# Deburring Stones: Is Convex Okay?



## Chips O'Toole (Feb 3, 2020)

I have been watching videos on precision-ground bench stones. For those who don't know, these are ordinary India (or other) stones that have been ground smooth with a surface grinder. They don't really have points on the abrasive crystals. Very strange. They remove burrs from stuff without scratching. They're amazing to watch.

To get them to work, you need a pair, because you have to rub them against each other sometimes to flatten the bits of metal imbedded in them. A pair will usually run $200 or so. The raw stones are $22.50 each on Amazon.

Robin Renzetti has a wonderful, informative video on these stones. I watched it, and now I'm confused, which is by no means unusual. On the one hand, he says the stones have to be flat (surface-grinder flat). On the other, he uses the stones on round work, meaning he applies them to things that can't possibly benefit from a flat stone. The side of a round object can't lie flat against anything. Some dude who commented on his video claims the stones don't really have to be flat, and although I know nearly nothing about the subject, it kind of makes sense to me, because when you look at a close-up photo of a ground stone, you can see that all the points have been polished off of the grains. You would think that the polishing of the grains is what really makes the stones work.

Adding to the confusion, machining guru Forrest Addy says that for deburring, he likes lapped stones that are slightly convex. Convex isn't flat. He said this in a discussion of burr files, however, so I don't know whether it's relevant to the use of precision stones.

For those who don't know, a burr file is a piece of a file which has been dulled deliberately on a flat stone. You can move a burr file back and forth on a metal surface, and as long as the surface is smooth, it won't remove anything, but if there is a burr, it will shave it off. That's the theory, anyway.

If you can get away with convex stones, then clearly, there is no point in paying someone a hundred bucks to grind them. Renzetti says it's extremely hard to create really flat stones without a grinder, and not all of us have one in the shop.

To make things more complicated, some stones come with a lip ground into them, along the long side. It allows you to put the stone up against a part and reach under it a little. I don't know how you would reproduce that lip without a grinder.

I kind of wonder what a local shop would charge to grind a couple of stones.


----------



## darkzero (Feb 3, 2020)

I've been wanting a set of precision ground stones. Found a couple of guys making & selling them, both are about the same price. Too much for me for a pair of stones, maybe one day. 

Hopefully I can make friends with someone who has a surface grinder & can do that. That's what I did before I had a lathe & mill of my own, made friends with a machinist but the best I could do was a friend of a machinist.


----------



## ErichKeane (Feb 3, 2020)

darkzero said:


> I've been wanting a set of precision ground stones. Found a couple of guys making & selling them, both are about the same price. Too much for me for a pair of stones, maybe one day.
> 
> Hopefully I can make friends with someone who has a surface grinder & can do that. That's what I did before I had a lathe & mill of my own, made friends with a machinist but the best I could do was a friend of a machinist.


I've got a nice surface grinder, but it seems that the diamond wheel to make these stones is pretty darn expensive.  You'd probably have to make a few dozen to make it worth buying the wheel :/  I had considered making my own (haven't pulled the trigger either way), but a wheel is as much as a set of stones, and I can't imagine you don't need to use 2-3 different grits to make the stones :/


----------



## Chips O'Toole (Feb 3, 2020)

Robin Renzetti uses one wheel.


----------



## Cadillac (Feb 3, 2020)

When the stones are surface ground you are leveling out all the particles to one plane. Yes it is taking away the bite of the individual grains but it still is abrasive enough to take off burs on your material. But since it is flat it will not take off anything but the high spots. If you use the edge or lift up so only the corner of the stone is in contact it will bite more into your material because you are decreasing the surface area of the stone. They are pretty magical and without his video and demonstration my guess is no one would of ever heard of them.
 You need some specific tooling to grind the stones on a surface grinder like hold down fingers which are pricey, then you need a diamond wheel which is also pricey. After acquiring all the tools you need the know how to do the job so I think the price for them is in reason. Yeah it's on the high side but I would gladly pay for robins work he is a master at what he does. I made myself a set about a year ago and I can say their is nothing that compares to them. Imo surface grinding work is where they shin they don't scratch the finish and take the minor burr off the edges. I'd be willing to do acouple sets but they wouldn't be given away the two stones alone are 50 bucks plus the labor.


----------



## ErichKeane (Feb 3, 2020)

Chips O'Toole said:


> Robin Renzetti uses one wheel.


Interesting! I see that Bob Korvas here on the board tried to do this a few years ago, but I didn't see the update.  Looks like he grabbed the Shars diamond wheel and some $20/each Norton stones and went at it!


----------



## tjb (Feb 4, 2020)

I have a pair of precision ground deburring stones that were given to me when I bought a very nice used Vectrax mill a couple of years ago.  The mill was ten years old but had only been used in a light, hobby context for one year, then stored in an atmospherically controlled environment.  The owner was a retiring electrical engineer who decided to set up his own for-profit small machine shop and wanted to upgrade all his equipment to a much heavier duty category.  (I showed up about a month too late to get a great deal on the lathe he sold.)  While at his shop, he had a pair of stones that he made on his surface grinder and showed me how he used them to debur the mill table and other surfaces.  I bought the mill and arranged to pick it up the following week.  I tried to get him to throw in the stones, but he wouldn't do it.  He did, however, give me information on how to make a pair - the Robin Renzetti video.  It was immediately clear to me that making these stones was going to be significantly beyond my skill level and equipment availability, so I just figured I'd live without them.  We had several friendly conversations over the next week - some with regard to deburring stones - and when I arrived to pick up the mill, he surprised me by giving me his pair.  Whoa!  He said it wouldn't take him too long to make himself another pair, so he just threw them into the deal - no charge.

I'm quite certain I am not using them to their full capacity, but I can assert from a near-novice perspective, these stones are amazing.  Based on his instructions, I always use the same stone for deburring (that one's in my right hand), and the other for 're-surfacing' if that's the correct term (always in my left hand).  Any time I remove the vise or rotary table from the mill, I will go over the table with the stone.  You can actually feel the grit coming off the table, and when it's smooth, there is an adhesion between the stone and the table top.  The stone will quite literally skate across the table when it's deburred.  Also, when rubbing the two stones together, the same adhesion phenomenon occurs.

I also use the stone on the underside of my vise and rotary table when I mount them on the table.  For my skill set and relatively low level of precision requirements, this is unquestionably over-the-top precision.  But I consider it a good habit, and it strikes me as a good subconscious exercise in observing and maintaining proper discipline in the machine shop.  All that from a free set of deburring stones.

Regards,
Terry


----------



## ErichKeane (Feb 4, 2020)

Previous discussion btw: https://www.hobby-machinist.com/threads/precision-ground-toolroom-stones.59776/ 

Bob ended up making quite a few, but that was the end of it  

I'm encouraged by his progress though, I might consider buying the Shars wheel and a few stones next time I'm looking for a project!  Just gotta get my coolant set up finally


----------



## Chips O'Toole (Feb 4, 2020)

I'm getting a stone, and I'm going to lap it on a coarse diamond wheel. I'm also going to make some burr files. I want to see what they can do. Meanwhile, I am looking into getting a couple of ground stones with lips on the sides.


----------



## Janderso (Feb 4, 2020)

I am one of the lucky ones that went to Bob's shop and had a set of Norton stones precision ground.
I use them all the time.


----------



## ErichKeane (Mar 2, 2020)

I ended up doing exactly as Bob did this weekend!  I picked up the shars wheel + 6 (actually 7 due to a broken one!) stones and ground them in!  I didn't do the step, but I did grind 4 sides and they turned out great.  It took me way less time that I expected it would, only ~2 hours for 6 stones with 2 people.


----------



## Janderso (Mar 2, 2020)

I just read on another site a guy that was asking about how to clean up/refurbish his ways. He was wondering if he could use India stones right out of the box?
I wouldn't want that lathe.


----------



## ErichKeane (Mar 2, 2020)

Janderso said:


> I just read on another site a guy that was asking about how to clean up/refurbish his ways. He was wondering if he could use India stones right out of the box?
> I wouldn't want that lathe.


Yikes! Even the Norton ones I got were no where near flat.  Now that I've seen them, I wouldn't want to flatten anything with them unground!


----------

