[Homeroast] Turning Bottomless Portafilter

raymanowen at gmail.com raymanowen at gmail.com
Sat Aug 28 01:58:49 CDT 2010

   1. "...use a hand drill / Vise to hold it and a hole saw to cut mine
   2. From now on I will do my own.
                             [When you see something done by someone
   experienced in doing it, the process always looks easy. Something looks
   simple like changing the engine or automatic transmission oil and filter
   when someone else does it-  or a flat tire? Looks easy when you see it done.
    Show of hands- How many people actually do their own maintenance on a
   regular basis?]
   3. The chrome coating is real thin. the core is bronze I think. [Bronze-
   like the Temple bells in Thailand? Stupid Marketing always says "Heavy
   brass" or "Marine grade brass"] Their specific heat is lower than that of
   the water they don't touch...
   4. Very easy to cut with a hole saw."

Maybe Not-
Did you see the machinist using a Hole Saw or claw hammer?  He did Not.
In order to do the machine work with precision, conserve tools and the porta
filter handle itself, the cutting tools and workpiece were firmly mounted on
the lathe.

If your friend did not fracture the porta filter handle between the two jaws
of the bench vise; did not break the pilot hole drill bit when the hole
saw seized in the work piece, crack his wrist, yank the pf out of the vise
as it broke and busted the drill motor when it hit the floor, he's either a
Magician or extremely lucky.

Luck and Magic may be entertaining, but have no place in a machine shop or
on the highway. (When you feel inclined to buzz it over 100, are all the lug
nuts torqued, tire pressures and engine oil just right? Do you Know so or
just Hope so?)

Cutting out the entire bottom of a portafilter might seem easy with a large
hole saw, bench vise and hand drill, but it's not the all-pro move. It will
probably cost you some painful surgery, a hole saw, a hand drill, a porta
filter and some CSA points.

At Colo School of Mines Earth Mechanics Research Institute, I used a 500-ton
press and 12,000 psi triaxial pressure to test and chart record the failure
point of 2" OD X 4"L oil shale cores, up to 480° C, (Love PID controller,
type K s/s sheathed grounded junction thermocouples) and my strain gage load

According to an ME/ Chem E friend, "Some of the stronger bronzes need full
hydrodynamic (pressure) lubrication, or they gall and seize. (That's what
grabs and breaks the hole saw, your wrist, and yanks the pf out of the vise
if you had a good grip on the drill)

The heavily leaded bronzes survive much better when lubrication is marginal,
if speeds are Slow Enough. Ever seen a slow hand drill? Hammer Mechanics
usually run full-speed.

The opposing material and finish can make a big difference in the wear rate
and tendency to seize and gall. (Break your wrist)

Poor geometry usually can not be compensated for with "better" as-cast or
annealed materials." The tabs of 58mm pf handles each have to withstand
almost 300 pounds in shear at 9 bar pressure. Things could go rong in a
hurry if the shot stalls and the OPV doesn't bypass and relieve the pump
pressure, like my former Crapesso.

Don't even think of using type J thermocouples around moisture- the couple
is Iron/ Constantan. Guess which one corrodes like Hell and destroys the
junction?  Type K couple is Chrome-Nickel/ Aluminum-Nickel or Chromel/
Alumel.(Was © Omega) The Chromel is a laboratory-grade Nichrome- the heater
wire in toasters- and can withstand ≤ 3000° F.  Alumel is a little lower and
type K is a stable Seebeck couple at 2700° F.

Cheers, Mabuhay -RayO, aka Opa!

Do what you always did, get what you always got- If you get back without an
ambulance ride or tow truck, maintenance was OK.

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