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Author Topic: Retrofit Recommendations for Sharp CNC Knee Mill Using Existing Components  (Read 2656 times)

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That ZlattePanda looks pretty cool!
it does at that! I've got a dual core Atom MiniITX board so not as kool (or powerful) as the LattePanda
but it does the business on my machine good-oh despite being such a low powered PC.

If you want a bit more grunt look at the UDOO series, they allow you to fit your own SSD or hard drive.


My wife left with my best friend...
     and I miss him!
sorry...wrong link:


My wife left with my best friend...
     and I miss him!
I've been doing some more calculations and have come to the conclusion that 800W is more likely peak power rather than continuous.
My reasoning goes like this:

The voltage feedback constant is 7V/1000 rpm.
The back EMF at rated speed is 7/1000 x 3500 =24.5V
The back EMF is in effect the electrical reaction to the motor delivering rated power to a mechanical load, therefore the current is:

power = voltage x current
800W =24.5V x current
current= 32.6A

The total terminal voltage when producing 800W into a mechanical load will be the sum of the back EMF and the resistive loss of the armature.

Vtot = 24.5 + (32.6 X 1.3Ohm)

The total electrical power delivered to the servo is:
Ptot=66.9 x 32.6
      =2180 W

Of which 800W is deliverd to the load so the remaining power is dissipated in the resistance of the armature:

Your servo would cook up BIGTIME if this were continuous. My guess is that the torque figure quoted is peak torque not continuous rated torque.
As peak torque is typically three to four times rated I would guess the true CONTINOUS RATED torque is  0.5 ft.lb and rated power is about 200W.
That would bring the dissipation in the servo down to 79W. That is a lot more realistic.

If my reasoning is correct then 200W servos could be used to good advantage on your machine and that may bring the price down somewhat.
I have heard CNCers say that very large and heavy and fast machines might have axis motors of 1 hp or 750W. It seems more likely that a smaller machine
like yours would have axis motors somewhat less than that, 200W is an estimate.

The power figures I quoted earlier from the DMM website are continuous power. It makes sense to compare your existing servos continuous rating
to the continuous rating of DMM's AC servos.

If you follow the proposal to replace the DC servos with AC servos then the biggest expense in the retrofit will be the servos and drives themselves. As you have seen
the DMM 400W 200V servos and drives are about $400 a set, or $1200 for the three axes. A 200W 200V servo saves you only $24 per axis, it makes sense then to get
the 400W units.

You may note that DMM list both 60V and 200V servos and drives in these power ranges. The 60V servos and drives are cheaper however you would then require high current
power supplies. The 200V units however are fed with 230V AC line voltage, no power supplies required. All up its a wash, they are both near identical in price given the power
supply. The 200V units require much less current and therefore are going to run cooler at elevated loads so the higher voltage units are preferred.

This proposal would cost you $1200 for the servos and drives and about $695 for Mach4, an ESS, two BoBs and a Lattepanda. This assumes you have a monitor and keyboard that you
could re-purpose. Not far off $2000.

Is this within your means? What would your machine be worth once its done?


My wife left with my best friend...
     and I miss him!
I see the DMM 400W 200V servos are $177 whereas the DHT 750W servo is only $194, an extra $17....

My wife left with my best friend...
     and I miss him!
That’s a good deal!
Also, what are the requirements on interfacing an MPG?
Any good, I expensive pendants or MPGs to recommend?
I was looking at the pics you posted and I think the data you posted about the existing servos is wrong.

V/1000RPM: 7

In the pic it shows the BEMF/1000RPM as 39V

Which is correct?

I found this:
which suggests 955W output. If that is the case you'd either have to consider gear reduction to use 400W servos or use 750W servos. Either way this
is likely to alter the cost of conversion.

My wife left with my best friend...
     and I miss him!
I don't know.  I guess I would say go with the plate on the motor?
I'll try to get a picture of the other motor.  Hopefully it is readable.
the spec, BEMF/1000RPM=39V, seems pretty plain. This is known as the Back Electro Motive Force constant, hence the acronym BEMF.

If the motor were driven by another motor at 1000rpm there would be 39V at the terminals. This is an important figure of merit for a servo.

Doing the calculation with a figure of 7V per 1000 rpm, which I suspect is in fact the tacho feedback device, then the shaft power of 800W could only be sustained
for a short period of time. With a figure of 39V per 1000 rpm that output could be sustained continuously. This is a more realistic and more probable than my previous
reasoning. I think it wise to consider the existing servos to be in the region of 800W continuous.

That has implications for your rebuild. You would ideally replace them with something of the same power but that is going to get expensive quickly. If you were doing
this for commercial purposes it would still be pretty cheap but if you have to convince your wife that you really really need it she might not believe you! You could use smaller
servos and have either a gear or belt reduction, that would increase the torque to that necessary to overcome friction and inertia but be a little slower. That would probably be
a good compromise for a hobby machine.

As I've pointed out the medium inertia 750W DMM servos and drives are  about $412 an axis. They could be relied upon to be a direct replacement for your existing servos.
You could save a bit by getting 400W servo and drives, $385 per axis. You'd probably need a belt reduction of 2;1 to match your existing servos, but that would be very doable.

If you choose to retain the two X and Y servos there are some choices to make. Given that the Gailil is only two axis its no good as a controller. If you chose a HiCon board from
Vital Systems AND paid the premium to get the analogue features enabled the you could drive your two servos and still have the capacity to add another or even more than
one for the Z axis. A HiCon with activations is about $1200-$1300. Note this would allow you to use the two servos AND your original drives.

If you replaced the drives with Geckos or something similar then you could use a much cheaper controller like an ESS or a UC300. Either controller would allow you to feed the two
Geckos with step/direction pulses and still have plenty of capacity left over for the Z axis. You'd still need a servo and drive for the Z axis, say $400 and two Geckos say another $300
for the pair, for about $700 total.

I think you'd be very impressed with AC servos but despite them becoming ever more affordable they are still somewhat of a premium. If you were to try to buy NEW DC
servos, then I suspect they would be even more expensive.

I was given a 100W DC servo off a customers plasma table, this table is $150,000 or more and the whole business of thirty or more people rely on its output to stay
working. It has a faulty line driver in the encoder. I can probably jury-rig a new line driver, this servo is now only a spare. But no, we want a new encoder, I found the
OEM and priced getting a new encoder made to suit, $585USD, prepaid, one month leadtime plus freight to New Zealand, and we want a markup....Funny that they didn't
seem that keen once the numbers were put in front of them. I could get TWO NEW AC servos and matching drives with more power than they could get one encoder!
Even more, the business is about 4 doors down from the local distributor of Omron/Yaskawa. I could walk in get what I need off the shelf and Yaskawa is serious good quality
but they cant get their head around AC servos......they insist that servos have to be DC, how else can they be closed loop they ask. I've given up trying to explain to them how they
work....When the table craps out we'll see how amenable to alternate solutions when they're out of action!

Sorry about my rant. The point is that DC servos are rapidly going obsolete.

My wife left with my best friend...
     and I miss him!