Re: [CAD_CAM_EDM_DRO] Conversion of 12 x 24 lathe
Posted by
Les Newell
on 2005-04-19 05:21:14 UTC
Hi Mike,
Determining the right size motor is always a problem. You need to work
out the cutting forces needed. Realistically the only way is to try to
measure the torque on the screw when taking the heaviest cut you are
likely to take. A spring balance on an arm attached to the screw is the
best way but rather awkward to set up. Double this value to give you
some safety margin. Now you need to decide on the maximum feed rate you
want while taking a heavy cut.
Select your stepper and gearing to achieve this. As stepper torque falls
off with speed you need to consult the torque curves for the motors to
find the actual motor torque at this speed. There will be a number of
options either using small motors geared down a lot or large motors with
a smaller reduction ratio.
As much less torque is needed for rapid moves you should still achieve
quite reasonable rapid speeds using these values.
It may be worth considering servos. They often work out a bit more
expensive but you get improved performance and no worries about lost
steps. For servos the calcs are slightly different. Decide on your max
rapid speed and use that to work out the reduction ratio so the motor
runs at full speed when moving at rapid speed. This gives you the
reduction ratio. From this you can work out the torque needed from the
motor. The motor's max continuous torque should be at least this value.
If you want manual control you can fit handwheels as well. If you do,
make sure the handles have no projections otherwise you could end up
getting caught up in one of them when the machine is moving under CNC
control. You need to have some way of disabling the drives when moving
by hand.
Another alternative offered by Mach3 (www.artofcnc.ca) is to use one or
more electronic handwheels. These can be set up to behave almost the
same as a mechanical handwheel though it does take a little while to get
used to the lack of 'feel'. The big advantage of this is that the
computer keeps track of the machine at all times so you can mix manual
and CNC operations. Trust me, this is very useful!
You can make a very nice electronic handwheel using the encoder out of
an old Epson inkjet printer. Some epson printers have a large 1200 line
encoder disc fitted in the left hand side. If you lift the lid you can
see the disc on the left hand side. I used one of these on my mill and
it works really well.
Les
sbsmwj wrote:
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Determining the right size motor is always a problem. You need to work
out the cutting forces needed. Realistically the only way is to try to
measure the torque on the screw when taking the heaviest cut you are
likely to take. A spring balance on an arm attached to the screw is the
best way but rather awkward to set up. Double this value to give you
some safety margin. Now you need to decide on the maximum feed rate you
want while taking a heavy cut.
Select your stepper and gearing to achieve this. As stepper torque falls
off with speed you need to consult the torque curves for the motors to
find the actual motor torque at this speed. There will be a number of
options either using small motors geared down a lot or large motors with
a smaller reduction ratio.
As much less torque is needed for rapid moves you should still achieve
quite reasonable rapid speeds using these values.
It may be worth considering servos. They often work out a bit more
expensive but you get improved performance and no worries about lost
steps. For servos the calcs are slightly different. Decide on your max
rapid speed and use that to work out the reduction ratio so the motor
runs at full speed when moving at rapid speed. This gives you the
reduction ratio. From this you can work out the torque needed from the
motor. The motor's max continuous torque should be at least this value.
If you want manual control you can fit handwheels as well. If you do,
make sure the handles have no projections otherwise you could end up
getting caught up in one of them when the machine is moving under CNC
control. You need to have some way of disabling the drives when moving
by hand.
Another alternative offered by Mach3 (www.artofcnc.ca) is to use one or
more electronic handwheels. These can be set up to behave almost the
same as a mechanical handwheel though it does take a little while to get
used to the lack of 'feel'. The big advantage of this is that the
computer keeps track of the machine at all times so you can mix manual
and CNC operations. Trust me, this is very useful!
You can make a very nice electronic handwheel using the encoder out of
an old Epson inkjet printer. Some epson printers have a large 1200 line
encoder disc fitted in the left hand side. If you lift the lid you can
see the disc on the left hand side. I used one of these on my mill and
it works really well.
Les
sbsmwj wrote:
>Hello,--
>
>A friend of mine, having seen my conversion of a Sherline lathe and
>the parts it makes, has decided that he really needs to convert his
>much bigger lathe. It's a Chinese made, gear head lathe (less than a
>year old), which can swing parts 12 inches in diameter. He wants to
>turn model locomotive wheels on it which are cast iron and 10 inches
>or 7 inches diameter. The lathe can be seen here http://www.warco.co
>uk/lathes/gh1224.htm
>
>Backlash has already been considered and various modifications have
>been either made or planned to correct problems.
>
>How do we determine the right size stepper motor for this two-axis
>conversion?
>
>Presumably, the best choice would be to run the steppers through a
>belt-reduction drive to increase the torque available?
>
>I can't convince him that once he's converted his machine to CNC he
>might not want to run it manually, but if he insists, are there any
>strategies that might permit this?
>
>Thanks for your advice,
>
>Mike Jones
>Cardiff, Wales
>
>
>
>
>
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Checked by AVG Anti-Virus.
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Discussion Thread
sbsmwj
2005-04-19 04:38:50 UTC
Conversion of 12 x 24 lathe
Les Newell
2005-04-19 05:21:14 UTC
Re: [CAD_CAM_EDM_DRO] Conversion of 12 x 24 lathe
turbulatordude
2005-04-19 05:31:37 UTC
Re: Conversion of 12 x 24 lathe