|Nicholas Hill||17/07/2019 18:52:04|
|23 forum posts|
I am not sure if this is a Beginners question or a Manual Machine Tool one,. So please tell me to go to the Beginners if my questions are too naïve.
Today I have been trying to mill a flat cast plate, and seem to be experiencing every problem going!
I’m using a Rodney milling Attachment, so rigidity is compromised, but even the lightest cuts cause an awful racket, and noticeable vibrations. The results are poor, and time consuming to get to. So I believe I must have something fundamentally wrong.
One of the causes appears to be the rough cast finish of the plate, it seems that every time a cutter hits a peak, it causes perturbations. It makes a thwacking noise as the cutter starts engaging the plate. Once it starts over the plate, the noise reduces, but a vast amount of friction starts – smell of burnt metal, and slipping drive belts causes lead screw judder. I have always been told that cast iron needs no coolant?
One area that I also know is compromised is the clamping – with the Rodney one has about 2 ½” of working space, so a proper clamp is not possible. But I have double sided it to the cross slide, so it is has direct metal contact, and it seems securely clamped at the edges. But due to the size of the Cross Slide, there is some overhang at the front.
Options – 1/ Change feed rate or spindle speed. 2/ Use a Slot Mill instead of the Facing Mill. 3/ Improve clamping – not sure how. 4/ Buy a Myford Collets system at silly money, and hope it allows space for a proper clamp. 5/ Buy some better cutters for my Facing Mill – the current ones are cheap that came with it. 6/ Another idea???
The results and set-up are shown in the photographs attached. Spindle speed was 200 RPM, feed was 0.0069” / Rev, depth of cut was 0.0025”
Any help most appreciated.
Clamping close up
|Dave Halford||17/07/2019 19:09:59|
|418 forum posts|
Assuming the plate isn't too hard, and it might well be given the heat you mention. Try a file and see if it marks the casting, if it doesn't then there's the problem. Inspect the cutter tips for damage. Good cast iron can be worked with HSS or a single point carbide cutter with lowish power.
The face mill might be too big for both the power available and rigidity of the Rodney, if the plate files try a smaller end mill.
|Douglas Johnston||17/07/2019 19:25:54|
590 forum posts
That shiny surface looks like you have hit a hard surface on the casting, which can rapidly blunt even carbide cutters. Too shallow a cut is the problem, you need to get under the very hard surface layer and into softer iron. I ruined a carbide cutter the other day with the same problem. By increasing the depth of cut all was well with another sharp cutter.
|4536 forum posts|
I suspect you have a list of disadvantages adding up to cause trouble. The Rodney and clamping are a bit wobbly, that facing mill is asking a lot of a small motor and - this is the biggy - cast-iron often comes with an extremely hard outer skin. Inside it's easy to cut, but breaking in can be an HSS blunting nightmare!
I'd try attacking the thin top surface with an angle grinder before trying to mill it. You might get away with cleaning off a slice so the cutter only has to deal with a thin edge as it ploughs across the rest, but if you've got the grinder out might as well do the rest.
The hard layer is caused by fast cooling and it's unlikely to be very thick, perhaps less than a mm. Not unknown though for castings to be deliberately or accidentally chilled making the skin much thicker. Once the top layer is gone try again with the Rodney; I'd use a single point flycutter rather than a face-mill because they don't need anything like the same power and rigidity of the milling machine. Give your face mill a try though; soft cast-iron is easy to machine.
If this is your first time with cast-iron beware of the mess! Awful black stuff that goes everywhere. It's worth spreading paper and magnets about and cleaning up as you go. The hard gritty top surface is particularly bad for lathe ways so give it a good clean when you've finished.
15773 forum posts
I'd say 90% chance of it being poor inserts and 10% hard casting.
can you post a good closeup of a used corner of an insert. My cheap 50mm facemill was transformed from junk to a usable tool buy changing inserts which cost more than the whole arbor/head and inserts from a well known far eastern selling site.
run at around 500rpm
Edited By JasonB on 17/07/2019 19:40:59
|Tim Stevens||17/07/2019 20:52:15|
1049 forum posts
The skin of cast iron can be nothing like the 'real' stuff underneath. The skin can be chilled - forming iron carbide, which is very similar to tungsten carbide, and just as difficult to machine. It has a silvery appearance, whereas the body of the casting is going to be dark grey, with included carbon which rubs off on a finger. This darker material can be machined using old fashioned tools of carbon steel, which is one reason why it was used so much in the 18th and 19th centuries for engineering.
It might be worth trying to remove some of the carbide by heating the casting to redness, and allowing it to cool in the ashes of a fire (such as a barbeque). This slow cooling should help to ensure that some at least of the dead-hard problem material changes back to iron, and carbon. The ashes serve as a blanket and prevent any sudden colling from drafts, etc.
Hope this helps
|old mart||17/07/2019 21:23:40|
|323 forum posts|
That shell mill is a bit big and sticks out a lot from the spindle. The skin on the cast iron is a real problem. I think you are exceeding the capabilities of the machine.
|901 forum posts|
I have in the past, used angle grinder to put small 45° angle cut along the edge to be 'hit' first by a carbide single point tool, to break thro the skin. It worked OK.
I wouldn't use a shell mill on such a flimsy set up as Rodney/Myford. No substitute for a proper mill and as hefty a type as possible.
Suggest you join a club with a decent workshop and if needed, a fellow member will advise / teach use of a big mill.
As said above, cook it in the ashes of a hot BBQ. Or solid fuel stove. Leave overnight to cool slowly, buried in hot ash.
|901 forum posts|
I have several new iron castings to take to next club wrinklies day, when we have a BBQ.
2426 forum posts
Definitely some chilled iron there; PIA unless you can get under the 'skin' or heat treat it as mentioned. The following pic shows the 'chilled' outer O/D portion of the frame off my S10,
|Nicholas Hill||18/07/2019 21:39:41|
|23 forum posts|
Many thanks for all the helpful suggestions. I had not thought of the case hardening and it makes a lot of sense. As you have guessed this is my first time at milling cast, and am somewhat of a novice as regards milling. So many thanks for being kind.
Several people have suggested a Model Engineer club, and I tried looking into the Notts branch. But have yet to hear back from them, and like most clubs, they seem very Railway orientated. My field is aeroplanes and motorcycles, so am not sure if I would “fit”.
I have been at work today, so have not had an opportunity to try the ideas mentioned. Of the suggestions the consensus seems to be:
1/ The Milling Cutter requires more power and rigidity than I have, so the first thing is to try another type – I have a fly cutter, and end / slot mills, so have options.
2/ In conjunction with this, investigate the surface via a file, and if required heat treat – what temperature should one aim for? Can I use my oven?
3/ If the above has improved things, persevere with deeper cuts – I tried one, and it made such a awful noise, I stopped, but it did work.
4/ Try at a higher speed – I guessed at my speeds based on the diameter of the cutter, and an old book on Milling. But as they this was talking about slot mills, it was always a conservative speed trying to compensate for the lack of rigidity in the Rodney.
I’ve attached a couple of pictures, one showing the cutter tips. Although cheap inserts, they seem in ok condition. I thought the main difference in cost was about chip clearance?
The other picture is of the “deep” cut I tried, which although making an awful sound is okish.
Oh, on the comment about cast iron dust…It was rather hot in my shed, so I worked shirt-less, and had a very black stomach after….
Oh, thanks for not mocking my Rodney…I have cursed it numerous times, and pondered if it was a good idea, on more than one occasion…..
Many thanks again,
15773 forum posts
1.Although it helps to have a rigid machine these cutters can work on light machines, I exchanged a few Wattsapp messages yesterday with someone happily using a 40mm facemill on a SX1LP. End mill eg 4-flute cutter would be the best of the three options you have but if it is from a similar source to the facemill and inserts then all bets are off.
2. No you need hotter than your oven about 875deg C and it needs to soak right through then cool slowly, On a limp like that I would say you may just have a harder skin not chilled a sthat mostly happens on thin castings and corners/edges very much like George's photo
3. Two videos at the end of this reply first is with the supplied inserts, see how the lamp vibrates, second is with better inserts on the same piece of iron, then steel and then aluminium all cut fine after changing inserts and making sure the head was running concentric to its loose fitting arbor
4. 1000rpm would be about right but I suggested the 500 as a conservative improvement also don't know what Rodney is able to run at.
When it's cutting right you will get small chips not dust so although your belly will be clean it will also get lots of little burn marks
Cost is about quality of the carbide and a keen edge.
Edited By JasonB on 19/07/2019 07:18:05
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