A Fixed Gantry Cast Iron/Steel Build

Hi all,

After 2 years of designing and building I’ve finally got around to creating a build thread for my new CNC. I’ve been meaning to write a post up on here for a while, and if I decided to start one at the time of the design stage I certainly would have saved a lot of time and effort in avoidable errors or design flaws.
My CNC is a fixed gantry type, it’s an iteration of my previous cnc that has served me well for 5 years which can be found here: http://www.cnczone.com/forums/uncate...29856-cnc.html

The goal of this machine was to ideally mill ferrous material whilst also being able to do the occasional pieces of wood and plastics (with a spindle swap). I understood from the onset that building a CNC capable of doing this well is difficult and I would be better off buying a C mill and converting it, but I didn’t have the room for a mill with the sort of table size and travel I wanted. I also liked the idea of having a machine that was capable of switching between materials such as wood and metals even though I knew this would mean compromises in performance. + it’s fun making one.

I’ve have since forgotten most of the fine detail with regards to design and some build aspects but I shall do my best to recall them. I have written a brief description about the design followed loads of pics of the build

Design Stage

My travel on the CNC are as follows:
X Axis: 1010mm
Y Axis: 450mm
Z Axis: 115mm
Attachment 384338

X Stage
Attachment 384340
X Stage - The Frame
Attachment 384342

The bulk of the frame is made from 6mm thick 100mm x 60mm cold rolled rectangular box sections. The original design (as shown in CAD) had 3mm wall; rectangle box sections running as cross supports but I have since got rid of them as I don’t think I can weld them in well enough to provide any meaningful benefits (this is mainly down to my lack of welding skill and lack of tools to cut the box sections accurately). This design has the spindle in line with the linear bearings that get bolted in the middle of the frame. This reduces some of the moments acting on the bearings as a portion of the milling force is in line with the bearings. It also means that most forces are transmitted through the middle cross member rather than any cross braces that would be present. This frame without the braces weighs 42.7kg.

The areas that mate with the gantry sides and linear bearings are all machined.

X Stage - Linear rails
Attachment 384344
The linear rails are supported TBR20, there are 8 of these bearings in total. The design is far from ideal and the open bearings when the table slides seems like it’s asking for trouble, but I have used this design on my previous CNC and its worked well so far. On my previous setup (which has 2 bearings in the middle of the frame) I see a 50-80 micron deflection when I push hard against the table. I’m hoping the use of an additional 2 (4 in total in the middle on the new design) will see this deflection drop. I am however planning on adding ‘proper’ linear rails (thk, hiwin etc) in the future to stiffen this further, I’m also toying with the idea of replacing half the ball bearings in the TBR20s with slightly oversized balls but I’m not sure if this is possible to do at home?

X Stage - The Table and Servo motor
The table in the CAD is from my previous CNC. I plan to change this as this table is currently welded box sections and aluminum flat bar. I'm considering casting a table in cast iron and machining the T slots, this may be cast in 3 or 4 parts. From initial designs it seems like it will weigh around 120kg. If I do go down this route I am considering using a 1KW Servo motor to drive the X axis coupled with a seriously stiff coupler. I’ve done some torque calcs and it seems like I can go with a lower power drive option however I still need to do some maths to work out inertia ratios between the servo and the table. Some work will be needed here.

The Y Stage
Attachment 384346
Y Stage -The Gantry Sides
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The gantry sides are cast iron, the biggest crucible that fits my furnace is an A35 so I’m limited to the weight I can cast these. They are shaped so any forces transmitted in the z direction do not impose any moments about the mating points about the frame. The sides have been through FEA and various iterations to optimize the design for stiffness and the weight of iron I can cast. In hindsight I should have performed some FEA to see if I could have got away with just using box sections instead of casting these. These sides weigh 28kg each
I shall at one point write a detailed post on alloy avenue about these as they were a bit of a challenge to make.

Y Stage -The Gantry Beam
The gantry beam is 8mm 100x150mm cold rolled box section, Weighs in around 18Kgs. The areas that mate with the linear rails are machined flat and so are the areas that mate with the gantry sides. There are 16 M5 bolts that screw this beam to the gantry sides, the beam and the sides have transition fits between their mating surfaces.

Y Stage - Linear Rails
These linear rails are THK SSR25. I’m using these as I got them at a bargain price on ebay however I believe they are not preloaded, which I think is a big part of some of the deflection I see in the Y Axis Assembly. I may consider changing these to preloaded bearings but will need to justify the cost and perform some calcs/tests to understand if this will help.

X and Y Axis Ballscrews
The ballscrews are 25mm in diameter with a 5mm pitch (2505), they are C5 rated. I managed to get them for a good price from Zapp Automation. The diameter is justified through torsional calcs vs inertial forces and the C5 rating is mainly due to error over the large X axis distance. At the time I did not consider that as these ballscrew are larger in diameter than say the popular 16mm ones it relates to a greater packaging area and therfore greater moments on some of the linear rails due to milling forces.

Y Stage - Stepper motor
I’m considering getting a leadshine 12nm closed stepper to drive the Y axis but this is not set in stone and I will probably need some advice to what motor variants may be best here.

The Z Stage
Attachment 384352
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The main mounting plate is cast iron, it’s currently 30mm thick however in hind sight I should have increased this to maybe 50-60mm. To overcome any deflection ill cast the spindle mount to stiffen the assembly.

The bearings are 25mm linear rails I picked up from Zapp Automation. They are mounted so the rails move up and down rather than the bearings. Although this imposes different moments on the bearings depending on how retracted the z axis is, it does mean the assembly can be stiffer than what’s possible with the bearings and rails mounted the other way. The ballscrew is mounted on the Z plate so it is not exposed to any swarf flying about during machining.

I shall be using one of my current 3N.m Steppers to drive the z stage, I believe the holding torque should be adequate to hold the weight of the stage plus an amble 30kg + of spindle (should it weigh this much). I don’t expect the stage to accelerate fast.

Design Calcs:
I’m not going to dwell on design calculations here too much, it’s something I have written up elsewhere and hope to post on my website one day. In short I have performed some calcs, such as moment analysis for the bearings to work out optimal positions. I have done ballscrew calcs to size up the ballscrews to maximize torsional rigidity vs inertial forces to accelerate them, I have also performed FEA to optimize the castings.

The design stage was mainly supplemented through calcs and understanding what else has been done, I have many useful report/docs/papers that I used lurking in my CNC folder and I can post these if anyone is interested.

Build Stage:
I have misplaced many photos I took during the initial stages of the build, but I have a few from later stages. I could probably talk forever about the problems, lesson learnt and how I went about manufacturing everything but I think I shall let the photos do the describing. In short I have not subcontacted any of the work, and all the casting, machining and welding/fabrication was done by me :)

Attachment 384484
Top Half Of the pattern

Attachment 384486
Cope half of the Gantry Sides, Using greensand with about 4% coal dust

Attachment 384488
Drag Half with no gates or runners, This was a pain to get right. Certainly will be making a split pattern if i do anything like this again

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Smaller Casting Part of the Z assembly. Small molds are lovely to make, takes seconds to ram up and they tend to come out well.

Attachment 384492
The cast iron here is from cast iron drain pipes, Great for fluidity (Due to the phosphor) as it just runs but not the strongest cast iron out there. I messed up this part, i unfortunately couldn't hold the weight of the crucible when pouring so I had to stop pouring half way, BIG mistake as the part is pretty much cast in two bits now (has a massive cold shut). One day I may recast this piece, but for now i have machined it and its being used. Another issue with this casting is the amount of metal penetration in the green sand. I added more coal dust in the subsequent green sand molds to accommodate for this.

Attachment 384494
This part came out and later machined beautifully, No visual defects

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The two sides, side by side. I used pig iron for the other gantry side and will be using pig iron for all my future castings. Its noticeable stronger than random bits of scrap and I know whats in it (providing the data sheet is accurate for the stuff)

Attachment 384498

I'm far from any expert but based on my experiences with mills and cnc machines, machining ferrous materials is very different to the run of the mill materials machined on most hobbiest machines.

It's a very nice looking design, but if the plan is for ferrous machining, I'd daresay for the gantry sides you'd likely be far better off with a solid steel inverted "V" panel to more rigidly brace the gantry/frame. To me the gantry sides currently are probably more suited to a moving gantry design based on lighter machining work, plus I don't see the point of producing and machining such a complicated casting as that, unless your doing it for aesthetics? Myself, I would also space the gantry rails a little further apart and go for a larger, heavier section to help counter-act the tendency for the Z axis to want to flex on the rails, which it will do. I think also you would maybe want to beef up the Z axis reduce the flex.

Machining ferrous, I find you really need mass and very high rigidity to help reduce the vibrations, and ensure more efficient machining and tool life.

I've got a 350kg 6090 that's incredibly solidly built, but I don't think I could machine steel on it in any meaningful way without ear shattering noise and lots of broken tools, the spindle most definitely wouldn't be happy either.

cheers, Ian

Hi Ian,

Thanks for the reply, Yeah I understand I wont be hogging steel all day. Tbh I don't see myself needing to mill steel often, although the goal of the CNC was to machine it, it wasn't to achieve huge material removal rates. As the CNC mill comes together, ill have a better idea of its capability. As for weight, I suspect this machine will weight over 300kgs when finally complete (cast iron bed etc) and potentially more should I decide to fill the frame with epoxy concrete or something similar.

Out of curiosity have you ever measured how much your spindle deflects when you mount a DTI to the bed and apply some force to the spindle? Just wanted to to get a vague idea on how stiff those moving gantry designs are.

Cheers,
Dejan

Attachment 384516
Machining the Y axis plate, This is most likely to be temporary part.

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A couple of defects in the gantry side castings.

Attachment 384522
The Machined Gantry Sides

Attachment 384524
The Cast and Machined Z Axis Plates, The plate has been surface ground.

Attachment 384526
The assembled Z Stage

Attachment 384528
Grinding the ends of the rails cas my angle grinding skills sucked at making a square cut.

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Showing the Y Stage linear rails and ballscrew.

A collection of photos from putting the frame together. Seems like I see a 0.08mm deflection if I push really hard against the z plate when its fully extended and the DTI is mounted on one of the gantry sides. However I really need to have the gantry properly mounted and the DTI on the machine bed to do this test.

My MIG welder can really only weld up to 3mm thick steel. So all interfaces are beveled.

Attachment 384548
Trial fitting all the frame pieces.

Attachment 384550
First couple of welds. Used a bit of vegetable oil on the milled surfaces to prevent any splatter from sticking, Also used some aluminium foil on top of this just to be sure. The vegetable oil seemed to do the trick but boy it stinks when you weld anywhere near it.

Attachment 384552
All welded up

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A close up of my pretty poorly looking welds. Lets hope they stay together ey.

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Degreasing, sanding and then degreasing this frame was an absolute pain. Got it looking really good after some hours of work, But those were some dull hours of sanding.

Attachment 384558
Masking up the machined surfaces for a spray paint

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After a couple coats of primer

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Hopefully the final coat of paint will look better still :)

That's pretty much all progress to date. Ill keep posting as I progress :)

Hi Dejan, what an awesome job you've done on the machine so far, the castings in particular are absolutely brilliant! I'm looking forward to see how it progresses.

With my machine, no, I never really went to too much trouble with tolerances other than some initial basic flex, runout, and spindle alignment checks, that confirmed the machine was true enough, and didn't show any real flex, for my purposes. I mainly machine wood, and sometimes aluminium, so I have no need at all of 3 digit precision.

There has been a couple of times though I've messed up the Z height and it's tried to bore through the table, there was some very noticeable flex then when the spindle hit the table and the stepper jammed, but in use it's barely measurable.

cheers, Ian

The last couple of weeks have mainly been painting the gantry components and frame. Progress has halted on the build as i'm at the stage of putting the machine together and wish to make a set of drawers to place them on. It may not be the best of ideas to place the CNC on a set of drawers but I need the space in the workshop. The drawers frame are all steel and should weigh in at 90kg, should the items in the drawers resonant whilst using the CNC I may consider filling the steel frame with concrete or something similar. I'm facing the drawers with black walnut which i'm hoping will look stunning with a bit of oil :)

It's been a while since i've posted and during that time not a huge amount has been done on the CNC. I have been spending most of my time making some walnut drawers, I've made a set of steel boxes, bolted them together and placed a plywood skin on top (I dyed the ply with some black ink and lacquered it). I suspect the drawers should be good for the weight of the CNC and I can bolt the CNC to the drawers steel frame if necessary.

I have started assembling the CNC, to my surprise the gantry components and Y axis fitted together really well, there is no noticeable binding of the ballscrew or the linear rails. Looks like all those dimensioning and careful machining paid off. Lets hope the servo fits with no dramas. Unfortunately the same cant be said about the z axis, however my dramas here are due to the cheap ballscrew bracket I bought being far from the dimensions stated. I suspect some machining here may solve this.

I've also spent some time machining some brackets for the Y stage. It should allow enough room connect the servo to the Y ballscrew and include space for the coupling. I've done a few calcs for the stages and opted to go for those cheap Chinese servos (aliexpress). I really don't know what i'm getting into here as i'm not sure how they'll perform. I've opted to go for a 1kw servo for the X stage and 750w for the Y stage, these are certainly bigger than they need to be but i've done this to keep the inertia ratio's down (X axis inertia ratio: 1:2.7 Y Axis inertia Ratio 1:1.838). These inertia figures are based on fact I plan to cast the bed in cast iron at one point (120kg+) and cast thicker beefier Y stage components.

Through some serious online digging it seems like a pain to tune these servos, i've gone for some disc couplings and am doing my best to keep all the brackets etc as stiff as possible to give the servos the best chances of moving the stages well and accurately. I initially planned to go for some closed loop nema 34 steppers to drive the stages however looking how quickly the torque dropped off with speed the servos would theoretically give me 3 times the speed. (ball screw calcs indicate any faster for the x stage and they'll reach their critcal speeds 3295rpm)

I am now in the stages of looking into how to interface the servo drives with mach 3. I understand doing this through the parrell port and using a standard BOB may produce a noisy signal and also limits my speed due to the pulse frequency cap. The drives do have gearing but i'm leaning more towards getting a Ethernet board such as a UC300eth or a smooth stepper. Does anyone have experience with these? The reviews for those Chinese Ethernet boards so far don't seem great?

Does anyone have experience connecting these servos to mach 3? Anyone use a differential line driver and is there much benefits in doing so?

Soon i shall also be purchasing one of those BT30 spindle blocks I did want one of those better built ATC BT30 cartridges but I cant justify the costs currently.

I also did some crude stiffness tests on the CNC. Pushing against the z plate when it was fully retracted with around 100kg of force (measured using a bathroom scale so don't count on this figure) I see around 0.09mm on the DTI. At 50kg this was around 0.05mm. The z plate doesn't extend to the bed (about 50mm off) so i would see this deflection go up with a tool fully extended. Not super stiff but I plan to beef up the Y stage and cast the gantry in iron. Hopefully these should see a stiffer machine.