Re: [CAD_CAM_EDM_DRO] Converting a Bridgeport Clone

Hi,

A big thank you to those that replied you have certainly destroyed my procrastination and given me something to think about. It would never have occurred to me to use a worm & wheel to drive the knee but it could be a very eligant solution, I assume control programs such as Mach allow for 20 or 30:1 reductions on an axis? When I bought my Harrison 280 CNC lathe it had a large tool changer bolted to the cross slide which blocked out the sun. I soon tired of trying to see around it and so it got removed. It appears to be a stepper motivated worm and wheel. I have no idea what size the stepper is (but I would think in the 400 in oz range judging by its size) or what reduction the worm and wheel is, so further investigation is called for. 15 ipm would be quite enough for the knee the prospect of more than a quarter of a ton of metal mass moving with greater speed is not something I want to dream about.

I have ballscrews for the X and Y axis and also for the quill. It is my intention to control all the movements of the machine by computer if possible (including a fourth axis) and I am aware I can use both Z axis' at the same time.

Regards, Terry

>Either will work but ... as it depends (grin .. like always).
>The limiter will be your acme screw efficiency.

>Neglecting screw efficiency:
>Lbs = pi * TPI * in-oz / 8
>3 Nm is about 400 oz-in, btw.

>Lbs = pi * 5 (tpi) * 400 / 8 = 785 Lbs = 365 kg.
>At 80% (mechanical losses x ballscrew effiency) this is 280 kg force.

>Using the smaller motors, for example with 3 Nm motors, at 1:3 >gearing,
>and a 80% efficient screw the thrust produced is 280 kg.
>I use this size, 3Nm, nema 23 motors, at 1:3, to move my milling >table.
>Table is 160 cm wide, 50 cm deep, 6 cm thick (on ribs), solid (F1
>calibrado) tool steel.
>I have a 0.750 inch ballscrew, 0.200 inch rise or 5 tpi, at about >90%
>efficient, and the table is on linear rails.
>(Linear rails are about 98% efficent, and HTD belts are about 97%
>efficient).
>Table can be moved by one finger (under 10 kg force required).
>Table mass is 240 kg give or take.
>Acceleration is excellent as is accuracy (better than 0.01 mm).
>Gecko251s, 48 V dc.

>HOWEVER, the acme screw is going to be about 10-20% efficient, and >you
>will have a lot of static start-up friction.
>Likewise, you are not on linear rails.

>The force achieved at 1:6 (very big secondary pullies at 1:6, btw), >with
>the smaller motor, will certainly move it.
>But I think you wont have a lot of margin.
>As in you will get slow acceleration if you program for worst case, >and
>missed steps otherwise.
>Acme screw stiction varies a lot with how clean it is, lubrication, >if
>you leave it to stand, cold/warm etc.
>I would definitely use the bigger motor for this app.

>However, I would not use belts but a worm gear.
>It is much more efficient, much more accurate, wont break, wont
>backdrive, is a better fit.
>IMO.
>A 10:1 -20:1 worm will work fine with the smaller motor, and will be
>much more accurate.

>I did my (stock) M-head quill with belts, and will now re-do it with >a
>worm, fwiw.
>Probably also preload it, upwards, get rid of backlash.

>> It has been a long time since I posted on this group and I notice
>> there is not so much activity these days. Anyway on to my question:
>>
>> I have a Bridgeport clone and want to fit a stepper to the knee >and I
>> am unsure of the size of stepper motor to do the job properly. I >was
>> thinking of buying the motors from Arc Euro Trade here in the UK >along
>> with their drivers:
>>
>> http://www.arceurotrade.co.uk/Catalogue/Stepper-Motors/Stepper->Motors
>>
>> If it takes 11.5 foot lbs of force to lift the knee and there are
>> 135.581795 newtons centimetre to one foot/pound, and I am thinking >of
>> a 6:1 reduction ratio on a toothed belt, what size motor would I >need
>> for reliable operation? I am confused about holding torque and >moving
>> force.
>>
>> Would I need the stepper 160-010-00500 @ 4.2A 650Ncm or will
>> 160-010-00450 @ 4.2A 350Ncm do the job?
>>
>> Regards, Terry