Why does increasing speed when taking very light finish cuts help things?

[digiex-chris]

I read about people taking finishing cuts on a lathe with a little higher rpm and a much shallower depth of cut, and a slower feed. I get the smaller DOC and the slower feed, but why the faster rpm? Is it just so you don't die of old age from the slower feed?

 

[TroyO]

Hah, kind of.

Think of it this way... if say you moved 1 inch per second @ 100 RPM, you would divide that inch in to 100 parts each one being a "groove" like in a record.

At 1000 RPM even keeping the exact same 1 inch per second feed, that same inch gets divided up into 1000 grooves.

10 times smoother.

 

[digiex-chris]

so, this only counts if I'm not powerfeeding (where the feed speeds up with the spindle)? at least on my lathe, where the only powerfeed is provided by the leadscrew. So really, this is more of slowing down the feed rather than speeding up the spindle?

 

[blighty]

if your using a geared feed from the head, you are basically cutting a very fine thread. so on the finishing cut you speed up the head and slow down the feed even more you are in effect cutting an even finer thread. i.e a smoother finish.

 

[blighty]

sorry, sort of said what Troy said. we must have been typing at the same time. ;D

 

[digiex-chris]

Glad there's no magic physics involved I couldn't bend my head around. Now I'm guessing, if I have a tool tip radius greater than the distance between those spirals, good things happen, right?

 

[Mosey]

I see that this is the place to pose my finish question.
I have been using some Nikcole minisystems cutting inserts because they seem to give a seriously wonderful finish, especially on brass or bronze turning. They are very narrow, usually .053" or less. They are grade C6 PVD TiN coated. Now when I use indexable carbide TCMM inserts in my Chinese holders the finish is just OK, not great. What is going on here? Is the USA made cutter better in shape or material?

 

[rake60]

I always leave at least two light finish cuts.
I guess it would be a pre-finish and a finish cut.

Both of those cuts are at higher speed and slower feed than the roughing cuts.
You are spot on about the tool nose radius. It should be bigger than the feed.

The first, or pre-finish cut, will remove any tool push off that may have been occurring
during roughing. It also allows a test for any taper in the cut that may be going on.

On the finish cut I go for the size, bumping the tool in or out .0005" at a time to eliminate
any taper that showed up on the pre-finish cut.

Rick

 

[TroyO]

Opps, yes I should have pointed out that I was talking about the feed/record groove thing when NOT using a geared feed from the leadscrew.

If it is a geared feed then upping it from 100 RPM to 1000RPM only serves to also increase the feed from 1 IPM to 10 IPM, which in effect keeps the "record grooves" exactly the same.

I hardly ever use geared feed on mine except for cutting threads so I kind of forgot that other folks do, LOL. I probably should... but unless I have a long straight length I tend to just hand crank it. I'm too lazy to set up the gear drive.

(Edit: I kan't speel gud.)

 

[mu38&Bg#]

Mosey, I think it comes down to sharpness of the tool. Those Nickole inserts are extremely sharp. They cut extremely well, so you can take a light cut with very little tool, part, and machine deflection. Most inserts do not have sharp cutting edges, they have a very small radius, to prevent chipping. If you were to try TCGX aluminum type insert with their much sharper cutting edges, you'd probably see similar results to the Nickole.

Greg

 

[tattoomike68]

For a Finnish pass on the lathe I go at least 1300 RPM+ on diameters up to 2" 4140 .010" deep and never a feed less then .005" per rev use a 1/32" nose radius and hone it with a diamond hone and get great finishes.

I dont ever turn with HSS unless it is snap ring grooves or big threads and olny use carbide to turn with. I tend to horse the piss out of my machines so HSS wont last but 20 seconds till it turns to soup.

 

[MachineTom]

The finish cuts are almost like art at times, one material with x feed, y doc, speed, then add the unlimited aray of cutters and everything changes, some 12l14 it likes fast with carbide inserts, but slow the feed down and it will look like a corn cob, faster feed and will almost shine, same everything else. With stainless too fast too slow SFM will lead to real problems, lite cuts need a positive insert designed for Finishing (LF). Lube counts as well. But when its right everybody notices.

 

[Mosey]

Mosey, I think it comes down to sharpness of the tool. Those Nickole inserts are extremely sharp. They cut extremely well, so you can take a light cut with very little tool, part, and machine deflection. Most inserts do not have sharp cutting edges, they have a very small radius, to prevent chipping. If you were to try TCGX aluminum type insert with their much sharper cutting edges, you'd probably see similar results to the Nickole.

Greg

Greg,
Can you point me to a reference for TCGX aluminum?
Mosey

 

[idahoan]

Hi Mosey

If you look at my build thread on the Morrison & Marvin vise you can see the Sandvik TCGX inserts that I use in some of the lathe pictures. They are wonderful for nonferrous metals, aluminum and plastic. They also work wonderfully on 12L14 and 303; I even use them for finishing harder materials after roughing.

Dave

 

[mu38&Bg#]

http://www.grainger.com/Grainger/SANDVIK-COROMANT-Carbide-Turning-Insert-4JKK7

xxGX is Sandvik's designation for aluminum turning inserts. Other manufacturers use some different designations. I use only these for aluminum anymore and have used them on finish passes for stainless. I can't say how they work in brass, but have a feeling they won't last long. With so many choices for inserts it pays to choose the recommended geometry and coating if it's important.

Machine Tom has a point though that finish turning inserts are also going to be much better than general purpose inserts.

I downloaded Sandvik's huge catalog a while back and studied it, bought some inserts and have been very happy. When you buy inserts you have to consider surfaces speeds, feeds and depth of cut. Some inserts want high speeds and at the diameters we are working with it could be well above what you're used to or your machine is capable of. Add to that rather high feed rates and turning can be down right scary. The other issues turning steel is than many inserts even finish turning inserts want generous depth of cut, something like .008" for the Sandvik's I bought. You have to plan well to get enough material to make that last pass at that depth. At .008" DOC (minumum recommended) the 1" round 4340 I was turning was beautiful, at 1500RPM and .004" feed. I ended up doing a final pass at .004" DOC and it was not nearly as clean. With lower surface speed it was the typical draggy cut and it was at the low end of recommended surface speed. This is where manual machining and CNC machining really show their differences regarding inserts. The latest and greatest insert might be no good at all in a manual machine.

 

[ShopShoe]

digiex-chris,

I know the subject has moved on to tooling, but there are discussions on the web from people have added independent power feed to their leadscrews to feed extremely slowly in order to get fine results. (This is only for finish, not thread cutting.) Kinda the same idea as adding a power feed to a mill for better finish. One way is to put a small gearmotor on the right end of the leadscrew: You disengage the threading geartrain and engage the slow-feed to use this feature. I have considered this as a future mod to my mini-lathe.

--ShopShoe

 

[Mosey]

Very interesting. I like to finish turn with power feed, but at high spindle speeds (1365 rpm)and low feed rate (.0015") (South Bend 10K), it loads the gear train to an uncomfortable level for my ears. I am unable to move the carriage by hand as smoothly as the power feed however to get those nice smooth final cuts.

 

[mu38&Bg#]

Feeding too slow also made the finish worse in my case. Minimum recommended feed was .002", and at the surface speed I was using it didn't work, .004" did and the finish was excellent.

The load on the gears is small. It's just noisy gears. I have shooting ear defenders in my shop and use them often.

 

[Mosey]

I'm afraid that I like music not noise from my machines. Too old to change. More grease in the train and better clearances might be the answer. as long as the pressure is consistent I get smooth.

 

[titex]

Mosey,
ANY insert with the designation ** GT are also for non ferrous metals and for finishing stainless steel [although they are often used for finishing plain carbon steel [at very small DOC] all these types have a very high rake angle and very sharp geometry and are usually polished and uncoated.
Titex.