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Author Topic: Setting up a 4th axis?  (Read 11509 times)

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Offline Hood

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Re: Setting up a 4th axis?
« Reply #10 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

Offline budman68

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Re: Setting up a 4th axis?
« Reply #11 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.
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Offline RICH

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Re: Setting up a 4th axis?
« Reply #12 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
Re: Setting up a 4th axis?
« Reply #13 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

Offline budman68

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Re: Setting up a 4th axis?
« Reply #14 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?
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Offline RICH

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Re: Setting up a 4th axis?
« Reply #15 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 

Offline Hood

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Re: Setting up a 4th axis?
« Reply #16 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

Offline budman68

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Re: Setting up a 4th axis?
« Reply #17 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?
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Offline Chip

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Re: Setting up a 4th axis?
« Reply #18 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
« Last Edit: July 05, 2008, 08:39:41 PM by Chip »

Offline budman68

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Re: Setting up a 4th axis?
« Reply #19 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!!   :)
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Just because I'm a Global Moderator, don't assume that I know anything !

Dave->    ;)