Originally Posted by
hanermo
I like the thread !
There are very few machines milling steel .. and there is a reason.
The max end mill your machine can use efficiently, is likely to be about 5-6-8 mm, for steel.
Compare your machine to am original Bridgeport, ie M head.
MT2 socket.
It has similar travels, 800 kg mass, max end mill 1/2" or 12.7 mm, 1/2 Hp was plenty.
Is your machine more rigid than the original bridgeport ?
Technical opinions:
Column is (far) too tall for the section size.
Table is not thick enough.
Base is ok, a bit thicker (50% more) in section would be better.
Technical data:
200x100 vs 200x200 - the latter is 4-8 times better.
Doubling the height of the column needs 8x more mass for equivalent rigidity, and it needs to be 8x more rigid for the same performance.
Example:
My mill is built to mill steel.
At the moment, I am converting to new linear guides (Hiwin 35 mm).
All sides wall thickness is 20 mm in steel (F1 calibrado a basic tool steel in spain).
My section sizes are 240x200 mm on ram: Wall 20 mm.Approx 140 kg.
Spindle mount:
300x240 mm- 530 mm talll: Wall 20 mm. Approx 100 kg.
Verticals (2 of).
250x400 mm- 1300 mm tall. 20 mm front wall, 10 mm side walls.Approx 300 kg. 2 of. => 660 kg total.
Why is the machine so heavy ?
There is a good reason.
All the build is modular, and bolted/tapped.
Biggest pieces are == 140 kg each.
I can move, fairly easily, 140 kg piece alone (2.3 m long).
Lifting one end at a time, with leverage, makes this easy.
Mass is your friend.
Heavier is better.
Nothing is too heavy.
Tiny motors (nema 23 steppers, old setup) will move a 240 kg table, with upto 200 load (capacity over 2 tons, theoretical 10 tons), effortlessly.
My current bridge will be about 700 kg, moving up and down (on 6 linear guides, overconstrained, preloaded).
The small servo motors (nema 23 size, 400 W) are far overpowered (and relatively cheap).
So, my advice is use big sections now, when its cheap.
And, my advice is based on building for milling in steel (my machine is large, thus I need larger section sizes. The ratios stay the same).
I have spent about 12 years at this, mostly full time, so this is my practical experience ...
I expect I might, maybe, be able to use a 20 mm end mill in steel, on the big machine.
(At 3 kW hopefully, with HSM strategies).
Spindle is industrial BT-ISO30, and auto toolchanger.
The larger your work envelope, the more rigid the machine needs to be.
And the mass/rigidity needed is cubed, because its x times y times z.
=> (relates to) section sizes.
A 320 x 240 spindle mount is approx 4x more rigid than 200x200. Its not a linear relationship.
Wall thickness is linear.
20 mm wall vs 10 mm wall => twice as rigid.
Hope this is useful ...
For tests;
Model deflection of 200x200 and 200x100 and look at deflection.
Now try 300x200.
300x200 is likely == 10x more more rigid than 200x100.
For real endmills, you are looking for worst case deflections in 0.02 mm range, or less.
Example:
HGH35-CA
I have theoretical strength of 10 blocks x 4900 kgf each == 49.000 kg.
I expect to get about 300 kgf milling force, or 3000 N (==1-2 kW).
Its possible I might be able to get 1000 kgf, IMO, IME.
I expect, based on lots of experience, to get to use 50:1 load/strength, at best.
This is an example, to illustrate why theoretical calcs are important, and what you should actually expect re: real world.
Using extra rails and blocks makes the system stiffer, averages errors, and makes it more accurate due to geometrical averaging.
Secondary (overconstrained) preload makes it more rigid.
All this for relatively low cost, vs SP / UP blocks and high preload at blocks, at very high cost.