"Reverse Boring" (Horizontal Mill) Sanity Check Requested

[Flynth]

Slight update: I moved things around and shortened up the boring bar quite a bit (at least an inch). I also switched to a CCGT 32.50 (1/128" radius) insert. I only had time to make two passes, but I got actual chips this time....still small, but chips rather than dust/sludge. I made a cleanup pass then another pass .010 DOC and there was still a bit of deflection through the hard spot (what I'm calling it for now) but nowhere near as bad, and the amount of taper is way, way better. I'm going to measure it with a telescoping gauge to get real number, but using calipers it seemed pretty straight going back about half way.

I'm going to make a few more passes this weekend and see if it continues to get better. If not, I'm probably going to wait until the CBN inserts show up. I'm still considering annealing the nose of the holder....submerge it in oil up to the flange and then heat it with an oxygen-acetylene torch. I already have a fair amount of experience heat treating and annealing tool steel with that method, so I think I can pull it off without turning it into a pretzel.

Thanks for an update. I'm very interested if those CCGT inserts work better. And aluminium inserts for hard steel? Who could have guessed it?


However CBN insert in a solid carbide boring bar wins hands down. The difference between carbide and CBN (when you try it first) is like between hss and carbide on a machine that can use carbide well. But you'll need to spin that tool holder at 1900rpm. Does your mill go that high in rpm? I know I can't t on my horizontal mill. CBN needs the high speed to generate enough friction to melt the metal. I never tried running CBN slow, but I doubt it will work. The speeds J mentioned are for hardened steel BTW (like the one in this thread). If anyone wants to use CBN for annealed steel for some reason sfpm is even higher (well into the "ludicrous speed" territory).

 

[G-ManBart]

Thanks for an update. I'm very interested if those CCGT inserts work better. And aluminium inserts for hard steel? Who could have guessed it?


However CBN insert in a solid carbide boring bar wins hands down. The difference between carbide and CBN (when you try it first) is like between hss and carbide on a machine that can use carbide well. But you'll need to spin that tool holder at 1900rpm. Does your mill go that high in rpm? I know I can't t on my horizontal mill. CBN needs the high speed to generate enough friction to melt the metal. I never tried running CBN slow, but I doubt it will work. The speeds J mentioned are for hardened steel BTW (like the one in this thread). If anyone wants to use CBN for annealed steel for some reason sfpm is even higher (well into the "ludicrous speed" territory).

I made a few more passes and it was getting better as far as taper and tool deflection, but very slowly (still with CCGT 32.50 inserts).

I decided to try softening the holder a bit rather than wait another 10 days or so for the CBN inserts. I cut a metal can to height, put the holder in it and filled it with canola oil then slowly heated the nose of the holder watching the color in the bore of the holder and checking the temperature with an infrared meter. I got it to 500*F which is dark brown in color, and held it there for maybe half an hour and then let it cool. At the peak the can never got much above 220*F and the flange stayed in the 300-350*F range so it shouldn't have softened enough to be a problem. That should have been enough to get the steel a touch under 60Rc but no idea how much since I don't know exactly what type of steel it is.

I made a number of passes afterwards and it's still quite hard, but clearly has tempered just enough I can get halfway decent chips out of it. Before I was getting a really high pitch sound when it was cutting and now it reminds me of the sound I get when parting on the lathe. I did mostly passes at .005" and after I'd made maybe half a dozen I cleaned things up and checked the bore with a telescoping gauge. The taper is down to between .001 and .002 compared with over .010 before. The insert still looked good to the naked eye, which is impressive.

Each pass seemed to get better and the hard spot was still there for the first pass or two but then seemed to switch to just a slight change in sound, but no pulsing when I hit it the last couple of passes. The last pass I could barely tell any difference when I got to that area.

I ran out of time, but I'm hoping a few more passes and the taper will be gone. If not, I'll set it aside and wait for the CBN inserts and see what happens with those.

I don't doubt the CBN inserts will be better. The mill tops out at 1,400rpm. I've been reading a bunch about speeds and feeds for CBN and some manufacturers cite a range of 200-600 SFM, which is doable with this mill.

Here's the simple setup to keep the tapered portion of the holder from getting hot. If you're into blacksmithing, the anvil is a 330lb Refflinghaus

 

[G-ManBart]

However CBN insert in a solid carbide boring bar wins hands down. The difference between carbide and CBN (when you try it first) is like between hss and carbide on a machine that can use carbide well. But you'll need to spin that tool holder at 1900rpm. Does your mill go that high in rpm? I know I can't t on my horizontal mill. CBN needs the high speed to generate enough friction to melt the metal. I never tried running CBN slow, but I doubt it will work. The speeds J mentioned are for hardened steel BTW (like the one in this thread). If anyone wants to use CBN for annealed steel for some reason sfpm is even higher (well into the "ludicrous speed" territory).

Update:

I've been trying to finish other projects so I didn't really go any further using the CCGT inserts. They were working, but it was slow so I decided to wait for the CBN inserts. At the same time (same eBay vendor) I ordered a solid carbide boring bar that uses CCMG/CCGT inserts and they showed up together.

I mounted the boring bar the same as the previous setup, popped a CBN insert in it, moved the table forward to check the center height, ran the insert into the work and chipped the insert....d'oh! I put in a new insert, and started making passes that were .001 or .002 and got nice, small chips and it was a consistent cut from front to back going off the sound. I tried running the RPM up high but the first pass over 1,000 and I could see the boring bar deflect, which wasn't happening at the lower speeds. I settled on a medium speed (I think it's 666rpm, which might be evil enough to cut hard steel). The surface finish was quite nice and darned if I didn't get a nearly perfect bore!

I was shooting for 1" and as best I can measure I got 1.001" front to back. Just going off telescoping gauges and cheap calipers I can't see any taper at all. I could probably take more time, get more careful measurements and use a micrometer, but for this purpose, I think it's good enough. I have a 1" ground dowel pin that mics a couple of tenths under 1" and I used that for a test fit. I can just get it in...a little bit of twisting and tapping are required. Now the big question is whether I'll be able to turn the end of the arbor close enough to fit nicely!

Of course in the middle of this a buddy of mine visited a shop that was closing down and picked up a pile of beautiful Bison 50 taper arbors for $15 each, including a 1" arbor which was the point of this whole project! I will say the arbor he picked up for me is shorter than what my fabricated arbor will be so it won't hurt to have both sizes on hand.

I'm looking forward to try turning hard steel with the CBN inserts and hopefully I'll get to see some molten silly string chips!

 

[Flynth]

Great, thanks for an update.

Pity about the chipped CBN insert, but thankfully you had another one. I never tried running cbn slow, but some people swear by it(with the right insert geometry). The usual geometry for "fast" speeds looks very dull. Imagine a chamfer where a cutting edge should be. Is your insert like this? If yes, and you got good results with lower sfpm this is very interesting for me.

I think I saw Robin Renzetti using CBN slow, but the edge was sharp (I don't remember if he said he sharpens them himself.). When my current "dull" CBN stops cutting I intend to try sharpening it too.

Regarding your chipped CBN insert. Did it chip in a way the CBN plate could be brazed to something? If it did, I wonder if one could braze it onto steel, or is carbide backing necessary.

 

[G-ManBart]

Great, thanks for an update.

Pity about the chipped CBN insert, but thankfully you had another one. I never tried running cbn slow, but some people swear by it(with the right insert geometry). The usual geometry for "fast" speeds looks very dull. Imagine a chamfer where a cutting edge should be. Is your insert like this? If yes, and you got good results with lower sfpm this is very interesting for me.

I think I saw Robin Renzetti using CBN slow, but the edge was sharp (I don't remember if he said he sharpens them himself.). When my current "dull" CBN stops cutting I intend to try sharpening it too.

Regarding your chipped CBN insert. Did it chip in a way the CBN plate could be brazed to something? If it did, I wonder if one could braze it onto steel, or is carbide backing necessary.

I'll have to look at the insert under some magnification to see what the edge looks like and report back. I bought inserts in 1/32, 1/64 and 1/128 nose radius and tried the 1/64" first so I could go up or down if I didn't like what I was seeing. The sharper nose radius might have helped.

The insert that chipped broke in a way that it took much of the top surface with it...the point broke and took a lot with it. Luckily I bought them in packs of four and they were about $20 per pack, so only a $5 loss give or take. The inserts were Pinnacle Tool branded, which seems to be the most common on eBay. For the price I'd say they are definitely worth having as an option!

 

[RaisedByWolves]

Just a suggestion, tighten all four screws. And a wild idea. Put a piece of kraft paper (brown butcher's paper) on both sides of the holder and tighten the bejesus out of it.

I was going to say emery cloth.

It be used emery cloth and clover compound to enhance grip in different situations and they both have their place.

 

[G-ManBart]

I was going to say emery cloth.

It be used emery cloth and clover compound to enhance grip in different situations and they both have their place.

The picture he was responding to was just the general setup so I didn't have everything tight. Ultimately, I don't think paper or emery cloth would have been enough to keep the bar from moving. I added a hold down to the back of the bar and it was rock solid.