Cherry

[Xtramad]

I'm making a cherry to cut a .78 calibre bullet mold. I started out with a 20mm ball bearing which I anealed and then drilled and tapped a hole for the stem. I'm in the process of finishing the eight cutting teeth, but I need some advice.
How do I best harden the teeth without overheating them?
Should I case harden the cherry afterwards?

 

[Zonie]

Being a ball bearing, I'm going to assume it was very hard when you started.
Ball bearings are usually made out oil hardening, or air hardening steel. (The better ones are M50 or 52100).
IMO if you were able to anneal it using the normal methods of bringing it up to red heat and then slowly air cooling it, it is probably oil hardening steel, as air hardening steel usually requires a very long, highly controlled cool down cycle to anneal it.

If it is oil hardening steel, then bringing it up to bright red heat for a while, and then oil quenching it will harden it up to the high Rockwell C60's.
If you get it into this hardness range (file hard), then polish it and heat it to 430-470 degrees F to obtain a yellow to straw yellow color and oil or water quench it.
This is the proper temper for cutting tools.

I would NOT recommend Case hardening it if it was very hard when you first started, before you annealed it.
Adding carbon to the surface of a tool steel will make it VERY brittle. Even tempering it won't be enough to remove this brittle condition.

If however the above heat treatment does not harden your new tool, it was probably case hardened to begin with.
If it was case hardened, then doing that to it again will be your only way of getting it hard enough to be a good cutting tool.

Have fun!

 

[wulf]

Zonie sounds like he has got some good advice...I could ask what material you are going to make the mold out of...aluminum???? It may not be necessary to harden the cherry. I make router bits out of common bolts and route hard wood..have for over 30 years and still use them. So,,Iron would require a hardened cherry mayby aluminum wouldn't..what do you think, Zonie??? Oh yea the hardwood that i router is barrel channels for octagon barrels...

 

[Metalshaper]

Xtramad,

Not to ask a smart-a** question,, but did you allow for the moulds coefficient of expansion??

When I was doing the research for mine..< I was using 6061 T6 aluminum, btw >.. I checked for the expansion rate at 650-750 degrees.. it amounted to .004-.006" So to make a .340 ball, the cherry had to cut a ball that's diameter was closer to .334-.335..( info from an old Machinery Handbook )

My first cherry was cut and had a final diameter of .339
I went ahead and made a mould anyway..cause I knew better! it couldn't make that much of a difference, right? :hmm: Yep, the final results, a .345 ball that fits real tight in my bore!!..The barrel is an old Numrich Arms .36, which Mic's at .358

I did better on my buddies .295 mould. cutting the cherry, and finishing it to .289. The aluminum blocks, cast a nice lil .295! although, he bitches at me for cutting only one cavity.. ::

If I remember right brass had a coefficient of .002- .003
and an iron block was something like .001-.002.. Something to think about!!

Respect Always
Metalshaper

 

[Zonie]

Metalshaper has a good point.
The size of the lead ball when it chills will represent the size of the mold when it is at the casting temperature (give or take 20 or 30 degrees).

The coefficient of thermal expansion for most aluminums is about 1.3X10E-5 inchs per degree F (temperature rise).

Just to simplify this, if the mold is made from 6061 (or any similar aluminum), and the casting temperature is about 740 degrees F the aluminum block will have to be undersize when at room temperature .0086 inch PER INCH.
Because the value is per inch, you need to multiply the mold size times this value. (The answer above is based on the casting temperature minus room temperature to obtain the temperature rise.)
(740-78)(.000013)=.008606 expansion per inch.

With a .780 dia ball, that would be .780 X .0086 =.0067 expansion, so after subtracting that from the desired .780 the answer is .7733 diameter for a room temperature mold.

If the mold is made out of steel, the coefficient of thermal expansion is 7X10E-6 or about half that of aluminum.

As for cutting aluminum with a non hardened tool, it partly depends on the aluminum being cut. Some alloys are high in Silicon which makes them rather abrasive. When cutting wood to inlet a barrel the results can be "close" and work all right, when machining metal to obtain a finished product with tolerances in the thousanths the tool really needs to be hard to minimize wear. :m2c:

 

[Xtramad]

I was going to cut steel with this, that's why tempering is so important.
The ball bearing is from the turbine shaft of an F-16, so I assume it is a very high quality steel. The ball bearings are Swedish (SKF). I had not thought about the thermal expansion but if I understand your figures correctly a finished ball out of a .780 steel mold would be .777 ? I can live with that, the bore of the musket is .826 so a patched ball would load quite easily.
I used the cherry to cut a mold in plaster (I'm impatient) and the ball came out very nice except for some shrinkage around the spur, is this common with big balls( :crackup?

 

[Zonie]

As your still talking to us, I assume you oven dried the plaster mold before you poured in the 700 degree F lead?

(Just a warning to others who might want to use plaster molds.
If they aren't fully dry, the remaining water in the plaster can create a little bomb when the molten lead hits it. A Cherry bomb? :

The bearings out of a jet engine will be M50 or 52100 steel, if your interested, so don't case harden your new tool. ::

Yes, for a .780 ball cast in a steel mold, the ball will be about .0034 undersize.
As you correctly point out, when it's patched it will be close enough for government work. ::

 

[Xtramad]

I've done the water and molten lead thing before. I used a previously made slab of plaster (I use them for carving moulds out of when making stuff in pewter). One of the first thing you learn when you start casting is that water and hot metal don't mix. I didn't have anyone to tell me, so I learned from experience :cry: :cry:

Thanks for the info on the steel.

 

[Zonie]

I'm sure you don't have the equipment, but I thought you would like a little more information on heat treating ball bearing steels like M50 for Jet engine use:

The process envolves several stages (if your making ball bearings).
Hardening
1. in a vacuum or inert gas furnace heat the material to 1450 F (cherry red) for 1/2 hour minimum.

2. Raise the temperature to 2025 F (bright lemon yellow) and hold at this temperature for 5 minutes.

3. Quench in 250 F oil to 400 F.

4. Air cool to room temperature.

Tempering
1. Heat to 1012 F and hold at that temperature for 2 hours min.
2. Freeze to -100 F.
3 Reheat to 1012 F and hold for 2 hours.
4. Refreeze to -100 F.
5. Reheat to 1012 F and hold for 2 hours.
6. Air cool to room temperature.

This will produce a hardness of Rockwell 62-64.


Of course I don't expect you to do all of this, but it does give a feel for the complex steps it takes to produce aircraft quality ball bearings.

The vacuum furnace or inert gas furnace is primarly to prevent scale. It doesn't add anything for what your doing although if you have access to an inert gas furnace it will keep the sharp edges from scaleing up.

IMO the main thing for you to note is you should probably preheat to the cherry red temperature for a little while and then heat it up to the bright lemon yellow temperature for a few minutes. Then quench it.
You can skip the freeze and reheat tempering cycles as these are more for assuring dimensional stability than for giving the steel a special quality.
You should probably temper your cutter up in the dull dark red (975 F)or hotter range as this will not hurt the hardness a bit.

Have Fun!

 

[Xtramad]

52100 steel: Heat treatment consists of heating to 1500 F followed by an oil quench for through hardening of the alloy. Temper at 400 F
52100 Properties

M50 is extremely susceptible to decarburization and must be protected during any thermal treatment.
M50 Properties

I think I have 52100 steel in this ball bearing because I did not see any signs of decarburization after annealing it. Decarburization can be seen by a thick brittle scale on the surface of the steel, right?