.311 hole in 4140 that needs heat treating

[Mayhem]

I've been asked by a friend to make a die body for him out of 4140, which will be heat treated.  The external dimension will be 7/8" diameter and about 3.5" in length.  A .311" hole will need to be bored approximately 2.5" deep.  

I have two questions:

1. How much distortion would you expect in the 4140 and would a wire cage and liquid soap in the bore reduce this and/or scale formation?  I've used the wire/soap with good effect on threads in O1

2. Most places I have looked (quick search) list .311" reamers as .3115 U/S, which I assume to mean under size.  They define D/P as being dowel pin but don't specify U/S or O/S.  My metric reamers only specify 5mm, 6mm etc.  I have some tollerance on the + size but the bore cannot be less than .311"

One option I have is to drill and ream, and then if the hole comes out undersize after heat treating, is to mount the Dremel and find a stone with a long shaft and put together a poor man's tool post grinder.

I think I am going to make a D-bit reamer, as I contacted a local tool supplier today and was quoted $110 for a .311" reamer, as they have to be custom made.  Enco wants less than $20 for a US one but postage and time are limiting factors.  Especially since the Australian dollar has taken a dive...

Thoughts, suggestions and comments welcome.

[TomG]

Hey Darren,

You can't depend on a dowel hole holding size or shape during heat treat. The normal approach is to ream the hole a thousandth under the finished dimension and then hone it to size after heat treat. All the die shops I've worked at had a Sunnen honing machine, but since you probably don't have one of those laying around, you'll have to lap it to size with some brass rod and lapping compound.

To protect the part from oxidation during heat treat (if you don't have an air tight furnace) wrap it in stainless foil with a piece of paper inside to consume the oxygen. A metal box sealed with fire clay will work as well.

Tom

[Mayhem]

Thanks Tom. SS foil is too expensive for my budget. The metal box sealed with fire clay sounds like a much more affordable option. I assume that after the heating, the clay is busted and the whole box dumped into water?

I also thought about filling the bore with hardwood charcoal. I could compact it in pretty firmly. Would that help?

[Sunset Machine]

Where did I see someone wrapping his part with iron wire and slathering a pasty solution of boric acid and alcohol on it for protection. Boric acid as used in silver soldering/brazing. Looked good to me, a fairly hard coating. Stuff steel wool in the hole.

and... how hard? Is machinable 4140HT hard enough?
and.. round the corners of a 5/16" (.313) drill bit and try that as a reamer.
That's what I do here. A radius similar to the rounded tip of a lathe bit.
Maybe stone the sides (flutes) to get that closer to .311

[edward.s]

Clickspring on Youtube uses wire/boric acid in some of his heat treats to reduce scale

[TomG]

Yes Darren, the box is opened before the part is quenched in oil. There is already adequate carbon in 4140, so adding charcoal isn't necessary.

Tom

[Dr Stan]

(03-01-2016, 09:54 AM)Sunset Machine Wrote: Maybe stone the sides (flutes) to get that closer to .311

Put the drill bit in your mill and run it in reverse.

[Roadracer_Al]

Ask a local cutter sharpening shop if they can modify an existing reamer to cut on the size you want.

SS foil isn't all that expensive and works ***great***. Good to have some on hand, IMHO. Makes great shim stock, as well.

[Mayhem]

Thanks for the comments guys.

Tom - I was thinking more of keeping oxygen away from the surface of the bore than introducing more carbon.

Al - I live in Perth, where everything is either unavailable or expensive. This is why I load up as much as I can when I get to the US. Having said that. There are some options now on eBay. Last time I looked (a while back), it was going to cost a couple of hundred bucks for a roll.

Edit - most stuff I see is .02mm (.0008") thick. Is that going to hold up to the temps required (~860°C / 1580°F)?

[dallen]

heat the part in molten salt to the temp you need when you drop it in oil the salt will peel off and you will have a nice clean maybe blue colored part, depending on the salt used. regular table salt melts around 1600 degrees F