Lathe Erratic Surface Finish

Not having much where to go I spent the afternoon in the workshop helping out a friend with a project he has. Part of the project required an 18" length of 3" dia schedule 80 pipe to be turned down to 68mm dia. I set up the pipe on my WM280 lathe, one end held internally on the 3 jaw and the other end supported on a revolving centre - I had to fit a cap to the end of the pipe to be able to support the pipe on the centre.

As there was quite some material to be removed and the pipe was quite long this was going to take quite some time so I set the feed selector on B (the highest).
Initially I started with 0.5mm passes at about 400rpm but this left a very poor finish for around 4" to the tailstock end. The finish was almost similar to steep angled knurling! I tried reducing depth of cut and use a finer feed selection but with same result, chuck side is close to mirror finish, tailstock side looks like a multi-start thread.

Out of frustration I cranked up the speed to 1500rpm with a shallow 0.1mm cut and the problem with surface finish disappeared - mirror finish all through the whole length. Not happy tho, I hate blue stringy swarf which does not break off. I was using a TNMG carbide insert which I normally use without any problems at all.

Most probably I was doing some or a lot of mistakes - I wonder...

Chris

Don't knock it, a good finish is a good finish no matter how it was obtained.

I suspect part of the issue may be that the tailstock centre allowed the pipe to flex slightly compared to the chuck end. Schedule 80 refers to the pipe size but not the material. I'd guess it's a low carbon steel and hence might be sticky. The key to machining sticky steels with inserts is high surface speed, as discovered.

Andrew

They say carbide loves speed & sparks tu boot.

Steve.

Pays to play with speed and feed to be sure. Pipe can be tricky material to machine. Welded seam down the length adds to its woes.

True, the variable speed lathe makes it easy as well as I can change speed on the fly, but in this case, turning a 3inch diameter at 1500rpm, I was saying to myself - this must be wrong!

The pipe was a seamless pipe, but not sure of the material, I think some kind of mild steel, we call them steam pipe over here. When picking up speed I noticed some vibration at the 600 to 700 rpm so maybe the 18inch length was at the limit for my lathe.

On small parts I don't worry too much to go up in revs, I often do and the results are always good (apart from the red chips) Larger parts do get me apprehensive to be honest!

Some materials are difficult to get a good finish on. Sometimes having the surface shiny may actually be a worse actual surface finish than one that looks dull but has no tearing. In general , a depth of cut of 1/4 the radius of the tool ,and a feedrate of 1/4 to 1/8 feedrate. So with a 0.4 radius tool, a 0.1mm depth of cut (0.2mm diameter) and a feed rate of 0.1mm to 0.05mm per rev .Sometimes an oil can help improve the surface finish, sometimes a soluble coolant can help as well.

If its anything like ERW tube then as Andrew says its nasty sticky stuff, I find **GT inserts work better particularly as the wall thickness comes down

I had a bit of tube "ringing " . Deafening it was. Stuffed it full of rag., end of problem. Nice finish too.

For the lathe I only have TN** insert holder, maybe I should get a different holder with positive rake. What would the triangular equivalent for the inserts you mentioned be?

I'm pretty sure the pipe was not the seam welded type, it machined pretty well for the 3/4 part closest to the chuck, it was the last quarter which gave me a headache, which is strange, but then again, I had never turned that length of pipe. Spring passes in the end solved the issue, I was more worried about the swarf getting at me!

Finished

Swarf!!

Personally, I'd be pleased with that, well done!

The multi-thread problem at the tail-stock end is likely due to movement; it doesn't take much to spoil things. Maybe a steady would have helped.

However increasing speed and depth of cut is often a very good idea with carbide. There's some combination of tool-shape, surface cutting speed and feed-rate that works best with the material being cut. There's no guarantee in a limited home workshop that all four can be satisfied at the same time. HSS may be easier because it works well at slower speeds than carbide, but grinding the tool is your problem!

With one exception, I've always found steel pipe difficult. Sticky and often gritty as well, yuk. Looking up Schedule 80 pipe on The Engineering ToolBox I see ' intended for mechanical and pressure applications. Can be used in steam, water, gas and air lines. Suitable for welding and forming like coiling, bending and flanging.' So, in choosing suitable steel for making Schedule 80 pipe, machinability isn't a consideration at all; they don't formulate it to be precision machined.

Certainly not impossible to turn nasty steel pipe in a lathe, but finding the sweet spot may be a right fiddle. Provided the finish was OK I'd put up with unbroken blue stringy swarf, but increasing speed again above 1500 rpm would probably spray chips. Like as not though, increasing speed would spoil the finish, making it necessary to repeat the fiddling stage again.

I find carbide insert catalogues difficult to navigate. What I need is an index that goes straight to the point: 'For sticky mild steel as used in Schedule 80 pipe, select Insert XYZ'. They don't do that! Instead, giant tables full of information I don't understand. If the professionals don't know what to buy, insert makers offer a service where you tell them what you need to do, and their expert makes a recommendation. Unfortunately this is for big spenders only, not the chap with a lathe in his shed who buys inserts off ebay. For ordinary purposes the simplified range of inserts sold by ArcEuro and the other Hobby Suppliers are suitable for machinable metals, but they're unlikely to be ideal for tricky materials like pipe.

Congratulations! Effective result achieved in tricky circumstances, I'd be smug for at least week.

Dave

TCGT

The only time I've seen frizzy blue swarf like that shown was when turning hardened silver steel with a CBN insert. The turning was done at high speed, shallow DOC and a high feedrate. The swarf was also very brittle.

Andrew

Thanks Jason, appreciated. Dave thanks for the encouragement!

The swarf was brittle when I was taking a deeper cuts at a higher feed rate, but there were also some sparks and thought the insert would not like that, so I reduced the feed and depth of cut.

I’m looking at the tailstock extension. Something I keep to the bare minimum, whenever possible. Every little helps....

I have TNMG 16 inserts in every grade from cast iron, stainless and aluminium. Its worth getting some of the very cheap Chinese ones, they are surprisingly good, the Drillpro ones get used more often than my big brands. The more different grades you have, the better chance there is of one being just right for the job. Resonance can be a pain at times, the only thing to do is try changing the feeds and speeds as you did, unless the flexibility of the work of tooling is to blame. The aluminium grade can be used for finishing steel when small depths of cut to creep up on the size are safer.

https://www.ebay.co.uk/itm/H-KORLOY-TNMG160408-HA-H01-Carbide-Inserts-CNC-TOOL/233348941822

Edited By old mart on 03/05/2020 19:21:06