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Materials, Finishes, and Treatments

This page contains my notes on the materials I've machined, how I found them, and on some finishes and treatments I've experimented with.

Materials
 

I started out mostly acquiring my machining stock on eBay. There's a bunch of it up there, the only question is price. My approach was to look for auctions nobody was bidding on, and go look up the price of something similar from one of the online suppliers. If I could bid half what the online supplier offered at, I would try to take the material.

You'd be surprised at how often this works. A lot of the material on eBay is coming from machine shops who can't use the material, so anything they get for it is gravy. Beware the shipping--that's how they make sure you don't get too good a deal. The best opportunities are small pieces that are the scraps left over from some small production run. These pieces turn out to be perfectly sized in many cases for our small home machine tools and projects anyway.

One of the really nice things about buying in this way is that you actually know what alloy you are getting, unlike scrounging metal from old machines and junkyards. The problem with the latter is that a lot of metal has very poor machining properties. Machine shops will order very little of that!

Spark Testing: This is an important help in identifying what kind of material you are working with. It is a comparative test, so its helpful to have some known quantities available to compare to. I have found pictures are not the same as seeing it yourself with this test. It's hard for the pictures to control for variables like the angle and pressure against the grinding wheel. Nevertheless, get thee to thy Internet and read up. Try to compare what you think you have with a known quantity.

 
Stainless Steel
 

Identification: If it looks like steel, weighs like steel, but is non-magnetic, that's a pretty sure indicator. I keep a welding magnet near my material storage area just for checking as I inventory the material. Sparking will be similar to mild steel, as there is very little carbon in most stainless. The exception is that there are stainless tool steels (4xx allow family) that respond to heat treatment. These would be rare to find unless you paid a premium, however.

Hardening and Heat Treatment: Most stainless will work harden, which means it gets tougher the more you work it. If you do wind up work hardening a piece, you'll have to anneal it to relieve the hardening so you can go on working it. I have not yet seen the work hardening effect turning and machining stainless. If you heat treat most 3xx stainless alloys they do the opposite and become softer.

303: I did my first cuts on the lathe on a little piece of 303. That original piece lives on today in the adapter I made for my Quick Change Toolpost. This is nice stuff to cut on the lathe, and finishes real nice too, although not as nice as 12L14. Some use of a finer grit sandpaper such as 1200 grit will develop a finer finish.

Other Stainless: I've got a fair amount of unidentified stainless. Most of it does not cut as nicely as the 303, and some is just darned ornery. You can't get away from a grooved finish on it no matter what I do, so I am left to file it if I want a nice finish.

 
Mild Steel
 

Identification: If it looks like steel, weighs like steel, but is magnetic, you're down to whether you have mild steel or a harder steel. On the spark test look for longer streamers from the wheel with fewer sparkles.

Hardening and Heat Treatment: Mild steel does not have enough carbon to respond strongly to heat treating, but it will respond a bit by becoming slightly harder. It is probably not worth your trouble to try to harden mild steel unless you do so by case hardening which involves providing additional carbon the part is packed in.

12L14: Following closely on the heels of my stainless experience was some 12L14 mild steel. I got a little more ambitous with this, and turned some hex stock to make a tailstock die holder. This material is a little softer than the 303 stainless was, but I don't think it produces quite as shiny a surface finish with my carbide insert tooling. Nevertheless, a fine finish is possible, and the material cuts very quickly and with almost no chatter. This is a nearly perfect steel for the home machinist to fool around with early in their career.

 
Hard Steels
 

Identification: If it looks like steel, weighs like steel, but is magnetic, you're down to whether you have mild steel or a harder steel. On the spark test, the sparks will tend to end closer to the wheel, and the more carbon (i.e. the harder the material), the more sparkles you'll see. A noticeably different spark, perhaps with unusual colors and shapes implies an alloy steel of some kind.

Hardening and Heat Treatment: Most hard steels respond well to heat treatment by becoming very hard. Experience dealing with hardening, tempering, and annealing will be a requirement to successfully manage these steels.

No experience here yet!

Internet Notes:

O6: Recommended by Evan on the HSM board, O6 is an oil hardening tool steel with 120% machinability.

D2: Air hardening. Designed to reach the upper 50's and low 60's on hardness. When you look at a hardness vs tempering temperature curve, you'll likely find D2's hardness will drop 15 or more points in a range of less than 100*F when you're shooting for hardness in the mid 40's. Accurate tempering becomes difficult if not impossible. Most of the high chrome air hardening tool steels are secondary hardening steels as well. The hardness drops initially, and then begins to increase again, usually peaking in the 900 to 1000*F range. Once you pass that peak, the hardness drops like a rock with increased temperature, hence the difficulty of accurately tempering to hardness lower than the steel was designed for.

W1 Drill Rod: Water hardening. Supposed to be hard to get a good finish. Prefers HSS to carbide. Particularly sensitive to machine rigidity. Some Asian lathe owners suggest you have to machine it with a plinth (no compound slide) to have much success. For carbide inserts, CCMT inserts are suggested at higher rpm (actually, pretty high--1200-1800 rpm for 3/4"). Lubricant beneficial: cutting fluid or oil. Resistant to shallow cuts (e.g. less than 0.010"), so finishing cuts will leave a rough finish. Prefer O1 for a better finish.

4130 and 4140: Ideal for making toolholders and the like. Designed to harden to the 40's. Easily tempered.

L6: L6 is a good choice if you need hardness in the mid 50's along with excellent toughness. Its not nearly so highly alloyed as the D's or A's, and heat treats more like common alloys than like tool steels. Costs a bunch less than A's or D's as well.

 
Cast Iron
 

Identification: Heavy like steel, and magnetic. Grey cast iron is usually a lot darker color than steel as it arrives, but after machining, it looks about the same as un-machined steel. You will note the tons of graphite (carbon) powder that goes everywhere when you machine cast iron. On the spark test, cast iron produces relatively few sparkles, and long dull reddish sparks compared to the colors and other characteristics of steel.

Hardening and Heat Treatment: I understand you can harden grey cast iron by heat treating but that there is so much carbon the material becomes so brittle as to be almost useless. Doubtless there is a process that works great if you can track down the secret!

Grey Cast Iron: I ordered a big chunk (too big!) of grey cast iron when it came time to make a backplate for my Buck 6-Jaw Chuck. What a project! The piece was right at the limits of what my lathe would machine, so there was lots of chatter to deal with. I am unsure whether this is a function of the cast iron itself or just the size of the piece I worked. Cast iron is also incredibly messy! It contains a tremendous amount of carbon which is trapped between the iron crystals and comes out as graphite dust when you cut swarf. To make matters worse, all that carbon leads to iron carbides, so there is a lot of abrasive potential in the dust. Be sure to clean your machine thoroughly when done! With that said, the grey cast iron machines easily (similar to the 12L14) and develops a nice surface finish (when you wipe away the gray dust). It is also very hard. I dropped my backplate off the table and a sharp edge hit directly on the concrete floor. It knocked a groove into the concrete but I couldn't even find a scratch on the backplate!

 
Aluminum
 

Identification: Very light weight compared to steel and a little different, whiter, shinier color usually reveals you're dealing with aluminum.

Hardening and Heat Treatment: There are a number of heat treatments for aluminum that are mostly designed to relieve work hardening and other undesirable characteristics. It's unlikely you'll be doing any aluminum heat treatments in the home shop, the material is typically purchased in a particular heat treated state.

General Remarks: I did my first milling on the lathe on a piece of aluminum to make a saddle lock body. The stuff cuts very easily and wants pretty high spindle rpms to even begin to have a decent finish compared to the mild steel. A big disadvantage is aluminum is soft enough that virtually everything leaves ugly marks on it. If you don't care about appearance, no problem. If you do, get ready for extra work. Note that aluminum gums up grinders. It also seems to clog drill bits. Long streamers of swarf literally cocoon the bit until it quits cutting. Higher speeds will tend to make for shorter streamers, but you really have to crank it up. Back out often to clear those chips! OTOH, it threaded real nice and easy for the saddle lock project. We'll see how well those threads hold in aluminum.

Relevant Threads:

Discussion of bead blast finishing

 
   
Finishes
 
Files, Sanding, and Hand Grinding
 

Filing is my first choice for quick finishing on the lathe, as it turns the file into a power tool <G>. While on the subject of files, a couple of thoughts:

  • Nicholsons are recognized as the best, although there are some European brands that are just as good.
  • Keep them chalked with railroad chalk (available from Enco and many other places), which helps keep them from getting clogged. I also find heavy sulfurized cutting oil works well, but it can be messy, so I general only use it on the lathe.
  • Learn how to drawfile. I won't attempt to explain here.
  • Keep a file card/file brush combination handy and clean the file often so it doesn't fill up.
  • I understand you can sharpen a file by soaking it in vinegar. I haven't tried it, but would do some more web research before trusting your file to it.

If not on the lathe, I use a right angle air grinder with a fiberglass reinforced cutoff wheel as a quick deburring tool.

 
Scale Removal
 

If you are working with cast parts, or want to remove scale from forged or hot rolled steel before machining, a variety of pickling treatments are well suited to the task. Look for names like Safe-T-Pickle and Sparex which are quite common. Note that virtually all dry pickling agents like Sparex are simply expensive trade names for a very inexpensive chemical compound called sodium bisulfate.

In some cases, sand blasting can also be an effective mechanism for scale removal.

 
Sand and Bead Blasting
 

I picked up a very inexpensive blasting cabinet off eBay. It looks identical to the ones sold by Harbor Freight, but was cheaper at $89. I'm very satisfied with it. I loaded it with glass beads, which leave a finer finish than sand. In practice it leaves a nice satin finish on surfaces. The beads are not a very effective way to clean, however, as they're a little to fine in their action. Beads produce more of a polishing action. A coarser grit will work better for cleaning or coarse work. Carborundum grain, pumice, and sand are all common media for the purpose. It's easy to swap grits. The grits sit in the bottom of the cabinet and there is a door that will release them into a bucket below. Dump the old grit, pour in the new, and you're ready to go. Be sure to run the gun a bit between grits to clear any residue.

Note that blasting can also produce hardening of the surface by work hardening it, often called "peening". Blast cabinets are cheap and any shop with room for one ought to have one.

 
Polishing
 

Lots of polishing methods are available ranging from hand polishing on buffs to vibratory polishing. The latter is a fast way to deburr and polish small to large production runs of parts, depending on the capacity of your vibratory polisher.

There is lots of good material on hand polishing from the gunsmithing and knife making fraternities, as they obviously value a very fine finish.

For vibratory finishing, Shor has some of the best information available. For example, this page on media.

See also my CNCCookbook page on Vibratory Deburring and Polishing.

 
Anodizing
 

Anodizing uses electricity to open up pores on aluminum and deposit a pigment inside those pores. It also hardens the surface of the aluminum. It is a process that is fairly straightforward to do at home. I recommend a book by Ron Newman as your bible for anodizing at home. I haven't tried the process yet, but the book is very comprehensive and Ron seems to get good results for his own business using these techniques:

Ron Newman Anodizing...

Anodizing's hardening properties can also be quite helpful in making aluminum more durable.

 
Cold Bluing
 

Cold bluing uses acids to put an oxidized finish on steel at room temperatures, or close to it. They're very easy to apply, requiring only a thorough degreasing before application. For fun I compared the finish from two Brownell's cold bluing products:

Dicropan T-4 and Oxpho-Blue Creme Cold Bluing Test

Both the Dicropan T-4 and the Oxpho-Blue Creme came as creams, which made them easier to apply. To perform the test, I took the spacers I had made for my new 6-jaw chuck backplate and compared the results. I simply cleaned them in some mineral spirits and then applied the compounds with a folded paper towel. Do this wearing gloves as the chemicals are fairly nasty. The steel turns blue quickly, after which you rinse it in cold water to remove and neutralize the chemicals. I then oiled the parts with Break Free, which is also an excellent anti-corrosive. The oil helps bring out the blue a bit more.

My conclusion was that the Dicropan looked darker, almost black, and seemed to go on more evenly. On the other hand, the Oxpho-Blue showed more blues and was a prettier finish despite it being less even. All things considered, I decided I preferred the Oxpho-Blue look. I'm not sure I'd want to do a whole firearm in it, but it's excellent for making tools look better.

Rear three: Oxpho-Blue. Front spacer: Dicropan T-4.

 
Hot Oil
 

Here is a machinist's jack finished with a hot oil treatment:

I like the look of the finish, though I don't know how protective or durable it may be. It was done by Dave Hylands who states that he simply "cooked" the jacks in his BBQ for 30 minutes and then plunged them into some 20W-50 motor oil.

 
Parkerizing
 

Parkerizing is a very durable treatment often used on firearms. It's pretty straightforward to set up a Parkerizing process in a small shop. Here are some picks from an HSM thread:

Stand for a Parkerizing tank is just welded from angle iron...

Shopmade propane burner...

Tank needs to be stainless. This one was purchased from eBay for $99...

Chemicals are from Lauer Custom Weaponry. 1 Gallon is $29 and will last a long time!

Mixing chemicals up in a big container like this is very convenient. Strain through a coffee filter and return to the container when finished...

"Cooking" a firearm in the solution. Rinse with clean water then apply the Lauer Post Treatment solution. When that's all done, a little oil and you have a military-grade nearly rust-proof finish!

 
Hot Bluing
 
   
Treatments (i.e. Heat Treatment, etc.)
 

This is definitely an area I'm very interested in experimenting with, both for the hardening properties afforded heat treated steel as well as the awesome temper colors that can be produced.

I have been interested in heat treating metals for a long time, and finally sat down to do a little web research on how to build a heat treat furnace. Industrial PID controllers are readily available on eBay cheap, and the rest of the materials required are not expensive either. I'm adding something like this to my "someday" project wish list:

Home-built Heat Treat Oven...

Links about making your own furnace:

The Home-built Heat Treat Oven Pictured Above: A nice project with lots of photos.

http://www.knifeforums.com/forums/showtopic.php?tid/752668/post/775813/hl//: Not much on pix, but some good tips and techniques. For example, there is a high temperature mortar/calk available at Home Depot to seal the bricks that is good to 2000 degrees. You want a "K" type thermocouple for this kind of project.

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