Lathe problem - seems to cut too much?

[loply]

Hi folks,

I've been having a problem with my little lathe - it always seems to remove too much material.

A few days ago I was turning a rod of stainless down to 10mm and as I approached the size I mic'ed the rod and found I had 0.32mm to take off, so I popped a dial indicator on the toolpost and wound it in 0.15mm and took a pass. Prior to this I had taken a few spring passes and the finish was even.

I mic'ed it afterwards and it was 0.03mm undersized

Just been out experimenting and I played around with some 8mm stainless bar and some 32mm mild steel and sure enough this happens every time.

In one instance on the 32mm bar I had 0.20mm to remove to reach my target, I dialed in 0.08mm and ended up 0.02mm under.

I'm using short lengths of bar, finishing with a high angle CCGT carbide insert, the bearings on the lathe have been warmed up fully, and in all cases I used a dial indicator to measure toolpost movement. The lathe is a heavily modified 7x14 Asian thing. I checked all the obvious things like saddle or cross slide looseness, but it's all snug.

Any ideas what might be causing this? Could this be something to do with head alignment?

Cheers,
Rich

[philbur]

Possibly your tool post tends to nod forward proportional to the cutting load.

Try to do the last three passes at the same or at least similar DOC.

Phil

I popped a dial indicator on the toolpost and wound it in 0.15mm and took a pass. Prior to this I had taken a few spring passes and the finish was even.

[chipmaker4130]

Is the cutting edge exactly on center height? Is the 'after' measurement the same on both ends of the bar? (taper?)

[Black_Moons]

hmm, Consider maybe putting a TDI onto your lathe bed, and checking the cross slide movement verus what the dial says.

Also, Most people usally 'sneak up' on the final dimentions.
ie if you had 30mils to go, your next cut would be 25 mils deep, then say theres 3 mils left after measurement, well your next cut would be 1mil (or maybe just a spring pass!), And see where that gets you.

Yea, its awsome just to see 'oh it needs 30 mils, i'll dial in 15 and cut the next 30mils in one pass and get a good finish and be done in 30 seconds' but it never seems to work that way for me!

[Scottike]

Before I'd be thinking something serious like head alignment, I'd look at some simpler, more likely solutions.
Have you tried using a fresh insert to make your finish passes with?
Are you using the correct speed, feed & doc for that insert?
Most inserts have a recommended feed, speed and doc range they work best at. Check the insert mfr specs for that insert.
Is your toolbit height centered properly? A toolbit that's too low can cause the work to try and "climb" the toolbit causing a deeper cut.
Some materials respond to a given doc & feed better at certain speeds.
And I,ve seen some materials (stainless and some steels) that have actually "work harden" while being roughed, then when you try to take a lighter cut the tool bit will actually grab the "hardened surface and end up "peeling" it off the softer material underneath.
You also might try using a HSS tool also, rather than an insert.

edit: As Black Moons said - Just turning the dial to a doc doesn't mean you'll get it, usually it ends up being more or less than you antipicated, for a variety of reasons. I usually try a few practice finish passes when I'm roughing, just so I know how things are responding, then I can allow for it when I make my final pass(es).

[gwilson]

Lathe is probably not rigid enough. Tool may be "sucking" into cut some. Try a less aggressively ground tool that won't tend to pull into the work.

[vpt]

hmm, Consider maybe putting a TDI onto your lathe bed, and checking the cross slide movement verus what the dial says.

Also, Most people usally 'sneak up' on the final dimentions.
ie if you had 30mils to go, your next cut would be 25 mils deep, then say theres 3 mils left after measurement, well your next cut would be 1mil (or maybe just a spring pass!), And see where that gets you.

Yea, its awsome just to see 'oh it needs 30 mils, i'll dial in 15 and cut the next 30mils in one pass and get a good finish and be done in 30 seconds' but it never seems to work that way for me!

^ that stuff.

[firbikrhd1]

As an experiment, why not try some HSS cutting tools. They tend to be keener and require less power to cut the same material depth and lower forces on the machine. If rigidity is the issue perhaps a keener edge will be the resolution.

Some here have said that Carbide today can have very keen edges and that may be so. I'm a HSS user exclusively so I haven't any experience with carbides, only can pass along what I've read, i.e. anecdotal evidence of what I said above.

[Forrest Addy]

I suspect part or tool deflection on the previous cut. You took a semi finish cut that resulted n X deflection, dialed in, then a finish cut resilting in a similar deflection.

Then you took a couple "spring passes" (Rant:: spring passes - I hate 'em. If you have the variables under conscious control you very seldom need them. If you don't, you use them like magic charms, hoping something works) machining away the deflecton and - surprise - the part is U/S by the amount of the deflection.

Using HSS is a good tip. Carbide takes 2 -3 times the force to penetrate the work compared to HSS. Dead keen HSS in mild steel takes very little force. Stainless is harder to penetrate and since it forms a resistant work hardened film from the passage of the tool, light cut finishes can be inconsistant. As a general rule 0.010" is the least practical finish cut in austinetic stainless. As ever, YMMV.

[saltmine]

It may just be the metal cooling off that accounts for the dimensional change. Aggressive cutting, especially with carbide cutters, can cause this.

The other thing is possible taper cutting. The spot where the measurement takes place might be closer to the axis of the taper, and may actually be the dimension you set it to.

I discovered a long time ago that I was always cutting things undersized, until I started "sneaking up on the final number, and then working the final dimension down with a file and emery cloth. Of course, if you're one of those CNC whiz-bang guys, hand filing and polishing with emery is out of the question (I have yet to see a G-code for hand filing or polishing)

Being an Asian clone, I would suspect either a mis-alignment or a lack of rigidity. Even though tool bit placement can play heavily into a tool "digging in"

[GadgetBuilder]

I use a 7x12 for much of my work and mostly use HSS because it takes less power, something the 7x12 lacks.

When size needs to be exact I get close to the target diameter, within 0.3mm say, and then switch to the vertical shear bit:
http://www.gadgetbuilder.com/VerticalShearBit.html

The vertical shear bit takes predictable small cuts (while leaving an excellent finish) so it allows sneaking up on the finished size in very small increments. I use the dial for this rather than an indicator because tiny infeeds are easier for me to judge this way (infeed varies slightly vs material), YMMV.

Edit: Fix link

[J. R. Williams]

Cut

Go for a HS tool and have it sharp. Carbide tools have a feed/speed cutting window for each tool. Work outside the window and you will have problems. Your problem may be the tool and not the machine.
JRW

[90LX_Notch]

Check the alignment of the indicator. It needs to be perpindicular in both planes. When you went to take .30, you were .03 under. When you went to take .20, you were .02 under.

[Alistair Hosie]

Please also remember the advance rule when requiring to remove 2mm you advance the tool 1 mm as it removes from back and front at the same time .Most metal lathes are set to take this into account .Ok I just wondered ? Alistair

[loply]

Hi folks,

Thanks for the very useful contributions.

Just to answer some of the queries-

I am using a carbide insert which is meant for aluminium, it has a very high rake angle and seems to require less cutting force than my HSS tools, I say that because it cuts more silently and leaves a better finish, I do have and use some HSS tools but the finish from these high rake carbides is a lot better, I will repeat my experiment with HSS.

Tool is definitely on centre.

Re the tool nodding forwards or the work climbing, that's what I thought and is why I tried on a 32mm bar, I figured that would be too stiff to climb the tool.

No taper is present and I did try to ensure the part wasn't hot enough to shrink it's way undersize.

I will take some of this advice onboard and have another play with it tomorrow.

Cheers,
Rich