Carbide tool mirror finish interrupted cut

I was facing off this block in the 4 jaw. It is about 44 mm square so an interrupted cut at the start. Knowing that carbide tools like to have the cutting speed up I was experimenting with the lathe speed and looking at the finish. First cut was done at about 450 rpm and result was nothing special though I had noticed that the corners seemed quite shiny. I had no immediate explanation and wound the speed up to 800 rpm and took another cut. The shiny area in the corners got bigger but the uninterrupted part of the cut was still nothing special. Putting the speed up to 1200 rpm achieved even more shiny area in the corners but again didn't improve the uninterrupted cut. So carbide does like the speed but is it the reduction in the surface speed as the cut gets closer to the centre that explains the poorer quality of the cut closer in. I really expected the outside interrupted cut to be worse than that closer in. Any explanations or ideas please

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.

I'll have to tread carefully here, but none of the finish looks particularly good to me. The central part in particular seems very uneven with ridges? Or is that a trick of the camera?

However, the basic premise that the surface finish can get worse as the surface speed of the tool decreases is correct. With aluminium you might see a shiny and very uniform finish to start with and as you move towards the centre the finish gets duller, as if it has been rubbed with scotchbrite.

Tomorrow, for work, I need to form some shallow recesses 200mm diameter in aluminium plates. I'll try and take some pictures.

Andrew

Thanks guys. The material is bright mild steel and I was brushing on coolant as well. The cut was taken with the power feed on the cross slide. No need to tread carefully Andrew, as somebody with extremely limited experience I'm happy to take well intentioned criticism The central part IS ridged which is what has thrown me as the outer corners are very much smoother in comparison and it is the marked difference that has thrown me. Should say that this piece is the first piece I have made on any machine that has more than one machined face or been made to fit; certainly the largest area I have faced off so far. More about the bored hole later.

If you crank up the speed you can get a wonderful finish with carbide insert, but you need to be in the 300-600 feet per minute. That means 600-1200 rpm with a 2" diameter workpiece. Generally with steel, if the chips are coming off close to blue, you are in the right ballpark.

With aluminium, there's a different problem. Turned at a low speed, you can get a good finish. Turned at an insane (2000 feet/min) speed you can get a good finish. Somewhere in between is a range where the chips weld on to the end of the tool causing 'built up edge'. Apart from causing a lousy finish, this can kill carbide inserts, because the end of the insert breaks off with the lump of fused swarf.

I had to tidy up a second-hand hardened arbour today (it was Brown & Sharpe and didn't quite fit the Jones & Shipman grinding wheels I was using it with). I started cutting it with a carbide insert and it went ping! Luckily, I'd scored some CBN inserts off EBay a while back, these are marvellous things . Taking off 2 thou of file-hard steel at 750 feet/min results in a shower of orange sparks and a mirror finish. I wish I'd got the camera in the shed at the time!

All the blather asside, don't be afraid to wind the speed up as far as it'll go.

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

I rarely bother too much wondering why something doesn’t work, I just try a different way. In your case why not try a HSS tool instead? Carbide has its uses but on a small lathe nice sharp HSS is hard to beat on stuff like mild steel. Just a thought.

Andrew: Thanks for that link, useful stuff. I have no idea of the grade though I bought it from a local professional supplier just as BMS from their short ends rack. Looking forward to tea and turning 😊

Vic: the bored hole in that bar I am particularly pleased with. It is the first 'boring' job I have ever done and I had to grind and modify an old HSS tool to do it. I ground it to cut both along the axis of the bed to open the hole up and to use as a facing tool in the bottom of that hole; very pleased with the whole process really as I applied quite a lot of knowledge I have been piecing together recently and tried different tools and techniques. Also happy that the blank arbor I bored the hole for is a non sloppy sliding fit, difference in diameters is .03mm as near as I can measure. Hoping somebody will tell me which 'sticky' engineering product will take up that bit of clearance and hold the arbor tight in that hole 😊 All in all a happy learning experience so far 😀

Loctite 603 and Loctite 648 are both good for holding stuff that doesn't want to come loose. With a 0.3mm diameter difference, they'll set pretty fast (5 minutes for hand-tight). after a few hours, you'll need a blowlamp to separate them.

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

Guessing you are going to fit an ER collet holder in the hole? No reason, if you want ‘belt and braces’ security, for not drilling and pinning after the loctite has cured (not before - or the piece may be pushed off-centre).

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.

Mark: Thank you, just what I wanted to know about the Loctite.

Hopper: no, I haven't checked the carbide tool tip though I will now. I understand what you are saying about rubbing and rough cutting. I'm using the carbide tools as I have them though I also have a load of HSS. I think I was beginning to think I just wouldn't need to keep 'sprucing up' a carbide tip as I might an HSS one and forgot that carbide can be quite delicate. I still have the piece mounted and will take an HSS tool with a slightly rounded nose to it.

NDY: the hole is for a blank end arbor on a morse taper. The end of the block you can't see has a 90 degree V slot milled across it to locate round bar while it is cross drilled. Drilling and pinning might not be necessary as I don't think there will be much torque on the block in normal use

Jason: Depth of cut was minimal I think as it was being applied with the saddle feed handle and done by 'feel' and I was being cautious as it was an interrupted cut. In hindsight maybe I should have used the compound slide for finer control of the feed. I was using one of the carbide tools that I bought with the lathe and hadn't thought about whether it might be suitable or not 😳

I have come to realise in just a couple of days machining that I need a much better idea of cutting speeds ! I might sit down and make myself a chart with a basic guide to cutter or drill diameter agains material surface speed for the material I am useing then stick it on the workshop wall !

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.

Not a good idea to dab coolant on carbide; in my limited experience that's far more likely to crack inserts than interrupted cutting. Carbide isn't HSS and treating it like HSS isn't a good idea. (The reverse is also true!)

Carbide is significantly harder and heat resistant. It removes metal with a technique HSS can't copy because HSS softens at high temperatures. Carbide can cut much faster, much deeper and far hotter. Inserts are identical and can be swapped in moments without resetting the machine. On the downside, carbide is comparatively brittle. When very hot, thermal shock caused by intermittently brushing coolant is likely to crack it. So cooling carbide is an all or nothing affair; if you must apply coolant to carbide, flood, don't dab.  (Usually I use carbide dry, and remove swarf with a dry brush.)

Not an attack on HSS - it has other virtues. Easier to get good finish, takes fine cuts on slow moving small diameter work, tolerant of dabbing and other abuse, and cheap (provided you ignore training, grinding, and tool resetting costs!)

Dave

Edited By SillyOldDuffer on 08/06/2018 10:00:58

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

Hi Nige,

It's a strange thing, but when I first got the SC4 I took a while to get decent finishes. It wasn't a case of bad adjustment, just getting used to the machine.

I suspect that the reason you are getting a poor finish on the uninterrupted cut is that the cut is too shallow. The interrupted bits whack into the tip and it gets under the surface and the cut is complete before it can escape. Further in, it can get out of the metal and once riding on the surface is less likely to get back into it, hence the ridges.

You should aim to be cutting a depth of at least 2/3 of the tip radius, especially with CCMT tips. CCGT are sharper and more forgiving.

Yesterday I was turning stainless at just ~450rpm using two different styles of tip, with DOC's between 2 and 1 mm I was getting a lovely finish but the swarf wasn't breaking up, just long, tight coils. I would have gone much faster, but I was cutting up to a shoulder.

FWIW these 45-degree SCMT tools from Arc are brilliant for roughing cuts, 3mm or more in even horrible steels and leave a good finish.

Neil

<edit> I see Jason has come to exactly the same conclusion!

Edited By Neil Wyatt on 08/06/2018 16:51:35

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.

Here's the result of a quick facing trial on hot rolled steel, essentially EN3, machined dry:

Done with a Korloy insert, 0.4mm radius, at 800rpm and 4 thou per rev. Depth of cut varied from zero to about 30 thou at the corners as I didn't get the block square in the 4-jaw. General surface roughness measurements are around 1.3µm Ra, consistent across the block. Interestingly the roughness just inside the zero DOC area was slightly better. Looks like the Wyatt/Ballamy theory has skidded off the road?

In the middle there is a small rough circle with a Ra of around 10µm. The diameter is about 5/8", so the reader can work out what the surface speed is when the finish worsens by nearly an order of magnitude. The insert is also a bit below centre height which doesn't help either.

Andrew

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