Strange surface when milling

[Roger B]

I have posted this already in my Junkers CLM thread however I will put it here as well for, hopefully, a wider audience.

The scavenge cylinder for this engine needs a 56 x 24 mm rectangular ‘cylinder’ 38 mm long in a block of cast iron. I decided that a true rectangle was beyond my equipment and skills (maybe make my own shaper??) but 4mm radius corners would just fit. It would then be possible to make the piston using a 4mm radius cutter. The middle of the block was cut out by drilling and hacksawing and the cavity was then roughed out with a 12 mm 4 flute end mill. This was cut in steps of 10mm to hopefully reduce the effects of deflection.

The finishing cuts were then taken with an 8 mm 2 flute end mill that was 38mm long, just enough. Taking small depths of cut and several bites at the corners this seemed to be ok. The dimensions between the top and bottom of the cylinder were within 0.05 mm but I could not check if things were at right angles or were parallelograms.

The first piston was not successful so I decided to polish the inside of the cylinder before making a replacement. I had square and 8mm diameter polishing/oil stones for this.
As soon as I started three stripes appeared, at the top and bottom of the cylinder as well as in the middle. The untouched stripes were 0.02-0.03 mm lower than the polished stripes (measured with a straight edge and feeler gauges). There are the same all round the ‘bore’.

Any thoughts on what could have caused these stripes, hard bands in the cast iron, defective cutter, vibration? The cutter was from a toolroom clearout at work and was still in it’s protective plastic. The cast iron is a slice of 90 mm round bar with the cylinder ‘bore’ on the axis of the bar.

[pirmin]

could be that the cutter was reground up to this mark and you would be left with this small protruding surface from the Old existing edge in the upper section of the cutter ?

could of course be a tram issue , or deflection ( you and my machine as i know are not iron pigs, rather light duty machines and i often have to face such things and refer it to deflection most of the times.

vibrations ( you should be able to spot it with a magnifying glass if it is vibration issue. you can see that on the surface when magnified

[pirmin]

oh, yeah now by looking at it , it is deflection and vibration ! you can avoid that my step milling .and then do only a 2 thou. finish pass with the whole lengh of your tool

[Jasonb]

Although I have not measured it I do sometimes see similar on the CNC and put it down to tool wear as the pattern often matched the depth of cut that the cutter has previously been use dat so even a full depth pass can show banding. Typically if I do the adaptive roughing cuts at 5-6mm and then the final full depth contour at say 20mm there will be a band 6mm up from the bottom. Change to an unused cutter for the finish pass and the step does not show.

I all cases it has been more visual than something than can be felt or even measured

[Vixen]

Plus 1 for microscopic tool surface wear.
I have often experienced this effect with my CNC mill, even with new(ish) cutters on aluminium. The surface banding is only visual and not feelable or measurable.

Mike

[john mills]

often if you take a number of different depth cuts the cuter will wear in steps may not be enough tome sure but you can see them so i have usually
use a cutter with has not been used before an cut full depth but only a very  light cut absolutely the minimum that is a cut .001 if that will clean the surface.a four tooth end mill would be stiffer  and solid carbide is even better and leaving  good surface.we don't always have the tools we would like ,can only use the best we have.I have done deep square holes in tool steels on  heavy machines with good bearings and you can get the same
problem the tool often is what is limiting the Finish  don't have the speed too high.
john

[jadge]

If I've understood correctly an 8mm cutter is being used to create an internal 4mm radius. If one thinks about the finish cut (small DOC) the cutter engagement on the flat sides is small, equals small forces and small deflection. When the cutter reaches the corner the cutter engagement suddenly increases to around a quarter of the cutter circumference which inevitably leads to chatter and cutter deflection. In the CNC world it is a no-no to use a cutter of radius X to form an internal corner of radius X, a smaller diameter cutter is used.

In the manual world one solution is to use multiple stepdowns, although there might still be some chatter. It would be better to use a short 8mm cutter for the first step down or two and then a long series cutter, but with short flutes. That way the plain upper part of the cutter can be guided by the existing radius. A carbide cutter may help as they are stiffer than HSS.

It's not difficult to create square internal corners with a set of decent files. Just don't use a square file, flat and three-square files are better.

Andrew

[Roger B]

Thank you all for your thoughts and suggestions 

I don’t think that it was cutter wear as I used a different cutter for the roughing and only used the 8 mm cutter at full depth.

I am not sure about the lack of ridigity / vibration. The marks are very consistent around the ‘bore’ and I would expect the machine stiffness to be different in different directions with such a light build.

The ripples were not visible or detectable by touch after the machining. They only appeared after I started polishing with an abrasive stick. It seems that the cutter was cutting consistently along it’s length.

The cutter is a good quality one from our toolroom. I classed it as new as it had a protective plastic coating but I now think that meant it had been sharpened, new cutters tend to come in plastic tubes.

The pitch of the ripples is around 15mm which is the same as the pitch of the milling cutter so I wonder if the sharpening  process was not quite correct and produced a diameter variation?

[Roger B]

If I've understood correctly an 8mm cutter is being used to create an internal 4mm radius. If one thinks about the finish cut (small DOC) the cutter engagement on the flat sides is small, equals small forces and small deflection. When the cutter reaches the corner the cutter engagement suddenly increases to around a quarter of the cutter circumference which inevitably leads to chatter and cutter deflection. In the CNC world it is a no-no to use a cutter of radius X to form an internal corner of radius X, a smaller diameter cutter is used.

In the manual world one solution is to use multiple stepdowns, although there might still be some chatter. It would be better to use a short 8mm cutter for the first step down or two and then a long series cutter, but with short flutes. That way the plain upper part of the cutter can be guided by the existing radius. A carbide cutter may help as they are stiffer than HSS.

It's not difficult to create square internal corners with a set of decent files. Just don't use a square file, flat and three-square files are better.

Andrew

Yes, there are chatter marks in the corners however the ripples that concern me pass through them as can be seen here.

[Jasonb]

Interesting that in that last photo the banding seems to run vertically on the short end and horizontally on the longer side

[uuu]

If the flutes have been sharpened as you suspect, the cutter would be a little less than 8mm.  Can you measure it?

Wilf

[Zephyrin]

poor chip removal during milling and rubbing can cause this kind of mark, especially with long, re-sharpened cutters.

[Roger B]

Jason, I think that was a trick of the light, the ripples are consistent around the bore. Here is a better picture of the end.

Wilf, it is difficult to measure but the end mill I used seems to be ~0.04mm undersize, a known new (but too short) end mill is ~0.04mm oversize. It may have been reground, I don't know how much would be removed.

As milled the bore looked fine, it was only when I started to polish it that these ripples were visible 

[Roger B]

Once again thank you all for your inputs 

[john mills]

i would see these patterns even with good HSS new cutters long smaller sizes .if profiling with cnc machines a smaller cutter than the radius
is required to produce the corner if profiling the shape just going to a corner the finish will not be good the machines i programmed need a move
to go round the corner.manually to get a better finish in the corner feeding vertically in the corner con produce a better finish.If you take a series
of depths of cuts you will get marks may not be able to measure but will be noticeable .a solid carbide end mill will be stiffer and can leave a
really good finish. I often used a carbide end mill for the finish cut to get a good consistent size and finish .
John