Small diameter (under 1 mm) orifices mounted in threaded holes are a
common part of hydraulic pump and motor controls. I could never find a
decent set of calibrated orifices on the local market, so I always
manufactured mine from socket set screws. I've been seeing that many
mechanics experience certain difficulties when they try to drill a set
screw with a small diameter bit, hence - this small set of tips about
making small diameter calibrated orifices for "hydraulic purposes".
Please note, that I am not a machinist - I simply enjoy working with
lathe and other metalworking machines - the technique I am describing
here has worked for me, but if you by any chance know a better one,
please do tell me about it.
So, to make a small orifice (or a gigleur, if you wish) what you need is:
a) a quality HSS drill bit set - don't buy cheap
sets, don't buy sets that have one bit of every size, buy the separate
one size ten-bit sets - you will break bits, especially in the
beginning. Don't go for carbide bits - they're very brittle, and a press
drill or a lathe don't have enough speed to drill efficiently with
these bits anyway. A Dremel tool might work, but common HSS bits are
easy to find, relatively inexpensive, and do the job just fine.
b) a lathe or a drill press with an adequate chuck. I
prefer the lathe - it's much easier to work with, but a good drill
press will do. Hand drills are no good for this task.
c) a quality center drill or a spotting drill (small and new).
d) a set of steel socket set screws (or grub screws,
as some call them). This is a tricky part, since you'll have to look for
the lowest grade you can get. You can use brass set screws, if you find
them, they indeed are much easier to work on, but I prefer steel screws
because they are stronger, and therefore there's a smaller chance of
the socket head "passing" when you are removing a "stubborn" or a
glued screw. If you didn't manage to find low grade steel set
screws - you can still lower the steel hardness though annealing
- just put all the screws on a thick steel plate, and hit the whole
thing up with a torch till everything is glowing red, then let it cool
down naturally - now the screws should be soft enough to drill. Some set
screws can remain very hard even after the annealing - that's why I
normally anneal several screws, and then simply choose a different one
when I see that the one I am drilling is too hard.
Now for the drilling itself:
a) Wear protective eyewear, if you don't want a
broken off bit end in your eye. You WILL break bits, that's why you
bought ten of each size - remember?
b) Center the set screw in the lathe chuck (or clamp
it in the press drill vice - make sure it is well secured).
c) Make a small starting point with a center drill
(or a spotting drill) - very important! If you don't have a well
centered starting point - most surely the bit will break.
d) Set the right drilling speed - I've been
successfully drilling small orifices (0.5mm to 1mm) with 1000-1500 rpms (
my lathe can go up to 1800 rpm).
e) When you drill the small hole, the most important
thing is the pressure you're applying to your bit. If you're bending it
(like many will do) - you're applying too much pressure! The key word
here is - Patience! Don't rush it, apply very light pressure, and drill
in short "pokes" to clear the chips. It may be hard to feel the amount
of pressure you are applying with a standard lathe tailstock - you'll
have to experiment with it. There are special attachments for small bit
drilling, that fit into the standard tailstock with a Morse Taper and
allow you to feel the amount of force you are applying to your bit very
precisely. These are very nice, and I surely will make one for
myself, but so far I've learned to use the standard tailstock
quite successfully - it's not easy, but it is doable.
In all - making small sized orifices for hydraulic purposes
is not hard. Get the right tools, get the right screws (or anneal the
hard ones), drill in small pokes with little pressure - and you will
never have to look for orifices again!!! Also - check out the video - this is is a hard M6 set screw, annealed, drilled with a 0.8 mm bit at 1000 rpm.
