Bridgeport Manual to CNC conversion...

[Davek0974]

Hmm, just had a thought...

Instead of dropping around £2k on building a mini-mill/router, what I should have done is use that cash to convert the Bridgeport and fit an adaptor mount that enables me to hold the 24k water-cooled spindle - best of both worlds??

I could even mount a complete Z-axis on the rear of the ram where the slotting head should fit!

The BP has a lovely 48" bed, ok maybe it only has 9" of Y axis but thats still not too shabby I think, certainly I've done a hell of a lot of big jobs on the old girl.

So, what would the drawbacks be here?

The new spindle has the speed I need, I think a BP conversion can reach 4-5000 mm/min rapids which is where the mini-mill is set so thats no issue, ballscrews would provide the same accuracy as the mini-mill has presumably.

Hmmm......

[komatias]

You lose Z travel when you convert the bridgeport's quill and another bit more when you add the adapter to hold the high speed spindle. Apart from that, I cannot see any issues except the speed at which your motors are able to move the bed. Good lubrication is key here. 4-5000 is way to quick for stepper conversions but possible with good servos. I have restriced mine to 3000mm/min actually.

[Davek0974]

Ok, but by building a custom Z-axis and mounting it on the rear of the ram I would retain whatever travel I build into the new axis, it means swivelling the ram round to change from heavy to light milling but thats about it, plus would need two z-axis drives - heavy and light.

The Z on the heavy side is a pain, a couple that i have seen today - one replaced the quill lever with a toothed pulley and drove that via a planetary gearbox reduction - seemed ok, another replaced the fine-feed hand wheel and drove that, the ones that mount to the front of the quill I am not too impressed with, have read a few tales of quills being trashed by the single bolt pulling out.

As for speed, so far most of my mini-mill work ranges from 600mm/min to 150mm/min - hardly blistering, I'm sure the old girl can match that ;)

I think this may be an interesting project.

[komatias]

Yes, that would do it. I have gone with direct drive on the quill and do not over drive it. The fine feed goes through a clutch while the backlash on the lever is too much for CNC.

[Davek0974]

Good points, thanks.

By direct, I gather you have the screw nut coupled to the front where the depth-stop used to sit?

[JAZZCNC]

It's slippery slope and quickly your into it $$$ before too far to stop.!! . . . Don't Do it Listen to John S.

[Davek0974]

awww, bum, damn that cold hard logic JAZZ ;)

In my defence, it would pretty much fit in with exactly what i want though!

So, the alternatives then -

Find a donor machine and hopefully just fit the electrics?

Downsides I can see...

space - nowhere to put it.
would like a big table again - this seems to mean a big machine (see point above)
transport costs
unknown wear & tear
different tooling probably

This seems not too bad, but its bed is quite small, although it does have an ATC - would that be the sort of stuff to search for???

http://www.ebay.co.uk/itm/1821213470...%3AMEBIDX%3AIT

[Davek0974]

Something that rarely gets mentioned...Power required...

I have a 3ph 2.2kW spindle motor on a vfd, 700w X servo, 700w Y servo, 400w Z servo, maybe 100w for pc and controller on this machine.

How do i calculate the supply capacity allowing for diversity - it will not be running 100% flat out all the time :)

I know i need to uprate the MCB's and supply cables as the build was originally for the small 1.5hp spindle motor.

Adding up the maximum ratings for all drives gives a figure way more than i can supply :)

[m_c]

A quick mental calc, and I'd say 16A would cover you.
2200+700+700+400 = 4000.
Divide that by 220 gives 18A, or 250 gives 16A.

Power up surges of the drives are likely to give a bigger problem than too much current draw during use.

[Davek0974]

Thanks, so there is no difference in current draw if a 3ph motor is running on 3ph or on converted single to 3ph via a vfd (apart from the internal vfd usage) ??