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Mach Discussion => General Mach Discussion => Topic started by: budman68 on July 05, 2008, 02:09:59 PM

Title: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 02:09:59 PM
Now bear with me since I'm not a very knowledgable person in this area-  :P

Ok, so I'm interested in setting up a 4th axis (horizontal) on my Taig Mill and I have my stepper and wiring all figured out, BUT, the machanical part of it eludes me a bit. Do I need a gear reducer of somekind? How come you can't just connect a 3 jaw chuck or a drill chuck to the motor shaft itself? Is the resolution too high and you need to lower it by reducing it in some way? I've seen many things used from a small rotary table to a small gearbox/reducer. How do I know what's right for me?

The range of diameters that I would use is probably between 1/4" to 2".

Please help me set this up if you can?
 
As always, thank you for your invaluable time!  ;)

Dave
Title: Re: Setting up a 4th axis?
Post by: Hood on July 05, 2008, 02:14:10 PM
You could just hook up a chuck to the motor but you would need a big motor as the cutting forces would be quite high. Gearboxes or rotary tables are used because the reduction increases the torque of the motor.
Hood
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 02:37:56 PM
Oh, I see, that makes sense, so do you have an idea of what I should be searching for? It seems the idea of a worm gear setup is best for keeping a handle on backplash, but I would have no idea what "ratio" worm and pinion gear to look for. Any ideas?

Title: Re: Setting up a 4th axis?
Post by: Hood on July 05, 2008, 02:55:16 PM
Not really, depends on what size motor, what type of motor etc. Have a look at some of the places that sell the 4th axis for the smaller machines and see what they are using, think Sherline and the likes do them.

Hood
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 03:01:30 PM
Will do, thank you-

Dave
Title: Re: Setting up a 4th axis?
Post by: Hood on July 05, 2008, 03:16:29 PM
Just had a look myself and from the sherline pdf it says it uses a 100 ozin stepper with 0.9 degree steps and a 72:1 reduction on their 4 inch rotary axis.
Hood
Title: Re: Setting up a 4th axis?
Post by: Overloaded on July 05, 2008, 03:30:05 PM
http://www.machsupport.com/forum/index.php/topic,6158.0.html
Budman, Here is a neat topic. Gearbox to Axis conv....In case you haven't seen it.
RC
Title: Re: Setting up a 4th axis?
Post by: docltf on July 05, 2008, 03:43:22 PM
DAVE

most 4th axis are done in degrees.so keep this in mind when you calculate your resolution.the rotory is done with (deg-min-seconds).with the dia you want to use,i would suggest the minimun steps per degree should be 16 full steps.
so figure your gearing with 16 full steps as your base number.then when you go microstep it gets smoother.

bill
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 03:58:48 PM
Thanks very much fells, been soaking up the info this afternoon and will take all your considerations, for sure. Thanks for the link, RC!

Doc, so a 30:1 gearbox would be fine then? Reason I ask is I may have found a reasonable nice small sized one.
Title: Re: Setting up a 4th axis?
Post by: Chip on July 05, 2008, 04:05:39 PM
Hi, Dave

Torque, Accuracy, Speed, Trade Offs.

If you use a 200 step stepper direct, It's 1.8 deg. per step Accuracy, If it's 10 Micro-step, It's 0.18 deg. per step Accuracy.

Depending on the material your cutting, Holding or Moving Torque could be an Issue.

Most rotary tables are 72 to 1, (72 turns to 1 table turn), Have a Sherline Hear.

Steps-Turns-Accuracy & Reduction Values to consider.

Steps 200 X 72 = 14400 steps, 0.025 deg Accuracy. (Sherline/Rot. Table Type)

Steps 200 X 7.2 (10\72 reduction) = 1440 steps, 0.25 deg Accuracy. (Belt/Geared/Stepper)

Steps 200 X 3.6 (20\72 reduction) = 720 steps, 0.5 deg Accuracy. (Belt/Geared/Stepper)

Micro-Steps 2000 X 72 = 144,000 steps, 0.0025 deg Accuracy. (Sherline/Rot. Table Type)

Micro-Steps 2000 X 7.2 (10\72 reduction) = 14,400 steps, 0.025 deg Accuracy. (Belt/Geared/Stepper)

Micro-Steps 2000 X 3.6 (20\72 reduction) = 14,400 steps, 0.05 deg Accuracy. (Belt/Geared/Stepper)

Hope this Helps, Chip
Title: Re: Setting up a 4th axis?
Post by: Hood on July 05, 2008, 04:18:21 PM
While we are on the subject of this,
 I have a fairly big Bristol Erickson indexing head that I am planning on converting to a 4th axis at some point in time. It has a worm driving it at the moment but it has too much baclash for my liking and no way of adjusting it without major headaches. I am planning on removing the gear and getting a big timing pulley to put over the spindle (2inch bore I think) I will then fit one of the AC Servos I have with a 6:1 backlash free gearbox that it came with and fit a timing pulley to it. The servo has a 2000 line encoder so resolution wont be a problem, it also has a continuous stall torque of 10Nm so with the 6:1 box that will be 60Nm continuous (8500 ozin approx) I will have a 10 inch chuck on the axis.
Anybody see problems in doing this?

Hood
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 04:23:10 PM
Now my head is spinning but I do think I'm starting to "get it",  :D So a 30:1gearbox is less than half of a 72:1 rotary table, correct?

Chip, that is indeed the size stepper I'll be using. It's a 282 oz. nema 23 motor that should be perfect for this.
Title: Re: Setting up a 4th axis?
Post by: RICH on July 05, 2008, 04:25:42 PM
Dave,
There are are probably 5-10 small rotary tables out there for a small mill. Three of us, over time, ended up getting the Sherline because we felt the quality was better. But only you can decide. Motors used were 100 & 125 in oz.. If you make your own base for mounting the motor you can save a bunch of $. You can mount it to a right angle plate
( maybe get one from Encho for $20-30).  Then treat yourself to a nice chuck on what you saved. I can post a picture of what i did if you wish.
RICH
Title: Re: Setting up a 4th axis?
Post by: Overloaded on July 05, 2008, 04:27:35 PM
That sound good Hood.
Only possible problem that I see is the belt being the weak link in that all of the cutting/tooling pressure during anX axis move will be against the belt instead of the "Metal to Metal" contact in a normal gear arrangement. Maybe use a brake of sorts ?
Should be fine for lighter cuts in softer matl though.
RC
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 04:30:37 PM
Dave,
There are are probably 5-10 small rotary tables out there for a small mill. Three of us, over time, ended up getting the Sherline because we felt the quality was better. But only you can decide. Motors used were 100 & 125 in oz.. If you make your own base for mounting the motor you can save a bunch of $. You can mount it to a right angle plate
( maybe get one from Encho for $20-30). Then treat yourself to a nice chuck on what you saved. I can post a picture of what i did if you wish.
RICH

That would be fantastic, Rich, I'd love to see a pic. As for the sherline, I'm considering buying one and have no problem retrofitting a motor to it but haven't I read somewhere that you can't just add a motor to the manual model sherline table? I could've sworn I read that you have to buy the CNC version of their table for that particular use? Or is that just mumbo jumbo and the worm drive is exactly the same?
Title: Re: Setting up a 4th axis?
Post by: RICH on July 05, 2008, 06:44:53 PM
Dave,
Will post some pictures in the show and tell ( tomorrow ) with additional info so this post dosn't get off on a tangent. Three of us  have converted the manual one. Here are some angular to linear conversions which you may find usefull in relating to angular positioning:

ANGLE     PER INCH      PER 3"         PER FOOT         
1 SEC       .000005"     .000014"     .00006"
30 SEC     .00014"       .00044"      .0017"
1 MIN       .00029        .00087"      .0035"

So if you were to drill a hole out from the center 3", then index 3 min, and drill another hole, the distance you have between the holes would only be approx .003".

I use mine primarily for positioning.
RICH 
Title: Re: Setting up a 4th axis?
Post by: Hood on July 05, 2008, 06:46:03 PM
That sound good Hood.
Only possible problem that I see is the belt being the weak link in that all of the cutting/tooling pressure during anX axis move will be against the belt instead of the "Metal to Metal" contact in a normal gear arrangement. Maybe use a brake of sorts ?
Should be fine for lighter cuts in softer matl though.
RC


Timing belts seem to be very hardy things, when I was setting the servos on the lathe up I was testing the stiffness of the settings by putting a long bar  on the ballscrew pulley and watching for encoder movement in the software. If I could feel movement on the bar the encoder was twitching, I could set the stiffness such that the only time I felt any movement in the bar was when I applied so much pressure that the drive faulted.
 This amazed me at how good timing belts were, I thought that there must be quite a bit of give in them, after all they are made of a fairly soft material but amazingly there seemed to be no noticeable movement at all.

Hood
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 07:01:29 PM
Thanks for the info and the future pic as well, Rich. What kind of backlash can I expect from the Sherline?
Title: Re: Setting up a 4th axis?
Post by: Chip on July 05, 2008, 08:26:29 PM
Hi Dave

30:1 (Based on 200 stepper) would give you 200 X 30 = 6000 steps per. 1 turn, .06 deg.'s per step, 16.666666 (steps per.) 1 Deg.

Depending on the Accuracy you need, This may be fine, Not Sure.

Mach can only move to within 1 (stepper/encoder step's), So 1 Deg move 16.666666 step's will be Rounded up to 17 steps, .00000001 to .5. Rounds Down, .50000001 Rounds Up.

Mach keeps track of the Remainders (+'es -'es), To 16 decimal places or more .00000000000000001, If my Memory is Correct.

There applied to the next move to the Nearest 1 step and so-on and so-on............

It's like your other axises, The combination of Stepper's or Encoders count pre. turn and the relationship with Gearing/Ball screw's Ratio's.

You need 10,000 steps (Per.) "Unit" (Inch or MM) to get .0001 Accuracy.

It's All a trade off, Speed and Accuracy, Best I Now.

Hope this Helps, Chip
Title: Re: Setting up a 4th axis?
Post by: budman68 on July 05, 2008, 10:48:01 PM
I can't imagine that not being enough and I just may grab that gear box to at least give it a try.

Thanks so much for taking the time, Chip!!   :)
Title: Re: Setting up a 4th axis?
Post by: Chip on July 05, 2008, 11:09:21 PM
Hi Dave

At least you'll get your feet wet.

Let us now.

Any-Time, Chip

Title: Re: Setting up a 4th axis?
Post by: docltf on July 05, 2008, 11:58:35 PM
DAVE

if you go with that 30-1 reducer, then you need to do another gearing of 1.5-1 so you can put your degrees on whole numbers. this is very important to using a rotory and mach3. so 30-1 X 1.5-1 X 200 =9000 fullsteps.
9000 divided by 360 gives you 25.0 steps. when you make your base set up like this mach3 likes it. then if you go microsteps you won't have problems. when you do rotory steps that calculate to numbers like 11.667 and 14.22333
at whole postions 1 degree or .5 degree you got trouble. you wan't to see numbers like this in key spots 0.00  0.05 0.025 0.5 12.5 then if you microstep at 1/2 1/4 1/8 you will gain resolution and solid numbers for mach.

bill
Title: Re: Setting up a 4th axis?
Post by: Chip on July 06, 2008, 02:38:00 AM
Hi, Bill  :D

Not trying to split Hair's hear, There some part's that Dave has sitting around, Now, On Hand.

6000, 11.666666 steps per, You get some rounding, .0143 .02 .03 deg, 12.5143.  ???

Even with 9000, 25 steps per, You'll still get some rounding, .02 deg. 12.5 is shown as 12.4800.

Now 7200, 36 steps per, seam's good, So dose 1440, 7.2, 144000, 400 steps per.,Hmm.....360 ?

73400, 367 steps per, 12.4986.

NO Hair Splitting Allowed, Chip  :D

PS: PI are round, Cornbread are square.  :-\
Title: Re: Setting up a 4th axis?
Post by: jimpinder on July 06, 2008, 03:11:58 AM
I can't understand all this.

Can you get a 36 to 1 gearbox - then all your theory is solved and all the sums cancel out. There are 360 degree in a circle - why bother with 30 unless you are forced down that road. Failing that, put a 5 to 6 step down between your motor and gearbox, using toothed pulleys - say using 10 to 12 teelth pulleys - this will also cancel out your 30 ratio gearbox and give you a simple figure of 36 on which to calculate steps per degree - and enough has been said about that in the past.

Hood - in my experience there is no backlash in modern toothed belts - look at car timing belts. The only error is that particulary tooth having a little bit of play with the corresponding gap in the belt, but since there are many teeth in contact with the belt, the resultant movement is non-existant
Title: Re: Setting up a 4th axis?
Post by: docltf on July 06, 2008, 05:29:13 AM
HI CHIP

Budman want's to do a rotory and i believe he said that his experiance with this is slim to none. he also mentioned that he is going to work with dia from .250 to 2.0 .which we both know is bogus, because as soon as he gets it working well
he will put somthing bigger in it the first chance he gets. we all do! but .250 to 2.0 is a good place to start. plus Budman has not mentioned if he has microstepping available. if he does it will be better for him. he also needs to know that larger
dia needs more resolution and more resolution means less speed. he just needs to have some info for his first build. how he plans to build and with what is his choice. i just want to give him something to think about.

Chip i diddled with your doodles.
6000=16.66(20 more)7 steps per
7200=20 
1440=4  i built one of these works pretty slick
144000=400 that's good

what i was trying to show budman about a good base setup and mach3 and if he has microstepping how it works out
if he builds with the 30-1 does the 1.5-1 gearing and 200 full step pulses, he will get 360 in 9000 pulses. that is 25 pulses per degree.
these numbers will be available as degree's on the DRO'S without rounding  1.0 2.0 3.0 etc.
now lets microstep.
F step 25=1.0 2.0 3.0
H step 50=1.0 1.5 2.0 2.5
1/4 step 100=1.0 1.25 1.5 1.75 2.0
1/8 step 200=1.0 1.125 1.25 1.375
1/16 step 400=1.0 1.0625 1.125
1/32 step 800=1.0 1.03125 1.0625  (mach3 DRO will round 1.03125 to 1.0313) but the number is still good.
these numbers show without rounding as the pulses increase.
doing big resolution with microstepping is kind of phony anyway,but it makes the DRO look good. big solid resolution is done with gearing.

P.S. my cornbread comes in round pie pans.

bill

Title: Re: Setting up a 4th axis?
Post by: budman68 on July 06, 2008, 07:42:42 AM
Bill, thanks for helping out and yes, I do have microstepping. As for doing any larger work, unless I win the lottery, and get more room available to me to buy a bigger machine,I don't think that's going to happen anytime soon!  ;)  I do know exactly what you meant though..it just often works out that way doesn't it?

Jim, I possibly might be able to get this gearbox very cheaply so that's why I threw that into the equation, but I'm thinking more and more about the sherline, and that may be the road I'm headed down eventually because it does seem "dumb" to go that way if I just wait a little longer to buy it. Seems like a really well built unit and I'm only hearing great things about it. Can't argue with success.

Again fellas, thank you for your input, I've already learned quite a bit and I haven't even started building this up yet. As Bill mentions, I know slim to none about this area, and that's why I came to you.

P.S. I LOVE Cornbread in any pan!!  ;D
Title: Re: Setting up a 4th axis?
Post by: RICH on July 06, 2008, 08:46:11 AM
- What kind of backlash can I expect from the Sherline? -

Mine has approx 5 min and it varies a little. I always rotate it to take it out before indexing. You can use the backlash compensation
in MACH ( haven't tried it for a rotary table / should work fine ) and if your using a SS backlash compensation is not available yet.

I have not checked accuracy of the gearing as it's very time consuming, not worth the effort, and you need the right equipment to do it. Have done some checking of my Klinger table but that's a different story on how and why it was done.

Maybe someone will post some experience on engraving / machining around the circumference as it relates to a rotary table and what they found to be desired. It all depends on what your going to do with it.

RICH



 

Title: Re: Setting up a 4th axis?
Post by: budman68 on July 06, 2008, 10:06:05 AM
Excellent, Rich, that's not too bad at all.  -  :)
Title: Re: Setting up a 4th axis?
Post by: Chip on July 06, 2008, 05:00:40 PM
Hi, Dave

My Sherline works very nice, Great value, Didn't think you wanted to split hair's, Just wanted to get something running.

Thanks, Chip
Title: Re: Setting up a 4th axis?
Post by: RICH on July 06, 2008, 05:07:35 PM
Dave,
I posted some pictures in show and tell. The "save good junk" is a picture of an extra mounting. Should you decide on a Sherline and want some good junk, for the price of postage you can have it.
 :)RICH
Title: Re: Setting up a 4th axis?
Post by: Chip on July 06, 2008, 06:01:48 PM
Hi, Bill

It was late, Sorry for the error's, I'll Blame it on my Calculator.

But your other comments,

"these numbers show without rounding as the pulses increase"

Mach Doesn't just put up values It thinks you'd like See,  Expand your DRO to show 00000.00000000000000000000 Accaurcy and look at it.

"doing big resolution with microstepping is kind of phony anyway,but it makes the DRO look good. big solid resolution is done with gearing".

I've herd some comment's on Micro-Stepping from some, There may be slight position error's between full steps, But after moving millions and millions of

moves, You make it sound like you'll never get even close to your Home Position, My rotary table is geared and Micro-Stepping, Returners to 0.0

Please think about it.

Thanks, Chip

 

Title: Re: Setting up a 4th axis?
Post by: budman68 on July 06, 2008, 07:42:26 PM
Thanks for the pics, Rich, and yes, I'll take the bracket (aluminum?) let me know an email to contact you at, and I'll take care of that for you. Thanks again- :)

Chip, the more I'm thinking about doing this, the more I think I should just do it the right way, the first time and I think the sherline is the way to go. It's funny that you posted that ebay screen shot, I have that bookmarked as of this morning-  ;D