Surface finish problems

Some very interesting responses here.

DMS, I made up on of those cutters and managed to get a smoother surface finish but I still had rings of rough patches on the material. I thought it worked well though, definitely going to play around with that cutter.

Ray C, thank you for the explanations and the pictures. I was using a live center when turning, the material was a very old bolt, probably poor quality steel intended for general construction. Also about 1/2" diameter, the picture was enlarged.

markknx, I've had good results with stainless steel, one of my preferred materials. I've yet to try aluminium but I have load of stock waiting.


I guess the next question is what metals do you guys like to machine? (I see Rey C has already listed some)
 
Aluminum is nice, if you can avoid the built up edge. Brass is probably the nicest, but of course the most expensive. For steels, 12L14 is a joy, but the lead content has some drawbacks. 1215 machines like dung in my experience, even though it is supposed to be "free machining". I like 4142; it is tough and gives a great finish, but you have to get feeds and speeds just right, otherwise it looks horrible (at least in my experience).
 
Carlos SA said:
Some very interesting responses here.

DMS, I made up on of those cutters and managed to get a smoother surface finish but I still had rings of rough patches on the material. I thought it worked well though, definitely going to play around with that cutter.

Ray C, thank you for the explanations and the pictures. I was using a live center when turning, the material was a very old bolt, probably poor quality steel intended for general construction. Also about 1/2" diameter, the picture was enlarged.

markknx, I've had good results with stainless steel, one of my preferred materials. I've yet to try aluminium but I have load of stock waiting.


I guess the next question is what metals do you guys like to machine? (I see Rey C has already listed some)
Click to expand...

Carlos, check that your cutting tool is not protruding excessively from the holder and that the work is well supported. Whenever I have gotten a bad finish it is usually because of these factors, or because I am taking too agressive cuts.

I have also found that for mild steel, a sharp pointed tool leaves a much better finish than a rounded tool. The rounded tool tends to push the material away and then tear it, while the sharp tool shears it out. It's strange but when I found this out, I looked at the surface finishes and didn't notice any "thread" type pattern from the sharp tool at all but rather a smooth finish.

Paul.
 
I have good results with low rpm, a fine feed rate, sharp tool, light cut (.002 to .004), and lots of cutting oil. IMO the higher the RPM the more likely you are to get material build up on the tool bit that creates a "false" cutting edge. Because this false cutting edge tends to build up as the tool feeds along the work and then breaks off this is causing the irregular looking surface finish. A couple of other considerations are the rigidity of your machine and set up, and the condition of your headstock bearings.

BTW - the surface finish on your part looks pretty good to me considering it's CRS.

Tom S
 
Someone mentioned supporting the work which I have found helpful. try different types of bits too. Check the contact point of the bit to make sure it is closely centered on the workpiece. I've had luck taking deep cuts rather than light cuts to achieve a better finish.
 
AlrightI have been noticing a twisted pattern arround the part. not threading but a very faint twisted or spiral effect in the finish. About 1 twist per 3-4" reducing spead from 560 down to300 made it less. pronounsed but sill there. The effect happens on both SS and mild steel. the test parts where 3/4 . Don't know ifyou can see it in pics I can but it is hard to see and I know what it looks like.
012.JPG014.JPG

012.JPG 014.JPG
 
That happens primarly from two things. Bit is sticking-out too far from the tool holder or, the piece is sticking too far out of the chuck and is unsupported. It could be other things too (such as a bad bit and overall looseness in the mechanism).

There is also a condition where you will see the imprint of the leadscrew thread pattern -which has a different set of typical causes.


Ray
 
might be a dumb question but are you retracting the tool before you return the carriage towards the tailstock end of the bed? this is of course if you are feeding the carriage towards the headstock when making the cut (general rule is that you should be cutting this way if you are not). if you return the carriage real quick without retracting the tool it will cut a small amount, especially if it was not a rigid setup meaning that all the material wasn't removed on the initial cut.

I know that will cause that type of spiral pattern. if you did retract the tool before returning the carriage to the tailstock end then just ignore this post

Todd
 
There are two uncommon issues with 7x type lathes (and possibly other types of Asian lathes) which crop up occasionally. Neither is obvious unless you're looking for them - not a likely cause of your problem but easy to check.

If you're using auto-feed the gears may be meshed too tightly causing a periodic binding - this causes the carriage to slow down and speed up periodically leading to a faint spiral imprint on the work. Ensuring a little space between the gears by using a slip of paper between them as they're bolted down will avoid this.

Another possibility is the carriage hand wheel's mesh with the rack is such that each gear tooth rides up on a rack tooth, binds slightly, then slips off. Diagnosed by holding slight forward pressure (aiding the auto feed) on the hand wheel where this will cause proper mesh of gear to rack. This doesn't show up when feeding with the hand wheel, just when using auto feed.

John
 
Years ago I had an old worn out Logan and just could not get even a mediocre finish on steel. Aluminum & brass were fine. After trying carbide just about every conceivable HSS grind I suspected the machine was just not rigid enough. I made a toolpost that attached directly to the cross slide and eliminated the compound all together. The old Logan instantly became a different machine finally putting a decent finish on steel. Not a mirror, but totally smooth to the touch even when using the same tooling the had previously failed so miserably.
 
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