Author Topic: The basics  (Read 13147 times)

Offline Will_D

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The basics
« on: July 19, 2016, 05:00:39 PM »
First of all thanks to the mods for creating this new area.

I have been maching metal as a hobby for many many years and am quite computer literate.

Now all this talk of CNC has got me interested in spending more money on the hobby.

I have a Sieg SX2 Plus from Arc fitted with 3 axis magnetic DROs and am getting tired of twiddling the X- axis on long cuts.

So some form of stepper motor control may be needed for X & Y and maybe Z.

First major question is:

1. What about backlash in the feed screws? Does CNC and steppers etc aleviate the problem or do I first need to get rid of the backlash? Looking at the price of some ball screws it would be cheaper to just buy a new Mill!
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Offline sparky961

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Re: The basics
« Reply #1 on: July 19, 2016, 05:24:29 PM »
Backlash is a bitc beast.

When machining by hand, you generally know which way the backlash is going and can compensate for it.  Most software allows for a backlash compensation but all that does is turn the crank enough to take up the backlash when you change directions.  You can't reliably cut circles and arcs with this because the cutting force will either push/pull the cutter into the backlash either before or after the compensation move happens.

That said, if you're taking light enough cuts that a bit of pressure on the gibs/locks can prevent the cutter from self-feeding, it can work ok.  Just don't get too excited about complex curvy shapes if you don't eliminate physical backlash first.  You can also use "CNC Assist" for some jobs, where you insert stops and comments like "LOCK THE Y AXIS" (do some machining with X) "UNLOCK THE Y AXIS" and so forth.

My own strategy will be to try to plan my machining cuts so that backlash adjustments aren't necessary while in a cut.  Instead, I'd take the cutter away from the part, change directions and then do another cut.  If this is unsatisfactory then I'll try anti-backlash nuts.  If I'm not too old to be machining by that point, I may install ball screws or just scrap the entire works and get a real machine.

Thanks for suggesting this area.

Offline PK

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Re: The basics
« Reply #2 on: July 19, 2016, 07:20:50 PM »
Whilst there are many strategies for backlash compensation, its a bit like owning an old car. You can prop it up and keep it going for years, but when you eventually buy a new one you realise you should have just replaced it much sooner.
In CNC that means replacing leadscrews with decent ballscrews. There really just isn't an alternative.

Offline awemawson

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Re: The basics
« Reply #3 on: July 20, 2016, 02:43:24 AM »
I do agree that ball screws are the way to go for a CNC conversion, however they can be a bit of a menace if you also want the machine to be able to be used by hand.

The free movement that they provide meaning that the carriage can more freely be moved by cutting forces, so locking a stationary axis becomes even more important.
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Offline SwarfnStuff

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Re: The basics
« Reply #4 on: July 20, 2016, 03:12:13 AM »
For what it's worth Will.
       Some with these small machines make and use Delrin / Teflon nuts moulded to the actual lead screw. Here is one link that will give you the idea.
http://www.instructables.com/id/DIY-Anti-Backlash-Nuts-Super-Cheap-and-Simple/

Naturally, there was another rather well written  build log but alas, no can find.

Possibly not for the purists among us but if finances are limited they seem to work for those who have used them. Ball screws are great though. I still turn handles too. Champagne taste on a beer budget is yet another of my difficulties in life.

    If you don't want to go the CNC route there are others that just motorize the x axis using a car windshield wiper motor and voltage control.

John B
Converting good metal into swarf sometimes ending up with something useful. ;-)

Offline Manxmodder

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Re: The basics
« Reply #5 on: July 20, 2016, 04:32:56 AM »
John B: Naturally, there was another rather well written build log but alas, no can find.

John,is this the one you refer to?  http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way

I have made about 4 of these to date for acme threads I have screw cut on my Harrison.

The Evanut is really simple to make and has no backlash at all.......Oz

Helixes aren't always downward spirals,sometimes they're screwed up

Offline Joules

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Re: The basics
« Reply #6 on: July 20, 2016, 06:32:17 AM »
Excellent THREAD Oz, I particularly liked the idea of using a glow plug in a short length of leadscrew.  My twist would be to put a thermocouple in with the glow plug to have accurate control of temperature.   I recon Evanuts will be going in my (distant) future desktop CNC mill project.

Will, a good starting point might be a cheap stepper motor and driver controlled by the new BBC Μicro Bit.  It would allow you to play with a smart power feed for the mill, be it X or Z axis.

« Last Edit: July 20, 2016, 07:37:47 AM by Joules »
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline Manxmodder

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Re: The basics
« Reply #7 on: July 20, 2016, 10:09:32 AM »
Yes indeed,very good thread. Evan sure hit the sweet spot with that idea,the thread is still being usefully contributed to 5 years from the first post.......OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline PK

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Re: The basics
« Reply #8 on: July 20, 2016, 05:37:22 PM »
One of the great truisms of DIY CNC is that everyone ends up using ball screws and linear bearings.
The interewebs are overflowing with "Cheap ballscrew alternative" and "Skate bearings just as good as ...." type articles. Ask those guys to show you 5 things they have made on those machines and the excuses start coming.

Offline nrml

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Re: The basics
« Reply #9 on: July 20, 2016, 05:58:19 PM »
Chinese ball screws and nuts don't cost that much these days. Unless you are just experimenting on a shoestring budget, there is very little reason not to spend some money for the massive improvement they will bring.

Chinese and other far eastern products get slagged off by a lot of people (sometimes with some justification) but they have done more to improve the average homeworkshop than most people could dream of a few decades back. The few examples of Chinese made ballscrews and Hiwin rail knock offs I've seen and handled have been excellent for the price. I would have no hesitation in buying or recommending them.

Offline SwarfnStuff

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Re: The basics
« Reply #10 on: July 21, 2016, 02:49:06 AM »
Thanks Manx,
   Yup, that was it. Strange how one finds such a thread, bookmarks it then looses the thing by some mysterious computer gremlin.  :scratch:
   In my case I think it was my junk removal software being a tad too efficient - like, all bookmarks disappeared as it seems to have been treated as a, "temp" file location.

John B
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Offline Manxmodder

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Re: The basics
« Reply #11 on: July 21, 2016, 04:06:42 AM »
Where many seem to go wrong with producing a successful 'Evanut' is they omit the final stage of putting it in the freezer to shrink the acetal co-polymer and using an end portion of the threaded rod as a thread relieving tap.

By not doing this action they end up with a nut that is far too tight to be functional in many cases......OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline Will_D

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Re: The basics
« Reply #12 on: July 21, 2016, 06:06:58 PM »
So after googling up some ball screws they seem well priced from China.

SFU 1204 seem they way to go, Tolerance is C7, and 12 as in 12mm od and 04 as in 4 mm pitch I assume??

Now what about the handle end. As already mention my SX2P has a fair amount of backlash at the handle (no thrust bearings). I posted a potential fix (http://madmodder.net/index.php/topic,10428.msg118625.html#msg118625)based on the ARC Myford cross-slide fix. If I just fit a stepper motor at the other end will this remove the backlash?

Also do I need to use those "bellows" type flexible couplings 'tween stepper and feed screw?

I already have 5 micron DROs fitted, can these be used as feedback for a CNC stepper controller?

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Offline John Stevenson

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Re: The basics
« Reply #13 on: July 21, 2016, 06:28:41 PM »
So after googling up some ball screws they seem well priced from China.

SFU 1204 seem they way to go, Tolerance is C7, and 12 as in 12mm od and 04 as in 4 mm pitch I assume??

Now what about the handle end. As already mention my SX2P has a fair amount of backlash at the handle (no thrust bearings). I posted a potential fix (http://madmodder.net/index.php/topic,10428.msg118625.html#msg118625)based on the ARC Myford cross-slide fix. If I just fit a stepper motor at the other end will this remove the backlash?

Also do I need to use those "bellows" type flexible couplings 'tween stepper and feed screw?

I already have 5 micron DROs fitted, can these be used as feedback for a CNC stepper controller?

Correct 12mm diameter [ nominal ] and 4mm pitch

For the handle end you want a pair of angular contact bearings mounted back to back.

For couplings use Oldham couplings, they are backlash free and the only type f coupling that can handle mis-ailgnment in two planes without binding.

This is how I like to mount motors.



Inside that block are the two bearings and the nut that retains them is the first half of the Oldham couplings. Don't mount the motor on pillars like many do, it's a machine tool not a bloody clock.

No you can't use your scales, none of the hobby controllers like Mach can accept feedback.
John Stevenson

Offline Brass_Machine

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Re: The basics
« Reply #14 on: July 21, 2016, 09:29:40 PM »



Inside that block are the two bearings and the nut that retains them is the first half of the Oldham couplings. Don't mount the motor on pillars like many do, it's a machine tool not a bloody clock.

No you can't use your scales, none of the hobby controllers like Mach can accept feedback.

John... I am trying to picture how that is setup in mind. Having difficulty seeing it (my mind is already full). Can you describe a little more?

Eric
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Offline Imagineering

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Re: The basics
« Reply #15 on: July 22, 2016, 04:31:15 AM »
In my experience, the use of a Flexible Coupling is redundant. If all the Bearing Supports and Stepper Mounts are square and perpendicular there is no alignment issue to compensate for. If the Thrust Bearings are on the Stepper Motor end of the BallScrew, a little Axial play is built into the System at the support Bearing end to compensate for thermal expansion.

Copied from my reply to another Thread;

My method for lining every thing up;
Make sure the BallNut is secured in the intended centre of the BallScrew Axis.
When fabricating the End Plates which hold the Bearing, make the Mounting Holes a mm oversized. (eg 6mm Bolts, use 7mm Holes).
Bolt everything together so that the End Plates can move laterally.
Drive the Axis to the Motor end of it's travel and nip up the Mounting Bolts at that end.
Drive the Axis to the other end of it's travel and nip up the Mounting Bolts at that end as well.
Drive the Axis back and forth between limits and check that there is no binding anywhere.
If all is well, tighten the Bolts fully, and repeat for the other Axes.

I've converted two Mills and built a CNC Router from scratch, and have used this method with success on all three.
The Dual Motor Y Axis on my CNC Router and both the X & Y Axes on my SX3 are directly driven without Flexible Coupling.

Online philf

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Re: The basics
« Reply #16 on: July 22, 2016, 06:07:39 AM »
In my experience, the use of a Flexible Coupling is redundant. If all the Bearing Supports and Stepper Mounts are square and perpendicular there is no alignment issue to compensate for. If the Thrust Bearings are on the Stepper Motor end of the BallScrew, a little Axial play is built into the System at the support Bearing end to compensate for thermal expansion.

Hi,

You say "there is no alignment issue to compensate for".

Everything is made to a tolerance. e.g. the stepper motor flanges may not be exactly perpendicular to the shaft and the bearings in the stepper probably aren't made to highest standards so will have some eccentricity. These errors may be very small but would have some detrimental effect. Using a solid coupling might seem to work OK but I feel sure that wear on parts would be accelerated and efficiency would suffer. I have used zero backlash couplings on my X & Y axes and timing belt drive on my Z axis.

On my machine I can push the table and turn the stepper motors (providing they are powered down). You may find that wouldn't work with a solid coupling.

I'm not saying that it won't work - it's just better to use a coupling in my opinion.

We had many machines at work (in the semiconductor industry) working to micron (or sub-micron) accuracy and, without exception, all the ballscrews were connected to the steppers or servos with couplings.

Cheers.

Phil.
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Offline Will_D

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Re: The basics
« Reply #17 on: July 22, 2016, 04:05:12 PM »
@John: thanks for the pics. That looks like a SX3 hence the big steppers.

I would also some more info (pretty please: a drawing) of the thrust bearings etc.

Another question has raised its head: While googling Expensive pro quality UK ball screws one company was showing how to anneal the ends (with an induction coil R/F heater) before maching to suit!

Do the unfinished Chinese screws be that hard??
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Offline spuddevans

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Re: The basics
« Reply #18 on: July 22, 2016, 04:20:55 PM »
@John: thanks for the pics. That looks like a SX3 hence the big steppers.

I would also some more info (pretty please: a drawing) of the thrust bearings etc.

I used something similar to John's method, but I used a nut to hold the angular contact bearing on the ballscrew instead of using one half of the oldham coupling, see halfway down this post for a pic of the coupling in place - http://madmodder.net/index.php/topic,3535.msg40859.html#msg40859

Quote
Another question has raised its head: While googling Expensive pro quality UK ball screws one company was showing how to anneal the ends (with an induction coil R/F heater) before maching to suit!

Do the unfinished Chinese screws be that hard??

They're not that hard, and the hardening is not too deep either. I used a tipped tool and had no problem - http://madmodder.net/index.php/topic,3535.msg39297.html#msg39297

Hope this helps,

Tim
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Offline Will_D

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Re: The basics
« Reply #19 on: July 22, 2016, 04:52:32 PM »
Many thanks for the replies -all are very helpfull

This is why the thread is called "The Basics"

I am really trying to 'drill down' to the fundamental knowledge we need!
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Offline Will_D

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Re: The basics
« Reply #20 on: July 22, 2016, 05:10:21 PM »
SoI will need steppers/drivers/psu so from the UK I see a nice priced package:

http://www.ebay.ie/itm/3-Axis-CNC-Kit-425oz-in-Nema-23-Stepper-Motor-Driver-CNC-Mill-Router-Lathe-/121787136378?hash=item1c5b14397a

Will the 425 oz N23's be up to the SX2P?

I'm sure X & Y will be fine but what about Z? (Uses counter weights/gas struts may maker for zero'ish loading on Z

And of course there's that USB to 4 axis controller box thingy that got me going down this path!

WONGA WARNING!! DO NOT FOLLOW THIS LINK!!

 http://www.banggood.com/50KHZ-CNC-4-Axis-Offline-Controller-Breakout-Board-Engraving-Machine-Control-System-Card-p-1021134.html
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Offline raynerd

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Re: The basics
« Reply #21 on: July 22, 2016, 05:11:52 PM »
Quote
am getting tired of twiddling the X- axis on long cuts.

So some form of stepper motor control may be needed for X & Y and maybe Z.


Sounds like a power feed may be better suited rather than a full on cnc. That said, my first ever project many years ago, was a stepper motor based power feed. This got me into the basics of steppers, drivers and cnc on one axis, and admittedly, it required very little accuracy so backlash wasn't an issue. It certainly introduced me to cnc ...

<a href="http://www.youtube.com/watch?v=oXwwqGBMdLs" target="_blank">http://www.youtube.com/watch?v=oXwwqGBMdLs</a>

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Offline Will_D

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Re: The basics
« Reply #22 on: July 22, 2016, 06:43:56 PM »
Indeed Raynerd,

I am not (yet - or even ever) loooking at full CNC capability.

What I am looking towards in 2 axis control form either manual or very simple g-code (like what is generated by the G-Wizard Simulator Wizards)

If I get the basic mechanicals correct then the full 4 axis g-code will follow.
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Offline PK

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Re: The basics
« Reply #23 on: July 22, 2016, 08:36:39 PM »
Here's how I did it on my X3
I used double shaft motors which let me keep the hand wheels, albeit with them sticking out a bit more.
The mounts were fabricated and then machined true.
The helical couplings are inside the tubes and you can see the access holes for the clamp bolts..

Offline Imagineering

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Re: The basics
« Reply #24 on: July 22, 2016, 10:46:48 PM »
This is how I did my X & Y Axes.
First Photo is the X Axis. Solid Coupled Motor and twin Thrust Bearings at the other end. This setup relies on the Stepper Bearings for thermal compensation. No issues in 6 Years of operation, 65micron accuracy.

Second Photo is the X Axis from above.

Third Photo is the Y Axis. Solid Coupled Motor with twin Thrust Bearings. This LeadScrew is so short it didn't require a Bearing at the far end.

Fourth Photo is the Y Axis from above.