B.B. u wrote in message
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In article ,
"Dave Baker" wrote:

[...]

In short, the less stock you remove with a reamer the better IMO. Also
the
slower the speed the better within limits. In rigid high speed production
setups it is often suggested to run at half the drilling speed but I
always
find a much slower speed does the job best for the home machinist. Maybe
80
rpm for a 1/2" reamer and 150 for an 8mm one. Much less chance of chatter
or
taper developing and less heat build up in the job.

Well, then. Mystery solved! To get down to specifics, I dug up my
notes. I needed to ream a hole .625", or 5/8. So, following the
instructor's instructions I wound up with a 19/32 drill, which the
project cabinet just so happened to have a plethora of in various stages
of devastation. IIRC, I wound up running the reamer around 700-800 RPM,

Holy crap Batman!

where the proper speed (according to the teacher) was closer to 600.
I'm guesstimating the "good speed" would have been more around a tenth
of that.

One thing you should bear in mind and which will stand you in good stead for
your future machining is this. The majority of the recommendations on speeds
and feeds for machining different metals have evolved from the needs of high
speed production to be as cost effective as possible.

That means the fastest way to remove metal consistent with not killing
expensive tooling too quick and maintaining accuracy. It's all about trade
offs. Let's say a carbide tip costs £5 and lasts for 100 parts at 500 rpm,
70 parts at 800 rpm and 50 parts at 1000 rpm. Factor in labour rates, wear
on the machine at high rpm, extra electricity at higher rpm and you can
calculate the most cost effective speed and feed to do each job at.
Generally that means going as fast as possible even if it burns tips out a
bit more than going slowly.

For hobby machining forget the lot. The best speed and feed is the one that
works for you and you find that out by trial and error. Most materials have
a wide range of speeds at which they'll cut just fine with the right
tooling. Cast iron needs to be turned fairly slowly because it's abrasive
and aluminium much faster or it tears but with a nice sharp tip you can get
a good finish on anything at fractions of the speed that a book will tell
you to turn at.

I always machine things slowly because my time is my own but carbide tips
and drill bits cost money and I rarely burn one out. My friend who's a CNC
engineer will run his lathe at 5 times the rpm and use 3 tips on the job
because taking an extra couple of hours costs him a lot more in wasted
earnings than a few new tips do.

Learn to machine things slowly and accurately and you can always speed
things up bit by bit as you gain experience.

Attempt #2 (which I think I'll be doing next Monday) will get a much
gentler treatment.
What would you say about feed rate? When he demonstrated he pushed
the reamer through a 2 1/4" deep hole in about two seconds' time. I
assume a much slower RPM would mean an equally slower feed?

It's a feel thing. Just push the reamer as fast as it wants to go. However
you don't want to leave a reamer turning in a hole any longer than you have
to or it'll rub and cut oversize. Aluminium cuts really easily so you won't
need to be in there long but if I were your teacher I'd be starting you off
on through holes rather than blind ones so the swarf can fall through.

If your only choice of tools is drills in 1/64 increments then get as close
to final size as you can. Ideally though you want to bore to within the last
10 thou before you ream but if you have no boring tool then manage without.
Try the drills at 400 to 500 rpm and ream at 100 to 150 and see how it goes.
It won't matter how fast or slow you ream though if you generate so much
swarf it clogs the flutes. You'll still chew the job up. You might need to
peck a few mm at a time and then pull back and clear them unless you're only
reaming the last few thou out of the job. That's a bad way to use a reamer
though and chances are you'll end up oversized at the top if you keep taking
it in and out of the job. Reaming should always be a one shot deal and then
back out of the job as fast as possible.


While I'm here, we had to lay out the part prior to machining it.
What, exactly, is the point of that?

To teach you to get your reamed hole in the right place as well as being the
right size. There's no point in getting the perfect hole but 2mm out of
position.

Is it really just so you can
double-check yourself and make sure you don't do anything stupid, or am
I missing something obvious? We didn't have to center-punch the part
before drilling, we simply used a center-drill first,

Good. I've never used a centre punch in my life other than for starting a
hand held drill in roughly the right place to fix a shelf up. On a machine
you should be laying out with a DRO or the crank dials and starting holes
with a centre drill or milling cutter. A centre punch has no place in
precision machining.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)