7:09:22 GMT -5
The pulleys sure are compact for a 1725 AC. A 3 inch and a 6 inch for ~900 rpm out. Go 3 and 5 for 1000+ out.
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Post by Rockoonz on Nov 30, 2019 14:41:29 GMT -5
I recently got 4 swamp cooler replacement motors. 2 are 1hp and 2 are 1/2hp, all are 2 speed 1050/1725. One will end up on my old Graves facetor so it gets a high gear. Variable would be better but I paid $130.00 for all 4.
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jamesp
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Member since October 2012
Posts: 36,154
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Post by jamesp on Dec 1, 2019 5:10:47 GMT -5
I recently got 4 swamp cooler replacement motors. 2 are 1hp and 2 are 1/2hp, all are 2 speed 1050/1725. One will end up on my old Graves facetor so it gets a high gear. Variable would be better but I paid $130.00 for all 4. A bargain Rockoonz. I believe the 1050 tap has reduced hp. This 1/4 hp AC motor was about right for powering this 8 inch lap. Larger than 8 inch wheel - a 1/3 or 1/2 would be better IMO. There is a growing interest in variable speeds used on tumblers too.
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Post by Rockoonz on Dec 1, 2019 22:50:40 GMT -5
I recently got 4 swamp cooler replacement motors. 2 are 1hp and 2 are 1/2hp, all are 2 speed 1050/1725. One will end up on my old Graves facetor so it gets a high gear. Variable would be better but I paid $130.00 for all 4. A bargain Rockoonz. I believe the 1050 tap has reduced hp. This 1/4 hp AC motor was about right for powering this 8 inch lap. Larger than 8 inch wheel - a 1/3 or 1/2 would be better IMO. There is a growing interest in variable speeds used on tumblers too. There will definitely be less torque but fixed speed FHP motors of facetors and laps are usually 1/4hp or less. If I recall correctly the current one in the Graves is 1/6 hp and I can't stall it or even slow it down.
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Post by RocksInNJ on Dec 1, 2019 23:34:54 GMT -5
Maybe this will help with your needed power estimates agatecollector . I really like this machine. Home built 8" lap uses a 1/4 hp AC 1725 geared down to 900 rpm at disc. When using 80-120-220 discs an 8 sq in rock face with moderate pressure does slow it down. Plenty of power for coarse shaping large cabs. Curious about the cost of 12 inch rubber diamond pads ? They are sure nice for 200-400-800-1500-3000-polish steps. At a good max speed of 3000 rpm for 4 inch pads it looks like 1000 rpm is the max speed for 12 inch pads. You are aware that the rubber starts to melt above these speeds ? I never found a need to run it slower or vary the speed. If anything faster. The build uses a 2:1 14" diameter 10 gauge tank head. The rugged tank head allows welding directly to it which made fabrication much easier. Fabrication photos: www.flickr.com/photos/67205364@N06/sets/72157696891131845What a great way to repurpose an old BBQ grill. Man I wish I was more mechanically inclined with motors, wiring and pulleys. This is to cool!
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jamesp
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Member since October 2012
Posts: 36,154
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Post by jamesp on Dec 2, 2019 5:36:45 GMT -5
Lol RocksInNJ, funny you mention barbecue grill. My business is fire pits(firepitsatlanta.com) and this machine was made out of a minature fire pit bowl and cover. Started with the mechanical gizzards and started welding till it became an operational machine. Very fun project and pleasant machine to use. It was the 14 inch diameter fire pit bowl that made the machine because it could easily be welded to. A bit of a compact and busy welded unit. I have bowls starting at 14", then larger ones at 24-26-30-36 inches if bigger machines were needed. It would be a good candidate for a shop built welding jig that would allow mass producing them easily and cheaply since the materials were inexpensive.
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jamesp
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Member since October 2012
Posts: 36,154
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Post by jamesp on Dec 2, 2019 5:49:59 GMT -5
Those motors should supply plenty of power Rockoonz. This machine is often used to square off cast 12 pound glass bricks. The glass bricks barely fit on the 8" wheel and require a lot of power to grind the faces flat. For general cab work it is way over powered at 1/4 hp at a 900 rpm wheel speed.
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