Re: [CAD_CAM_EDM_DRO] Positioning

Jon, I think you're forgetting that there is already a lot of friction in the system. You'd have to calculate the force required to move the table and subtract that from your value. With the screws disconnected I think you'll find that it is pretty hard to move the table by pushing it (x or y) by hand.

--CS

>On 06/03/2014 12:38 PM, 'CS Mo' cs@...
>[CAD_CAM_EDM_DRO] wrote:
>>> I have read on another forum that retrofitting ball screws to a
>>> convention mill, say a Bridgeport, reduces the power needed to move the
>>> table - both by the operator and by the cutter. One comment was to the
>>> effect that with ball screws, hand machining became quite difficult.
>>> What I am interested in is how does one determine the necessary holding
>>> torque of stepper motors (or other drivers) to avoid the table being
>>> moved by the cutter?
>>>
>>>
>it is a bit difficult. But, I tried to figure this out,
>once. Assuming the worst
>case is a slow-turning cutter (so it has a lot of belt
>reduction in the spindle
>drive) you could take the motor torque (you can get an HP to
>torque
>conversion online, all you need to know is RPM). Then
>figure out the
>belt ratio by the speed reduction. So, if it was a 1725 RPM
>motor and
>you had 300 RPM on the spindle, that would be a 5.75
>reduction in speed,
>so you get a 5.75 multiplication in torque. Assume the
>motor was 2 Hp, 1725
>RPM, that gives 6.09 ft-Lbs. Converting to inch-Lbs is 73.08
>now, multiply by 5.75 for the belt reduction, and you have
>420.2 In-Lbs.
>So, if you had a 2" diameter cutter, it would have a radius
>of one
>inch, and so be exerting 420 Lbs of cutting force against a
>side-cutting
>surface. Now, to convert that to the torque exerted on a
>leadscrew to
>counteract this force. These screw conversions are
>complicated, but
>for a 5 TPI ballscrew, it is a linear advance equivalent to
>a drum of .0318"
>radius pulling on a string. So, 420 Lbs * .0318 = 13.4
>in-Lbs of torque.
>So a motor with a holding torque of 13.4 X 16 = 214 In-Oz could
>resist this force. BUT, you need additional torque so the
>motor can
>move while resisting this force, so I'd at LEAST double that
>for steppers.
>
>Jon
>
>
>------------------------------------
>Posted by: Jon Elson <elson@...>
>------------------------------------
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