Introduction: Make a Hot Knife Using a Soldering Iron

Picture of Make a Hot Knife Using a Soldering Iron

Are you having trouble cutting plastics with an ordinary x-acto knife? Then here is a simple tool mod you can do, turn an old soldering iron and a x-acto blade into a Hot Knife!

This hot knife idea isn't really mine, I found this idea made by somebody from MAKE:, I can't remember who made it. And so I decide to make one for my self to make my life of cutting hard plastic easier.

Disclaimer: This is a fairly dangerous work tool, it can burn and/or cut you if you misuse it. And I am not responsible what ever happens to you with this tool mod.

Step 1: What You Will Need...

Picture of What You Will Need...
You will need:
  • Any soldering iron will do - the higher the wattage, the better
  • A nasty old soldering iron bit you don't want to use.
  • A spare x-acto blade

Because I don't have a welder which is most recommended for this mod, I will have to use a different technique to attach the blade to the soldering iron bit...

My idea is to cut a groove into the soldering iron bit with my dremel with a cutting head. Then put the blade into the groove and squeeze the soldering iron bit onto the blade using the vice.

Step 2: Cut Out a Groove

Picture of Cut Out a Groove

Cut out a groove onto the bit!
You will need something to hold the bit tightly while you cut out a groove with the dremel. I used my vice to hold the bit. I wrapped the thread part of the bit in cloth to prevent damage to it.

Step 3: Squeeze It!

Picture of Squeeze It!

After you cut out a groove, make sure the blade can fit okay into the groove. Cut a bit more if necessarily. Then squeeze the bit onto the blade tightly with the vice, but be careful not to squeeze it too much otherwise the bit or the blade might break.

Before you squeeze the bit onto the blade, it is a good idea to add a little bit of glue to hold the blade at the right angle,

Step 4: And You're Done!

Picture of And You're Done!

Thats it!

I tried to cut some plastics with my hot knife, but it did not work out quite well, my soldering iron wattage is too low (15 watt)... Or it might be the cold weather. :-)

Be careful not to burn and cut your self with it!


castresana made it! (author)2016-04-05

Great idea! I had an old soldering iron (30W) with a copper tip laying around, I've followed your instructions and this hot knife works like a charm. I've only made a slight modification: I've cut the bottom part of the blade in order to maximize the surface area with the tip in order to get better heat exchange. Thanks!

Cormaxzyz (author)2013-10-12

I'm not sure if 15W is enough. My Xacto 30W soldering iron came with a hotknife, and it cuts fairly well, although the blade loses heat quickly.

I wonder if you could add a hotknife to one of those huge 130W soldering guns...

huck alexander (author)2012-06-18

This is a great idea! Have been looking for a more efficient way of trimming plastic parts. This idea will adapt nicely to my requirements. Thanks!

mrmerino (author)2011-09-27

At least if you cut yourself with it, it'll cauterize itself.

ilpug (author)mrmerino2012-02-01

Yeah, a built-in "Safety device."

aireal (author)2010-05-03

The flat blade might be causing to much heat loss (due to a larger surface area).

I am also worried about how much downward pressure you can put on this when cutting.

But great idea. Thanks for the insperation.

killbox (author)aireal2012-01-26

pressure is reall ynot needed, just go slow and let the melt do the work.

awang8 (author)2009-02-24

I'll give this a try when my current soldering iron tip is rusty enough to be qualified as a "A nasty old soldering iron bit you don't want to use.".

Plasmana (author)awang82009-02-26

LOL! I am not trying to say use a soldering bit that you don't use that much anymore because you are going to destroy it. You can use a new one or an old one.

awang8 (author)Plasmana2009-02-27

Yeah, I realised but for two things... *I'm the kind of guy that hates destroying things that you can still use, even if it is destroying it to make something awesome *The local electronics shop only sells Weller tips ($29.99!) and I hate buying online I'm not going to use a new one. PS: If you're wondering how I'm supposed to get a new tip when my current one is a knife then yes I will buy a new new iron ($10).

killbox (author)awang82012-01-26

I just posted an instructable about this. about the one ive been using, for one out of the two irons i used i didnt have to modify the iron at all. I found the cheapo exacto knifes you can buy at places like ace hardware (with a plastic handle) fit where the tip was in an old iron. let me replace blades. and also if i need to solder with it pop out the tip and put in the non knife one. (the other iron i did have to tap out a little bit to get it to fit. works best on irons where there is jsut a screw on the side to hold the tip in.

comander01 (author)awang82009-03-03

If the new Iron is $10, why would anyone buy a $30 tip?

ARJOON (author)comander012010-07-16

THat u pretty expensive for a tip. in maurititius a tip cost me Rs5 or $0.2 yeah the big one

Plasmana (author)comander012009-03-04

Good point...

xproplayer (author)Plasmana2009-03-31

i could just see someone a huge pile of old irons in ur workshop plasmana

xproplayer (author)xproplayer2009-10-18

wow i just realized that is the worst grammar ever!!
i mean to say i can just someone like plasmana having a huge pile of soldering irons laying around in ther work shop if you hook em all up a once u have a heater!

lordofthedonuts (author)2009-02-23

I've tried to make something like that with and old solerding iron, but this one is way easier. I can't wait to injure myself horribly with that!

awang8 (author)lordofthedonuts2009-02-27

Quote from lordofthedonuts: I can't wait to injure myself horribly with that! Isn't that kinda emo?

Foxtrot70 (author)awang82009-03-02

Hi Awang8 - Please forgive me, I am not well versed in internet shorthand so, what does EMO stand for?

xproplayer (author)Foxtrot702009-03-31

emo is not shorthand but is a term for people who like to be goth dress all black talk about death and most commonly cut themselves purposly

Emo is shorthand.

awang8 (author)Foxtrot702009-03-02

Nothing. It means you really hate yourself and you love trying to hurt yourself. Search YouTube for "how to be emo".

klingonprins (author)awang82009-06-20

h2b emo is 1337ness like never before!


lordofthedonuts (author)awang82009-02-27

I hope you understood that it was a joke. It's more ironic than Emo, it's just that every time I use a knife I end up cutting myself, accidentally obviously. Maybe the formulation isn't clear enough.

A burn and a cut, OUCH!

Plasmana (author)Yerboogieman2009-04-13

A big ouch, lol

bug on fire (author)Yerboogieman2009-03-03

the burn will melt the skin back on the cut so no harm done lol just kidding dont cut yourself ( or burn )

xproplayer (author)bug on fire2009-03-31

i got it u cut urself then pull the blade out and put the flat side on the skin and it will melt it back in place

comander01 (author)Yerboogieman2009-03-03

Pre-cauterized, too!

awang8 (author)lordofthedonuts2009-02-28

Yeah, I understood the joke. I was being sarcastic.

Plasmana (author)lordofthedonuts2009-02-26

You can't wait to cut your self with that?!? LOL!

jtobako (author)lordofthedonuts2009-02-24

Crank up the heat for instant cauterizing ; )

techno guy (author)2011-03-04

I was thinking that you could attach the blade by soldering it but that would fall apart as soon as you turn it on.

dombeef (author)2009-04-13

No It means you really hate yourself and you love trying to hurt yourself.

Xankahr (author)dombeef2011-01-31

no it dosent

dombeef (author)Xankahr2011-02-01

Yes , it does.
I was wrong with the music genre, though.

Emo does'nt mean emotional. It's short for emotive hardcore, which is a music genre that branched out of punk rock. Some notable bands of this genre are The Get up kids, weezer(early), and Sunny Day Real Estate. It has nothing to do with hating yourself, or self mutilation.

MrHanimi (author)2010-12-22

Ah yes! :)
now i can cut through plastic like a hot knife through....

I'll get back to you on this one.
nice instructable :)

Plasmana (author)2009-03-06

Oh dear... The lead was once on the bit from soldering and now it is in my lungs!! AHHHH!!

sk8erdude (author)Plasmana2009-03-06

Actually there isn't lead in the air that is the flux boiling off. lead melts at very high temperatures let alone evaporate so there isn't actually anything to worry about! Keep up the good work, -SK8ERDUDE

Lhtrf (author)sk8erdude2010-09-26

actually, theres lead in solder to make it melt at LOWER temperature, but still, nowhere near enough heat to make it evaporate.

BluTiger (author)2010-08-01

I like the warning cut yourself and cauterize at the same time LOL

wolfs2424 (author)2010-03-30

Good idea,  has many uses like removing insulation from thin wires and saves money

punkhead58 (author)2010-03-13

A little 'bit of epoxy couldn't hurt either.

Plasmana (author)punkhead582010-03-20

Wouldn't it burn off?

karnold70 (author)2010-03-09

If you have the equipment and the patience, I'd recomend using a tap to thread one half of the 'hole'. use the next size larger drill bit on the other half so it will squeeze.

tiekietemboe (author)2009-04-19

What is a x-acto blade and where do you get it?

Aggrieved (author)tiekietemboe2009-09-06

x-acto blades are just a name for the general craft knife blade. any art supply store will have them and they will all fit into a standard handle.

xproplayer (author)2009-04-04

i have to funny stories for this first when i tried to take out my old soldering iron bit it broke leaving the threads still in then when i was clamping the blade it took like the hulk to get it even after practically braking the vice it still is not rock solid

About This Instructable




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