Surface finish problem - Could you please help me overcome this problem?

[DMBGO]

Hi I have an Emco style Chinese bench lathe, that I am about to convert to CNC.
I thought that before I started the conversion I would get the machine set up as nicely as possible, so I have made a new toolpost to overcome some of the issues with the standard toolpost, which was pretty rough.

The new toolpost works well, but when I am facing I get a lovely finish on the outer circumference of the work, but as I progress towards the centre the surface starts to tear and gouge. the centre inch or so is aslo deeper than the rest of the surface.
Here's a photo of the best I have been able to achieve.



I am running the 4 jaw chuck at 1500 rpm and 0.0008" per revolution feed rate. I have tried different feeds and slower speeds, but this combination produces the best results so far.

Here are some photos of the toolpost, the lathe and the way piece of steel that I was machining. The steel is 4140, which I know can be tough to machine.

I'd really appreciate some helpful advice.

Cheers
Dave

[trevj1]

Sit down and work out what the surface speeds are in the areas that the cut is working for you, and calculate the surface speeds in the area that it is not.

As you approach center, the speed that the material moves past the cutter, slows, a lot. This slower surface speed causes the cut dynamic to change, the chip formation and surface finish change as well.

With a cnc lathe, the answer is to program for a constant surface speed cut, using a variable speed motor on the lathe. On a bench lathe, the use of a variable speed motor, and raising the rpm as you approach center, is an option.

Otherwise, it becomes a matter of finding that happy medium and making it work out.

Using a quality insert is a real plus too, if you are using inexpensive Chinese inserts.

Using a cutting fluid? Even applying some with a brush will make a difference, flood is better.

A sharp HSS tool can do wonders at slower rpm too. Hog off what you want moved, then finish pass for pretty with a HSS bit.

Cheers
Trev

[DMBGO]

Thanks Trev, I'll give your suggestions a try. I am using flood coolant and the lathe is flat out speed wise, so I'll give the HSS toolbit a run.
Thanks again.
Dave

[cnc_swe]

Hi, I do not know your background as a lathe, but have you set the tool at the right height.
If the machine is not loose, the tool is sharp and right grinded it sounds as if you have the low
speed when you come to the center. Thin tag does not necessarily mean that it always becomes
smoother surface. Most types of steel have their properties.
Steel 4140 is a tough-hardening steels in Sweden is the same as the 2244th
Are you running with carbide inserts. Find a table so you can see what is required of your tool.
Maybe you need higher speed the last 10mm of the center.
Please be sure that the tool is not the center of the external turning.
In your case, you must be at the center of 0.5mm otherwise you may get strange phenomena in your case

Good luck

[DMBGO]

Thanks for the suggestions, the tool is dead centre and is exactly at centre height and all of the ways except the cross slide are locked. I think it must be surface speed. I have a 3 phase motor and inverter, so I will fit these and try raising the speed as I come towards the centre.

Cheers
Dave

[cnc_swe]

Fine, try again with a more soft steel and if you found with Lead (plumbum)
manganese alloy with lead is often used in hydraulic pistons and it is fine to work in.
Next try to adjust anything down your steel. For the right chip removal is not
tool to be exactly in the center and last speed up to 2000 in the last 10mm.