Different speeds=Different cut depths?

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?

brian: in an ideal situation you have a roughing tool and you rough with a heavy DOC and a slow speed. and hog of the material . then switch to a finishing tool and take the last couple of light cuts with a faster speed. to get the nice finish. I think in reality most of us use the same tool . but saving the finish tool for finishing saves it and extends the life and the ideal geometry is different for each application.

Results cam be unpredictable the metal may be growing a bit due to heat hard to say . you are not the only one that things do not always go exactly as planned.
Tin

Hi there,
Keep in mind that during heavy cuts, your entire machine deflects slightly. When you go for your finish cut, your machine springs back slightly, thus giving a deeper cut than you would like.This is especailly true of small machines. I know that seems impossible, but it is quite true and easily proven with a dial indicator. The answer is to rough cut no closer to your final size than about 10 thou or so. Then mic your part, calculate how much more you really need to take off, then finish to size. I personally like to have my final cut no deeper than about .002. Unless I'm turning something that work hardens really bad like Hastalloy.
Hope that helps!
Chris

Hi Brian,

This happened to me a couple of weeks ago while I was turning the register boss of a back plate for a Toolmex chuck on my Taig lathe, everything was spot on untill the last cut of 0.002" to bring the registrer boss to the correct diameter but it all went wrong, it was over cut by 0.002" and a perfect fit became a loose one. I think that I have spring in both the cross slide feed and the nut as this happenes from time to time. Perhaps the best way is to repeat the last cut but one to take out the gremlins.

Regards and Greetings,

A.G

Hello Brian,

I would say to you that when roughing you need to leave on 0.020" ( diameter ) to allow for a separate finishing cut.

It is never really a good idea to use a roughing tool to also do the finishing cut, although it is convenient not to have to change tools, in most cases one can get away with a resultant "acceptable" finish.

If you are hoping for a fine quality finish, and accurate finished diameters, it is best use a different tool specifically for the finishing cut.

For Aluminium, a razor sharp piece of High speed tool steel with a steep cutting rake angle and a 0.3mm tip radius, and use a squirt of soluble oil mix from a washing-up liquid squeezy bottle.


You did not mention the diameter of the piece that you are turning, which is important for working out the optimum finishing cutting speed in revs / minute.

Finish cuts on ( most ) aluminium grades give a good mirror finish at 150 metres / min surface cutting speed, and a feed rate per revolution of half the cutting tool point radius.

So for example a piece of 50mm diameter bar, at 150 metres / minute surface cutting speed, will need to run at 950 revs / min, with a feedrate of 0.15 mm / rev.
( 144 mm / minute )

For 150mm diameter, to achieve the same cutting speed and finish, you will need to run the lathe at 380 revs / minute, with the same feedrate of 0.15 mm / rev.

Obviously you are held within the limiting constraints of a "Home" lathe, where choice of spindle speeds and selected feedrate options are few.
You will have to experiment with the available saddle feed rates on your machine.

It is well worth delving into the dark scary world of working in Metres / minute, Ha Ha...
for this will give you the correct revs / min speed value you need for the same and consistent results at any diameter that you are working on.



Metres / min = ( Diameter in mm x 3.14 x Revs / min ) \ 1000

150 = ( 50 x 3.14 x 950 ) / 1000

~~~~~~~~~~~~~
Revs / min = ( Metres / min x 1000 ) / ( 3.14 x Dia )

950 = (150 x 1000 ) / ( 3.14 x 50 )

~~~~~~~~~~~~

So your 970 rpm spindle speed is about the correct spindle speed if you are turning some bar around the 50mm diameter.

Of course 150 metres/min for aluminium is not a fixed number, there is a wide variation
to play with, especially with the softer materials, and every other type of metal has it's own particular range.

You can find recommended Metre / min turning charts for different metals on Google.

Picture : A 12 x 12mm square hand ground finishing tool in HSS tool steel.
5 degree relief angles on three planes.

Nice smooth groove radius gives a good material flow and prevents material binding on
the cutting edge.

dave

When you work in a job shop, speed will keep or cost you your job.
Some machinists take medium rough cuts and depend on one finish cut being accurate.

I always took the heaviest rough cuts the machine would push and allow stock for 3 finish cuts. The first pre-finish cut was just a spring cut that would hopefully remove the effects of tool push off. The second pre-finish cut was to check for taper in the cut.
The third, and REAL finish cut was cutting for the size.

The only for sure thing in machining is nothing is for sure.

Rick

For aluminum especially, make sure the work is cool before measuring and taking finish cut(s). My lathe can take a pretty big roughing cut, and that heats up the work quite a bit, be it aluminum or steel.

Its also helpful to remember that a lathe will "cut away from the stock" whereas a mill will "cut into the stock".

lensman57 said:

Hi Brian,

This happened to me a couple of weeks ago while I was turning the register boss of a back plate for a Toolmex chuck on my Taig lathe, everything was spot on untill the last cut of 0.002" to bring the registrer boss to the correct diameter but it all went wrong, it was over cut by 0.002" and a perfect fit became a loose one. I think that I have spring in both the cross slide feed and the nut as this happenes from time to time. Perhaps the best way is to repeat the last cut but one to take out the gremlins.

Regards and Greetings,

A.G

Click to expand...

Machine and tool deflection come into play. A heavy cut has more of both and the actual cut is smaller than what was dialed. Then when you dial in a light cut afterward, cut the actual cut is much closer to the dialed in amount and if there is less deflection in the machine and tool it will cut more than dialed from the last heavy pass. Making a spring pass after the last heavy cut will give you an idea of how much deflection there is.

Brian,

When you're pushing what the machine can do, the machine limits the speed which can actually be cut due to deflection, chatter, or power limits. In production where equipment is stout and not worn, surface speed is fixed based on material and inserts to give the best tool life. Tool geometry can certainly result in different cutting characteristics with surface speed. I would avoid changing both speed and DOC for a tight tolerance part. I usually leave enough material for 3 passes at finish depth of cut. I measure after the first two to see how much is actually coming off. This is especially important in steel where the insert may not allow a very light pass with good finish. So when cutting steel, I have to plan ahead to make sure that last past is within tolerance. In aluminum, with aluminum cutting inserts I have no trouble making a .0002" pass, especially with some cutting lube, usually paraffin oil. If I blow the size on steel (oversize by a hair), the sharp aluminum cutting inserts will allow fine cuts, but it's murder on the inserts. Now, you didn't say if you're using inserts and I don't remember if you do.

So, no you're not the only one.

And, whether or not a mill cuts into or away from the part depends on whether you're climb or conventional milling.

And, whether or not a mill cuts into or away from the part depends on whether you're climb or conventional milling.[/QUOTE]

I was referring to a boring operation where a rotating tool will cut more where there is more stock. Been doin' this for 30 years. Happens every time.

The original post mentioned changing speed on the final cuts. I believe, all things being equal, changing the speed alone will alter the resulting diameter. As several have mentioned, take the final cuts in increments, BUT, take the final cuts at the same feed/speed. To a degree, even changing the feed rate will change the depth of the cut.

generally finishing feeds are slower and speeds are higher for better finish and to save time(since generally the feeds are set relative to revolutions). but you should take more than one Finnish pass so you can recalibrate for different amounts of deflection. but it is not always necessary to change speeds. in a hobby scenario on a small machine you are not likely to be using aggressive feeds that leave a poor finish and higher speeds can cause irregular surface Finnish if the cutter radius is large or the cutter is dull. basically if you want to adjust the speeds and feeds you may also want to grind Finnish tools with smaller radii and change over. otherwise yay may be fine with you roughing tools feeds and speeds for everything.

any time you cut metal there is deflection. it can be in either direction in both the cutter and the part making the part bigger or smaller than ideal. any time you change your feeds, speeds, depth of cut, or any part of the cutter geometry you change the deflection. part of machining is figuring out all the little things that make a good part. it doesn't come over night.

a thing to avoid is too much rake and relief that could cause the cutter to dig which will make diameters less predictable as you will have to deal with leads screw backlash if its enough to suck the cutter into the part. I've seen people say you should have 10-15 degrees rake and relief! that's total nonsense. my cutters probably don't have more than 3 degrees of either but you may need more rake if chip ejection is a problem or things are getting hot.

a good tip is to try taking a "free cut" when you move on to the finish cuts. just take a pass without dialing in anything to see how much deflection you had and to get you setup for taking finish cuts so the effects of deflection on the roughing passes don't mess with the finish diameter.

teach your self to read your chips. and figure out how to alter he cutter and the techniques you use to get good results.