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View Full Version : How to stop edge chipping when turning hard steel



DJones
05-14-2009, 10:12 AM
Guys,

I am trueing some large cast steel wheels that carbide will just cut. When the tool enters or leaves the cut I get serious edge chipping with large flakes flying off and pitting to a depth of .125 in some cases. Any ideas on how to prevent the chipping?

McNeillMachine
05-14-2009, 10:31 AM
What are the cutting conditions, SFM, DOC? What grade of carbide are you using?

Evan
05-14-2009, 12:12 PM
Sounds like a problem with the material, not the tool. If it's that hard that carbide barely cuts it and it's spalling off flakes then it has too much carbon near the surface of the casting.

I doubt there is an easy answer but you might try going a lot slower if you haven't already. If it is a problem of carbon being drawn out of the melt during solidification then the answer is to normalize the casting by heat treatment.

Forrest Addy
05-14-2009, 12:36 PM
I don't know the size of the work or the machine your working with. but I havve offer some general hints.The tricks for hard turning are fairly sumple but some may seem counter-intuitive.

Naturally the machina and tooling have to be up to the job with overhang minimized and support maximized.

Use negative rake tooling with an edge radius of about 1/8 the feed rate. That's edge radius not nose radius. Use at least 30 degrees of edge lead angle. Use an old insert to "strop" a radius on the edge of a fresh insert. Those silocon carbide pocket stones are what's reccommended but stropping one edge with another works for me and it doesn;t shower abrasive all over.

The l30 degree ead provides a gradual tool engagement in the cut.

Nose radius has an effect too. Use a radius at least 6 ytimes the feed rate. If the tool fails regardless of the nose raius go to round button inserts

The carbide insert should be thick as practicable to withstand crushing effect.

Use a relatively heavy feed. If the machine has limited HP, reduce depth of cut. If going slow doesnt seem to work go fast.

Don't let the tool dwell in the cut. Either make chips or back the tool away.

Try a ceramic insert. These historically are delicate and won't take the heavy feeds of carbide but modern developments may have improved them. I've had good luck wityh Kyrocera's line of high hardness inserts.

You may have to screw around with cutting parameters until you find the "recipe". I used to turn locomotive drivers having migh maganese steel tires. These were real tool eaters but a modified ceramic insert worksed quite well. I got several cuts from each corner and a hell of a lot of almost impossible to break ribbon chips. .

DJones
05-14-2009, 12:38 PM
What are the cutting conditions, SFM, DOC? What grade of carbide are you using?

I calculated the SFM at 23.8, depth of cut is .030 as it requires too much force to go any deeper and caused the chuck to slip. I have guess at the grade of carbide as C-6 TIN coated as it's not my shop or tools. I should add that these are 26" diameter railroad heavy boxcar wheel sets. The lathe is a Lodge & Shipley 32" and as you might guess is a bit worn and springy. I can't turn any slower as I'm already in low speed, first gear and back gear.

lane
05-14-2009, 03:26 PM
An old machinist trick,is to put a 45 chamfer on the exit edge when cutting something like that ether mill,lathe, are shaper. Make 45 at least as deep as cut . Keeps edge from chipping off.

DJones
05-14-2009, 04:51 PM
An old machinist trick,is to put a 45 chamfer on the exit edge when cutting something like that ether mill,lathe, are shaper. Make 45 at least as deep as cut . Keeps edge from chipping off.


Been there tried that. It still spits schrapnel.

Evan
05-14-2009, 07:39 PM
Try preheating the casting to about 250 F before machining. Many cast steel alloys have an extended glass transition temperature range. In some cases the charpy impact strength at 212F can be as much as 130% greater and the hardness 20 % lower at that temperature. With that large a workpiece heating with a steam cleaner before mounting it up for machining should work as it will retain heat for a long time.

Bet it works.

airsmith282
05-14-2009, 07:54 PM
what about taking lighter cuts, sometimes when iam trying to turn somthing if i take to deep of a cut i get problems like this and so i take lighter cuts and seems to go alot better..just bcause you can take deep cuts dont mean you should in fact it is dangerours to take to much at once all kinds of bad things can go wronge.. i have lately been learing how to part off SS at a much faster rate at 230 rpm and so far so good but its still touch and go at the best of times ,, any how my 2 cents ,,

Patch
05-14-2009, 11:14 PM
Actually, there are 2 methods used to perform such restoration/repair.