SAE all-metal hot end, how to taper the melt zone?

I'm planning on making a stainless steel hot end and of course I'm just going to go to the local hardware store which doesn't really have much in the way of metric stuff.

So I'm going to be drilling out my stainless bolt with a 1/8" drill bit. Do I need to get a 5/16" bolt or will 1/4" do? When I tap a 1/4-20 hole, I like to use a 13/64" drill bit, which implies that the wall will be 13/64"-1/8"=13-8/64=5/64" or about 1mm wall all the way around, less any eccentricity. Which I think should be enough, even given that I'll be using a drill press instead of a lathe. A 5/16-18 bolt uses a 17/64 tap size, which is 1/16 larger, or about 3.5mm extra, so 1.7 wall thickness under the threads, assuming an exactly centered hole. (which there will be, b/c I'll start the hole on that end.) That should be enough to ream?

if I want to ream it out to give a tapered hole, do I need to get a 5/16" bolt? Nophead's bolt was reamed almost completely out, I don't think I want to get that close, as I need to torque the acorn nut and aluminum heater block onto the end.

Do I need to a tapered hole? Nophead seems to think that tapered holes are just magic, and it's hard to argue with his results, so I'm inclined to say yes, I do need to have a tapered hole for reliable operations.

I suppose that it depends on the slope of the reamer. The reamer I found says 1/8" to 1/2", and the cutting edge is 3 3/4" long. My heater block is 1/2" thick. The depth of thread need for the acorn nut on the end will be greater for a 5/16" nut than a 1/4" one.

The length covered by the nut and heater block will be the molten zone, with the melt zone (which is what I'm trying to taper) just above that. So the tapered area should extend, say, 1/2" above there. So about 1 1/2" of tapered area. ~3/16" reamed out on the end of the bolt. Okay then, Looks like 1/4" is out, b/c 3/16>5/64. How deeply can a 5/16" bolt be reamed, assuming it's exactly centered and we touch the thread on the outside? Only ~3/4"! Something's wrong somewhere?

I'm trying to make the melt zone shorter, not the molten zone! Do I need to find a reamer with a lesser slope then? I'm not sure that such a thing exists. Could be emulated on a lathe probably though. Except I don't have a lathe, and wouldn't know how to do that if I did, it seems like it'd be fairly difficult. Ideally I suppose I'd make a custom tapered bit, drill out the end to 2/8" instead of 1/8", and then use the custom bit to add a taper just to the melt zone, above the heater block.

I have no idea how I'd go about making a custom tapered drill bit though! Or where I'd look for a reamer with a gentler slope, something like 1/16" per inch. Maybe I should drill partway through the heater block before threading it so the acorn nut sits up inside of it? That'd maybe get the total length (acorn+heater=molten zone) down to 1/2", but even that would only leave 1/4" of taper beyond that, and I think the melt zone would extend above that?

If I can't taper it, I may as well just use 1/4", but will it work as well?

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I'm building it with Baling Wire

If I can't taper it, I may as well just use 1/4", but will it work as well?

Official policy: Try it and see, and stuff the answers into the wiki,
http://reprap.org/wiki/SAE_All-Metal_Hot_End
at which point it may become official documentation.

Also, what about using a tapered end mill to drill it out?

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-Sebastien, RepRap.org library gnome.

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Remember, you're all RepRap developers (once you've joined the super-secret developer mailing list), and the wiki, RepRap.org, [reprap.org] is for everyone and everything!

I broke off an 11/64 drill bit, then ground the taper on the end using a bench grinder by turning the bit slowly in a hand drill against the side of the wheel to get the overall shape. I then had to regrind the flute tip and land profiles with a mini grinder to make it cut.... What a pain

drill.jpg

Not sure how it would manage drilling any distance through stainless. I only drill the full diameter of the bit three full threads into the rod to attach the hot end. The drill bit that I use is very hard and I use lots of coolant and drill slow. Stainless has a nasty habit of burning drill bits to bits. 1/4-20 is a real tight fit. The taper is not very long, maybe 3/16”, and it's actually a cooling zone as apposed to a melt zone, which is why it's so thin at the bottom. The thin area provides a bit of a thermal break. My understanding is that if the taper stays cool enough then the plastic will harden against the tapered wall where it can easily be dislodged and before it manages to migrate up the tube where the hard filament can wedge it into a jamb..... Hope that made sense.

Edited 1 time(s). Last edit at 12/19/2011 09:36PM by Bruce.