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Reamers

Jim Hawkins

01-15-2008

Re: Reamers

Re: Reamers

Re: Reamers

Re: Reamers

Re: Reamers

Re: Reamers

Posted by Dave Baker on January 16, 2008, 8:44 am

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> Question from a beginner:-
> When should one use straight-fluted reamers and when
> should one use spiral-fluted ones ?

You need to understand the purpose of the flutes in any machining tool and
then most of it will become apparent. Let's start with a drill bit. Drills
must be able to drill into material from scratch so they are by definition
creating a blind hole. They could also equally be enlarging a blind hole
already made by a smaller drill. This means that the swarf must be pulled
back out of the hole by the tool and therefore the flutes must operate in
the same direction as the tool rotation i.e. right handed flutes.

Reamers are always enlarging an existing hole and also not removing much
stock so the swarf is less of a problem. Also most reamed holes are through
holes. The flute direction is therefore usually left handed to push the
swarf through the hole and make sure the flutes do not clog which would
abrasively create an oversized hole. The other advantage of a left handed
flute is it doesn't pull the tool into the work which could lead to chatter
if the tool 'overtakes' the driving force behind it. In hand reaming this
could also mean the tool jams in the hole. Both of these things lead to
better hole sizing and finish. They also indicate that reaming works better
in the vertical direction than the horizontal so that the swarf falls away
from the tool. However this should not prevent lathe reaming in the
horizontal plane if the stock removal is correctly sized.

For reaming blind holes a straight flute is usually better although if the
stock removal is correct the reamer should still not clog in short holes
because the flutes have sufficient void volume to contain it. It does tend
to indicate that you might want to drill closer to finished size with blind
holes than through holes.

In no case does the flute direction indicate whether the reamer is a hand or
machine (chucking) reamer. This is determined by the 'lead in' to the
parallel section. Hand reamers (fixed or adjustable) have a fairly long
tapered lead in (an inch or so) which self centralises the tool. Chucking
reamers have a very short lead in because the tool is centralised by the
machine and the action of the bevel. In both cases the tool only cuts on the
bevel of the lead in and not on the flutes which are there solely to
position the tool in the finished hole. Regrinding a blunt reamer therefore
means that only the lead in is reground and not the full length of the tool,
as with a drill bit.

In summary therefore you shouldn't be concerned with the flute direction
other than sometimes for blind holes.

The most important thing to get right is speed. Traditional wisdom was to
ream at half the drilling speed but IMO this is usually far too high,
especially on non rigid equipment. Even with small reamers 100-200 rpm is
fine for most purposes. High speeds create chatter and oversized holes.
--
Dave Baker
Puma Race Engines

Posted by Mr Crane on January 17, 2008, 3:18 am

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Can I ask if there is a "rule of thumb" regarding drill size for a
particular reamer size? For example, if reaming to 1/8", what should the
drill size be?

Cheers,
Paul

--
Mr Crane
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Posted by Peter Neill on January 17, 2008, 4:02 am

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On Thu, 17 Jan 2008 02:18:50 -0600, Mr Crane

>
>Can I ask if there is a "rule of thumb" regarding drill size for a
>particular reamer size? For example, if reaming to 1/8", what should the
>drill size be?
>
>Cheers,
>Paul

Useful chart here:
http://www.cjtkoolcarb.com/catalog/Technical/reamertechnicalguide.htm

Looking at the allowances though, I'm sure that these are for machine
reaming, i.e under power. For hand reaming I would allow less than
these figures, perhaps by as much as 30-40%.

Peter

Posted by Austin Shackles on January 17, 2008, 2:32 pm

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On or around Thu, 17 Jan 2008 09:02:26 +0000, Peter Neill

>On Thu, 17 Jan 2008 02:18:50 -0600, Mr Crane
>

>>
>>Can I ask if there is a "rule of thumb" regarding drill size for a
>>particular reamer size? For example, if reaming to 1/8", what should the
>>drill size be?
>>
>>Cheers,
>>Paul

>
>Useful chart here:
>http://www.cjtkoolcarb.com/catalog/Technical/reamertechnicalguide.htm
>
>Looking at the allowances though, I'm sure that these are for machine
>reaming, i.e under power. For hand reaming I would allow less than
>these figures, perhaps by as much as 30-40%.

reaming under power, I have no hassles with taking a 3/4" drilled hole out
to 20mm.
--
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
------------------------------------------------
>> http://www.schlockmercenary.com/ << ...and Kill them.
a webcartoon by Howard Tayler; I like it, maybe you will too!

Posted by Dave Baker on January 17, 2008, 5:50 am

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>
> Can I ask if there is a "rule of thumb" regarding drill size for a
> particular reamer size? For example, if reaming to 1/8", what should the
> drill size be?

It depends a lot on the quality of the drilled hole. The Dormer handbook
distinguishes between pre drilled holes and pre core drilled holes (a core
drill is a special multi fluted drill designed for high accuracy). The
handbook advises against leaving too little stock for the reamer to remove
which is meant to promote rubbing and wear of the reamer.

In practice however for low volume and home shop applications none of this
is likely to be relevant. As long as there is some stock left and the hole
is straight enough to clean up I find any reamer works just fine. The less
stock removal the better IMO. For small sizes (up to 10mm) I always double
drill and finish off at about 0.1mm below finished size at a slow drilling
speed, say 200 rpm. The key to accuracy with a drill bit is slow speed for
the final cut so it doesn't chatter and cut oversize. If the drilled hole is
not dead straight you might need a tad more than this but careful double
drilling usually works well enough. For larger holes, up to say 25mm I'll
leave about 0.2mm in.

Recently I had to enlarge some existing cast iron valve guides from 8.0mm to
8.04mm for use with different valves and had no trouble reaming them out and
the cut was clean and sharp.

As for speeds the handbook suggests half the drilling speed but even so says
"in fact present ideas tend to reduce this even further." Well they
certainly do and IME if you use anything like half the suggested high volume
drilling speeds you'll run into a lot of trouble on light equipment with
chatter and oversized holes. I find 200 rpm to be fine at 8mm for example
and half that maybe for much larger reamers. Basically it's whatever works
best for you and your equipment but start slow and work up to higher speeds
if the equipment and material will take it.

Remember that most advice you'll see on speeds and feeds for anything are
biased to high volume production work on rigid machines to get the job out
as fast as possible with 'acceptable' tool wear. For shop use if you go
slower you won't be wasting much time anyway and the tooling will work
better and last much longer.
--
Dave Baker
Puma Race Engines

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