Mild steel vs cast iron - machinability on small machines

[MarkoAnte]

Hi,

I ran into a need to machines a larger heatsink from aluminium and I can not hold it in my normal vise. Long story short I remembered a this old tony video and want to copy his two piece vise. It will give me good access to mill all the features.

But I have a small milling/drilling machine (https://www.bernardo.at/de/bf-25-l-super.html) that is not the happiest milling steel. I did some modifications on cast iron and it seemed happier.

Is gray cast iron (generally) easier to machine than mild steel? I mean for small machines like mine. Or was I just imagining it? Normally I use soft stuff like aluminium, copper, brass and steer clear of steel.

Best regards,
Marko

[liveaboard]

Iron is very easy to machine.
But look out for the waste that flies away; it makes a terrible mess if you don't clean it up before it turns rusty.

[Harold_V]

Gray iron machines quite nicely, assuming it isn't contaminated with sand on the surface, or that it is not chilled. Sand will be very hard on cutting edges, thus the first pass is generally recommended to be deep enough to get below the surface a reasonable amount, which can be anything from a few thou to 1/8".

Because gray iron machines freely (assuming it is not hardened, or contains sand), rake angles are reduced to limit self feeding (hogging is the term often used).

I agree with the comment in regards to the resulting chips/swarf. Cast iron is the dirtiest material I have encountered, aside from graphite, to machine.

If one machines gray iron and finds it to contain sand, or it is chilled, the use of carbide can be of benefit, even on less robust machines, as it has the ability to perform longer than HSS before degrading to the point of not working. A C2 grade is recommended. The grade of carbide normally used for steel (C5) fails to perform well.

H

[LouStule]

Gray iron machines quite nicely, assuming it isn't contaminated with sand on the surface, or that it is not chilled. Sand will be very hard on cutting edges, thus the first pass is generally recommended to be deep enough to get below the surface a reasonable amount, which can be anything from a few thou to 1/8".

Chilled cast iron is almost impossible to machine. I picked up some steam engine castings on E-bay and thought they were authentic Stuart Turner but they just wouldn't machine at all. Later, I found out they were likely cheap Chinese knockoffs. I learned my lesson and only buy from the source now.

[pete]

That mill is just a rebranded Sieg X3 sized mill. Metal cutting is at the basic level a function of machine rigidity, tool sharpness and available horse power. I've got light years better now, but when the only mill I had was a tiny Emco C5 round column mill I still didn't hesitate machining steel, stainless or even titanium on it. Tool sharpness and very light cuts had to fill in for it's almost zero HP and rigidity. I wouldn't think twice about machining steel or any home shop level material on the mill you have. But no matter how large and rigid the machine is they all have limits, if yours doesn't like steel you might be trying for too much at once. Back your depths of cut off until it is happy and that's the limit for your machine. How well adjusted your mill is can also have a large effect. I've said it many times before, but I highly recommend pulling the tables on any off shore mill and THOROUGHLY cleaning the ways, feed screw and nut of all the anti rust preservative they use and then properly re-lubricating, readjusting the X,Y axis nuts and gibs. Doing so makes a missive difference in how the machine reacts when cutting harder materials. As Lou mentioned chilled cast iron if you were to get any is just about impossible and it can even wipe the cutting edge off any carbide type I've tried if it's really bad. Re-heat treating it can sometimes help. He's also right about buying from the real source and paying for that known quality. So if your buying cast iron blocks or plate don't buy at the lowest possible price.Other than the mess already mentioned, cast iron does cut a lot easier than steel since it fractures easier and has it's own built in lubrication with it's carbon content. But even machining your own two piece vise is going to need at least steel jaw plates. Aluminum soft jaws could be substituted, but hardened steel jaws are going to last a whole lot longer. If it were me? I'd see about ordering replacement jaws for a more standard milling vise and design your two piece around those. It looks like yours has about a 6"/ 160 mm wide table, so even jaws for the industry standard 6" Kurt type mill vise could work and there real easy to find.

But those two piece vise designs have what seems to be a not quite fully understood design issue by most. Yes they can and will work, but they also require a bit of logic about how they actually clamp the work piece. The higher the clamping force you use and the higher the work piece is within there jaws the more those forces get transmitted down into the mill table. Newton's third law,"For every action there's an equal and opposite reaction" At a certain point depending on the mill size they can and will flex that table no matter how rigid your vise design is. That clamping force will bend the table away from those forces. Enough so that it's not impossible to fully take up the clearances and have the table lock up to where it can't then move on it's X axis. So once you know that then keeping those clamping loads to just enough would be the best approach. My own personal opinion is those two piece milling vise designs for vertical mills were an idea borrowed from the same thing used on the large old school planers. They work, just not as well as they do on those massive planer tables. Those 6" and larger industrial milling vises have good reason to weigh what they do, there built heavy so there vise beds absorb most of those clamping forces.

[John Hasler]

I agree with the comment in regards to the resulting chips/swarf. Cast iron is the dirtiest material I have encountered, aside from graphite, to machine.
H

Recycled tire rubber is a close second to graphite. Great shock absorbing properties, though.

[MarkoAnte]

Ty everybody for you input.

@pete Ty for the additional help. I already cleaned everything but the problem (rigidity) that I see with my machine is the y axis.
On the x axis on the whole travel of the machine the same length of dove tail is engaged. But on the y it goes form 1/2 to full to 1/3. I made some modification that I have more gibe screws and it a bit better. When I got the machined and if you pulled the table all the way to the operator side only one gibe screw held the gibe in. And no matter how I adjusted it it always went loos. If I would buy another one of these small mills, I would definitely check the dove tail engagement length at the end of travel. But I had to order one unseen because nobody had any to show me.
All this means that when I mill close to the ends of y travel (only 150 mm travel so you get there quick) the mill is not happy milling hard stuff.

[pete]

Without one of us having the same exact machine it would be fairly tough for anyone to make suggestions on what might help to help with your Y axis problems. Any chance of posting a scan from it's user manual showing the Y axis parts assembly and maybe a few detail pictures?

[MarkoAnte]

Hi,

so I was mistaken last post, it is 2 screws that hold the originally hold the gibe but form one side only. I added the silver one, that was originally only a braking screw:

So originally there would be screw no. 1, 2, brake, 3, 4. And if I tighten the screw 3 so that the table would not have slop at the end of travel the table would bind when going past screw 3. So originally the table would be supported by cca 1/3 of the dove tail because only 2 screws would securely hold the table. The other end it would flex the gibe.

Now That I added a another screw it is better. But still the dove tail length is small compared to others : on x the dove tails engagement length does not change and is 23 cm. On z it does not change and is 22 cm. But on y it goes form 24 cm to only 12 cm.

Its a shame that they did not make the y dove tail longer. they had space on the back. There is 12 cm of empty space till the column. It tried playing with this gibe multiple times but I just cant get it so its tight at the end of travel without binding in the middle. Maybe it just requires more expertise that I have. Or that it would benefit form scraping the ways, but this is all to advanced for me XD

[pete]

I see what you mean. It's very difficult to compensate or even improve things when the original design faulty to start with. It does sound like the dovetail widths may not have been machined as accurately as they should have been. And yest to fix that is fairly advanced. There's a lot of scraping and machine alignment videos here, https://www.youtube.com/channel/UCD1jVj ... PHw/videos but I'd also highly recommend this book. http://www.machinetoolpublications.com/ not cheap at around $100. But also not really optional for improving the basic machine tool alignments.

[MarkoAnte]

Mah, I will leave good enough alone. I can mill 95% of what I need. I also don't have a surface plate or a straight edge like they use for scraping. 

The next mill will probably be something older, maybe older and refurbished. Right now, I have a problem with the space that the machines are in. The landlord said that the strength of the floor it's max 100 kg /m^2 XD I know that can't be right and it was just a number he pulled out his head, because if it was right I would already have broken the concrete floor just by sitting in my chair. But I don't want to give him a reason that he finds a crack on the other side of the building and now it's my fault that I brought a 1 tone mill in. Mind you these is a old industrial building made form reinforced concrete that they used to have manufacturing in 30 year ago. If you want to drill in a wall anywhere to mount something its always concrete and I always hit rebar XD

[jmpharrington]

I had a mini-mill that poorly fitted gibs and machined dovetails.

This may be unorthodox, but I lapped the gibs (which frankly looked like pieces of rough scrap metal)on progressively finer wet/dry automotive finishing sandpaper down to either 600 or 1000 grit (can't remember which)with WD40 on a sheet of glass, as I didn't have a surface plate. With the leadscrews removed, I then installed the gibs with a piece of the fine sandpaper held on with doubled sided tape between the gib and the dovetail, and with the gib screws loose enough to allow it to slide freely, lapped the surface of the dovetail. This took some time, including replacing the strip of sandpaper many times, The results were worth it and and produce a much smoother, tighter machine.