Is 80/20 too light for a metal cutting mill?

[digits]

Hi guys - having been inspired by the stunning DIY CNC mill on 5bears.com, I am considering building a metal-cutting mill (mostly Al, but some steel) using aluminium extrusion and linear rails.

I have done some back of envelope calculations based on the cutting rates and accuracies I'd like, worked out the cutting forces they'd produce and used the deflection calculator to determine the size of profile I'd need to keep the deflection acceptable.

I am still working on the exact structure of my design, but so far, it's looking like it will come out at about 40-50kg for the structure without the milling head/motor which will add another 20kg or so. This is for a table size of about 750x250mm (30x12") and travels of about 500x300x300mm.

My worry is that it seems a bit light for its size. I have tried to ensure that my design is naturally balanced - i.e. no overhangs or massive cantilevers, so that it doesn't need a massively heavy base to keep it upright, but I am no mechanical genius, so I can't help thinking I've overlooked something!

Should I just trust my sums, or is there a good reason that mills are all rather heavy?

Cheers.

[ZipSnipe]

Hey Digits, I,m in the planning process to build a gantry cnc mill. One of the things you need to consider is how much metal are you planning on removing. I,m doing steel for the base and outside frame parts. but all the axis's will be 1/2 alum. Here's a good link from the Widgitmaster( this guy knows his stuff) Building A CNC Router - A Few Hardware Things To Consider!
Building A CNC Router - A Few Hardware Things To Consider! - CNCzone.com-The Ultimate Machinist Community

Good luck and happy building!!!!!!

[digits]

Originally Posted by ZipSnipe Hey Digits, I,m in the planning process to build a gantry cnc mill. One of the things you need to consider is how much metal are you planning on removing. I,m doing steel for the base and outside frame parts. but all the axis's will be 1/2 alum. Here's a good link from the Widgitmaster( this guy knows his stuff) Building A CNC Router - A Few Hardware Things To Consider! Building A CNC Router - A Few Hardware Things To Consider! - CNCzone.com-The Ultimate Machinist Community Good luck and happy building!!!!!!

Thanks for the link ZipSnipe - I am rather unambitiously aiming for 10x the performance of my current mini-mill

According to my calculations, a full width 1/4" cut, 0.1" deep in aluminium at 20 IPM will present a force of about 200N on the cutter - so I'm designing for 350N

My current cost projections are very,very scary, so it may never happen, but I have already shelled out for an X-axis and I bought an R8 mini-mill head today, so who knows?

[digits]

Hmmm, well, after a bit more design last night, I ended up with a weight of 84kg in 40x80mm and 80x80mm profile alone - which would cost about as much as a complete X3 mill here in the UK!

Back to the drawing board, I think - my last design had X&Y in the table and Z on the gantry. I am going to have a go at putting X&Z on the gantry - it might not save me any money, but at least the footprint won't be 6-7x the XY cutting area!

ZipSnipe, what sort of Z-travel are you aiming for? I am after about 300-400mm, which is causing me no end of pain. I might go for adjustable gantry height + 100-150mm of Z in the quill instead.

[ZipSnipe]

Well Digits after all the designs I,ve looked at I,ve favored one that where all axis's will be over the table(overhead).I would show you a pic that I found here on CNC Zone but it was old and no longer can be found.With the Z axis with at a minimum 6"(152.4 mm) travel and for the X at 48"(1219.2) Y at 24"(609.6mm) . For spindle speed I,m aiming for 0-5000 rpm. 1/2 to 1hp motor.I do want to use the r8 spindle but not the minimill head so I,ll just buy the r8 spindle and fab my on spindle enclosure with dual bearings(the minimill R8 spindle only has one bearing to my surprise). I plan to do wood cutting and thin gauge stainless steel cut outs so I want a machine that is flexible in cutting abilities. My plan as far rigid machine is to make the base frame with 1/2 (12.7mm) steel plates which will hold the X axis rails which will then hold the Y and Z assembly which I was also thinking of using the 80/20 aluminum or 1/2 (12.7mm) aluminum plates for that part of the construction. Also your calculation for taking .1 out in aluminum sounds a little heavy cutting. I started working at a tool and die shop several months ago and I was expecting to see big chunks of metal flying off these big VTC machines they have but thats not the case. But we don,t really mill aluminum here(lots and lots of drilling) but they do p20 steel and I,m amazed at how light of cuts that they do here. I think they rather it run all night on light cuts rather than crash or wear out the tool. I don,t know your expierience level(I,m only in metal working now for about a year but been in the construction industry 20 or more years) but anyway I think people(including myself) when starting out have the tendency to want to make heavy cuts and when ya got a cnc machine to do it for you theres no need for heavy cuts you can just press the button and let her run all night. Good luck and keep us posted(with pics)

[digits]

Cheers ZipSnipe - you're probably right about 0.1" being quite a deep cut - I don't intend trying to cut out huge swathes of metal, but I do need to be able to cut faster than I can currently. I made a very simple part on my X1 - basically a 25mm wide, 200mm long bar with rounded ends from a 1" wide, 1/2" thick aluminium bar. At 300mm/min I can only cut 0.5mm/0.02" deep - it took 2 hours! The part is nice but life is too short

I am currently thinking about rehousing my CNC'ed X1 in an enclosure made from 80/20 profile - to allow me to run long jobs without ear defenders, and to give me some idea of what the profile is really like to work with.

I have been balking at the costs of screws and rails, so I'm going to try and work on a couple of design variants that could be put together with the same 80/20 and linear components. I do like the idea of everything over the table, but it seems to require a lot of very careful alignment along relatively long distances.

I doubt I can cut 80/20 well enough to make very precise butt-joints and I am limited to about 5"x12" for CNC'ed joining plates etc, so I need to design out things that need too much accuracy. 80/20 is however pretty straight and pretty flat - if I build structures that don't need to have the cut-ends butting against anything, I should be able to achieve decent accuracy with a bit of t-slot twiddling

Best of luck to you as well - though having seen your manual mill, I'd say my money's on you building a better machine than mine with far fewer headaches!

[Zumba]

80/20 has plenty of strength and roughly zero rigidity. Seriously, it's one of the worst materials IMO for building a precision machine.

Keep in mind that a 2000lb Bridgeport mill is considered a featherweight in manufacturing industries. If you want any kind of rigidity at all, make it out of steel, preferably welded. Cast iron would be even better but it's not really an option for most of us.

80/20 machines are great for cutting wood in light passes. Cutting steel would be a nightmare. And the stuff ain't cheap. The 3030 series (3"x3") is by far the smallest I would go, and it's more expensive per foot than 3"x3"x1/4" steel tube. Put the two side by side it's obvious which one you'd want to use.

[digits]

Originally Posted by Zumba 80/20 has plenty of strength and roughly zero rigidity. Seriously, it's one of the worst materials IMO for building a precision machine. Keep in mind that a 2000lb Bridgeport mill is considered a featherweight in manufacturing industries. If you want any kind of rigidity at all, make it out of steel, preferably welded. Cast iron would be even better but it's not really an option for most of us. 80/20 machines are great for cutting wood in light passes. Cutting steel would be a nightmare. And the stuff ain't cheap. The 3030 series (3"x3") is by far the smallest I would go, and it's more expensive per foot than 3"x3"x1/4" steel tube. Put the two side by side it's obvious which one you'd want to use.

It looks good in deflection calculators - where does it lose all its rigidity - in the joints?

I'm not going to suggest that a Bridgeport isn't well built, but how much of that 2000lb is in the base to keep it from toppling over when the table's off to one side, and to counterbalance the overhang of the head?

[jpelosi2002]

Bracing is your friend. My current design is using all 6x6 and 3x3 extrusions. Box all your corners and keep the lengths to a minimum. Most machines I see made out of 80/20 are designs that are better suited to steel or cast iron as a material. Not much thought is put into the weakness of the material OR it's strengths.
Aluminum is actually a very rigid material and if you take the extrusions internal honeycomb design into consideration with your design in regards to force loading you will have no problems building a rock solid machine.
When designing for strength remember to keep lengths to minimun. You'd be suprised how much rigidy can be gained by taking an inch off a moment arm when dealing with high forces created in material removal. That being said, do you really need 6 inches of travel on your Z axis?
Bridgeports are great machines but what happens when you get to the end of your travel on the X axis? Shudder city. Leaverage is a powerful force multiplier. Use it your advantage. See the forces working againt you and gain leverage againt them. If you're able to keep that mental picture in your mind while you're designing, the design will tell you exactly what it wants to be. It sounds simplistic, but machines design themselves, just be watchfull of what the design is telling you and don't be stubborn if the machine is not wanting to be what you originally had in your mind.
I design filling machines for a living and I'm not proclaiming that to raise my stature here or anywhere else, it's just a job. But a lot of what I'm saying comes from some hard lessons as well as some great triumphs in a professional environment. Good luck to all. Use your imagination but listen to the design.
Jim

[Stepper Monkey]

I would tend to very much agree there, and I too have had some great results making smaller metal cutting machines with 80/20. It takes coming at it from an engineering perspective though and not a trivial amount of time in calculations. It also isn't good for everything. It definitely has its advantages though, especially when coupled with ali plate to make stressed panels and simple box sections.
I think your arguments would definitely make sense to a crowd of engineers, but around here we have a large and vocal subset that opens cans with dynamite because can openers aren't manly enough, so I don't think it will have much of an effect, especially if calculations are needed to express the concepts.