# At a complete loss for ideas



## Scott-ak (Dec 12, 2020)

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.


----------



## Cooter Brown (Dec 12, 2020)

I think the edge radius might be to big on that carbide insert..... Have you tired a tool with a sharp edge?


----------



## Scott-ak (Dec 12, 2020)

I have tried CNMG, WNMG & TNMG cutters


----------



## Cooter Brown (Dec 12, 2020)

Are you cutting 1018? 1018 almost always does this and its fairly difficult to get a good finish..... Also try a HSS tool.....


----------



## Scott-ak (Dec 12, 2020)

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


----------



## extropic (Dec 12, 2020)

Use a sharp (cutting edge), slightly radiused, HSS tool, set at center height.


----------



## davidpbest (Dec 12, 2020)

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:


----------



## Cooter Brown (Dec 12, 2020)

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....


----------



## mikey (Dec 12, 2020)

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.



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.


----------



## Scott-ak (Dec 12, 2020)

Back in the shop, ground an HSS cutter, much better, thanx!

I'll make some notes of the carbine related comments for later review


----------



## Bi11Hudson (Dec 12, 2020)

HSS shear tool. . . 

.


----------



## Canus (Dec 12, 2020)

+1 for Bi11Hudson comment.  I use my HSS shear tool regularly to achieve fantastic finishes.  Remember to take VERY light cuts .001-.002 with a slow feed rate.


----------



## ShagDog (Dec 12, 2020)

Scott-ak said:


> ... Anything less than about 5 thou depth of cut results in a surface with grooves and such with 2 thou differences over the length.  ... This pic is of my 4th attempt to cut a 3/4" diameter for an arbor.
> 
> ...
> 
> The pic is 2" round bar. The left side was cut with the same carbide tool as the right. Same DOC & feed rate.



That looks like a short piece. Are you saying you were getting a taper of .002 over that short distance? Was that also resolved with the HSS tool?


----------



## Pcmaker (Dec 12, 2020)

I generally get better finish with HSS with a large radius

That could be hot rolled steel also


----------



## GunsOfNavarone (Dec 12, 2020)

Smaller radius for sure BUT...hot rolled anything, not to mention 1018 and I challenge anyone to get a mirror finish!


----------



## jbolt (Dec 12, 2020)

Also when turning on the PM1440GT run the compound parallel or perpendicular to the travel. If you have the compound set near 30 deg for threading, one of the t-nuts that hold the compound to the cross slide is in the access hole slot for the t-nut which affects rigidity. Generally the compound is the weak link for rigidity on this lathe. I eventually removed the compound on mine and replaced it with a solid tool post riser. I rarely use the compound except for short tapers I cannot do with a form tool. Carbide insert cutters for steel need more rigidity than properly ground HSS. I can get great finishes on 1018 and A36 with inserts and the solid tool post riser but they need to be run deep, hard and fast. 

This post shows the solid tool post riser and the finish improvement.


----------



## Pcmaker (Dec 12, 2020)

I wouldn't have thought you'd have rigidity problems with something as big as the PM1440.


----------



## davidpbest (Dec 12, 2020)

The compound really is the weak link in the rigidity - even if you position it so the t-nuts are not straddling the mounting holes.  Like Jay, I implemented a solid tool post and wow - what a difference in rigidity.  It really helps with parting and finishing on 1018.  *This post *shows the implementation for the BXA solid tool post on the 1340. It's been over a year since I had the compound on the machine. Thanks Jay for the encouragement to go this direction. It was actually a very fun build.


----------



## jbolt (Dec 12, 2020)

Pcmaker said:


> I wouldn't have thought you'd have rigidity problems with something as big as the PM1440.


It's not limited to the PM1440GT but more a function of the style/design of cross slide and compound on a lot of the import machines. At the end of the day it is still a hobby class lathe, though quite capable for it's size, there are areas that can be improved to squeeze out more performance.


----------

