At a complete loss for ideas

S

Scott-ak

Registered
Registered
Joined
Nov 14, 2020
Messages
42
I've got a new PM1440GT. I can not take light cuts on small diameters, like 1" and down. Anything less than about 5 thou depth of cut results in a surface with grooves and such with 2 thou differences over the length. I have made a few parts of larger diameter w/no problems. This pic is of my 4th attempt to cut a 3/4" diameter for an arbor.

I have tried everything I can think of including locking down the gibs on both the cross slide and compound. I have tried many different speeds, DOC and tools and still get this unusable finish on the right.

The pic is 2" round bar. The left side was cut with the same carbide tool as the right. Same DOC & feed rate.
 

Attachments

  • 20201211_190816_resized.jpg
    20201211_190816_resized.jpg
    480.4 KB · Views: 194
I think the edge radius might be to big on that carbide insert..... Have you tired a tool with a sharp edge?
 
Are you cutting 1018? 1018 almost always does this and its fairly difficult to get a good finish..... Also try a HSS tool.....
 
I don't know the grade, It's just material from the local steel yard. I don't have any trouble with it at the larger diameters. I'll see if I have some other material I can try
 
1018 is difficult to get a good finish on to begin with, but the depth of cut should be at least half the nose radius, and the insert you are using does not have a honed edge so is not particularly sharp. Unless you have an insert with a honed edge, the the most expedient approach might be to switch to HSS tool. For this operation with carbide, you'd be best off with a CCGT and a #0 nose radius (0.004"). The CCGT will have a positive angle of attack instead of negative, the G tolerance will ensure the faces of the insert have been honed to get to the 0.001" tolerance, and with a 0.004" nose radius it should cut well at 0.002" or larger depth of cut. Difference shown below:

CCMT v CCGT.jpg
 
Try to find some 1045 steel, you will be able to get a much better almost mirror finish on it very easily with only a lathe, its also a little stonger than 1018 steel and can be heat treated if you need to..... Its usually pretty low on the price list at most suppliers....
 
Scott-ak said:
I've got a new PM1440GT. I can not take light cuts on small diameters, like 1" and down. Anything less than about 5 thou depth of cut results in a surface with grooves and such with 2 thou differences over the length. I have made a few parts of larger diameter w/no problems. This pic is of my 4th attempt to cut a 3/4" diameter for an arbor.

I have tried everything I can think of including locking down the gibs on both the cross slide and compound. I have tried many different speeds, DOC and tools and still get this unusable finish on the right.

The pic is 2" round bar. The left side was cut with the same carbide tool as the right. Same DOC & feed rate.
Click to expand...

I suggest you consider going back to basics. You're using a finishing insert with positive rake, which is good. I cannot tell what the nose radius is but as David just posted, the nose radius affects the minimum depth of cut the insert can take. I'm going to guess your nose radius is on the larger side, maybe 0.015 to 0.016 or so. If that is so then the minimum depth of cut the insert will take will be about 1/2 the nose radius so, at best, it will take a minimum of 0.008" DOC. If you try to less than that the insert will skate and deflect, which may be part of the issue you're experiencing.

I also cannot tell if the nose of your insert is just dirty or if there is a built up edge on the tip or if the radius is chipped but I would look at it under magnification to make sure it isn't damaged. If it is, index it.

Finally, speeds matter. In fact, nailing the correct speed is more important than depth of cut or feed rates. If that is some kind of mild steel then most inserts will have a cutting speed somewhere near 800 sfm. For your 2" OD, that gives us about 1500 rpm but for your 3/4" OD, that is over 4000 rpm.

So, I would evaluate the condition of the nose and index it if necessary, then dial in about 1/2 the nose radius, then boost speed to the max your lathe will run and try the 3/4" OD again.
 
You must log in or register to reply here.
Back
Top