Tips On Turning Hard Steel On Lathe With Cemented Carbide Toolbits

[Pmedic828]

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.

 

[JimDawson]

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!

 

[randyc]

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

 

[T Bredehoft]

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.

 

[JimDawson]

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

 

[countryguy]

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.

 

[randyc]

...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:

...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.)

 

[Pmedic828]

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

 

[randyc]

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

 

[T Bredehoft]

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