Author Topic: supporting mini mill head,,  (Read 7019 times)

Offline stirling lad

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supporting mini mill head,,
« on: February 28, 2014, 06:58:19 PM »
Hi Gents, please dont think that this is a newbie trying to re invent the wheel,  :loco:  :palm: but after discussions with a fellow modder I was told about the spring supported mechanisms on some mini mills not being as strong as the gas strut ones like Chesters conquests and the likes..I know some people have used counter weights to stop the head coming down accidentally which seems wise but has anyone considered using one of these type of door closers underneath the head to stop them coming down .
 http://www.ebay.co.uk/itm/WHITE-Surface-Mounted-AUTOMATIC-DOOR-CLOSER-Spring-Arm-Auto-Self-Closing-NEW-/191063718525?pt=UK
ive used them on doors before and they have a very wide power range ..you just wind the centre part with the holes back a hole or two at a time until you have enough power loaded on the spring to close fire doors ect,,so you shouold be able to do the same with it mounted underneath the head until you have nullified the downward force but not enough to make it go up the way.
i'm sure one of you guys must have considered this before so could you please put me out of misery and tell me why it would'nt work. :scratch:.I'm a complete and total beginner with mills and  i'm just trying to think of ways to avoid these problems before they happen to me :doh:  ...these closers are dirt cheap especially when you compare them to the gas strut mod kit...personally i'd be afraid to go drilling holes in the head or column as i dont have a clue whats under the skin ...

I'll shut up now and crawl back into the ''be quiet'' corner..  :wack:

...Mike...

Offline Lew_Merrick_PE

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Re: supporting mini mill head,,
« Reply #1 on: March 02, 2014, 01:12:12 PM »
I'll shut up now and crawl back into the ''be quiet'' corner..
Mike,

1) There is no be quiet corner here.  We all get to make fools of ourselves from time to time.  This is part and parcel of the fun here!

2) I do not have mills without gear-driven head support, so I am speaking somewhat out of turn here.  The problem with springs is that the force they apply is proportional to their deflection.  This problem can be minimized if you can pick a spring with a low enough spring rate (force/deflection).  This is often not possible.

The solution I often use when I need a specific force being applied through a long throw is to mount an air cylinder with a moderately large reservoir (call it 2X the maximum volume of the air cylinder) with a precision pressure regulator feeding the reservoir.  This gives me a very constant force over the (almost) entire stroke of the cylinder.  I have used this extensively in machines that pack pyrotechnic propellants (which have the characteristic of going boom when over-loaded!).

If you join http://www.GrabCAD.com and go to https://grabcad.com/library/closed-not-ground-end-spring-design-for-geomagic-design-1 -- you will find a spring design routine (customized for the Geomagic CAD system) that will give you a decent overview of how compression coil springs are designed.  There is a similar utility I created for double-hairpin torsion springs.  The spreadsheets associated with these utilities may help you see the variables in a spring's design and how they interact.

Offline philf

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Re: supporting mini mill head,,
« Reply #2 on: March 02, 2014, 02:32:41 PM »
The solution I often use when I need a specific force being applied through a long throw is to mount an air cylinder with a moderately large reservoir (call it 2X the maximum volume of the air cylinder) with a precision pressure regulator feeding the reservoir.  This gives me a very constant force over the (almost) entire stroke of the cylinder.  I have used this extensively in machines that pack pyrotechnic propellants (which have the characteristic of going boom when over-loaded!).


Hi Lew,

I used to use air cylinders to balance safety guards on test gear but simply used a relieving pressure regulator (just a cheapo, non-precision one) without a reservoir. It was so simple but so effective.

What is the idea of a reservoir? Is it because you're using non-relieving regulators?

Cheers.

Phil.

Phil Fern
Location: Marple, Cheshire

Offline Stilldrillin

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Re: supporting mini mill head,,
« Reply #3 on: March 03, 2014, 03:53:07 AM »
Mike.
In reply to your question.

I think it would be a bit "messy", and not give any real advantage over the maker's spring. Still giving variable lift, depending on head position......

I can understand you, not wishing to modify your brand new machine, until you've gained some confidence.

How about, installing the machine in it's working position.
Then lasso the head, and run a line/ cable upwards over a wall or ceiling mounted pulley, with a balance weight on the end.

My workshop door closer, has worked on this principle for some 10 years now.........

David D
David.

Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!

Offline stirling lad

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Re: supporting mini mill head,,
« Reply #4 on: March 03, 2014, 11:03:55 AM »


    Lew, err ehm,, probably yes??? or maybe no?? :scratch:( way over my head)

   David,That seems to be a tried and tested way of supporting it,,But i thought that perhaps if you did'nt have space for that route them maybe you could mount something along the lines of one of these things, say higher over your mill with a longer arm to give more lift,, but the main thing that i was unsure off was how it would effect backlash,,for instance, if it was premanantly lifting the head up would that mean you had backlash on every turn of the fine feed or is that back to front and you would have permanant backlash if you were just short of constant uplift ..I imagine having the downward pressure and upward lift nulified would result in the head kind of floating which i dont think would be as good as it first sounds as there would be a little backlash all the time then,, or am i misunderstanding these rack and pinion all together,,? now that would'nt surprise me...

...mike... 

Offline Lew_Merrick_PE

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Re: supporting mini mill head,,
« Reply #5 on: March 03, 2014, 11:49:43 AM »
I used to use air cylinders to balance safety guards on test gear but simply used a relieving pressure regulator (just a cheapo, non-precision one) without a reservoir. It was so simple but so effective.

What is the idea of a reservoir? Is it because you're using non-relieving regulators?

No, I am using relieving regulators.  The ullage volume provides more consistent pressure control as force is applied to the cylinder.  This is very important when the material being pressed can become highly exothermic if the applied force/pressure exceeds some critical level (which is why I often use +/-0.5% regulators).  The last such device I made is used to press shaped charges for skin cutting on spacecraft.

If your application is not particularly critical for force/pressure, then you can reduce the requirements.  I live & work in a somewhat different universe.  My current project is a vacuum test chamber for NASA.  My previous project was a sidearm for the USMC.  Such work puts a different slant on criteria.

Offline Stilldrillin

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Re: supporting mini mill head,,
« Reply #6 on: March 03, 2014, 12:31:15 PM »
Mike.
That's a nice idea! The door closer, mounted above the machine, connected to the head!  :clap: :clap:

The mill head needs to be asserting an upward force against the rack, at all times. This takes out any backlash when feeding downward.

If I remove the pinion from my mill, the head rises to the top of the column, all on it's own........ :thumbup:

David D
David.

Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!

Offline stirling lad

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Re: supporting mini mill head,,
« Reply #7 on: March 04, 2014, 12:00:11 PM »
I know this is in the wrong place but i don't know the right place to put it so if the mods want ot take it down.....

I found this yesterday lying beside my mill after i had been moving things around,, i checked the gib strips on the mill and all 3 have a gib visible at at each end so is it possible that my gibs are little critters like this and the middle one has fallen out somehow.???I have never seen it before in my life till last night it just appeared... :scratch: :scratch: does anyone recognise it ?



...Mike....

Offline spuddevans

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Re: supporting mini mill head,,
« Reply #8 on: March 04, 2014, 12:08:39 PM »
does anyone recognise it ?

"I do!!"

It's the Gib strip for the mill's head depth stop collar thingy. ( you know the black squashed "U" shaped piece that, on my mill resides permanently at the bottom of the column )

Tim
Measure with a micrometer, mark with chalk, cut with an axe  -  MI0TME

Offline Stilldrillin

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Re: supporting mini mill head,,
« Reply #9 on: March 04, 2014, 12:19:08 PM »
Well done Tim!  :thumbup:

I remember the same conundrum, around 5 years ago.........  :scratch:

David D
David.

Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!

Offline stirling lad

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Re: supporting mini mill head,,
« Reply #10 on: March 04, 2014, 04:34:21 PM »
oh Brilliant !! thanks a million Tim... :smart:
I really had'nt a clue where it had come from.... :scratch:
But I could see it was the same colour as the others and the grub screw dent looked to be the same size....

what a relief that is as i did'nt know what was going to fall off!! :Doh:

  phew!!

...Mike...
« Last Edit: March 04, 2014, 10:39:49 PM by stirling lad »