[crl1_1]

axyzmachines.com has ditched the x2 cast iron and they are now making mineral castings.
I found these videos one seems to be doing 100ipm and theres another one doing 200ipm on those linear rails,

[crl1_1]

heres the one at 200ipm rapids

[joeavaerage]

Hi,
mineral castings have about 1/3 to 1/4 (30-40 GPa) Youngs modulus of cast iron(100-120 GPa), thus the sections need to be much bigger to match the rigidity of cast iron.

I got the axis beds for my new mill cast in iron, then machined. Expensive but I got what I paid for.

Craig

[crl1_1]

so by comparison let's say for the x2 for the column how much larger would it need to match the rigidity?

[joeavaerage]

Hi,
the flexure of an object under stress is equal to the product of the first moment of inertia of the object times the Youngs modulus of the material.

As a general rule and approximation the first moment of inertia increases as the square of cross dimensions of the object. Thus an object of a given shape
has a fist moment of X, then if the object were scaled to twice the dimensions but the same shape then the fist moment would be 4X

Thus if the Youngs modulus of the material is 1/4 that of cast iron then doubling the dimensions of the column would counteract the loss.

This is the rub with epoxy granite or any other composite, the cross section of the various parts has to increase so much, it's not that it can't be done, but it
quickly becomes overly large and expensive. There are main stream manufacturers using the technique, but looking the the prices for their machines its not for price advantage.
I believe it's dimensional stability under temperature variation that is the stand out feature. Note the the machines that use composites are almost invariably the highest
accuracy machines, and prices to match and thermal stability is a must at any cost.

Craig

[crl1_1]

thanks for the response Craig, Now I am not sure how this mill they are offering stacks up as a benchtop cnc?

[joeavaerage]

Hi,
composite materials have become very popular with hobbyists and I suspect the principle attraction is that "I can do it all myself', and that
is an appealing thought for many. Another quoted benefit is cost....but I'm very unsure that its correct.

If you follow the argument I posted earlier then the parts become so big that any cost saving goes out the window. It would be cheaper
but only at the sacrifice of rigidity.

The most cost effective way to produce a rigid structure is to use a material with the highest Youngs modulus. Carbon fiber would be great if you could
afford it....but steel, even just mild steel has a Youngs modulus of 200-210GPa. A welded/bolted/stress relieved steel structure of good design will result in the lowest
cost, and probably lowest weight for a given rigidity. Its the process of deep penetration welding, then heat treating and then machining the critical surfaces
that puts most hobbyists off...its not something they can do themselves and they don't want to pay.

Cast iron is another good material for machines, it has a good Youngs modulus of 100-110GPa. Consequently to match the rigidity of steel requires more mass
of iron, but this is not usually a problem as cast iron sections tend to be much MUCH thicker than steel sections. Ideally the cast iron should be heat treated and machined,
again this puts off hobbyists.

As I posted earlier I had cast iron axis beds cast for me for my new mill. I really wanted to cast the frame (approx 450kg) as well but I ran out of money, and used a manufactured
steel frame instead. The axis beds are 'U' shaped channels 700mm long x 250mm wide x 130mm high with all sections 50mm thick. They weighed 115kg each and cost $3300NZD
(2300USD). Then I had them rough machined, then stress relieved, then finish machined for $6000NZD (4200USD).

Is it expensive?....Hell yeah its expensive.
Is it a good result?.......Hell yeah its bloody superb. I nearly s****t my pants when I was quoted for the machining but every single surface and cut is perfect to a 'a few tenths',
so I got what I paid for.

Craig

[joeavaerage]

Hi,

Now I am not sure how this mill they are offering stacks up as a benchtop cnc?

Nor I. Any fool can get a machine to move around as in the video but want I want to know is what happens when you put a mild steel block in there and
with an 8mm tool and start boring in a making chips. If the machine is less than rigid it will be all over the place like a 'mad womens s****t'
When it comes to making useful parts there is NO substitute for a rigid machine.

Craig

[crl1_1]

I suppose those speeds are good for rapid moves but yeah I am curious to see, at the same time cutting feeds are limited by that 6000rpm spindle

[joeavaerage]

Hi,

at the same time cutting feeds are limited by that 6000rpm spindle

No, you just put a bigger diameter tool in there. I suspect the cutting speeds are limited by the power of the spindle and the flexure
of the machine. If you push it the machine will flex, you'll get a God-awful vibration, all accuracy and surface finish is lost. It does not matter
how much spindle power you've got or what rpms it can pull, if the machine cannot contain the cutting forces you'll have to back off until it does.

That was the problem with my first mini-mill. It was quite good with a 750W 24000 rpm spindle, but when I put a 1.8kW 3500 rpm 6.2Nm(cont) spindle
on it I had to back off the cutting speed because my mill would start to flex. It was all cast iron and steel too, just not thick enough.
There's only one component of my machine that thick enough...that's me!!!

Craig

[crl1_1]

i found this SYIL X7 looking at the castings one is cast iron and the other is epoxy, but the size does not look much different?

[joeavaerage]

Hi,
I don't know whether you've ever had any machining experience on Drill Mills? They go by different names but are all variations on a theme,
a RongFu 45 is a good example.

If you haven't I would avail yourself of someone who has and study it closely. They are not over-built by any stretch of the imagination but they are
rigid enough to satisfy buyers. If you are going to build or buy a machine you need to ask yourself 'how rigid is this compared to the RongFu or Precision Mathews
or whatever, I saw last month?'

You may come to the conclusion, that many others have also, that they would make a good donor for a conversion. The main attraction is that all the difficult cast iron
parts are already done....and after all is this not the point of this post....'how do I avoid all those tricky cast iron bits?'

Craig