[Newbie] Looking to buy a mill, I need alot more insight!

[tjschweizer]

Hey guys, first poster here. I'm a college student. I'm almost out for the summer. I used to be in college to be a machinist, that was last year, and now I'm in engineering. Long story short, I miss machining. I want to buy an x3 mill. Basically, it seems small enough that I can handle it in my house, yet big enough it can handle most of my projects (stirling engines, etc.) It's hard to find an exact answer on the internet, but I was wondering exactly which brand I should go with for the x3 mill? Sieg or Grizzly seem to be the main two options, but that's all I could pull with a Google search. Also, I need to know how well they can cut steel. Not any huge cuts, just cut it period. And, as I am a college kid, I'm broke. Preferably, I want to spend less than $3500. It seems I can get the mill and some basic tooling for $2000. Maybe a little more. But here's the catch, I want to convert it to CNC. So I would need an estimate on how much it would cost. I have access to SolidWorks and MasterCam through my college, so I don't need to spend any money on software. If anyone has any information, and links, and good places to buy, I need to know as much as I can before making an investment like this.

Thanks for the time!

 

[DMS]

I have seen more X2 and BF20 CNC conversions than X3s, not to say there aren't any. Cost of converting to CNC varies widely, depending on the way you go. At the very least you will need

* A PC for the control software (free for me, since I had an old one sitting around, or you can probably find an old clunker for less than $100, it doesn't take a lot of HP to run Mach or EMC2)
* Control software, the most popular are Mach 3, and EMC2, Mach is commercial, and I think the price is around $150, EMC2 is open source, and is free. I am using EMC2, and have been impressed with it. Lot of fans of Mach as well.
* Motors and drivers. These can be steppers, or servos. Most of your money will be here. I got mine at Keling (http://kelinginc.com/NEMA23Motor.html). I actually went for large servo's. If you are doing a benchtop mill, steppers will be cheaper, and should give you enough performance.
* A power supply to power the motors. You can buy new, or find some used switching supplies on E-bay, etc

You may also want to convert to ball screws. These can be expensive. I have not done this yet on my mill (I am using backlash compensation) and am getting decent results.

For $3500 you should end up with a decent machine. If you look through youtube there are lots of people who have done conversions, they may vice you some ideas. There are also companies that sell conversion kits, which may save you some time.

Last but not least, check out http://Hossmachine.info he has done a lot of work with cnc conversions on these small mills. He also sells plans for some.

 

[tjschweizer]

@dalee
Thank you Dale, that was exactly the kind of info I need. I have a few months before I make any purchases, and I more or less needed a "starting place". I will look for some CNC kits, and see what I can come up with. Thank you very much!

@DMS
I will look at both of these sites. Thank you very much for the information. I do know what backlash is, and I do know what ball screws are, but how exactly do you compensate for backlash in the CNC?

@anybody who reads this
The only manual mills I've used were knee-mills, so obviously I just used the knee for adjustments with my quill set all the way up when I needed accuracy. How exactly does the z axis work on this? Is it just a quill? If it is, how accurate are the measurements? For instance, on the quills that I had there was basically a ruler with 1/32" measurements, and that was judged by eyesight. I would hope it's more accurate than that . And will putting a servo or stepper on it make it more accurate? I apologize if these seem like newbie questions, but you gotta learn somehow right? I know my way around computers, software and hardware, so I can hopefully save a lot of money on buying parts and doing a bit more work myself. Also, where would I go about buying ball screws? Are they machine specific, or just basic bearing dimensions? And where do I find the specifications on the mill's shafts and such for the ball screws? So many questions, answer only one and I will still be happy Thank you all for commenting! I appreciate it.

 

[DMS]

Backlash is the lost motion in a system when changing direction. If you can imagine a screw driving a nut forward as the screw is turned. When the screw changes direction (to drive the nut the opposite direction) there will be a period where the screw turns without moving at all. If you have used a manual mill or lathe, it is the reason you always approach the part in the same direction, and if you "overshoot" your measurement, the reason you back off half a turn or so before feeding in again.

Ball screws are pretty much what they sound like. They consist of a screw (the ball screw), and a nut (the ball nut). These have small ball bearings that ride between the nut and the screw. Like ball bearings this reduces friction. You can also do something called "pre-loading" which allows you to (for all intents and purposes) get rid of backlash in the system.

Backlash compensation in EMC2 is pretty easy. First you measure the backlash (say, with a dial test indicator), then you type it into the config file. You still have to be careful of certain operations, because the backlash is still there. You didn't get rid of it, you just compensated for it. It is especially important to keep this in mind when climb milling.

 

[tjschweizer]

@ DMS

I will probably take the cheap route to start off. Unless I run across a good deal on ballscrews. I looked at that site, and found he also owned g0704.com . There he is converting a G0704 into cnc. I also discovered the Precision Matthews PM 20 and 25. They look like pretty good deals too. They seem to have less shipping time and less chance on being out of stock.

@ anyone

Does anybody know where I can find a clear-cut step-by-step basic list on what needs to be done for the conversion? I have a pretty good idea, but I would like to know a little more. I will keep searching, and I would recommend the site DMS said to anyone. That Hoss guy is a genius.

 

[tjschweizer]

I think you're right. I've been weighing the pros and cons, sent some emails to Grizzly and Precision Matthews, and I'm going with the G0704. A while ago, I made an account on www.kickstarter.com for more or less the same idea, but with much less knowledge, and expecting a much higher goal. I think I will retry it to aid in purchasing just some of this equipment. I'll make sure I post when the account is launched, in case anyone wants some ideas. If anyone comes across this and has the same questions I've been posting, pm me. I can help you avoid staring at a computer screen for hours Thank you everybody!

 

[DMS]

...
If you do the conversion, I would very seriously consider getting the ballscrews right away. Software backlash comp works OK for straight-line moves, but is all but useless for cutting a radius since the slack when the screws change direction at the quadrants cause flats in the arc.

dalee

I hear people say this frequently, but i have actually been really impressed with backlash comp on my mill, especially with circular interpolation. Sure, if I try to take a heavy finishing pass in an arc, it's not going to work out well (ask me how I know) but if you finish up at say, 10thou things go good. I have done small features (1/2") that were within 1thou. Backlash in my x is .0125, and .0120 in my y. I should probably take some pictures for show and tell. What I'm saying is, if you can't afford ballscrews up front, don't let that stop you. You can do good work without them, and you can always add them later.