# Tips On Turning Hard Steel On Lathe With Cemented Carbide Toolbits



## Pmedic828 (May 6, 2015)

I was wondering if anyone can shed some light on turning to a shoulder using a cemented carbide tool bit.  The tool bit that I am using is an AR 1/2 size in a QCTP aligned with the center of a cylinder that is about 5/8 inch in diameter with a 3/8 inch by 1 inch extension.  the speed is at about 275 to 300 rpm and the feed is approx. 0.005.  Each pass I remove about .020.   I only let about 1 inch past the shoulder to stick out beyond the chuck.  I am using a 3 jaw chuck and need to turn an additional 0.500 off of the 5/8 rod.  This will increase the 3/8 portion from 1 inch to 1.500.  Not sure if my hardness of the carbide is C2 or C5 - I think it is C5.

The question I am having is as I approach the shoulder, should I stop the lathe and crank out the  saddle out and reposition to take an additional .020 or stop short then use something to square the shoulder.  I keep fracturing the tip of the cutter after turning about 3 of them.  If I try to align to the shoulder and feed in, the cutter doesn't want to advance easily.  How would you approach a shoulder maintaining speed and feed and get a square shoulder without fracturing the bit.  I have 50 of these pins to make so reducing the damage on the tool bit would be greatly appreciated and/or more reasonable cost.  Thanks.


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## JimDawson (May 6, 2015)

It really sounds like your tool doesn't have enough front clearance and side clearance, sounds like it's rubbing and not cutting.  Another trick that I do is to set the tool bit below center, as much as 0.125 below center, but it depends on the diameter, try it, it works!


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## randyc (May 6, 2015)

If I understand how you're doing this, I wonder why you don't extend the workpiece far enough so that you can completely turn the diameter to the desired length?

Is there some reason why you can't center drill the end and support it with a tailstock center ?  Why are you not running the spindle speed up to around 1500 RPM which would be more appropriate for carbide tooling ?

OK, sorry about the diversion so let's address the problem of breaking down the carbide cutter.  I hope that I'm getting it right that this is your main concern.

If your lathe is fairly small (like most HSM machines) a technique that I find useful is to turn off the motor as the cutter nears the shoulder but with the carriage feed still enabled, allowing the tool to "coast" nearly up to the shoulder.

At that point, rotate the spindle by hand (carriage feed still enabled) until .003 - .005 from the final shoulder dimension.  This will minimize stress on the cutting edge of the tool as it encounters the shoulder.  As mentioned previously, the lathe must be small enough so that the spindle can be rotated by hand.  Disengage the carriage feed and resume turning the workpiece.

Continue reducing the diameter until the finish dimension is obtained.  At that point, two choices are available - the first is to hand-rotate the spindle with carriage feed engaged to remove the last .005 of material of the shoulder.  Disengaging carriage feed, the shoulder can be faced by moving the cross-slide outward.

The other choice is to note the reading of the cross slide when the finish diameter has been turned, then retract the cutter.  Move the carriage the last .oo5, turn on the lathe and plunge the final .005 until the cross slide reaches the previously noted number.

There are a half-dozen other methods that would also work just fine.  You didn't specify the hardness of the workpiece material but if it is under about Rc 30, get rid of the carbide cutter and use HSS instead - no breakage problems


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## T Bredehoft (May 6, 2015)

In my humble opinion, cemented carbide tools are not satisfactory for serious machining. They once were all that was available, but soon were replaced with index-able inserts of varying shapes. Use a triangular insert with a small radius and undercut the shoulder to eliminate the radius.


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## JimDawson (May 6, 2015)

randyc said:


> There are a half-dozen other methods that would also work just fine. But the most obvious thought is to get rid of the carbide cutter and use HSS instead - presto, problems go away



+1


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## countryguy (May 6, 2015)

I myself. Would love a pic or two ...even a short you tube vid.  .  All sounds really neat.  The son and I are trying to get into the local lathe class this summer.   Time to turn!     Of course the first thing he asks me if  we can turn a rifle barrel....  Kids. Lol.    I see gunsmith in his future.  Haha.


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## randyc (May 6, 2015)

Pmedic828 said:


> ...If I try to align to the shoulder and feed in, the cutter doesn't want to advance easily...



Oops, I sort of skimmed over this part of the problem   Here's some good advice suggesting that you may want to re-configure the geometry of your cutter:



JimDawson said:


> ...It really sounds like your tool doesn't have enough front clearance and side clearance, sounds like it's rubbing and not cutting...



Forget about my suggestion of using a tailstock center, BTW.  There really shouldn't be a problem with 1.5 inches "stick-out" from the workholder with the DOC you are taking.  You haven't yet told us the hardness of the material which influences everything.

If the material is similar to HRS or CRS, a sharp tool and a sturdy small lathe would gnaw off 5/8  diameter to 3/8 diameter in one roughing cut + a small finish cut.  (Wouldn't be at all unusual to take it all off in one pass, depending on material hardness.)


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## Pmedic828 (May 6, 2015)

Thanks to all who replied - Maybe I did not make myself clear enough - I have a pin that is 6 inches by 5/8 - one side has a 30 degree point and the other has a 1 inch X 3/8 "tenon" that I need to make a little longer - it is now 1 inch and needs to be 1.500 inches long - I have the 5/8 end of the pin out a little over 2 inches as 1 inch is not being turned and only the next 0.5 inches next to the chuck will be turned down from 5/8 to 3/8.  the intersection of where the 3/8 meets the 5/8th rod has a square shoulder.  I don't want to run the cutter into the shoulder so I stop short about .015.  As I attempt to feed the cutter from the large diameter to the small at the shoulder, the corner of the carbide fractures - there is enough clearance both on the shoulder and on the cylinder.  I just don't know how to clean out the shoulder without either advancing the carriage which the cutter won't cut or running the cutter along the axis and hitting the shoulder.  hope this makes it clearer.  I don't want to turn down the whole pin.  Hardness from what I am told by person that purchased the steel  400 - 450 Bhn  Maybe I am not turning fast enough but I don't want to crash the cutter into the shoulder - the chips are curling ok and are not blue, but slightly brownish.  I haven't tried HSS because I believe these pins are too hard - maybe wrong though - that is why I am posting this question.  Thanks


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## randyc (May 6, 2015)

But one of the most important questions hasn't been answered yet:  _hardness of the workpiece_.  IMO your helpful, detailed description doesn't substantially alter the advice already offered


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## T Bredehoft (May 6, 2015)

Burnell hardness of 400 - 450 Bhn equals approximately 45 Rockwell C., probably harder than HSS can easily handle.


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## randyc (May 6, 2015)

Oops, OP provided hardness information while I was fooling around somewhere else.  Not HSS territory at all - need good carbide insert tooling, as Tom mentioned previously, rather than the $.90 brazed carbide throw-aways.


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## JimDawson (May 6, 2015)

When I'm cutting to a shoulder like that I normally grind nose of the tool to an 80 to 85 degree angle, they normally come out of the box at 90 degrees and while I'm at it I normally add some front and side clearance angle.  It has also been my experience that the cheap AR bits that I buy won't even cut out of the box they have to be ground to be useful at all.


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## Tony Wells (May 7, 2015)

I've always referred to those as "tool blanks". It is true that they might cut as purchased, but nearly every case proves some grinding and or honing improves things immensely.

For what you are doing, my tool of choice would be a 80° diamond inserted tool. The most common holder gives you a -5° lead angle while still providing 5° clearance on the trailing edge. That tool will present a robust cutting edge but retain front clearance for facing. I use that profile more than any other for general purpose turning, up to and including a shoulder. Plus I have a inserted milling cutter that uses the 100° corners, so I get more use from that style insert than most. A turning tool is available to use that corner also, but it has a low lead angle and cannot turn up to a square shoulder. It's great for roughing, especially if you can run off the end of the part.

Generally speaking, I dislike triangular inserts, except for threading, either laydown, or on-edge.


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## mikey (May 7, 2015)

I would use a parting tool to cut the diameter at the shoulder down to the desired OD, then turn the rest down from there. A carbide parting tool would be best but I bet a sharp cobalt cutter will do it without any problems.


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## Wreck™Wreck (May 7, 2015)

T Bredehoft said:


> In my humble opinion, cemented carbide tools are not satisfactory for serious machining. They once were all that was available, but soon were replaced with index-able inserts of varying shapes. Use a triangular insert with a small radius and undercut the shoulder to eliminate the radius.


All carbide tools are sintered (or cemented, grains of carbide bound together by a third material under heat and pressure in a mold.), inserts, drills, endmills and the like. I believe that you are confusing cemented with brazed tools, which is sintered carbide brazed to steel tool shanks.
See here http://todaysmachiningworld.com/magazine/how-it-works-making-tungsten-carbide-cutting-tools/


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## T Bredehoft (May 7, 2015)

Yes, I was confusing cementing and brazing.  Its brazed carbide tooling Henry Ford used when he first machined his V8 engines in 1932.


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## Wreck™Wreck (May 7, 2015)

I also dislike brazed carbide tooling, have never found it to work well without careful preperation yet comes in handy when you need to grind a form tool for use on a manual lathe.


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## epanzella (May 7, 2015)

As I understand it, you have a 5/8 shaft with the last inch dropping to 3/8 forming a shoulder. You want to push the shoulder back .500" for a total length of the 3/8" portion of 1.5". I would set a carriage stop .005 short of where you want the shoulder. Then machine .020 at a time by hand feeding. Spindle should be at least 1000 rpm. When you get to the last cut (.020 or less) readjust the carriage stop to get the last .005 at the shoulder. Feed the last cut in by hand and when you hit the stop back the tool out towards the OD with the cross feed cleaning up the shoulder on the way out. While backing out the cross slide with your right hand, keep the carriage hard against the stop with your left hand on the carriage wheel.


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## mattthemuppet2 (May 8, 2015)

if it's one of those 90deg brazed carbide bits, most likely what's happening is that as you hit the shoulder your cutter is jammed into the corner. Instead of cutting on one edge, it digs in and cuts on both, springs back and chips the tip. Either use a non-90deg cutter or angle the cutter so that there's some trailing edge relief, then use a parting tool or LH 80deg cutter to "face" off that shoulder.


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## Jester966 (May 8, 2015)

You are leaving too much on the shoulder - leave .005" max.  After your finish pass on the 3/8 diameter, manually move the carriage to the finished shoulder position, lock the carriage then clean up the shoulder by manually feeding the cutter out of the part.


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## Pmedic828 (May 8, 2015)

Thanks to all - I will look into a 80° diamond inserted tool.  Any suggestions for someone who is on a limited budget and a small 2 hp lathe.  I managed to cut down 6 of these monsters but the person that I was turning them for did not want to pay more than $2 each to have them turned down so I didn't take the job!


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## benmychree (May 13, 2015)

T Bredehoft said:


> In my humble opinion, cemented carbide tools are not satisfactory for serious machining. They once were all that was available, but soon were replaced with index-able inserts of varying shapes. Use a triangular insert with a small radius and undercut the shoulder to eliminate the radius.


I think that the term "cemented" carbide is an archaic term that refers to the method of manufacture of the carbit bit only, not the method of holding them to the shank; when referring to tools such as the writer refers to the correct term is "brazed tools"  In that sense these would be cemented carbide blanks brazed onto steel shanks.  I quite agree that insert tools are far better and more convenient than brazed tools, which possess perhaps greater durability in the case of interrupted cuts than insert tools.  The shop where I apprenticed back in the early '60s used lots of brazed on tools, up to 1" square shank size, and they would hold up to interrupted cuts much better than insert tools, but lots of time wasted in re sharpening them.  I think most beginners should learn to grind HSS tools and learn to use them before graduating to carbide for those jobs that require their use, such as hard and abrasive metals.


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