Diamond grade for polishing ccgt inserts?

richard newman · Mar 9, 2021

I use ccgt and scgt inserts, high polish uncoated ones for aluminum, on my patternmakers lathe for turning wooden banjo rims. They cut great, but really need to be dead sharp to get the best finish. I have an Accu-Finish grinder, and would like to touch them up, wondering what grit diamond wheels to use to get the absolute keenest edge possible.

My shop made insert tool holders are tilted to reduce rake (helps avoid tearout with super curly hard maple) so I have more clearance than the 7 deg. the insert is ground with. Thinking that would allow me to just create a small micro-bevel at the cutting edge rather than having to grind the entire flank of the tool.

implmex · Mar 9, 2021

Hi Richard:
The finest wheel I have for my Deckel SO grinder is 600 grit and it does a pretty nice job, even looking under the microscope.
It could never be described as "polished" though.

You certainly don't want to take off a lot of material with it, so my rougher is 150 grit.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

dian · Mar 9, 2021

i have a 3000 grit disc. i think the trick is to take out a fraction of a degree each time you go finer.

Conrad Hoffman · Mar 9, 2021

Our Accufinish grinder has a smooth ceramic wheel that you squirt with diamond slurry. Finest polish a person would want. I think ours is 1 micron, but they sell various grades. Give 'em a call and see what they recommend.

michiganbuck · Mar 9, 2021

I guess if often grinding such inserts I might set up a bench grinder with a 120 gt OD diamond wet wheel to rough-in at 20* fixed simple table, grinding just the heal up to about 3/4 way up+, and then finish the cutting edge on the Accu, desired clearance with a 500 to 1500 grit.

Yes likely need to true-in a bench grinder wheel hub to run better (wobble) and then knock in the diamond OD wheel to rin .002 or so/less.

I have not used the Accu grinder

richard newman · Mar 9, 2021

Thanks for the info.

The Accu-Finish ceramic lap is $323, would a cast iron lap work as well? I have some durabar that i could make it from.

Conrad Hoffman · Mar 9, 2021

I'd think it would work fine, but you'll have to be very careful not to catch and gouge it. Don't let anybody else touch it!

richard newman · Mar 9, 2021

There isn't anyone here but me. Well, there's the pixies who come in at night and hide my tape measures and 6" starrett scale...

That brings up a question: if I'm just kissing the edge, should the wheel be running into the work or away from it?

eKretz · Mar 9, 2021

Always into the edge on carbide. And I've used 9 micron diamond grit slurry for a decent edge (this would be a little over 2,000 grit in the more familiar rating). You can certainly go finer; though if you're touching up a dulled tool you should start coarser and work your way up. Diamond lapping on carbide goes pretty quick.

Screwmachine · Mar 9, 2021

With a lap you would run it away from the edge. No risk of digging in, and loose abrasive won't be flowing over the edge you're trying to make.

Orbital77 · Mar 9, 2021

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eKretz · Mar 9, 2021

Screwmachine said:
With a lap you would run it away from the edge. No risk of digging in, and loose abrasive won't be flowing over the edge you're trying to make.

Not for me. I always see chipping at the apex when going away from the edge. I generally run my laps after pressing the abrasive in with a rolling bearing, so not a lot of loose stuff on there.

CarbideBob · Mar 9, 2021

Assume you are grinding the flanks.
400 grit good if grinding in compression. (wheel into part edge)
600-800 if the other direction or safe.
I would never go finer that a 1200 as that slow and no improvement on edge.

This all grinding wheels information and not charged laps.
Charged laps (diamond paste) should not be done rotation into the cutting edge so backwards only here which means finer grits to avoid particle pullout from the binder.
A free slurry or lap leaves a rounded entry due to abrasive loading and piling up front.
Sort of the edge shape that is known as a waterfall hone is produced. This may seem razor sharp up top but it is not in cutting action.
Bob

hanermo · Mar 9, 2021

Em, whats your take in time on removing about 0.01 mm from the edge, or half a thou ?
10 secs, 30 secs, else ... ?

eKretz said:
Always into the edge on carbide.

And I've used 9 micron diamond grit slurry for a decent edge (this would be a little over 2,000 grit in the more familiar rating). You can certainly go finer; though if you're touching up a dulled tool you should start coarser and work your way up. Diamond lapping on carbide goes pretty quick.

michiganbuck · Mar 9, 2021

I had a trick for grinding carbide as for Multi tooth cutters and high precision reamers.
I would grind into the edge up to the last .001 .002 then reverse my work direction with the wheel still into the edge. Yes, I would be coming from the heal first into the part for finishing. Doing this I would achieve a much better edge finish, and would not have the crash effect at the cutting edge. To see the crash effect you run a needlepoint micron indicator point up to the very cutting edge.
At the big shop, I would take photos under an arc microscope and see grind fractures on the very best grind Carbloy and Valenite could make on inserts. We had one insert that was print stated no fractures at the cutting edge. ..they looked terrible but were the best edge of all the competing insert brands. Carbloy and Valenite ran this job/insert special for us.

Someone might say I was slowing the surface speed of the wheel and that is true but when the work is going perhaps 250 to 380 or so RPM the change in surface speed is hardly changed.

eKretz · Mar 9, 2021

hanermo said:
Em, whats your take in time on removing about 0.01 mm from the edge, or half a thou ?
10 secs, 30 secs, else ... ?

Depends on the surface area, grit level and process. Lapping is generally only for finishing - I don't try to remove any real volume of material, only remove the deeper scratches from whatever grit or process was used previously. Rough in with a grinding wheel, finish with a lap.

I will definitely revisit lapping direction, there are a lot of votes for lapping away from the edge. My laps are pretty much of the embedded variety.

dian · Mar 16, 2021

i wonder why everybody is messing with the charged laps. you never know if the grit is embeded or tumbling, the lap is oily/wet and collects dirt. i grind, then lap on two weels dry (resin or plated ) and am done with it in under a minute. no problems with "waterfall edge".

btw, i doubt the edge has to be extremely sharp, it would break, no? we really want the flank polished.

Screwmachine · Mar 16, 2021

dian said:
i wonder why everybody is messing with the charged laps. you never know if the grit is embeded or tumbling, the lap is oily/wet and collects dirt. i grind, then lap on two weels dry (resin or plated )and am done with it in under a minute. no problems with "waterfall edge".

btw, i doubt the edge has to be extremely sharp, it would break, no? we really want the flank polished.

OP is cutting wood, and says a super sharp edge is best and what he's after. I do very tiny work, just made 20 little tubes 1.35mm total length max diameter 0.63 and a 0.25 hole, I was definitely glad my tool was keen as heck and mirror finish (I use a D7 wheel to finish).

Otherwise I agree, it seems to make good sense to use real wheels instead of laps. The D15 on my Agathon already gives a really good finish, it's about twice as fine as the typical finish wheel on those machines. I think if OP got a similar grit wheel he'd leave the laps behind.

richard newman · Mar 16, 2021

Thanks for the replies. Yes, I am cutting wood, need a mirror finish for best cut.

I've just ordered a 3000 grit wheel to see what kind of finish it gives. Would be great if I don't need the lap, the 3000 wheels are less than 1/10 the price.

michiganbuck · Mar 16, 2021

Do set up a way to use coolant...

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