I will be machining a fair amount of S-7 this summer and am a little concerned about it. I have had components machined for me from S-7, and the machinists commented that it was "different", "difficult", and "gummy". Another stated that he had struggled with it for some time before he learned "the tricks".

So, I thought I'd ask about it here. Is there a specific tool geometry, cutting speed, chip thickness, or coolant chemistry involved to get good results with this stuff?

I'd ask the guys who cut it for me before, but that was, unfortunately, long ago and far away.

Thanks for any kinks.

RG

Unless you're into production machining you'll do fine with almost any tool...but clearly carbides will benifit you....

With Hi-Speed steel keep the surfacefootage to about 40 and feed to suit your power available

With carbide try to keep surfacefeet to about 300 or so.....if the tol life isn't good try faster rather than slower....and again feed rates at a minimum of .003 per rev!

S-7, D-2, A-2, O-1, and other "tool steels" are normally alloyed with some nickel and chrome and the chrome is a very abrsive molecule.....tool life is never good but can be extended with reasonable feeds and speeds....

The strength of your lathe will also be a big determiner of the speed you can cut.

Thank you very much, Vapodog.

This gives me solid starting points, and I'll proceed with a little more confidence.

What I'm mostly worried about is a large form cutter, to run in the horizontal mill. It's HSS and with about 3.0 to 3.5% chrome, it's sure to wear. As luck would have it, there's some folks in nearby Prescott that will sharpen it for me overnight, so I'm good to go.

Thanks again.

RG

Comparing S7 D2 O1 and A2 is like apples to rocks to cars to space shuttles.
Yes S7 is a tool steel but it is by far one of the easiest materials to machine thats not a free machining alloy of the tool steel variety.

You can cut S7 just as fast as mild steel just use a good cutting oil. Turning and drilling are relatively easy just use sharp tools as it will work harden if you you try to be easy with it.
As for your form tool They don't put chrome in HSS Do you mean Cobalt?? by any chance.
That tool if it Cobalt will last nearly as long as carbide. And once again a good carbide tool is best C6 or better preferably coated Tian or Tin.
Keep in mind that when using your form tool it not really the diameter that maters as much as how much surface to tool is making contact with the steel. Form tools are a pain and should be run at reaming speeds. Slow with good depth of cut.

Thank you for the good advice, KC.

My reply to Vapodog was poorly worded. I meant to say that the S-7 formulations I've been looking at contain from 3% to 3.5% chrome, enough to cause wear. For instance, Crucible S-7 XL, a premium grade, comes in at 3.25%. The cutter itself is straight HSS.

Your suggestion to go slow on the speed, but with a good depth of cut reminds me of machining stainless, and I'm wondering if a stainless-specific cutting fluid would be of any benefit. I have been using a general-purpose soluble oil.

One of the parts I had made in S-7 required the reaming of some holes, and while they gaged OK, the surface of the bores showed evidence of tearing, which I put down to cutting oil selection.

Would you have any recommendations as to cutting fluid?

Thanks again.

RG

S-7 is a high quality steel used in injection molds.....What are you building with it?

Bacon grease for tough materials.
Other then that relton tap fluid works great just a dab

Believe it or not the stuff works great. As for the surface finish of the reamed hole. That was typical of S7.
If a better finish is desirable try reaming a few thousandths undersize then lap to finish dimension.
When we had to make parts that required a nice finish. You had to use a machine you can trust and on the last pass take no less the .025" depth of cut at a good feed rate like .006-.008" per rev on a lathe. As for the mill. Carbide cutters and blue chips is the only way to get a nice surface finish. High RPMS.
All of this within reason of course.
Examples
1/2"-3/4" carbide endmill .030-.050" depth of cut 1200-1700 RPM. med feed rates
Fly cutters with HSS tool 1200 RPM .030
Low to med feed rates.

Is the S-7 hardened ?. I know it's a tool steel which can be hardened as it's kind of common in drill

rod and such . I was just curious .

Doc,
I doubt he is cutting hard S7.....It is hard milled,ground,turned and edm'd every day in mold shops around the word though. Usualy 52/54 C scale.

Thanks I was curious ; As I've read some outfits that make actions use hardened 4130 Steel .

They harden it or buy it that way in rectangles . Then machine the action from it rather than heat treat

after the fact . I think it was Surgeons ??. Of course not everybody does it that way .

I believe that's what I read . I'm not a machinist I'm more of a composites epoxy person .

injection moldmaker is kinda what I am...or what I do...something like that

To each there own but the shop I worked for never built molds out of S7. not even inserts.
S7 is a common drill rod. Easily heat treated with basic tools i.e. no vacuum furnace required. It just has a shock resistant quality add to it. But still a common material.
Our material of choice is P20 pre-hardened to Rc 35-40. P20 is a modified 4140 P= polishing steel. finer grain structure and better polishing quality. and a royal pain to machine. Carbide tools and lots of coolant.

And just because S7 can be hardened to Rc 54 doesn't mean it is. You can draw it back to what ever hardness you want.

Doc
Yes you can order a piece of pre- hardened 4130 specify the hardness you want and machine your action out of it. It's done that way in a lot of shops. People can buy the machines and CNC software fairly easy but setting up to heat treat your parts is all together different. So most shops just send it to a heat treat shop or order it pre hardened.

Ordering pre hardened material also has another advantage. If machined properly the part will not require any secondary machining.
Most of the time when you machine a part then heat treat depending on the material depends on how much extra to leave to grind or machine off later due to growth or shrinkage, then you run into warping too.

Machining pre hardened material is a pain but at the hardness we deal with in molds and rifle actions it is far better to machine it after heat treat then before.

Thankd for the interest, gents. Picking up a lot of good info here.

I chose S-7 for it's combination of mechanical properties and the fact that it experiences no volume change when heat-treated, meaning that no distortion occurs during the process. Heat treat is also simple- bring it to about 1750F and let it cool, followed by tempering. If drawn at 400F, we get a hardness of about Rc 57, and at that hardness level, the Charpy notch test comes in at about 125 ft. lbs. This is very good toughness at that hardness level.
It is used for parts subjected to heavy impact loading, but also for shear blades.

I particularly like the fact that no post-heat treat second ops are needed. What you put in the furnace is what you get out.

I will be cutting it in the dead soft annealed condition.

And yes, it's for a rifle action.

I have flood coolant set-up on my mill, and wonder what would be the best type to use with with this S-7.

Thanks again for all the commenmts. This is a big help.

RG

Plain old moose milk works fine i.e. Soluble oil. Noting special. If needed you could get the EP grade of soluble oil But in the annealed condition I wouldn't worry about it. The stuff actually machines very well.
But be advised just because it says "no dimensional change" doesn't mean it won't warp.
Shrinkage is minimal but it could still curl up on you. Thin sections need to be brought up to temp slow. as in over six plus hours. If you have the time and material make two and heat treat one first then make adjustments as needed. And if it come out ok first shot then you have two actions.

As for material selection Well it depends on the action design. You could make an action out of aluminum if designed correctly. AR15 and the Thompson SMG have Aluminum actions. A bolt action needs to be made from a high tensile material. Personally I'd use 4140 or 4340 for the action. Just because that is what is used in industry and I have a lot of experience with it.

I made 5 actions with 4140-50 great to machine !

quote:

Originally posted by Concho42:
I made 5 actions with 4140-50 great to machine !

I have to agree
Chromolly is just plain good stuff to work with.

RG
Just curious as to why you chose S-7 to make your action?

I worked for a few moldmakers in San Antonio and built a few myself. S7 is a fantastic metal for injection molds because of it resistance to abrasion and it is a thermal shock resisting tool steel. In normal condition it is more machinable than 4130-4140. We used HSS cobalt endmills almost exclusively. Any water soluble cutting oil works great. Inserted cutters and about 300 surface feet with .003-.0035 cpt sounds about right. Make sure when youo heat treat it you wrap it tightly and double layer the foil. I ran across a knife maker awhile back that has patented a process that allows S7 to be used with a Rc of 59 and still be flexible enough for swords. It is a two week process that involves heat and cryo treating but the results are incredible.

Andy B

H2O- re your q. about why s-7-

Well, mainly bacause of good mechanicals, decent machining properties, and with little or no distortion in heat-treat. This would mean machine, H.T. blue, done. I'd like to avoid putting the piece back in the mill to fix distortion.

I recall reading about a fellow making double rifles from S-7 for the above reasons.

A couple of years ago, I designed some complex parts and had them made from S-7, and they came back from H.T. net on all surf's, and ready for survice. Hardened and cooled in inert atmos, tempered (450 F) in air. Tuned out great- was very happy with the experiment.

I had one test part made in the form of a ring, and sent it along with the parts mentioned above and when everything came back from H.T., I got after this ring with a hammer on the anvil, to get a feel for it- This stuff is TOUGH!

RG

Andy, KC, Vapo-

Thanks for the great tips.

I won't be gettng to this experiment until the middle of summer at the earliest, so I'm printing out this thread and filing it so it'll be ready when the time comes. (not remembering things so well these days.)

I did want to ask about the foil wrap. If hardening and cooling take place in an inert atmosphere, is this wrap required?

All in all, the input here is very encouraging.

Thanks

RG

Foil won't be required if heat treating in a vacuum furnace or an inert gas filled oven.
Foil is for heat treating in a basic oven.

S-7 is what George Hoenig uses for his rifles. he has them air tempered after working and says they require very little fitting afterwards.

quote:

Originally posted by BaxterB:
S-7 is what George Hoenig uses for his rifles. he has them air tempered after working and says they require very little fitting afterwards.

Well just to be clear All the generic tool steels. i.e S7, A2, D2, D7, H13 all have minimal shrinkage. They were designed to be a quality tool steel that could be heat treated with a minimal investment in heat treat equipment.
Therefore in particular A2, S7, and D2 can be machined then wrapped in stainless foil twice and heat treated with little to no change in dimensions And by little I mean as small as .00005" in dimensional change. So basically no change at all.
They are great materials there are better materials but at a higher cost to process

I'm a writer, not a metallurgist so I thought I;'d add that comment based on what I know from george. Am happy to learn more from thiose who know more.

Cheers,

_Baxter