"Reverse Boring" (Horizontal Mill) Sanity Check Requested

[G-ManBart]

Your toolholder is somewhat off center in the vise. Typically that will put more pressure toward the center of the vice. I'd put parallels under the tool holder, and/or put something the same thickness (such as another toolholder) on the other side of the vice.

Good call on having another toolholder on the other side to even out the vise pressure. I had another identical holder and set it in place and tightened the vise...seemed pretty secure and locked down tight.

 

[Flynth]

Have you determined that the end mill adaptor is machinable? Generally, they apr pretty hard.

I'm not certain, but looking down the bore it appears to show evidence of turning past where the end mill would be (I assume they grind that area for exact dimension). I figure I'll find out quickly if it can't be machined.

I would hit it with a file to see if it slides off or bites. I would hate to run an insert just to find out the metal is hard.

One tip I have is that even if it is hard, but not super hard (let's say 35hrc up to 45hrc) it should be machinable with carbide, but to do it you need to lower sfpm well down. As if you were cutting alloy steel with hss(up to 15m/min or 45sfpm) and take light passes - up to 20 thou. Another method is to speed it up to ~90m/min (270sfpm) and take really light passes (few thou). This method turns hardened metal into "string of fire" same as when machining with cbn. You need coated carbid and it will not last long, but it is doable. I would go slow instead.

Also if it is hard and you go slow the forces on the tool will be a lot higher than with mild steel.

 

[G-ManBart]

I would hit it with a file to see if it slides off or bites. I would hate to run an insert just to find out the metal is hard.

One tip I have is that even if it is hard, but not super hard (let's say 35hrc up to 45hrc) it should be machinable with carbide, but to do it you need to lower sfpm well down. As if you were cutting alloy steel with hss(up to 15m/min or 45sfpm) and take light passes - up to 20 thou. Another method is to speed it up to ~90m/min (270sfpm) and take really light passes (few thou). This method turns hardened metal into "string of fire" same as when machining with cbn. You need coated carbid and it will not last long, but it is doable. I would go slow instead.

Also if it is hard and you go slow the forces on the tool will be a lot higher than with mild steel.

It certainly won't hurt to hit it with a file, so I'll do that.

Slow has been my plan since it's an old horizontal with a lot of low speed range options and I really don't care how long it takes. If my memory is correct there is a 135rpm setting and with the .875" bore that would put me right around 30SFM.

Everyone's comments have been very helpful, and that's why I figured I would post a few days before taking a cut!

 

[pontiac428]

So, have you had a chance to try it yet? I'm waiting to see if you go for it with the insert (and post pics) or if you change your mind and decide to get at it with a stone using some form of rotary grinder in that vise instead.

One thing I know about Germans, they are on a hundred fifty year tradition of world-leading steel metallurgy, they will not let a piece of tooling leave the factory if any part of it is still in the annealed state.

 

[G-ManBart]

So, have you had a chance to try it yet? I'm waiting to see if you go for it with the insert (and post pics) or if you change your mind and decide to get at it with a stone using some form of rotary grinder in that vise instead.

One thing I know about Germans, they are on a hundred fifty year tradition of world-leading steel metallurgy, they will not let a piece of tooling leave the factory if any part of it is still in the annealed state.

I did try it a bit today. I first tried my basic setup from above with the addition of another tool holder at the back to keep the vise jaws parallel. It cut, but the boring bar did very slightly move nose down after a few passes so I made adjustments. I added parallels under the holder with the boring bar, went up one size in boring bar since I had opened the bore just enough to allow that, and then made a hold down for the back of the boring bar. That all seemed to lock things down. I also switched to a CCMT 32.51 insert since the cuts are light.

The holder is definitely hard. I kept the RPM low (93RPM) and a slow feed rate of around 3/4" per minute. It's cutting, but I'm getting some taper to the bore (smaller as I go in) and I'm seeing the boring bar deflect rhythmically sideways through one section. I ran multiple passes through that area without changing the table position and it got better, but not completely....almost like it's a hard spot.

I'm going to have to confirm the insert is on center, see how much I can reduce the stick out and start trying slight variations. I almost think the DOC is a bit too shallow as the chips are very fine and mix with the cutting oil to make a slurry.

I'm pretty sure the end mill holder is a U.S. product, but the arbor is definitely German. A file did bite into the end mill holder, but not much. I'm worried the arbor is harder than the end mill holder!

 

[Flynth]

The holder is definitely hard. I kept the RPM low (93RPM) and a slow feed rate of around 3/4" per minute. It's cutting, but I'm getting some taper to the bore (smaller as I go in) and I'm seeing the boring bar deflect rhythmically sideways through one section.

Yes, that's what turning hard steel slow with carbide does if the setup is not rigid enough. In your case the bar is the limiting factor most likely.

It sounds like prime candidate for cheap Chinese CBN inserts, but in their absence you do need a fairly sharp, coated carbide insert for it to work well with very light passes and slow cuts. The insert you had in the first picture looks like the one that worked well for me.

If slow cutting deflects even at low DOC (I don't think you mentioned the DOC other than it being light). If it is under 20 thou and it is still deflecting I would try another insert (as you did). If the insert is capable of taking such a small cut I would try 10 thou. If that deflected too I would sacrifice an insert for a fast cutting test. I suppose you absolutely need no taper in this holder as much as possible or the tool will wobble badly.

First a little disclaimer. This "fast hard cutting method" is discovered accidentally is definitely not the correct way to use carbide. Who knows, peroI have freak inserts that it works with, but will not with any others. So please be aware there is high chance of failure. Possibly even more so than with slow hard turning with ordinary steel carbide.

So for fast hard cutting with carbide you need a geometry that's opposite to slow cutting. A negative rake is OK, it doesn't need to be sharp much (but not chipped). Definitely needs at least yellow coating (as yours do). I probably would try the same insert first. For a roughly 1in bore you need at least 1000 rpm. I would start by taking a 1~2 thou cut, maybe increasing it to at most 10 thou if it worked well. The feed rate 2 thou per rev if possible.

I have a theory why it sometimes works. I believe the coating is doing all of the work and carbide beneath is just providing rigidity. You need crazy rigidity for it to work. That's why I used 1~2 thou cuts. Instead of being cut the hardened material rubs on the tip and heats up to yellow heat becoming soft, then it is just being pushed away just like with CBN. I don't know what kind of coating my inserts have (I have to check). I beleit is the "standard yellow one" you find on normal steel inserts so likely TiN. Just for comparison. Single crystal CBN has up to 5000hv hardness. The TiN coating gas half of it (2500~2600hv). Carbide typically used in inserts up to 1800hv. To bore that hole with cbn I would start with 1900rpm. I'm not sure if it works that way that half sfpm equals half as hard cutting material, but it is somewhat neat it just happened to work like this for me.

For sake of completeness I should mention there are also special carbide grades formulated for hard turning. I tried two Iscar ic908 and Mitsubishi vp15tf. I had no luck with both, but the material I was cutting was very hard cro-moly steel. Perhaps with low alloy carbon steel it would work fine? Who knows. In my use they chipped a lot sooner than my cheap steel insert. Also those "hard turning" inserts appear not to have any coatings.... So I can't understand how they are supposed to work. I did run them at prescribed feeds and speeds.

There are also ceramic inserts (sometimes called cermet) which many people have success with. They are very brittle, but for a continuous cut that should be fine. A youtube channel I watch (cutting edge engineering) uses them for hard turning with great success. He uses Ab30 grade from Ingersoll.

If everything fails (tapered bore etc) perhaps you can buy a single cbn (if your mill spins that fast) or that Ingersoll ceramic insert to avoid scraping the holder.

Coming back to ordinary steel carbide there is a lot of variability between simingly similar geometry inserts when hard turning. I had one last for 15min and another for 1 minute. The difference is just a chipbreaker(the good one had a simple one like on your first photo, the bad was a fancy one) . It might have been just a coincidence too.

I ran multiple passes through that area without changing the table position and it got better, but not completely....almost like it's a hard spot.

I'm going to have to confirm the insert is on center, see how much I can reduce the stick out and start trying slight variations. I almost think the DOC is a bit too shallow as the chips are very fine and mix with the cutting oil to make a slurry.

I'm pretty sure the end mill holder is a U.S. product, but the arbor is definitely German. A file did bite into the end mill holder, but not much. I'm worried the arbor is harder than the end mill holder!

Hopefully it is not very hard. If the file bites that may be anything from 35hrc to who knows how much (depending on the file, how hard you press etc).

 

[mattthemuppet2]

If you're getting taper that spring passes won't get rid of, try a ccgt insert and light cuts. It'll take you longer but those inserts work just fine on hard steels

 

[G-ManBart]

Yes, that's what turning hard steel slow with carbide does if the setup is not rigid enough. In your case the bar is the limiting factor most likely.

It sounds like prime candidate for cheap Chinese CBN inserts, but in their absence you do need a fairly sharp, coated carbide insert for it to work well with very light passes and slow cuts. The insert you had in the first picture looks like the one that worked well for me.

If slow cutting deflects even at low DOC (I don't think you mentioned the DOC other than it being light). If it is under 20 thou and it is still deflecting I would try another insert (as you did). If the insert is capable of taking such a small cut I would try 10 thou. If that deflected too I would sacrifice an insert for a fast cutting test. I suppose you absolutely need no taper in this holder as much as possible or the tool will wobble badly.

First a little disclaimer. This "fast hard cutting method" is discovered accidentally is definitely not the correct way to use carbide. Who knows, peroI have freak inserts that it works with, but will not with any others. So please be aware there is high chance of failure. Possibly even more so than with slow hard turning with ordinary steel carbide.

So for fast hard cutting with carbide you need a geometry that's opposite to slow cutting. A negative rake is OK, it doesn't need to be sharp much (but not chipped). Definitely needs at least yellow coating (as yours do). I probably would try the same insert first. For a roughly 1in bore you need at least 1000 rpm. I would start by taking a 1~2 thou cut, maybe increasing it to at most 10 thou if it worked well. The feed rate 2 thou per rev if possible.

I have a theory why it sometimes works. I believe the coating is doing all of the work and carbide beneath is just providing rigidity. You need crazy rigidity for it to work. That's why I used 1~2 thou cuts. Instead of being cut the hardened material rubs on the tip and heats up to yellow heat becoming soft, then it is just being pushed away just like with CBN. I don't know what kind of coating my inserts have (I have to check). I beleit is the "standard yellow one" you find on normal steel inserts so likely TiN. Just for comparison. Single crystal CBN has up to 5000hv hardness. The TiN coating gas half of it (2500~2600hv). Carbide typically used in inserts up to 1800hv. To bore that hole with cbn I would start with 1900rpm. I'm not sure if it works that way that half sfpm equals half as hard cutting material, but it is somewhat neat it just happened to work like this for me.

For sake of completeness I should mention there are also special carbide grades formulated for hard turning. I tried two Iscar ic908 and Mitsubishi vp15tf. I had no luck with both, but the material I was cutting was very hard cro-moly steel. Perhaps with low alloy carbon steel it would work fine? Who knows. In my use they chipped a lot sooner than my cheap steel insert. Also those "hard turning" inserts appear not to have any coatings.... So I can't understand how they are supposed to work. I did run them at prescribed feeds and speeds.

There are also ceramic inserts (sometimes called cermet) which many people have success with. They are very brittle, but for a continuous cut that should be fine. A youtube channel I watch (cutting edge engineering) uses them for hard turning with great success. He uses Ab30 grade from Ingersoll.

If everything fails (tapered bore etc) perhaps you can buy a single cbn (if your mill spins that fast) or that Ingersoll ceramic insert to avoid scraping the holder.

Coming back to ordinary steel carbide there is a lot of variability between simingly similar geometry inserts when hard turning. I had one last for 15min and another for 1 minute. The difference is just a chipbreaker(the good one had a simple one like on your first photo, the bad was a fancy one) . It might have been just a coincidence too.




Hopefully it is not very hard. If the file bites that may be anything from 35hrc to who knows how much (depending on the file, how hard you press etc).

All good stuff...thanks!

For the most part the cuts were around .010" after the first couple to get things cleaned up.

I'm pretty sure you're right that the bar is the limiting fact right now. I'm going to try a couple of things. First, I know I can move things around and probably shorten the stick out an inch or so. It's a 3/4" bar and I only need to go about 1.8" deep, so I'm going to shorten it up as much as reasonably possible. Second, I've read that some folks have had good success using CCGT/CCGX inserts meant for aluminum when turning/boring hard steel. I have some in 32.50 profile and the sharper 1/128 radius tip might help.

I've been considering adding solid carbide boring bars to the arsenal and been looking at the inexpensive ones that ship direct from China. I'm pretty sure I can find a source with both the bar and CBN inserts as well. Those shipments might take a while, but I'm not in a rush.

I've also thought about annealing the nose of the holder. I've got some blacksmithing experience hardening and annealing tool/alloy steels. I could submerge the taper portion in oil and flame heat the nose probably enough to soften it a touch. I'm not overly worried about dimensional changes since I'm still going to bore it in place and that should clean things up.

I'm pretty sure the arbor is harder than the end mill holder based upon the file test....more reason to get some CBN inserts.

 

[G-ManBart]

If you're getting taper that spring passes won't get rid of, try a ccgt insert and light cuts. It'll take you longer but those inserts work just fine on hard steels

Funny, I was just typing that I'm going to try 32.50 CCGT/CCGX inserts and reducing the stick out next. I don't remember who recommended them to me first (may have been you actually) but they're handy to have. I was actually turning some 7075 aluminum to make an outboard spindle spider for my Sheldon over the weekend and was shocked at the finish I was getting.

 

[Mill Lee farm]

Is the arbor Morse or Brown & Sharpe? You can tell with a couple quick measurements

Dimensions of Standard Tapers

This page give the dimensions of Morse, Jacobs, Brown & Sharpe, and Jarno tapers.

littlemachineshop.com

Is an NMTB 50 to 'whatever the arbor is' an option? Seems much easier than trying to turn down that hardened German arbor.

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