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Author Topic: Bridgeport Knee Mill Conversion?  (Read 159009 times)

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Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #580 on: October 23, 2016, 10:04:48 AM »
Future thoughts...

Machine is working very nicely so far. However I still have big doubts about the weakest link in the system - the Z-axis drive, its a pretty tortured setup, the connection to the quill is poor being just one bolt, the block face is only around 15-20mm square where it bolts on, there is around 75mm from the quill to the ball-screw so there is a massive twisting moment acting on the joint and the ball-nut.

There is already movement visible between the nut and the quill - you can see the connecting arm twisting when nudging the ball-screw pulley by hand so it would be pretty rough under drilling or plunge-milling forces. I can see a very short life for the ball-nut as they don't like twisting moments, axial or radial but not twisting.

It was assembled with loctite but with only the one bolt I doubt that would do much.

I have a feeling it would have been better to go with the drive idea in the pictures below, the quill rack is designed to take all the Bridgeport can do, there is not much backlash and what there is could be helped with a gas-spring maybe to keep it under positive pressure.

No idea what the pinion gearing ratio is but likely something like 3" per turn so would use a bigger reduction from motor.

Just looking ahead, but as i said, I can see issues here already and thats only after a few jobs....
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #581 on: October 24, 2016, 04:28:58 AM »
Thinking out loud...

With the drive to the quill rack, probably from the left side as done in the FlashCut kits etc, without measuring i  think the quill pinion does 2 full turns for full travel so about 75mm/turn, with a 4:1 or maybe 5:1 belt reduction from servo to pinion shaft, would that give enough resolution/torque - the motor would only ever do 8 or 10 full revs??

Just working things through...
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #582 on: October 26, 2016, 02:53:18 PM »
Quill drive reduction idea, mk1, pictured.

Gives a 16:1 reduction, takes up little space, relatively easy to build.

Mounts to left side of head onto the 6 screw holes no longer used.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #583 on: October 27, 2016, 08:52:24 AM »
Anyone have a video or link to some pics of a powered knee mill conversion?

Seems a lot of people rate it but i cant find any proof!  :)

Still deciding which way to go.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline RICH

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Re: Bridgeport Knee Mill Conversion?
« Reply #584 on: October 27, 2016, 10:55:30 AM »
Dave,
Here is a link to what I did on my Atlas mill for what it's worth.
http://www.machsupport.com/forum/index.php/topic,7396.msg47306.html#msg47306

I originally wanted to be able to use the quill both manual and CNC driven.  That worked but there was backlash and could not get rid of it and
frankly that just sucks for CNC and backlash compensation leaves something to be desired.

So I redid it such that I could move the quill with almost zero backlash and a good resolution. I still can disconnect the ballscrew nut
and use it manually but frankly haven't done that in years.

There is thread in here about using the knee of the mill...do a search.

BTW, never ends does it!  :D
RICH

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #585 on: October 27, 2016, 01:48:50 PM »
Thanks Rich,

So you had it first on the quill drive and then moved it to the quill direct? I have several good ideas coming in regarding strengthening the joint but all mean losing z travel and i can't do with any less of that.

I am looking at powering the knee instead - the attraction of 15 odd inches of Z axis is appealing but the big drawback is cost, i reckon i'd be looking at £600 at the end of the day with a big ball-screw, servo motor and drive, pulleys, extras etc. Maybe more if it needs a brake for when the power is off.

I'm drawn to splashing about £100 to see if driving the quill is a possibility or not.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #586 on: October 28, 2016, 02:19:29 AM »
This seems a pretty neat way of backlash control - its a brake cable from a motorbike, one end is mounted to the quill and the other has an air cylinder on it with variable pressure.

The guy that fitted it also fitted a very tidy gear reduction system in the aluminium casing on the left, 15:1 ratio, double reduction.

He reports zero backlash and full quill travel :)

I think he had a small air receiver as well to provide a cushion effect.

On cost alone i feel it worth trying a drive from the left of the quill, topper the knee at any speed is going to take some time to save up for and its a big job to fit.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #587 on: October 29, 2016, 05:24:39 AM »
Measuring on the existing adaptor from the Z screw to the quill, near the quill, I have well over 0.5mm of flex - thats a bad figure as it equates to backlash at the tool, so doing nothing here is not an option.

So, looking at the options -

Power the quill via the left side of the pinion shaft, this has slop in it so will need some clever way of preloading the quill upwards - this could lead to rapid rack/pinion wear as it will need enough preload to counter cutting forces from pulling the tool downwards.

Power the knee - this is an option but will cost a lot as it needs a ball-screw, servo, drive plus full tear-down to fit it all.

Alter what i have got - looking at the picture below, is there any harm in removing the web of metal circled?? It would mean i can get a bigger connecting arm in there with much more contact below the bolt to resist the down-forces, without losing any Z travel. Means stripping the head again.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #588 on: October 29, 2016, 11:02:44 AM »
After some thought I reckon the easiest option based on what i have going already is to modify the quill housing and build a better connector arm - this should give more rigidity and still maintain the full Z travel.

In the attached picture, on the left is the existing setup, on the right is my next version - as you can see, in the existing one a lot of twisting moment is placed on the bolt when the arm is pushed down (heavy lines).

In the new version, the twist is converted to an axial pull on the bolt and the force transferred to the quill body via the longer contact face below.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline Davek0974

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Re: Bridgeport Knee Mill Conversion?
« Reply #589 on: October 30, 2016, 12:45:21 PM »
That was a long day!

Stripped the head, cut away the small web of cast at the bottom of the quill slot, altered the screw connector by welding on a 13mm thick bar underneath, the end of the bar was milled to the curvature of the quill before stripping, fitted up, refitted the Z drive and shimmed the connector, power head back on, motor back on, encoder back on, tested.

The connector now has an extra 25mm of contact below the quill bolt which is a massive improvement over the first attempt, this makes a big difference to the twisting effect on the quill bolt and is now far more rigid in the downward direction which is what matters.

Total backlash is now down from 0.6mm+ to 0.1mm and I also discovered that most of that backlash is caused by the BK screw block not clamping the screw correctly - this was always there but never spotted it before. I will strip that block and see whats wrong with it - either crap bearings or sleeves too short and the nut not reaching properly.

I don't think this one is getting any stiffer (ooh err!)  :)
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives