Gecko Stepper on Bridgeport

Hi

By way of introduction we were a volume retrofit company in the UK in
the recent past but we now sell PC based retrofit kits into the
industrial market using Ahha software. We sell to other retrofit
companies and direct to end-users. Our best selling kit is for the
Bridgeport Series 1 BOSS machines using the original drives.

We have a number of customers interested in 4th Axes so we bought
some Gecko drives both stepper and servo. We have been contacted by a
number of list members who are interested in our experience with
Gecko drives. We have finally wired a G201 stepper to a Bridgeport
Series 1 X-axis motor.

This is a UK built Series 1 Boss 6 with metric ball screws. The
pulley ratio is such that 1 full step gives a table movement of
0.01mm (close to 0.0004") The Motor is a Superior Electric M112
rather than the more common Sigma and we have always considered the
Superior motor to be slightly more powerful (As an aside there must
have been some supply problems into the UK when these machines were
first built as there were 4 volume manufacturers of Stepper driven
CNC machines in the UK: - Bridgeport, Matchmaker, Beaver and Acton
and they all used a mixture of Sigma and Superior motors)

The Transformer on the Bridgeport is too high a voltage for the Gecko
so we are using an off the shelf transformer giving 40 Volts AC. The
Drive is a standard G201 not the G201 optimised for the size 42
motor. We originally mounted the drive on a heat sink as is required
for the full 7 amps but were still concerned at the temperature it
was reaching so we finally fitted a small fan blowing through the
sink.

The smoothness of the axis compared to the original drives is
OUTSTANDING. We have been experimenting with rapid traverse rate to
find the stall point. We have set the acceleration at 40, 000 steps
per second per second which is reasonable. We have found we can get
100% reliable operation up to 13,000 steps per second tested over a
period of 8 hours. At 14,000 steps per second we lost the odd step
here and there. At 15,000 steps per second we get stalling. On our
set up 13,000 steps per second is about 30 inches a minute. With the
original drives we normally set the machines at 100 inches a minute.
We understand from Mariss that size 42 motors will run out of torque
as motor speed increases when driven by the G201 so we would expect
different results on an earlier Series 1 machine with imperial screws
and 1:1 belt ratios. Whether this would be better or worse I would
not like to say as although the revs are lower so is the torque
available at the ball screw. Incidentally with UK Series 1 machines
you can tell if they have the belt ratio by the speed control and
brake operation. If they are air operated the machine has the
reduction ratio giving .01mm per step. If they are manually operated
the steppers are 1:1 giving .001" per step. We have squeezed a
2:1 ratio into some of these earlier machines to give better
resolution and torque but it does require 1 special timing pulley
manufactured, some re machining on the X Axis nut and stripping the
head to change the timing belt.

We would have no hesitation in using these drives with 42 motors on
4th axes, as these typically require less power. As an axis drive the
slow rapid rate is frustrating if you need to jog the length of the
table but the machining performance is fine at the feed rates I
normally use.

The Servo drive has only been connected to a motor on the bench at
the moment using the same 40-volt transformer. It is a 42 size Baldor
rated at 5nm with a 1000 line encoder and the bench performance is
VERY impressive. When we get time we will fit this to a machine and
find out just how far we can push it.

Ian Eagland
Eagland Machine Tools Ltd