I've got some old thread roller dies that I need to regrind. I'm not rolling threads, but simply rolling a .050R groove and forming a chamfer on the end of some 1008 mild steel pins that have been cold headed.
I have no idea what the material is, but it is definitely hardened. I presume it's A2 or D2 or something similar, but some die shops do make them out of M4.
Right now I'm simply trying to square up/flatten the die faces on my surface grinder, before grinding in the raised groove. I'm using a 46 grit H hardness white grinding wheel (AA46-H8V40) is what the wheel says. I also have a finer Norton 38A80 that I tried. Neither wheel wants to cut worth a darn. I'm getting black lines (burn marks) in the part even with very light cuts, .0002", and on a freshly dressed wheel. I'm also using flood coolant. It also appears that there is not that many sparks when grinding. Further reinforcing the feeling that the wheel is pushing off and not cutting.
It's as if the wheel is loading up immediately. I'm totally stumped. I've ground heat treated A2, D2 and S7, mild steels, stainless etc... before without any problems at all. The material these dies are made out of is killing me though.
Any recommendations on a grinding wheel that will ease my pains (grit, coarseness, hardness etc...) Thanks for any help.
Norton 5SG You can get from MSC. Great Wheels. You'll have to decide on grit/hardness.
Since you mention "not getting many sparks", I suspect the rolls may be made of a Tungsten tool steel like T-5 which is a bear to grind at best.
A suggestion; try dressing the AA46 wheel with a fairly rapid traverse across the wheel and don't make a dead pass back across the wheel.Use a .0005" downfeed for the last pass. Then carefully pick up the contact with the part and use .0002"to.0004" depth of grind and .050" per pass, crossfeed. Try this before you invest in an "SG" wheel as they are relatively expensive,although VERY good as
A good , simple rule of thumb for wheel selection is the following: soft work,hard wheel;hard work,soft wheel. This is very general but the logic is this: a freshly dressed hard wheel will retain it's shape and sharpness longer on soft material, but a soft wheel, while losing shape will break down quicker, thus exposing fresh, sharp grains, avoiding "burning"the hard part.
Just my 2 cents worth.
Ive tried all kinds of wheels on the "kryptonite" tool steels. Including borazon and borazon/diamond mix. IMHO you are using the right wheel. You might find one that will make a 5% difference,big deal not worth the trouble. The stuff just dulls the wheel right away. The only thing that I have found that works is if I want to take off say .010 from a surface is to touch it with a sharp cornered wheel and drop the wheel .0085 over to the side of the peice. Then feed the table in small amounts (.003) at a time with the table reciprocating. Plunge and drag is what I think they call it. Its loud and messy,tears the wheel up and just not my style of grinding but,seems to get the stock off.
Sounds like you have some nasty stuff.
We grind a lot of CPM 15V, 9V, and 420V and use a CBN wheel exlusively. Doubt a different grit wheel would make a big difference. Some stuff is just not meant to be ground with a grit wheel. Not to say it can't be done though.
Make sure you are putting a really coarse dress on the wheel. I would say if it takes more than two seconds to run the dresser across the wheel it's too slow.
Let us know what works.
You can hunt around for the optimum wheel for the stuff you're grinding but wile you're hunting the job is going stale and the customer isn't getting any happier. Chances are the wheel you have may not be perfect but trust me; they are adequate for the work you have given a small order.
The no-sparks lead me to believe the rolls might be one of the more bullet proof HS steels. If that's the case (I hate it when sharpenable tooling doesn't have a permenent material ID on it) you have to dress and dress the wheel to ensure the as-ground surface is free of heat checks and metalllurgical damage.
I've ground D-2 shear knives where I had to re-dress the wheel every 4 to 6 table strokes. It's plain a bi#ch to grind and the price quoted reflected the time and wheel wear. If the machine has a tunnel dresser you can save some time. Dressing from a block at the end of the table can go quick too if you get proficient at the post dress work pick-up.
Thanks for all the information people. I'm more experienced with machining than grinding, so I thought I would see what others had to say. That's the thing about metalworking, as soon as you think you know something, you find out you don't know sh*t [img]smile.gif[/img]