Rockwell/Macinability

[Bob Quale]

At what rockwell hardness is it not possible to machine with HHS cutters?
Say I want to make a precision thread and want to heat treat the rod and then cut the threads. This eliminating the distortion of heat treating. What rockwell should I aim for. I don't want to harden the rod so much that I can't cut it with HSS cutters.
I see that MSC sells acme threaded rod. Am I safe to assume that I can machine it?
Thanks

[SGW]

MSC sells that pre-hardened 4142, which is 26-34 Rockwell C. I assume that the stuff is hardened about as much as is practical; if it could be harder and still be machined effectively, they would make it harder. Based on my experience with it, you can't hurry it any, but with sharp tools it machines to a beautiful finish -- I got a finish that looks ground.

I'd also guess that if you're not a commercial shop, "practical" doesn't have quite the same definition, so I'd guess that as long as you are willing to go slowly, you could get away with somewhat greater hardness...maybe 40 Rockwell C? My end mills didn't show any particular distress after dealing with the pre-hardened 4142, so clearly 26-34 is not the upper hardness limit for machinability.

Probably some of the other folks whith more experience than I have can give a more definite answer.

Oh -- the Acme rod. Yes, unless it says "hardened," which I don't think it does, it ought to be readily machinable.



[This message has been edited by SGW (edited 03-08-2002).]

[bspooh]

I agree with SGW...I have used HSS in Stuff that was 40-42 rockwell...It is ok to do maybe one part this hard, but usually one part will dull out the hss....

brent

[Rotate]

Can you machine harder material if you are using carbide tool, or is it just for speed?

Albert

[bspooh]

Yes, you can do harder material with Carbide...Their are many grades of Carbide..

All the grades have different characteristics...you have to use the right grade sometimes to achieve any satisfactory results...

Some grades of carbide are tough, but not too hard...while some other grades are hard, but not so tough....

Coating is everything with carbide...For example lets say we are using indexable carbide inserts...Uncoated inserts are great for aluminum, they will have a ground edge.(J-polish)

But the old technology for coating, is TiN..This is great for providing lubricity...

I have an example of what I am currently doing...The part I am machining is D2 tool steel...this is approximately in the 28 rockwell c range..Not very hard, but very tough to cut....I am using an insert that has 3 coating on it...

HSS doesnt cut this material worth a crap...and regular carbide doesn't work either...So I am using an Iscar cutter with Inserts designed for Tool steel...I am running at 400 sfpm..which equates to 1141 rpm and a feedrate of 26 in/min....beautiful results...

bottom line is that sometimes carbide is the only way to go...

brent

[Herb W]

Was comparing some cutting speed charts recently, and found that the recommended speeds for carbide were about double in Mach. Handbook (26th ed.) what they were in a Moltrecht book (20+ yrs old). Is this due to advancements in carbide technology during this time frame?

[Thrud]

Brent, Everyone

Have you used any CBN inserts? They use them in Europe to finish turn hardened crankshafts instead of grinding. (They rough the cranks with Silicon Nitride Wisker reinforced Cermet inserts). The only problem with CBN is the abnormal wear experienced with material softer than about 38-42 on the Rockwell C scale. No one knows why they wear so fast with softer materials (unless someone has figured it out lately).

The Screw could be hardened and then finished to size with CBN - having the threads precision ground ground would be cheaper - CBN inserts are big bucks ($100-200/each up here). A cermet insert might also do a one-off job but I would not count on it (also expensive).

A cheaper aternative might be to make the screw in A2 then have a shop harden it in a controlled atmosphere oven. Or case harden some free machining stock.

[bspooh]

Yes thrud, CBN is the second hardest substance known to man, right behing diamond...I do occasionally some hard turning and I think a good coated carbide works really well with about a sfpm of about 200 seems to work well with 30+ Rc to about 38 Rc....

brent

[Thrud]

Brent

I was not lecturing you (you sure are touchy for an olde farte), I was just mentioning it FYI. And, "next to Diamond in hardness" is misleading. Oh, sure - it is hard, but it is still about 10,000x softer than Diamond. And it ain't nearly as hard as my head...

Did you know that Diamond is an excellent conductor of heat and an insulator to boot? Did you know that one of the semiconductors of Diamond has been discoverd and they are now looking for the other one? If they find it, they could make intel CPU's with Diamond semiconductors on a diamond insulating base plated onto a coper heatsink - maybe then they could catch up to AMD in performance without burning up from overclocking (like intel has done before).

Geez, that felt good, I have not vented about intel for a long time (intel sucks). Dennis Miller would be proud of my rant!

[Thrud]

Brent

I was not lecturing you (you sure are touchy for an olde farte), I was just mentioning it FYI. And, "next to Diamond in hardness" is misleading. Oh, sure - it is hard, but it is still about 10,000x softer than Diamond. And it ain't nearly as hard as my head...

Did you know that Diamond is an excellent conductor of heat and an insulator to boot? Did you know that one of the semiconductors of Diamond has been discoverd and they are now looking for the other one? If they find it, they could make intel CPU's with Diamond semiconductors on a diamond insulating base plated onto a coper heatsink - maybe then they could catch up to AMD in performance without burning up from overclocking (like intel has done before).

Geez, that felt good, I have not vented about intel for a long time (intel sucks). Dennis Miller would be proud of my rant!

[bspooh]

geez thrud, you only had to tell me once...do I stutter or something??/

I know you werent lecturing me, but occasionally i need to be lectured...some of my information might be Bogus.....

All the stuff i said earlier about me being really ugly??

It is not true...I look just like Brad Pitt...

brent

[Thrud]

Brent
I heard you really do look like Brad Pitt - when he is bent over... (har har)

That's OK Brent, I sing like a parapalegic - can't carry a tune or hold a note!

[bspooh]

The only note I can carry is one written by my mother....

I love you Mom....

brent

[Ragarsed Raglan]

Let me tell you of an experiance I had.

I had a repair job to do on a cast iron fence off of a combination Planer/thicknesser for a woodworking buddy. I was asked to skim up the fence flat and square after it had been welded. The welder had gone and used hard facing rod for the repair (Colbolt/Manganese/Nickel alloy) - I used a flycutter in the mill using a lathe tool with a hard grade TCT tip ~ got to the weld and lit the place up with sparks! After a search thru my stock of tips I found a CBN tip from the 'sad stall' at the local J&L (paid about $5 for it). Cut thru the hard facing like butter ~ but blunted up on the C.Iron!!

Things one does for a buddy!