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brian Rupnow
12-23-2012, 02:02 PM
Does this only happen to me, or is it a typical machining gremlin? I do most of my aluminum turning at 970 RPM. I don't have read-outs, so I go by the dials on my crossfeed for depth of cut. I know that, infallibly, one mark on the dial to the next mark on the dial will tale .002 thou off the diameter. And it works consistently. 10 marks on the dial takes .020 off the diameter. However, on my last cut, I turn the speed up to 1620 rpm, which is the next higher available speed (in fact its the highest) to give me a really nice finish----And my formula seems to go out the window. This happens whether I am turning an o.d. or boring an i.d. And it happens for the worst, it seems. All my bores get "just a tad" bigger than they were supposed to be---enough to make an intended "light press fit" into a "pushes in with finger" fit, and if its an outer diameter, the part turns out "just a tad" smaller than the damned thing was supposed to be. Does that happen to everybody, or have the machining Gods singled me out for weird punishment?

lbhsbz
12-23-2012, 02:11 PM
At lower speeds, your tool is deflecting more than it is at higher speeds. Try to take 2 or 3 spring passes (do not advance the tool) before taking your measurement, then dial in .010 on the dial for a theoretical .020" diameter reduction, and see if you really get .020" off the OD.

Example...you take a piece of 2" stock and dial in a .020 cut (10 marks on your dial). Do to tool deflection, you're probably only really getting a diameter of 1.984 or so, instead of 1.980. Another .020 cut results in 1.964", etc... When you take your light finish pass at high speed, your going to have much less tool deflection so you'll take off the remaining .004", plus however much you've advanced the tool.

beanbag
12-23-2012, 02:16 PM
Are you one of those "turn by RPM" instead of "turn by SFPM" guys?

brian Rupnow
12-23-2012, 02:25 PM
Are you one of those "turn by RPM" instead of "turn by SFPM" guys?
Yes, since 99% of what I turn is 3/4" dia. or less, and my lathe is not variable speed. I have 6 speeds to choose from with 115 rpm being the lowest and 1620 being the highest. I generally turn aluminum at 970 rpm, and steel at 550 rpm. I use tool steel cutters, not carbides. I generally kick the lathe up one speed range for my last cut, to get a better finish.

MichaelP
12-23-2012, 02:26 PM
If the bores become larger and ODs smaller, I'd blame overheating. Rough it leaving some meat for the finish cut, let the work cool, measure it and take the final cut.

J Tiers
12-23-2012, 03:15 PM
heating, and possibly an effect of less "fuzz".

When you turn at a faster speed, the "phonograph grooves" are smaller and closer together if you do not alter the feed, so their "average height" is less. That will do exactly what you say.

beanbag
12-23-2012, 03:46 PM
I generally kick the lathe up one speed range for my last cut, to get a better finish.

Why not have the same speed for all cuts then?
I agree with J Tiers on the grove theory. Try grinding a wiper flat on your tool.

Jaakko Fagerlund
12-23-2012, 05:45 PM
Simple: Take the last two cuts at the same settings and try to get them to be same DOC. This way, when you take the second to last chip, you measure how much needs to be removed and take that. Always spot on, especially if you take even three "finish" cuts.

As for normal turning, I always rough the piece to about 1.5 mm bigger than the print calls out, cool down the work and take 0.5 mm, measure, take half of measurement, check again and take the final pass. Hasn't failed ever.

And spring passes? Nope, not needed and are generally a waste of time, tool and a poor practice to begin with.

becksmachine
12-23-2012, 05:58 PM
When you turn at a faster speed, the "phonograph grooves" are smaller and closer together if you do not alter the feed

Now wait a minute, this would be true if the feed was in "inches per minute", but I believe that most lathes would feed in "inches per revolution"?

Dave

brian Rupnow
12-23-2012, 06:18 PM
I don't use a power feed nor traverse. Its all manual.

topct
12-23-2012, 06:25 PM
If the bores become larger and ODs smaller, I'd blame overheating. Rough it leaving some meat for the finish cut, let the work cool, measure it and take the final cut.

Perfect. Depending on the alloy and cross section aluminum can really move around when hot.

Jaakko Fagerlund
12-23-2012, 07:29 PM
I don't use a power feed nor traverse. Its all manual.
That is one more variable as to why one gets variable results. Using power feed is highly recommended, gets consistent results and consistent work quality.

mike4
12-23-2012, 08:58 PM
Aluminium can be variable ,depends on the alloy a lot , as some have said second last cut undersize allow to cool and measure then two or three light finish cuts .
Occasionally I have found that a much higher speed than what is recomended with good coolant for the last passes gives a better finish , and I often change the tip to a honed one for better cut finish.

Depends on the job , as some ask for an as machined finish , others want a polished surface.

Michael

becksmachine
12-23-2012, 09:30 PM
I don't use a power feed nor traverse. Its all manual.

Ok, this would essentially be an inch per minute feed, depending on how consistent you are turning that hand wheel, which would equate to a much finer inch per revolution feed, making the scenario described by J Tiers very likely. Possibly made even more so as you may be turning the handle even slower to achieve a nice finish with the final cut.

Increased RPM + ≤ ipm feed = finer ipr feed

Jaakko's comment is very relevant.

Dave

demerrill
12-23-2012, 10:28 PM
Consider the mechanics of chip formation. The point of chip seperation from the workpiece does not necessarily coincide with the cutting edge of the cutting tool. Plastic deformation preceeds the advancing tool edge. This deformation produces significant localized adiabatic (means doesn't have many microseconds to be conducted away) heating. Depending on a number of factors including workpiece thermal properties, tool geometry, built-up edge and machining speed the softening effect in this 'heat affected' zone MAY alter the effective location of the chip separation point relative to the cutting tool edge.

Some flavor of these phoenomena may be gleaned from some of these U-Tube videos: http://www.youtube.com/results?search_query=machining+chip+formation&oq=machining+chip+formation&gs_l=youtube.3...33705.36303.0.36951.10.10.0.0.0.0 .52.474.10.10.0...0.0...1ac.1.SP9zqbZCmTo

This is a complex field of academic study that has been pursued for years. I pretend no expertise in this specific subject nor do I know whether it is the explanation for Brian's practical observation but I can see it as a possible contribution to one.

David Merrill

JEZX
12-23-2012, 10:30 PM
if you are cutting aluminum alot i assume you have tried a lil more angle on you cutting tool , if not i suggest it , it lets the chips flow away alot nicer then just a flat or 5degree or what ever is normal . . the flater cutting tool seem to build up a lil drag piece where it sticks to the aluminum .

J Tiers
12-23-2012, 11:26 PM
Now wait a minute, this would be true if the feed was in "inches per minute", but I believe that most lathes would feed in "inches per revolution"?

Dave

Precisely..... if you manual feed at the same rate you will get less fuzz and "depth of furrow" at a higher RPM.

Obviously if you use a spindle-linked power feed, it won't be any different. But I NEVER use power feed, my Logan variety HAS no power feed, just the half nuts, and I refuse to wear out my new leadscrew doing turning feed.