Smithy CNC conversion, how much torque?

[jester69]

Hi all,

I am new to machining but a very old hat at computers. The part where you have to know machinist type stuff I get lost, and I can't find too many mill/lathe conversions to go from.

the smithy has a 12" swing, 20" bed & the cross slide has 8" travel. I plan to cut mainly aluminum, but a fair ammount of steel, so that needs to be accounted for. I won't be in a huge hurry, but won't complain about speed.

I can't for the life of me find anyone else that has done one so I can just copy their motor specs, and the formulas are a bit daunting. I know mill/lathes have a bad rap, but I got a really good deal, and figure deal with the devil you know etc. My experience (frustration?) will transfer to my next project should I decide this is not doable.

So, what should I be looking at motor wise. Of course I want to use 60 in/oz printer motors, but am guessing that might not work. I am not totally opposed to belts or reduction boxes...

take care & thanks,

Jester

[2muchstuff]

For that size of machine 60 oz. motors just wouldn't do. If you did want to use them, they would have to be belted down so far that it would take forever to do anything. If you really want to use printer motors, the HP Laserjet 3's and 4's have motors rated at 100 oz.

[fyffe555]

Is your smithy the cnc prepped version with ballscrews or the std acme profile? I was ( still am) looking at a granite series and did the calcs to direct stepper the acme version with the help of smithy. Minimum torque for the X to give the correct cutting speeds and chip loads in steel was 400oz/in. Lathe Z axis would be similar. Mill head even more to be able to lift the head since the std screw has quite a course pitch. Note that trying to run smaller motors and gearing to produce torque will mean the machine would run too slow to get the cutting speeds necessary for the spindle speeds and typical tooling.

As an aside if you can afford a smithy then the steppers are almost trivial.

Andrew

[jester69]

Originally Posted by fyffe555 Is your smithy the cnc prepped version with ballscrews or the std acme profile?

Mine is from the early 90's sometime I think. from the reading I have done since mine is a mm unit instead of inch, it is not the current midas series, but the previous iteration. Mine is closest in specs to the non-XL midas 1220. I would assume that it is pretty basic, so probably no to anything CNC readyish.

Originally Posted by fyffe555 Minimum torque for the X to give the correct cutting speeds and chip loads in steel was 400oz/in....

Cool, that is kind of what I was looking for. Wishful thinking that I could get away with printer motors I guess, unless I can stack 4 together or something, heh. Looks like NMEA23 may be a bit small. Powermax motors seem reasonably priced on ebarf, would 280-300in/oz maybe be enough, or is 400 the magic number? Is that brand OK?

Originally Posted by 2muchstuff the HP Laserjet 3's and 4's have motors rated at 100 oz.

Hmm a bit small it sounds like, but good info to have in case I run across one. Thanks. I'm hoping to run across one of those old wide carriage IBM mainframe printers. Bet those are pretty stout steppers.

thanks,

Jester

P.S. Sorry to ask so many stupid questions, I really hate to be a newbie at stuff, heh. I like to be the one with the answers. Anyone want to know anything about VW vans, unix or networking & firewalls?

[miljnor]

I had a smithy and similar since i was 16yrs old. If you cant get anything else they are great... many a hot rod project was done on mine!

The major down side of a smithy is the milling head. If you can figure a way to double clamp the column like the HF mill drills it would be awsome. I personaly wasn't ingenious enough at the time to do this but It would have allieviated huge amounts of problems and frustrations. That being said the only other problem I had with the smithy (or equiv, had 3 dif. types) is: under heavy load on the lathe sometimes the locknut on the spindle would kick out and spindle would stop. filing the flats (so that they were flat!) stopped this.

the 400oz is probably overkill if my mini lathe is anything to judge buy. but if you find a way to lock down the mill head you might need it.

[jester69]

OK, do you guys think I could get away with 280 oz? found some reasonably priced NEMA 23's with that power. Any more and you are getting into 34's. It would be nice to have the smaller form factor.

The major down side of a smithy is the milling head. If you can figure a way to double clamp the column like the HF mill drills it would be awsome. I personaly wasn't ingenious enough at the time to do this but It would have allieviated huge amounts of problems and frustrations.

I have seen a few references to the milling head rigidity on the Smithy. So, is the one column lock the main stumbling block, or are there others?

thanks,

Jester

[fyffe555]

Jester, the ~400oz was calculated as a result of the required chip load on an endmill cutting steel with std (imperial acme) leadscrew.I can't remember the details but if you're not going to be cutting 1/2" em .25" cut at full spindle speed then you can use smaller steppers. Remember the torque required is a sum of the torque needed to move the cutter through the part at the required speed, the torque needed to overcome the inertia and stiction of the machine and the intertia of the motor itself. The second will not change.

[miljnor]

to the milling part that was the only thing that i realy had a problem with. For what the machine is (a 3 way compromise) it is real quit handy. but if you could make the swivel on the head become more secure you would have a realy nice machine.

I have had the cutter just take off in the part because of that damn column and basically thats the only flaw i realy had problems with..

some way to preload the leadscrew would be the next thing to change. Rigidity Rigidity Rigidity! Thats the key

[Mrsonso]

I ran across this page after doing some reseach to convert a Smithy to full CNC control. Hope this helps. http://www.novalab.org/cnc/