Thread: CBN Grinding points life span?

So i have a customer that needs the splines inside a very very hard tool-steel block opened up a couple of thou. I have a fair qty of these to do. Problem is spline id currently is sub 5/8" and a little over 1 1/2" long - deep, and blind at that :-(

So far the best i have come up with - done is grind them out the required amount. Its not a precision job, just gotta get a supplied part to fit. Current approach is going through mounted abrasive points at a alarming rate (like 3 parts per point), the splines really hard on them. That said for once the price is right and im making money, so its not too bad But the killer and why im going through the points is the dead end at the bottom, to get to within a 1/16" from the end that i need i am having to frequently dress the point to keep my corner.

Would a plated CBN grinding point be a viable option? If so anyone care to take a guess on how much material one can remove before being worn out? Based on the cost of them im more than happy to give a few a go, even if they wore a bit faster than std points i could well be ahead, its more time that's the issue here than just tooling costs for once. The dressing is the current killer.

I only have a max of 30Krpm on the point spindle and 500 on the work piece, work piece can counter rotate though, so its accumulative :-) Oh probably not going to be larger than a 1/2" diameter grinding pin.

Yes. I haven't ground that exact size hole but used to do 8mm holes on a jig grinder with 5-6 thou removal in 440C at 60-62Rc. Sometimes do 100 or more per point. Get carbide shank if you can. If it clogs up spray with WD 40 and go in the hole, they cleaned right up.

If i got ten per point i would be well chuffed. If i got over 50, well it would become a very profitable job ;-) Will have to look into carbide shank versions, for now all i have been able to find is steel ones, got a few on order a couple of days ago, here is hoping they arrive tomorrow or im out of luck till middle of next week due to double bank holiday next week :-(

Funny, when I Jig ground with CBN, we needed to dull the CBN grinding pins first before we would run them on parts where surface finish in a hole or on a contour mattered...most times.

If finish can be course, CBN is excellent for the job you have to do. The bigger the wheel and shank you can fit, the less bounce and more cutting surface you will have.

Traditional vitrified wheels dont hold edges for shit. redressing for corners is a totally standard problem. Tis' a reall pain in the ass, but in grinding, as they say, dress for success...

Finnish is not a requirement here, just get rid of it, but i can't get away with hard turning it, already tried and failed there, hence the cbn approach. Just out of curiosity how did you dull them?

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Can't see a burr lasting long, firstly im only removing a little over a nats, secondly these are so damn hard.

To dull them we would chop grind (jig grind up and down recriprication) along a piece of hardened scrap. following the scrap for a few inches seemed to knock down the high points well and give a better surface finish while actually grinding for size.

I still love thinking about the giant reflective chips that would "float" off of a part when grinding a hole with a course CBN wheel (the plated type)

Originally Posted by nateman

Funny, when I Jig ground with CBN, we needed to dull the CBN grinding pins first before we would run them on parts where surface finish in a hole or on a contour mattered...most times.

If finish can be course, CBN is excellent for the job you have to do. The bigger the wheel and shank you can fit, the less bounce and more cutting surface you will have.

Traditional vitrified wheels dont hold edges for shit. redressing for corners is a totally standard problem. Tis' a reall pain in the ass, but in grinding, as they say, dress for success...

I've dressed away way more wheel than I ever thought of grinding with trying to hold a feature, but I still prefer a vitrified wheel to CBN. I've noticed that the worn CBN's do produce a better finish than a new one, as you described, but it's still pretty rough. One thing that I've done before is to rough out holes with CBN, then go back & finish them out with a standard wheel. I feel like I get the best of each that way, both speed & surface finish. The #3 Moore at the shop has seen better days, but it's repeatability is still top-notch for doing something like this.

Originally Posted by adama

Can't see a burr lasting long, firstly im only removing a little over a nats, secondly these are so damn hard.

According to this PDF, burrs can cut steel in the 40-60C rockwell range.

www.maford.com/pdf/metric%20burs.pdf

Gene

Originally Posted by easymike299

According to this PDF, burrs can cut steel in the 40-60C rockwell range.

www.maford.com/pdf/metric burs.pdf

Gene

Maybe 1 piece. And it will look like shit. Would you try to turn 60Rc with a carbide insert? Per the OP that's about how hard this is .

I don't have hardness testing capability, but its pretty much as hard as non Hss type steels get. On a par with silver steel at a very light straw temper. I can turn small bits of that with carbide, have done before, but its costly insert wise, especially across a hole let alone a spline. Add in a internal operation at 3-4 times the bars diameter deep and carbides not good here i tried, creates a major burr issue. But thankfully i priced it allowing for this. On od work the Finnish with just std carbide can be good, just hard to hold precision! Add in single part insert life spans too.

Would be ideal to hone out, but according to my customer they cant do blind (to bottom of hole as required) + hate the spline also :-) Its orignaly 2 pieces and a plug is very securly pressed into a counter bore in the far side just to add to the fun the plug has got timed features (ground post assembly) on it in relation to the spline. Hence they cant easily be popped out and honed + reassembled.

Cant you do this on a sinker with a rotary head? Just thinking out loud...

Me personally, no, don't have one but yeah, would probably make pretty quick clean work of these. As is this lot are done, CBN works well, did not have enough left to get a real feel for life span and such, but its way more profitable than conventional abrasives, just need to fine tune a little.