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Mill Spindle Speed Control - DC Motor
« on: August 18, 2017, 04:27:18 AM »

I have recently purchased a 1984 Denford Triac. This was marketed as a "desktop" mill. It is larger and heavier than I would put on my desktop. It has 300mm X travel and weighs 250kg

I am in the process of replacing the existing under powered (1/2 Hp) spindle motor with something with more power and torque. The original motor is AC and I am replacing it with a DC servo motor manufactured by callantechnology.com I am looking at the 746W (1Hp) M4-2950 B which uses 0-10VDC for speed control. I will also need a DC Drive - Speed Controller, something like the Parker 507 (http://ph.parker.com/us/en/dc-motor-speed-controller-dc506-507-508-series).

I have a C11 NC board (cnc4pc.com) and a USB SmoothStepper BOB (warp9td.com). I am planning on using either Mach3 or Mach4.

My question is

How does Mach3/4 control the motor speed. I am guessing that Mach3/4 sends 0-10VDC to the DC drive which then sends it on to the motor. I would appreciate some detail of how this works. I have solid mechanical background but limited electrical experience.

I would appreciate some advice on a suitable DC drive for the motor. I have not found one that matches the motor specs which are: 115V, 10A, 0-500RPM. Power input is single phase 230V 15A



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Re: Mill Spindle Speed Control - DC Motor
« Reply #1 on: August 18, 2017, 05:22:15 AM »
Hi Keith,

Welcome to the forum.

For spindle speed control Mach3 sends a 0% to 100% PWM signal at TTL level (5V or 3.3V peak). The PWM duty cycle is controlled by the Gcode S### command and turned on / off with the Gcode M3,M4 / M5 commands.

It is up to a connected board (BoB or motion controller) to convert this PWM signal into the 0 to 10 Volt (or whatever) signal required for a VFD.

Hope this helps.

Re: Mill Spindle Speed Control - DC Motor
« Reply #2 on: August 18, 2017, 05:27:32 AM »
if you require speed control only then all you require is a variable DC voltage. If you wish to have an indexable spindle for rigid tapping say
then you will require a DC servodrive that can accommodate the encoder fitted to the servo. The latter option while desirable for future operations
or bragging rights does severely limit your choices and possibly makes such a drive expensive by virtue of rarity.

Is there a reason you've chosen this particular servo for a spindle? The over whelming majority of servos these days are AC servos, no pesky brushes
to wear out. I suspect that due to the abundance of choices they would be cheaper too....I would recommend a matched servo and drive.

I adopted this strategy when I bought a servo for a spindle, I bought secondhand a 1.8kW Allen Bradley servo and matched it to an Ebay Allen Bradley
drive. I did have to buy the servo setup software from Rockwell Automation (current owners of Allen Bradley) but it made combining the two was easy.

I now have (at least) three choices for driving the spindle:
1) Set up 8 distinct speeds in a table and select a speed entry by asserting three servodrive input pins in binary fashion
2) Have the servo follow an analogue voltage applied to one input pin, the gain and slope are programmable
3) Use position control, with or without electronic gearing which allows superb resolution and accuracy for rigid tapping

Despite having such a variety of control modes I find that in practice most milling jobs require the spindle speed set once at then left alone for the
rest of the job. Unless you use a wide range of tool diameters like fly cutters and facing tools don't get to hung up on speed control, you may find you've
invested a lot of time, effort and money which in practice don't get used much.

My wife left with my best friend...
     and I miss him!
Re: Mill Spindle Speed Control - DC Motor
« Reply #3 on: August 18, 2017, 06:04:17 AM »
Hi Craig

The reason that I chose this particular motor is that the machine manufacturer dropped the AC motor and replaced it with an SEM www.sem.co.uk branded motor. This has now been taken over by Callan callantechnology.com . The two big draw cards for me are

1. The physical size (Ø108mm) is a good fit for the space in the machine head
2. Motor speed is high at 5000 RPM, most of what I have found on the net has a max RPM of 3000

I had not found Allen Bradley, so will have a look into this brand.

I had not though of using a range of preset speeds, something I will also have a look at. My day job is NC programming and am used to the luxury of outputting any speed that is calculated from the CAM software. It is also valuable to be able to over ride the speed at the machine to tune the cutting conditions

Re: Mill Spindle Speed Control - DC Motor
« Reply #4 on: August 18, 2017, 04:15:28 PM »
I think the frame is a 'standard' size, just what standard...its the great thing about standards is that there is so many to choose from!

You are right, 3000 rpm is a common rated speed for servos in this power output. There are a number of manufacturers who regularly produce
fast servos, Baldor comes to mind. They tend to be expensive tho. While Baldor servos can be had on Ebay pretty cheap the drives are
eye-wateringly expensive, Baldor is owned by ABB and they traditionally are really expensive, I avoid ABB on principle!

The Allen Bradley servo I bought is rated to 3500 rpm. Its listed max is 5000 rpm but  without trying to hoodwink the servodrive software I cant
get it to go beyond 3500 rpm. The extra speed would be nice for 1/4 inch tools but I'm not stressing, this is a hobby machine, if a job takes an extra
5 min then so be it.

I would suggest that you consider the price of a replacement spindle motor be the cost of the combined motor and drive. Depending on the
sophistication of a DC drive it may tip the total cost to be more than an AC servo and drive. An asynchronous AC motor (ie induction motor) and
matching VFD will be cheaper than any servo but probably 50% bigger for the same power.

As far as servos go... there are servos and servos! Some brands like Fanuc and Seimens are just over the top price wise and then there are brands like
Baldor and Allen Bradley and many other US or European brands which  are just expensive. Delta servos from Taiwan have a good reputation and are
well priced, not bargain basement but not bad. There are cheaper Chinese brands and a lot of those have been rebranded to sound US/European and
for hobbyists probably good enough.

The further you get from known brands the more online research you should do BEFORE you stump up your cash, once they've got your money
its too late for you to decide that the unit is not good enuf!

The range of control strategies is so broad that its hard to specify beforehand. I personally have found that very simple speed control is adequate.
I will at some stage use position control for indexable operations like rigid tapping, that capability is built in and just waiting to be used.

I note that in your original post that you have a USS. At the current time the ESS has a Mach4 plugin but the USS does not. Warp9 are working on
producing a Mach4 plugin for the USS but you may be frustrated waiting. I use an ESS/Mach4 and VERY happy with it.

My wife left with my best friend...
     and I miss him!
Re: Mill Spindle Speed Control - DC Motor
« Reply #5 on: August 18, 2017, 10:44:39 PM »
Thanks for the reply Craig, that's all really good sound advice. I was guessing that a new AC motor with a new VFD would be a more expensive option and like you say research is the sound way to find out. Interesting that it may now give DC a run for its money. I have another reason to go to DC and that is I am remaking the spindle to go from R8 taper to BT30 taper. This will also enable me to add a support bearing on the tail of the spindle. The original spindle only has a pair of angular contact bearings at the nose end of the spindle. This is an improvement that the manufactured also made to the more recent models. With this modification there is now limited space for the spindle motor and the form factor of the DC motor is a perfect fit for the machine. So I am committed to using a DC motor. I tried to embed an image of my machine in my first post but it did not work for me. I see that there are very few images in the forum threads which is a shame as a picture is worth a least a thousand ...


Re: Mill Spindle Speed Control - DC Motor
« Reply #6 on: August 18, 2017, 11:09:24 PM »
Hi Keith,
pics often need to be compressed somewhat before they can be attached or alternately linked to a suitable site like photobucket.

I used a RegoFix ER25 cylindrical tool holder and turned up my own housing to accommodate matched ABEC7 angular contact bearings at the nose.
I used plain deep groove bearings at the tail, mainly due to budget....ABEC7 bearings are bloody expensive! It worked out pretty well.

I am quite obsessive and want much better than I can afford or have the skill/machinery to make. In this case I just had to buckle to the fact I couldn't
afford what I really wanted and so made the best I could from what I could afford. I'm glad I did, its by no means perfect but its certainly good enuf to
use and I take satisfaction from its making. That is after all what the hobby is about.

My wife left with my best friend...
     and I miss him!
Re: Mill Spindle Speed Control - DC Motor
« Reply #7 on: August 19, 2017, 02:41:09 AM »
Yes it is always nice to have the good gear, its hard to beat quality tools. When I buy tools I like to buy quality as I know I will have use out of if for a long time. Great that you have made parts to improve your machine. It always feel good when a project works out well.
I am lucky that I work in a toolroom and have access to use the machine tools. This is coming in handy to make bits for my project.

Re: Mill Spindle Speed Control - DC Motor
« Reply #8 on: August 19, 2017, 03:01:33 AM »
years ago I was working in a workshop and we had an old lathe, big sucker but pretty worn. I had no problems turning bushes and things like that
provided I didn't need to get closer than a couple of thou.

When a boat propshaft came in the boss would call Bob, a local guy, retired fitter and turner trained in the UK old school. He had a little notebook that
he had made for this lathe and he would get it out and turn tapers to PERFECTION! It was my job to blue and lap the prop onto the shaft and I never
had to work hard for it... I asked Bob any number of times how he did it but the old bastard would just smile and tap his head with his finger.

I've tried for years ever since trying to improve on the basic accuracy of the machine tool I'm using at the moment. The real trick is not just the quality
of the tool but the smarts that you need to add to get really good results like old Bob did.

My wife left with my best friend...
     and I miss him!
Re: Mill Spindle Speed Control - DC Motor
« Reply #9 on: August 19, 2017, 07:43:33 PM »
Hi Keith,
I have open in front of me the drawing of your DC servo M4-2950  -the main body of the motor 108mm dia and motor body excluding shaft
215mm long...correct?


This motor is shorter at 165mm and the body of the motor is 80mm square or fits in a 113mm circle. 730W at 5000rpm.


This motor is a bit longer at 215mm but the same frame size, ie 80mm square and 1.8kW at 5000 rpm. This is in fact a slightly faster but otherwise identical
servo that I used for my spindle.


This is the matching drive. Note that you still need the cables to join the drive and the servo, not trivial, one I Ebayed cheaply and one I made. I also bought
the Ultraware software required to program the drive at $200 NZD , about $130 US.

For your information.

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