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Scraping hardened ways with a CBN insert?

drogus

Aluminum
Joined
Aug 12, 2018
Hi,

I’ve seen quite a lot of discussions about restoring hardened ways. The consensus seems to be that you should either use a surface grinder or abbrwsive disc grinder as a last resort (like when a machine can’t be moved).

I was wondering about alternatives and as CBN is used for hard turning I wonder: has anyone tried “hard scraping” with a CBN insert? Assuming you could find one big enough, cause a lot of them seem to have CBN only at a tip of the insert.
 
Most CBN inserts are rounded at the cutting edge, due to the brittleness of the material and to help them "live" in the cut zone. You'd have to grind the insert to a true sharp edge, then see if it remains intact during use.

If it does, and if you can generate the needed force to scrape harder material, then it might work OK.
 
How would you generate the tool pressure required for CBN when scraping?


Good question to which I don't have an answer (which is why I asked if someone tried it). I'm using a power scraper, so I was hoping that with enough pressure on the scraper it will bite at least enough to have a good result, but I have no idea how much pressure CBN needs.

Milland said:
Most CBN inserts are rounded at the cutting edge, due to the brittleness of the material and to help them "live" in the cut zone. You'd have to grind the insert to a true sharp edge, then see if it remains intact during use.

I was afraid someone will say exactly that :D

I mean, I was hoping that someone already tried so I don't have to, but I guess I might have to try myself. I know about the brittleness, but then carbide is also brittle (although again, no idea how much it differs from CBN).
 
You might try doing a little hand-scraping with a properly sharpened CBN insert first before going to a power scraper. At least, it may save you from some digs or a quickly-fractured tip.

You might even need to change the cutting edge angle, perhaps Rich has some thoughts on that?
 
You might try doing a little hand-scraping with a properly sharpened CBN insert first before going to a power scraper. At least, it may save you from some digs or a quickly-fractured tip.

You might even need to change the cutting edge angle, perhaps Rich has some thoughts on that?

Good points! I don't think that Richard tried this though, he was one of the people that told me about using an abrasive discs as a last resort option (on his workshops in Austria, the owners of the rebuilding shop that co-run the workshop also did such a thing once, but said it was basically a pain in the ass)

Any pointers on what CBN inserts would be easiest to use for this purpose? I haven't used any CBN inserts so I'm not sure what's the easiest way to find solid pieces of CBN (as opposed to just having a CBN tip), ideally in a bigger form.
 
Well, I've never tried it, so the best I could suggest is look for a CBN grade that's biased towards "toughness" so you have a good chance of it holding its edge. We have some tooling guys on the forum, maybe one of them could make a suggestion - if you don't get any answers here, post a new question specific to that.
 
I've scraped with abrasive discs and even angle grinders, it works fine, I wouldn't even call it particularly last resort. I was taught by someone who's scraped many many square feet of turbine housings with abrasive discs.

I always understood the primary benefit of CBN for hardturning is its capability to hand the high heat of hardturning cuts, and if the cut isn't hot CBN quickly loses its effectiveness (and edge). Given that, I don't think it's going to provide any benefits over carbide for scraping, even hardened material.

Personally I would scrape with abrasives, moving progressively finer in grit. Then finish with lapping, which for example is what Moore does to tune the hardened ways of their jig bores and jig grinders (they skip the scraping step).
 
I think of scraping the same as any other metal cutting operation. For starters, your cutting tool of course needs to be harder than the surface you are cutting. How much harder your cutting tool is than your material dictates how long it will last.

The ability of the tool to dig in has less to do with the material the cutter is made out of and more to do with the power and rigidity with which you are using the tool. You can mill hard materials because the machine has lots of power and rigidity FORCING the cutter into the work. Old school HSS cutters didn't last long because they were softer, but were much more forgiving because they are not as brittle as carbide. Carbide cutters can last a very long time, but only in the most optimal conditions. Any deviation from those conditions and your cutter dosn't cut, it chips and/or deflects (breaks) because it is so brittle.

Grinding by hand works because you have millions of cutting edges taking itty-bitty amounts of material off individually, and it also self replenishes its cutting edges. The abrasive media is just strong enough to scratch, but as they break off from being forced too hard, there are more abrasives behind them.

Scraping requires a single cutting edge making perfect cuts every time. So in theory, yes you can scrap hardened surfaces with a carbide insert... if you had the strength and accuracy of Superman.:)
 
CBN chips so easily, as in look at wrong easy.
Yes there are tough and there are brittle grades or classes but not like the world of carbide and nowhere in the world of HSS.
Scraper blades in general are very wide with a gentle radius. How many have a CBN insert with a full 3/4 inch side?
Yes it can sort of work but you will eat though a very expensive tool quickly and the front rake and T-land is very fussy with usage.
I can see it in very special work or if you have lots of free CBN laying around in the size .....Mostly I give 90% chance of failure.
I do do it but the tools are free. Scrap from runs.
Also will use cermets or silicon-nitride on occasion in the handle. All these need a different grind from carbide.

Base carbide also comes in grades and hard, soft and the very hard and brittle. Most just stick with the standard stuff.
The right CBN for the material being attacked can save time for sure. Getting it right in grind and edge is a nightmare or sometimes just plain ole luck.
Bob
 
Thanks for all the info! I thought that CBN doesn't chip as easily. I guess I will stick with carbide for cast iron and if I have to scrape a hardened surface I'll try with abrasives. For now I don't own any machine that I'd plan to scrape that way, at the moment I'm restoring a very old Deckel FP1. But in the meantime I'm also looking for some bigger machines and some of them have hardened ways (like Deckel FP NCs or some Avia machines)
 
I used a die grinder/dremmeloid on the bed of the Hardinge HLV. Roughing with bullet Al-Ox points and finishing with bullet cratex points. The technique I used was to swipe the point sideways, taking small square-ish cuts. It works very well with a bit of practice. Pattern size and shape is similar to what wyou would see from a pull scraper rather than a Biax.

I wouldn't use an angle grinder for this unless I was working on something more suited to a shipyard or had a very fine abrasive disk in it. Possibly one of the 2" disks? YMMV.

I believe that the Biax advice is to use an HSS positive rake blade for hard steel. I tried both positive and negative rake carbide and found that it was almost uncontrollable, either skidding off the work with normal pressure or digging in when I used enough pressure to cut. The prime effect was that I damn near dislocated my shoulder.
 








 
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