Advice for slabbing large Royal Imperial Jasper nodules?

Thanks to both of you.  The two real big ones are, of course, oddly shaped, which is part of the problem.  And my desire to cut slabs with the largest face possible limits the options for clamping them in the vise.  

I am going to buy that clamp off ebay.  I am sick of struggling with big Brazilian agates, and using something like that to cut them in half would be a good first step.  I will be interested to see if it will hold roundish rocks off center.  With Brazilians, my favorite part is usually the outside 1/2" to 1" of the stone because the matrix often caused cool designs when the agates started forming.  It would be nice to be able to slab off all the sides.  I don't think that will be possible, but I will experiment.  

Slabbing rocks is one of my favorite activities, and yet, fighting to get them clamped into the vise securely is a constant, frustrating struggle.  Hopefully that clamp is a good addition to my arsenal of wedges and slab grabbers.  

@hankrocks

I bought that clamp.

I am mostly using a 14" slab saw at the moment. I also have a 10" slab saw and an 18" slab saw that I will have running again shortly.... Other than slab saws, I've a 16" MK Diamond drop saw, a 10" high speed lapidary trim saw, two 6" lapidary trim saws, a 10" tile saw, a 6" tile saw, a 4-1/2" tile saw, and one of these, although I've yet to figure out a good lapidary use for it.



All of my slab saws have auto stop, so if I set it just right, is shouldn't be a problem. I get your idea about raising it up, so that is something else to try if needed. I bet I can figure out a good way to use that clamp with my drop saw as well. Would be a fast way to cut rock in half since it has a huge 2 h.p. Baldor motor on it and eats through rocks when set up with a good tile blade.

Rockindad

I might just try that. I have a few rocks that are way too large to fit into any of my slab saws, including a 40 pound chunk of Maligano jasper, a 50 pound chunk of Graveyard Point Plume agate, and a chunk of Australian lace agate that is shaped like a USPS large flat rate box. Maybe I could use a couple ratchet straps to secure the rocks to a big piece of wood, with a strap on each side of the cut. If I cut through the rock and into the wood that won't hurt the saw. I have also thought about partially buying the rough in a hole filled with crushed limestone and plunge cutting the rock that way.  

I need to either find it a lapidary use or find it a new home.  

You could do the lapidary version of this:



I've watched a few demonstrations of this (not this particular guy) in person, pretty amazing.

As far as securing a large piece I started with a piece of 3/4 plywood. I placed the stone on it and screwed some scrap 2x4's around the piece to create a cradle so it could not slide around, I left an opening where the cut(s) were going to be. I drilled four 2" holes (2 on each side of the cut) and ran a couple of ratchet straps through the platform and around the rock on each side of the cut, rocks weren't going anywhere. 

@jadedvision
HankRocks

I finally found the bravery and patience to start slabbing my large royal imperial jasper nodules. They were larger than I remembered -- 5-6 pounds each and oddly shaped.

The way I had to position them in the vise, most of the rock was sitting on the carriage and only about an inch was hanging off the carriage. So I could only get 2 or 3 slabs from each rock. At this point I am going to glue them to 2x4's to finish slabbing.

Both rocks have some potential.

The first would be amazing if not for the green bands, which are all too common in these rocks.    I could have tried to slab it parallel to the green bands, which were visible on the rock exterior.  But since the bands are not exactly straight or parallel, that would have been easier said than done. The hole in the center seems to be closing up as I cut into the rock. The largest slab is about 4"x5"




I am more excited about this one. I have only cut about 1" into it the rock and there is about 4" to go. I am hoping for more eggs and that the orange area takes over the soft white part.  This one is big, about 7-1/2" x 3-1/2"

The first one is going to be made into a whole bunch of long, skinny cabs,  I suppose, in order to work around the green stripes.  

I am excited about the second one as well.  Slabbing these is quite a cliff hanger.  

Oh I would love to slab some of those just for the fun of it!

Exactly.

Yes, it all makes sense.  

My slab saws use a motor to turn the feed screw so there is just one speed -- about 3/16" per minute.   I believe that this is roughly equivalent to the slow speed on saws with belt drive feeds with three pulley slots.  If there was a way to adjust the speed I would love to know, as I get impatient slabbing large chunks of obsidian that could definitely be fed faster if I had the option.  

I agree with your logic -- a slow feed speed will allow the blade to straighten itself out rather quickly since the blade core is stiff steel.  Centrifugal force comes into play as well to help the blade self-center despite initial deflection.  Ideally, the blade will self-center within the first 1/4" or so of the cut.   The diamonds on the sides of the blade are what allow the blade to grind its way back to a centered position.  If the centering does not happen quickly enough, the blade can no longer self center once the blade has cut to a depth greater than the height of the diamond segment.  Blade that have diamond segments significantly wider thn the core of the blade do allow for some additional centering of the blade, but the MK diamond blades I use have steel cores that are only slightly narrower than the cutting edge, so pretty much a self centering must occur before the blade has advanced into the rock beyond the diamond segment or rim.  

So that's the theory, but here's my experience.  

When I use a wide kerf blade, like a 0.110" kerf on a 14" saw, I can run the rock into the blade at pretty much any angle and it cuts just fine, exactly as you have described.  Because I hate cleaning saw oil and a wide kerf eats up a lot of rock, I have been using MK Diamond blades with a 0.065" kerf.  The blades make very clean cuts and last a long time, but they are not nearly as stiff as wider blades, and I believe that the core is only 0.01" narrower than the rim.  Nevertheless, everything still works as you have described UNLESS something is out of adjustment on the carriage or feed mechanism. 

After I cut with my brand new slab saw for about 50 hours, many things related to the carriage and feed mechanism needed tightening.  I consider this normal break in maintenance.  Unfortunately, HP does not offer a manual or any specific advice about what to adjust or when to do it, and as a first-time slab saw owner, I was not aware of all of the points of adjustment.  The first thing that needed adjusting was the gap between the carriage bearings and the rails.  If the bearings are not kept very snug (but not too snug) to the rails, the carriage can be pushed up and to the side slightly when the rock comes into contact with the blade at an oblique angle.  That amplifies the initial deflection, making it much harder for the blade to recover and self center.   That is exactly what happened to me within the first few months of me owning my 14" saw.  I was trying to cut at an oblique angle and the blade did not auto-center quickly enough.  As the autofeed advanced, it continued to deflect the blade until the blade was pushed into the metal "shelf" that runs from behind the blade to the front of the saw.  Eventually, the advancing rock severely warped the blade, which was pinned between the rock and the steel inside the saw.   I ruined a $200 blade.   I also had to replace the v-belt and the drive pulley on the motor, which had becomes glazed over from spinning against a stalled belt.  I am lucky that I did not burn up the motor as was extremely hot as well, although the the motor's thermal protection circuit was never tripped.  

I don't want to turn this into a novel, so I will just briefly say that I recently had a similar problem before making adjustments to the drive screw bearings and the split nut mechanism to keep everything very tight.  I didn't ruin a blade this time, but I was experiencing other problems when trying to cut at an oblique angle.  

When everything is tight and properly adjusted my saws cut perfectly smooth, even thickness slabs every time.   The angle that the blade hits the rock does not seem to matter much.  But given my expensive learning experience, and the fact that I am not constantly inspecting the carriage / bearing / drive screw / split nut tightness, I am now gun shy about cutting outside of the 80-100 degree or so range. 

My expensive, blade-destroying learning experience was with a huge chunk of polish flint.  The rock was right at the limit of what my 14" saw can cut, and chert is a very hard rock.  Harder than most agates and jaspers, with the exception of things like Montana and Brazilian agates, which are also fairly pure chalcedony.