Ok, anyone figured this out yet? Having an ATC is the last impediment to me achieving machining nerdvana - something I know you all lose sleep over . I just watched a video on the Tormach ATC and it is dead sexy but I'm not a huge fan of the belleville washer trick for a power drawbar. It SEEMS that the NM-200's slick drawbar design can do a better job. I'm thinking a small impact wrench or such that goes up and down. Khai told me a few months ago this was in the works but he also said to give him a ring if I figured out anything on my own which did not make me feel any better about it's chances
I've got a good bit of robotics experience (far more that than machining) and would love to collaborate with one of you experts on here. The initial goal is to make an effective power drawbar inexpensively for the NM-200 mill. If it works for other Novakon mills, the more power to us. Some requirements then:
1) Be cheap - we didn't buy Haas for a reason
2) Be easy to source parts - IE: no lucky finds required to make it work
3) Be 100% effective every time. If it ain't reliable, it ain't worth it.
4) Be totally digitally controlled (necessary for future ATC use)
To kick start the discussion, here is an electric drawbar I thought was pretty slick in it's simplicity:
http://www.cnczone.com/forums/bencht...o_drawbar.html
Here is a cheap and pretty solid electric impact wrench:
1/2" Electric Impact Wrench
This wrench in particular is of an interest to me because it's cheap enough that I'd gladly tear it apart to give it a vertical housing and lose the handle. By losing the plastic casing, it would be possible to use the already provided screw holes to attach it to a fixture. I imagine the "indents" used to recess the casing screws would serve double-duty as a way to restrain the tool to keep it from rotating. In short, it seems very doable, especially since we all have CNC machines at our disposal to mill up a fixture for it.
In case anyone is wondering, I'd rather not use an air impact wrench because I have enough crap in my tiny shop already and running a compressor just to mill is not ideal BUT with that said, this is clearly a working approach:
CNC Cookbook: Powered Drawbar for IH Mill
The thing to look at in that page is all of the tricks with the way to guide the socket down. Nice and simple solutions to a potential issue.
Another advantage to electric is that it's quite easy and cheap to control an electric system and a bit more of a hassle to computer-control a pneumatic system as good quality electronically activated air valves cost a lot more than relays. If the ultimate goal is total automation, I think we should stick with what's easiest.
Now I personally think the whole mechanism should "float" ~ 1/2 to 1" above the nut and assume about a 1/2 to 1" engagement on the drawbar to avoid stripping. The floating would be handled quite easily with simple springs on a pair of risers. This poses the biggest issue though. Sticking with the whole "electric" theme it means a few options - motors w/gears and the like, solenoids, or linear actuators. Motors with gears and such are likely a great choice but a pain in general for custom tasks like this. Linear actuators are incredibly strong but also generally dog slow. If you are willing to wait 10-20 seconds to release and re-engage they are great. Personally, I think the best approach is a solenoid. Sadly, I don't know where to source these cheaply so we have to work with something more reasonable like McMaster Carr. Part #7723K5 in particular seems reasonable with it's 21 lbs of force. The biggest issue with it is the depressing 1" throw but that's solvable.
Next we need the control electronics. I'm a whus here and would just stick with what I know. The EASY solution is an Arduino - the cheaper solution is a raw ATMega chip. Both would need to use some relays to control everything. The advantage to using a microcontroller is one of intelligence - this same controller can manage the ATC flawlessly too which gets away from all the nasty Mach 3 macros so many people need. Additionally it lets us do smart things in the future like detect if the drawbar didn't engage, etc. So let's assume Arduino but we can switch to the raw ATMega later to keep costs down if needed. This is a nice tiny one that is perfect for us:
Arduino Pro 328 - 5V/16MHz - SparkFun Electronics
And the relays:
Relay SPST-NO Sealed - 30A - SparkFun Electronics
We'd need some transistors, wire and the like but all too cheap to worry about for now. Additionally we need some sort of button to toggle engage/disengage and all that but we'll skip those too. So the pricing looks something like this:
Arduino - 19.95
relays - ~15
linear solenoid - 45.60
impact wrench - 49.95 (can get it for $40 with the monthly 20% off coupon)
Everything else - ~$70 (aluminum, wire, springs, etc)
Under $200-250 pretty reasonably. I think this is pretty realistic and very acceptable all told.
Here are some commercial options to see the range:
http://cgi.ebay.com/POWER-DRAWBAR-BR...4#ht_500wt_922
$135 w/shipping - this is still effectively a manual setup so not going to cut it for ATC but it is a good design and one we should "borrow"
http://cgi.ebay.com/Air-Power-Drawba...ht_1840wt_1139
$455 w/shipping - this appears to be very nicely done but it would need to be retrofitted with an electronic valve for ATC use. Still a clean and solid piece of work and if it fit my mill easily I'd very seriously consider it.
Any thoughts on this from the experts? Anyone willing to work with me on this? Thanks!
-Mike