agatemaggot
Cave Dweller
Member since August 2006
Posts: 2,195
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Post by agatemaggot on Jan 8, 2009 22:08:19 GMT -5
I have been buying quite a few slabs / strips/ of gem grade Obsidian from a man in Pine Creek Oregon for pressure flaking arrowheads. He cuts a LOT of this material and while we were chatting the other day he mentioned a problem he was having with one of his blades. It seems that the blade on his 18 inch saw has developed cracks running from the center hole outward for about 2 inches . Being as Obsidian isn't all that hard to cut compared to something like a Brazilian , he couldn't come up with any reason as to why ! Not binding or walking , just a nice easy cut straight thru. I was wondering if any of you have had this happen , or have a clue as to why it did ?
I ventured a guess as to what caused the problem , I told him that the blade was probably made on a Saturday night and the guys in the blade shop were thinking more on the lines of drinking beer and chasing loose women rather than what they were doing with the heat treat furnace. That theory probably won't hold up so any thoughts here would be appreciated !
Thanks,
Harley / Agatemaggot
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Post by Bikerrandy on Jan 9, 2009 4:06:46 GMT -5
The only possible explanation I can think of is a loose arbor. Excessive vibration from this is the only thing I'd guess would crack a blade from the inside out. Maybe his bearings are wore out? Blade out of balance?
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Post by Jack, lapidaryrough on Jan 9, 2009 5:49:14 GMT -5
And the other possible reason maybe the collier plates on the arbor. The collier`s need to be hollowed out for 3/4 of the radius of the collier to allow pressure out on the colliers edge, on a highland park 24'' saw the colliers are about 4.5 inch and the ring edges that tightens to the blade is about 5/8s of a inch where the surface tighten to the saw`s blade.
Jack
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fanatic
starting to spend too much on rocks
Member since October 2007
Posts: 233
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Post by fanatic on Jan 9, 2009 6:19:35 GMT -5
Is it possible the blade isn't running perfectly perpendicular to the vise and stressing one side of the blade?
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drjo
fully equipped rock polisher
Honduran Opal & DIY Nut
Member since May 2008
Posts: 1,581
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Post by drjo on Jan 9, 2009 8:20:09 GMT -5
I agree with lapidaryrough, but know the parts as flange support washers. If he doesn't have large enough washers the blade will flex near the arbor and stress fracture like your describing.
He can have the blade welded if he can find someone who knows what they are doing (warpage problems).
Dr Joe
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Post by Jack, lapidaryrough on Jan 9, 2009 10:19:26 GMT -5
do not weld on the blade, if it has any life at all lift, drill a 5/16 holes at end of cracks, and run the rest of the life out of the blade.
then do a complete Clock aliment of Blade to arbor shaft. you`ll need to use a Dial inductor vised into the vise, use a piece of flat stock drill arbor hole into ( In the middle of stock ). then set the vise carriage to the front to ( 0 ) then run vise to back of saw carriage you should have the same reading front too back.
Then re-vise inductor to top of vise and check the ( 12:00 ) reading, this should be the same as you turn the arbor shaft ( 180 ) degrees.
you`l need shim stock sheeting, Bearing supply store have it. the shim stock is to be used on the arbor. ( Pillow block bearing`s )
Checking the readings with the blade wont work as most Lapidary blades have a slight wobble to them.
Never have i every seen cracks at the arbor on any blades i have cut the life out of. well Congo's, MK- 297 - 301 - 303, Star blade`s, Felker`s.
I have a Lortone 18'' and it cuts to fast, i had to change out the pulley to slow the feed screw down. went with a pulley 1.5 in. bigger, change the feed by 2 minutes slower. ( the there speed sheave ).
Jack
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Post by bobby1 on Jan 9, 2009 10:55:29 GMT -5
I have had a similar problem with a blade. It comes from "dishing". Dishing is caused by overdriving the rock past when it has cut through and the tip of the breakthrough rides on the blade distorting it. The point of most stress is at the outer edge of the blade flanges. This stress can cause fracturing of the blade. If a dished blade is reversed then the stress is just applied from the other side. The answer is to very accurately adjust the drive screw stop so that the rock stops as soon as the blade breaks through the cut. If the dished and cracked blade has enough dishing it might make a good frisbee but that is about all it is good for. Bob
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Gem'n I
freely admits to licking rocks
Member since March 2008
Posts: 980
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Post by Gem'n I on Jan 9, 2009 12:07:07 GMT -5
One other possibility is over torque of the flange washer..there is a happy medium to how much torque there should be and really clamping down on the washer can lead to the dish effect bobby1 alluded to in his post thereby contributing to the crack problem. I also had a post that talked about blade run out and someone posted a very interesting blurb about blade care and pitfalls which may cover some of what we are talking about. Larry
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Post by catmandewe on Jan 11, 2009 1:17:35 GMT -5
I have seen it caused by dishing, also. Need to set the shut off to stop the cut just before it finishes the slice. Not much you can do about the cracked blade, I would still run it until it became apparent that it was dangerous to continue doing so my self. The drilling the end of the crack idea may work, kind of like stopping a crack in a window, but I have never tried it.
Tony
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Rogue Trader
freely admits to licking rocks
"Don't cry because you are leaving, smile because you were there."
Member since December 2008
Posts: 839
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Post by Rogue Trader on Jan 11, 2009 5:11:19 GMT -5
Would drilling the blade not remove weight (albeit minimal) from that part of the blade and therefore make the blade out of true and cause slightly (not visible to the naked eye) more vibration?
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Post by Jack, lapidaryrough on Jan 11, 2009 9:08:05 GMT -5
NO
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docharber
has rocks in the head
Member since October 2008
Posts: 693
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Post by docharber on Jan 11, 2009 14:59:55 GMT -5
Throw it away. It's DANGEROUS and you have no way of knowing how long the defect actually is. If it fell apart in use, you have a projectile. Usiong blade stiffeners like you do on woodwroking saws, won't help. They'll just transfer more stress to a smaller area of the blade. I have more experience with woodworking saws, but the principles are the same. Warpage, like Dr. Joe said,is likely if attempts are made to weld or braze the blade. This is commonly done to repair band saw blades. But, chances are you'll loose tyhe temper in that part of the blade and the metal will become brittle, making it more probne to cracking again under any deflection stress. So, for safety's sake, I'd ditch it. Something else to think about, is whether the blade was properly tempered to begin with. It should hve some flexibility if it was, and cracking would not be very likely under foreseeable operating conditions, with such a large blade. I might contact the manufacturer and ask their advice. Who knows, they might just offer to replace it free or at reduced cost.
Mark H.
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Post by Jack, lapidaryrough on Jan 12, 2009 12:05:45 GMT -5
Most wood saw blades RPM 3400 - 5200
Most Petrified Wood saws 24" 480 - 960, RPM
Lapidary Tools Lapidary Blades Cup Wheels Core Bits Hand Tools Polishing Supplies Accessories Library Tool Talk Blog Cut N' Strut Search Technical Library Lapidary Blade Speed Recommendations Blade MK-297 MK-301 MK-1000 MK-303
Recommended RPM Operating Range in Surface Feet Per Minute
3000 - 3500 3000 - 4500 4500 - 6000 4500 - 6000 Blade Diameter Approximate Arbor Shaft RPM Range 4" (102mm) 2860 - 3340 2860 - 4300 4300 - 5730 4300 - 5730 5" (127mm) 2290 - 2670 2290 - 3440 3440 - 4580 3440 - 4580 6" (152mm) 1910 - 2230 1910 - 2870 2870 - 3820 2870 - 3820 7" (178mm) 1640 - 1910 1640 - 2460 2460 - 3270 2460 - 3270 8" (203mm) 1430 - 1670 1430 - 2150 2150 - 2870 2150 - 2870 9" (229mm) 1270 - 1490 1270 - 1910 1910 - 2550 1910 - 2550 10" (254mm) 1150 - 1340 1150 - 1720 1720 - 2290 1720 - 2290 12" (305mm) 960 - 1110 960 - 1430 1430 - 1910 1430 - 1910 14" (356mm) 820 - 960 820 - 1230 1230 - 1640 1230 - 1640 16" (406mm) 720 - 840 720 - 1070 1070 - 1430 1070 - 1430 18" (457mm) 640 - 740 640 - 960 960 - 1270 960 - 1270 20" (508mm) 570 - 670 570 - 860 860 - 1150 860 - 1150 24" (610mm) 480 - 560 480 - 720 720 - 960 720 - 960 30" (762mm) 380 - 450 380 - 570 570 - 760 570 - 760 36" (914mm) 320 - 370 320 - 480 480 - 640 480 - 640
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