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Sieg X3 Milling Machine - Cnc Conversion


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I think an important way to start this is to go over some basics, some lessons learnt and almost a summary view of the project as a whole. It really has been a learning experience for me; I am not an engineer although, like most of us, I'm not afraid to get into the technicalities of things and I'm not against figuring some stuff out along the way. Armed with a PC and the internet, you can figure out a lot of things but you also can fall foul of a lot of spouted *bleep*e. So...


What is it?


It's a 3-axis milling machine. Originally a Sieg X3, these are fairly small but rugged milling machines aimed at the high-end home hobbyist or very low-entry workshop work, maybe small garages who need a small amount of milling capability. Hand cranked, obviously, with a big 600w variable-speed motor driving an extendable quill (like a pillar drill). The bed area is roughly 550x160mm in size, so you'll mill a fair size piece in there - aluminium, steel, no problem. The Sieg's go under various other brand names too, so you'll find them as "Grizzlys" or "Harbour Masters" in the US, or Axminsters in the UK. The X3 has been superseded by the Super X3 (SX3) and "SX3 HiTorque" now, which really just adds a more powerful motor, an RPM gauge to accurately see what you're spinning away at, and power tap features.


When you start off in your mind thinking you want a CNC milling machine...


...you really start with a lot of options, and it greatly depends on what you want to do with it. Ignoring cost for the moment, you need to ask yourself what capacity you need out of it (what material do you want to work with), how fast you want it to work, and that age old question - what sort of accuracy and repeatability you want out of it.


In short, you could be any of :

  • milling wood and MDF of several feet in diameter to a tolerance of a few mm
  • milling soft metals to a 0.05mm
  • milling small PCBs to a few thousandths of an inch

...and each one would take you in a different direction. For me, I was stuck with wanting to do 2 things that are pretty much opposite ends of the spectrum - I want to mill aluminium (and maybe steel occassionally) for the usual car parts and stuff, but I also wanted to be able to mill small, electronic circuit boards at fine accuracy.


I was aware of a few options, and these are the common ones spouted around on the internet...


  1. Buy one in kit form from China. Loads on eBay; seach CNC 6040 or 8060 and you'll find these tabletop, aluminium-extrusion jobbies as complete kits for under a grand.
  2. Design and build one completely from scratch yourself
  3. Buy a manual machine and convert it (like the Sieg, or something bigger like a Bridgeport)

I looked long and hard at option 1, and convinced myself (along with many others on CNC forums) that it was not the route to go. The China machines are really aimed at mucking about with acrylic, wood, MDF or soft aluminium - mainly for engraving. They are built to a price, and that means poor electronic components with a history of showing failure, and flimsy construction that can't take serious loading, so if you're looking at milling aluminium for example, you can't do anything more than "nibble away at it", slowly and gently. And the accuracy of the devices is, well, fine for mucking about but really if you want accuracy it can be a bit "variable". I'll come on to accuracy/repeatability later on in another post as there's quite a bit to it.


I started off down option 2 but it soon became apparent that it would be very hard for me to make my own machine from scratch without some machining capability in the first place... and the accuracy of the build would be limited by what I could achieve. How can I machine or build a truly "flat" surface, really, in my home workshop? I didn't fancy my chances. After going and chatting with a chap locally who invited me off a CNC forum to "go see what it was all about", it was clear that buying a cheap 2nd hand machine like his 2 amazing Bridgeports, then converting it, was not only cost effective but would give you a machine with incredibly accuracy and power - probably far more than I was ever going to be able to fabricate myself. I didn't need a large capacity in terms of material size, so it didn't really warrant building something custom.


Much as I'd have loved a Bridgeport, I simply don't have the room. As Steamer will attest, they're not light buggers :D, and I really had nowhere to put one. So something like the Sieg just worked out "right". I wasn't sure whether I'd successfully get it doing both my tasks on a single build. A lot of people on CNC forums would say it can't be done, or that I was expecting too much, and you start getting that "noob" feeling like you're out of your depth, getting pulled from pillar to post between a sea of keyboard-warrior opinions.


My advice? Say "*bleep* it" to the lot of them, and just do it the way you want to anyway. If no-one can say with absolute certainty that X will work, or Y won't, then put it down to a learning experience and just find out for yourself. If you're willing to sit down and read up on stuff as you go along, and manage to sift through the advice to find the good stuff, the take-it-or-leave-it stuff and the outright crap, you should be able to get to something that will do what you're after. "Opinions and arseholes..."


You might not get it right, particularly if you do something out of the ordinary, but if you go with what's generally accepted as the norm chances are you'll be fine ;)

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One of the first things I did was set about costing it up against the alternatives, planning out all the items I needed - in particular the high-cost bits like the ballscrews, the motors, the motor drivers and electronics. I kept a Google Drive Spreadsheet going with all my workings as I went along - notes, calculations - which I'll bring in here and reference along the story. Attached is the final costings, which has been updated over the course of the project with real numbers so it's a spot-on guide for what it will cost you to do - however it doesn't include a few items of importance...


  • PC to drive the machine. I have PC's coming out of my ears so... yeah, if you don't have a spare one, add it on. You won't need anything high-end or fancy.
  • Software - there are freebie solutions out there, or costed, it's up to you
  • Costs for getting the ballscrew support brackets machined - this is a case of claim a favour from someone who already has a machine, or pay someone to make them. I called in a favour with someone I knew so it just cost me a bottle of single malt and some biscuits ;)

Some resources on the internet for you also.


www.mycncuk.com - a forum dedicated to DIY CNC. Useful as a research tool and source of information, but I wouldn't get too tied up in finding the answer to everything on here. You'll just get lots of opinions, all of which are valid to one degree or another, and you'll tear your hair out thinking which one to follow :)


www.cnc4you.co.uk - a retailer, based in Milton Keynes, and fairly well priced for certain things. They sell a lot of the bits you'll need and, being UK based, it's quick delivery. Once you've shopped around on eBay a lot of the times you'll find their prices are not all that bad, and the low hassle factor means you can just end up buying from them.


zappautomation.clickforward.com - these guys are very expensive, but their product listings and website provide lots of detail on ballscrew and ballnut specs, including 3D models you can download to build up and design your brackets and parts. So you might not end up buying from them, but their wide range of screws is helpful to browse.


fbtmotion.com - this is where I sourced the ballscrews from. Firstly, just forget getting ballscrews at comparable prices in the UK. A lot of them are just buying in from China anyway and selling on, but the price difference is just astronomical. Just go Far East for all your mechanicals (ballscrews, nuts, supports) and also your motor drives. It's the bulk of the cost of the conversion, and to do it with UK-sourced screws would have pretty much doubled the price of the project. Fred at FBT was an utter professional, they custom-made my ballscrews for me to my spec and even accomodated my awkwardness for C5 screws (more of that later), and did me a specific low-profile ballnut that I needed. It did mean the delivery time was ~6 weeks as they were made to order, but such if life.



CNC Machine Calculations - Costings - Convert Sieg X3-1.pdf

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So probably one of the central and key decisions you'll need to make early on is what ballscrews you intend to use. The ballscrews are the long, precision-machined rods with a screw ground or rolled into them at a known pitch. They take a ballnut on them, which is fastened to one side of a sliding axis arrangement - and then the ends of the ballscrew are held in position by ballscrew mounts. Once fastened, you twist the ballscrew and the ballnut is forced along the axis, hence moving the machine (the table, in this case). Pretty obvious stuff. But there are various specifications of ballscrew to be aware of.


Firstly, the Sieg X3 as it comes from the factory uses what are called Acme screws. These are not really suitable for CNC control because they have inherent backlash in them - fine when the machine is operated by hand because an experienced operator will know to take up the backlash when he's cutting a part, but for CNC it's no good - from memory I think I measured about 0.4 to 0.6mm of backlash on the machine as I got it. Even with a bit of tuning of the brass ballnut blocks you'd never eliminate it, so a CNC controlled machine would never be able to accurately place itself with any repeatability better than this figure.


When you spec up a ballscrew, you'll obviously have a total length in mind - whatever is needed for the machine in question. You'll have various diameters available, but also "grades" from C0 to C10 - the most common for DIY'ers being "C7" spec. C7 to C10 are rolled ballscrews, and C7 grade has an error margin of 50 microns over 300mm. So this basically means that, in moving the screw over what you believe to be 300mm, you might actually get 300.05 or 299.95mm of travel. C5 drops that down to 18 microns, C3 is more like 8 microns, and so on.


There is a wealth of opinion on screws, and most told me C7s would be fine for my requirements but I kept thinking "Why not go for a C5 just to be sure"; why not give yourself the best chance of getting repeatability as you can? Faced with the advice that C5s were an extravagance unnecessary on a home-converted Sieg X3, I ignored it all anyway and went with C5 spec screws. At the very least, they weren't all that more expensive than C7s anyway. The only difference was on the Z axis (the up-down), where I did stay with a C7, but this was more due to screw pitch and the necessary leverage/torque I would need to lift the mill head up along the axis - it's bloody heavy. More on this later - calculating motor torque values and so forth. There's some work there.


Here's the old Acme screw that runs left to right driving the X axis :-




No fancy bearings, it just sits in bronze bush at each end, and you hand-crank it left and right old-skool. In comparison, here is the replacement 1604 C5 ballscrew in mocked-up end brackets...




In this early picture, I had designed the end brackets (all for free, in OnShape) and had mocked them up with 3D printed parts. This was utterly invaluable, as I effectively built the machine back up with plastic mock parts just to verify that my design was right. It's essential that you get the ballscrew running straight and true across the table, as the slightest bend will see it bind up. I made a slight cock-up in my understanding of how the end supports - the black mounts - go, and had the threaded part of the ballscrew too long, so they ended up turned outwards rather than sleeving inside the brackets. One day I might re-order the X screw shorter, and invert the supports properly. But for now, it works, so I won't worry about it too much.


A quick word on screw specs : "1604" means 16mm diameter, and the 04 is the pitch of the screw. The pitch is quite a key part to the spec. 4mm pitch means that one complete, 360 degree rotation of the screw equates to 4mm of lateral movement. Naturally you can get a wide range of pitches, but these are limited depending on both the diameter and the grade of the screw. A small pitch (say, an 02) will give you much more mechanical leverage from the motor to turn the screw, so if you've got heavy loadings you can run a less torquey motor and use the pitch of the screw as a mechanical advantage... BUT, your machine speed will suffer, as you need to turn it twice as fast as a 4mm pitch screw to achieve the same speeds. Likewise, a 5mm pitch screw needs turning less to move a given distance, but your motor will need to deliver more torque against a load than an equivalent 2 or 4mm pitch screw.


On a machine this size, it's not all that big an issue, but I mention it just in case. I'll do torque calculations in another post some time - there's a formula I found for it and put in my spreadsheet, but I'll find the source for reference and link it.


Here's a quick pick of the machine stripped down....




There's a very good maintenance manual for the X3, including detailed instructions on how to strip it, so there's no hardship here. But the mill head is fricking heavy, so you'll need something like an engine crane to take the weight of it while you remove the rear Z-axis mechanism. As you can see above, I placed a block of wood underneath it to stop it slipping down, and then strapped that wood to the pillar along with some suitable text to warn people not to knock it out!!


At this point I hadn't removed the Z-axis wheel from the front of the machine, and you can also see the Y-axis Acme block on the bed of the machine, yet to be removed.

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Fair enough, each to their own. People said the same thing to me about my 3D printer...


edit: I'll qualify that. For some people, it may be a toy, and if that's your thing then go for it.


Me, I do a lot of electronics projects on the side and etching PCBs is a messy business; accurately drill them afterwards is even more of a ball-ache. On top of that I have plans for several enhancements to the rally car which will require various aluminium bracketry to be made - cam position end covers, gearbox mounts, gear selector mechanisms, alternator mounts, radiator mounts - stuff that can't be 3D printed. And probably some blingy bits too.


The Zero also has a few things that I could probably do better now with a mill, so I might re-do some brackets over the years and/or make things slightly differently.


There's also a few PC part projects I'd like to do in the future, not stuff you guys would be interested in - I need a handbrake for my driving simulator :) why buy one when you can make it yourself :D ;) but that's longer term really.


Now I know a lot of these don't necessarily need CNC control in the hands of a well-skilled engineer. But I'm not one of them. I'm on of those generation that can design it quicker in CAD than I can operate a machine to do it manually ;)

Edited by brumster
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