What is the material – can't really tell from the photo. Is it aluminium?

If it is, I had the effect this evening myself. I speculated that in the uninterrupted cut the metal chips were getting warm enough to adhere to the tool tip on my carbide face mill. In the interrupted parts I speculated the tips cooled off and the chips never got hot enough to adhere. Testing this hypothesis, I reduced the feed rate a great deal, and ended up with a near mirror finish every cut thereafter. Even if the hypothesis was wrong, the result was good.

May be the case with you if it was aluminium.

Regards

Richard.

Dooooh, dunno why I thought it was aluminium. BMS can be one of the more difficult materials. Do you know exactly which grade? EN3B is particular can be a bit "sticky". I wouldn't bother will coolant, brushing on is really neither one thing or the other. Unless parting off I turn dry with carbide tooling.

Have you seen this thread:

**LINK**

As and when we meet we can try some turning experiments, after tea of course.

Andrew

Have you checked the tip of your carbide tool with a magnifying glass? Quite often an interrupted cut will break the tip off a carbide tool. The now-flat end of the tool will often rub on the job, giving a mirror finish as seen on your outer corners, but also often creates a rougher finish as seen in the centre part of your job.

The higher surface speed and interrupted cut on your outer corners would probably encourage rubbing of the tool and the resulting mirror finish, whereas the slower surface speed and non-interrupted cut in the middle with a chipped tool will give a rougher finish.

I always use a HSS toolbit with a radiused tip for interrupted cuts. Although, today's carbide inserts are a lot more impact resistant than they used to be so your problem may lie elsewhere. Worth a look though.

Also, hard to tell from the perspective of the photo, but it looks like the job has a fair bit of overhang sticking out past the rather small chuck jaws. This might allow a bit too much flexing or movement under cutting loads. I would take a length of threaded rod and two nuts and washers and run it trhough the lathe spindle with the nut and washer down inside the recess on the job and tighten up the nut on th eother end of the spindle, just to help hold the job good and firm against the chuck. Rigidity is the key to good surface finish and little hobby lathes need all the help they can get in this area.

Edited By Hopper on 08/06/2018 02:46:41

Nige, you don't say what depth of cut you were taking. The wrong carbide tip and a fine cut can cause the tip to ride up over the surface and give a poor finish, the bright corners may be caused because the length of cut is short and the tip does not have time to ride up before it stops cutting.

If you do find the surface speed getting too slow as you approach the middle just wind up the wick on the variable speed as you go, I don't bother doing this most of the time but would use it on a large dia.

Quick rule of thumb for cutting speeds is 400 divided by the diameter in inches. This is for mild steel with a HSS toolbit. So 2" diameter steel would have an RPM of 400 divided by 2 = 200 rpm. A four-inch diameter job (or flycutter) would need an RPM of 100. And a 1" diameter job would rev at 400rpm, a 1/2" diameter at 800rpm and so forth.

If using carbide tools, double the rpm.

If machining brass or aluminium, double the rpm.

If machining tough alloy steel such as 4140, or cast iron, halve the rpm.

If cut is interrupted, halve the rpm.

Nigel ,

Just as an exercise try starting the cut in the middle and feed out to see if the finish improves but first check the carbide insert for damage as hopper recommended and double check that it is on centre height . What shape insert are you using ? Are you able to lock the carrige on your machine when doing facing cuts ? If you cant you can use a packing block between the saddle and the talistock then lock the tailstock down , this will stop the tool pushing off the workpiece and if you use a sash clamp to clamp the saddle to the tailstock it cant move in either direction , Sometimes the shock and vibration from an interupted cut will make the saddle creep alond the bed .

It could come down to insert geometry , small lathes like sharp tools not the various radii built into carbide inserts which they need to strengthen the cutting edges . Generally i would use carbide as a roughing tool then switch over to HSS to give the finish i want , they are also better for very light cuts that i may need to creep up on a dimension that has a tight tolerance .

Just to add to what SOD mentioned with cutting fluid on carbide , you either use  it from the start or not at all , adding cutting fluid halfway through a cut can crack a tip so if you must use it apply it before you start cutting and don't add to it during the cut . One of the benefits of carbide for us model engineers is it negates the need for cutting fluid for most jobs , i still use wd40 on aluminium and this is applied before each cut .

Edited By XD 351 on 08/06/2018 10:08:11

I'll post a couple of pics from the Surrey Turning Trials a bit later to show how I got the similar finish to disappear.

I took a few cuts off the end of a bit of 44mm sq EN1A bar starting with cautious cuts which is what Nige said he was taking and worked upto some heavier ones to see how that affected the finish. Tip was Kennametal CCMT with 0.2mm tip radius, my weapons of choice for shifting metal are a pair of holders that make use of the obtuse corners of the CCMT inserts that don't get used in the usual SCLCR/L holders. Cut dry.

0.020" DOC, 0.0015" per rev, 800 rpm. Bit rough all over with the corners looking slightly better.

Same cuts but at 1000rpm, slight improvement and showing the brighter corners like Nige was getting.

0.030" DOC, 0.0015" per rev, 1000rpm, getting better but still gloss/matt

0.040" DOC, 0.0015" per rev, 1000rpm

0.040" DOC, 0.003" per rev, 1000rpm almost even finish right across

0.040" DOC, approx 0.005" per rev as this was hand feed as I would have needed to alter change gears, 1000rpm that's about it

And from another angle, picking up a little in the middle but at least my tool looks to have been on ctr height

Changed to a bit of 50mm round EN1A and took a 0.050" DOC at the same sort of 5 thou per rev. You can see the reflection of the tool is better towards the outer edge where surface speed was higher but still not bad and you can't feel the difference if aperance though I don't have surface comparitors just a dirty finger.

I think Nige has one of ARCs SCLCR holder so did a 0.040" cut with one of those and got a similar finish to the other holders.

Finally did a 0.005" DOC, 0.005" per rev, 1000rpm cut with a CCGT 0.02 tip rad (on the right), no tearing but not the reflective surface that the other cut gave (on the left).

So it looks like feed rate may be the main factor to avoid the shiny corners and matt middle, Andrews cut was around the same sort of feeds as I was getting the even finish. The main problem with this on the smaller vari speed machines if that you can't take these heavy fast cuts as the diameter gets much bigger without starting to stall them and as Neil mentioned the tool can start ot move a bit quickly which can be off putting for a beginner particularly when working to a shoulder.

Edited By JasonB on 09/06/2018 08:26:42

I meant to say that during those test cuts all the chips from the CCMT were breaking into small pieces and being thrown away from the finished surface BUT it was only the 0.005" per rev ones that saw them coming off blue or straw coloured which is said to indicate that the tip is working around its ideal speed/load.

The fine cut with the CCGT came of in longer coils.

Also I was belt the slower speed range for the lathe which put the motor revs to almost flat out where it gives its highest power. Running at half that motor speed with it on the fast belt setting would have put it closer to stalling point.

What is 303 stainless like for lathe reaming please.