VFD and Belt Drive conversion for my Sieg X3 mill, Part 1

VFD and Belt Drive conversion
for my Sieg X3 mill, Part 1
By Christian Banninger, Australia
August, 2009
Copyleft protects this article
From this.....
........To this
Background
Shortly after purchase of the X3 back in 2004 I began entertaining this idea. The X3 always
made too much noise for my liking. The motor sounded like a vacuum cleaner, and the gear
drive like a skeleton walking on the metal roof. The drive had a very rough feel to me. Vibrations
were evident.
I carefully de-burred and polished all gears, took the spindle completely apart, the noise did not
change. Also I would have liked a slightly higher top speed for the spindle, and certainly a
smoother running spindle at very low speeds. Alas, it took me 5 years to really do something
about....
Earlier X3 drive modifications done by others
Back in 2004, there already was a VFD conversion for the X3 documented on the net by a
Japanese home machinist. It was a very well done conversion. It even had a two speed belt
drive. I did however not like the belt tensioning system which required tools for speed changes.
This is it:
The Japanese site has a detailed documentation with plenty of
pictures on this VFD upgrade. See this webpage and the following
for more information:
http://homepage3.nifty.com/rockhill/furaisu-10.htm
Text is Japanese, so you may want to translate it into
English, this link uses the Google translate feature to
open the above webpage:
http://tinyurl.com/r88erg
Also back in 2004, there was already a conversion for the X3 that kept the existing motor and
controller, but replaced the entire metal gear drive by one single 560-8M timing belt. This
resulted in a single speed setup, with spindle speeds from 500 to 4000rpm. According to its
designer Jure Spiler from Ljubliana, the noise problem remained unsolved. See pics here:
http://www.basic.si/x3/X3_Spindle_Horror.htm#_Toc81198733
Back in November 2005, Dick Stephen published in “Model Engineer's Workshop” issue #111, a
modification that replaces the final gear set to the spindle only with a timing belt. Thus retaining
the 2-speed metal gearbox and all the other drive components unchanged. He claimed to notice
some reduction in noise and head vibration.
My Drive Design
I wanted it to be a VFD drive. No more stuck and arcing brushes in cheap DC motors, no more
turning the scored collector over, no more carbon dust, no more ozone smell.... And I decided to
use an endless Polybelt of exacly the same type and length as used in my EMCO Compact 8
lathe: a Gates 5M690. I know this belt will keep any vibrations away from the spindle. Since I
only used 3 belts after owning and using my lathe for 22 years, I know it can handle the power
and is going to last several years in the X3 too. And the idea of having to stock only one belt
type for both lathe and mill is appealing.
Having had my lathe upgraded to VFD long ago, and used it extensively for milling before
purchasing the X3, I knew that a 0.5HP drive was going to be all I ever need. A French
Telemecanique Altivar 28 was found “new old stock” on eBay Germany for 89 Euro including
postage to Australia. It is a “sensorless vector” technology drive of 0.37kW and 240V single
phase input. The motor of my choice was found on eBay Australia, it is a 3-pase Teco dual
voltage 4-pole flange mount motor in IEC71 frame, again “new old stock” for under 60AU$
including postage.
With VFD and Motor bought, next I had to make up my choice regarding spindle speed ranges.
The original X3 motor has 3/4HP and a two speed gearbox to cover speed ranges of 0-1000rpm
and 0-2000rpm. Being a dc drive it is constant torque from zero to its base speed, which also is
its maximum speed. See the graph below. And yes, I did make sure that the original SIEG DC
motor really delivers 3/4HP. I used a power meter instrument to determine how much power
goes into the original DC drive with the motor fully loaded, and assuming a 60% efficiency of the
Treadmill type SIEG motor, it really has the 600W shaft power claimed!!
Power vs. RPM graph for the original 3/4HP SIEG X3 motor (assuming the motor torque curve
is nearly linear). At slightly over 1000rpm spindle speed, when shitching into 2nd gear, the motor
produces about 300W power. Also, in first gear at just below 500rpm at the spindle, about
300W are available. The motor cannot be driven above 1000rpm spindle speed in 1st, or
2000rpm in 2nd gear, without risking damage to the armature by centrifugal forces.
I decided that with VFD and a 3-speed belt drive, I could cover a speed range of 0-3000rpm.
Being a VFD vector drive, it is near constant torque from near zero to its base speed (50Hz),
then its near constant power up to about twice its base speed (100Hz), then it rapidly looses
power to the point where at about 140Hz it can just overcome the friction losses of belt and
spindle bearings, and no more power is left to drive the cutting tool. The graph below shows
what I mean:
Power vs. spindle RPM graph of my proposed VFD drive
(much simplified, the real curves are indeed curves, and different for each motor)
Please notice that this is my personal choice and preference. The drive could be designed
single speed, or with 2, 4, 5, 6-speed pulley speed settings with or without laty shaft. Also the
final top speed could be choosen anywhere up to some 5,000RPM (that may need some better
quality spindle bearings and low viscosity grease, but the current spindle design as such should
cope with 5,00RPM...). I thought that for me and what I do, a top speed of 3,000 RPM at 120Hz
would be fine. In practice, with the pulley diameters I did choose, I get up to 3,400RPM @
150Hz with just the power to run a smallish end mill.
My last graph today shows the two previous graphs overlayed on top of each other. It serves to
get a feel how the original X3 DC drive and my new VFD drive compare:
The dashed burgundy lines are the two speed settings of the original SIEG X3 DC drive. It can
clearly be seen that the orginal 3/4HP drive has more power in the speed ranges of 7001000RPM and 1500 to 2000RPM. In all other speed ranges it is either same or inferior to my
VFD drive – and this despite that the original drive is not even capable at all of speeds above
2,000RPM. Whilst my VFD drive reachers up to 3,000RPM. In practice, the graph does not say
it all: the VFD drive is also very much smoother and quieter. It is particulary much smoother at
lower speeds, where the original DC drive suffered badly from commutator torque vibration.
The Pulleys
To achieve above spindle speeds with a 4-pole motor and a 3-speed drive, the following gear
ratios are necessary: 1: 3.51, 1: 1.8 and 1: 1.2.
To achieve these ratios with a 5M Polybelt drive, it is necessary to consult the Gates Website
www.gates.com and register with them, then you click on “power transmission” and click on
“drive design engineering” and “drive design manual online” and download the “Light Power and
Precision Drive Design Manual”. Page 97 is the most important with the dimensions for the
Polybelt grooves, the following pages help a lot if you want to design a slightly different drive
from what I did.
page 97 from the Gates “Light Power and Precision Drive Design Manual”
After some reading and a little math, I came up with the following pitch diameters for the pulley
grooves:
Spindle pulley: 131.3mm, 111.3mm, 95.3mm
Motor Pulley: 38.3mm, 62.5mm, 78.5mm
With these diameters, the fixed length belt should fit all three pulley positions with exactly the
same tension. It is critical to get this real right, otherwise changing the belt speeds will be a
nuisance for many years to come. The groove angles for the 131.3 and the 111.3mm pulley are
64 degrees, the other grooves under 110mm are to be made 62 degrees.
Above pulley groove diameters will provide the following spindle speeds:
frequency Hz
10
20
30
50
60
80
100
120
1st gear RPM
85
170
255
425
512
680
850
1024
2nd gear RPM
175
350
500
875
1050
1400
1750
2100
3rd gear RPM
250
500
750
1250
1500
2000
2500
3000
The whole idea was to have three overlapping speed settings, with at least 1/4HP being
available at any speed. Much like this:
1st gear 0 to 1,000 RPM (ideal for large diameter drilling, flycutting, boring, slitting)
2nd gear 500 to 2,000 RPM
3rd gear 750 to 3,000 RPM (ideal for small diameter tools in plastics and aluminium)
Spindle pulley, being made from a billet 150mm round Aluminum stock
Spindle pulley, being made from 150mm round Aluminum stock
How to check that all three belt positions will have the same tension (if you have 3 identical
belts handy)
Makeshift setup to cut the splines on the X3
Spindle pulley, almost finished, being slotted
Motor pulley, being slotted (the liquid is WD40)

It is actually very important that the surface finish of the grooves is impeccable and
smooth. It is normally done with a form tool. But form tools only work well on heavy
industrial lathes. All I have is a light 8” hobbylathe. With a form tool, the surface finish
would very likely show chatter marks. So I used a narrow pointed tool, and set up the
compound to the groove angle, once left and once right for each groove. It is very easy to
cut a single groove into a pulley this way. But I can assure you its real challenging to
make a 3-step pulley that way. That is, because the next steps shoulder wants to foul the
tool. I had to make an extra slim and long tool especially for this task (6mm shank).

despite careful calculation, some fine tuning is necessary whilst cutting the last grooves.
Look at the pictures how I use 3 polybelts to ensure the pulleys will once installed have
the same belt tension on all three positions. This is crucial, because if this is off you
would need a different distance between motor and spindle for each belt position. The
spring loaded belt tensioner can only compensate for very minor errors.

Cutting the splines into the pulleys is actually the easiest and fastest part of the job. All it
takes is some 50 short strokes with the quill, not much force is needed. I used a spare
carbide lathe grooving insert brazed onto a steel shaft – it will probably last me for the
next 10 years, since I rarely need to cut internal splines. Notice the wood to clamp the
top gear (yes, one day I will make a real locking collar for this....).
Motor Modification
The IEC71 flange motor needs its rear shaft mounted fan removed for three good reasons:
–
Once because I intend to spin the motor at well over twice its 50Hz base speed, and at such
high RPM's it would sound like a turbine.
–
Second, because I want to be able to run the motor for extended periods of time at speeds
blew 30Hz without overheating, and this calls for a separate constant speed fan.
–
Third, I intend to install the motor in the same position left of the milling head facing
downwards, and thus an as short as possible motor body is desirable. I did at first look at
mounting the motor to the top of the head, but did not like the idea of making the X3 both
taller and more top heavy as necessary.
The nameplate
Notice the rear shaft extension for the fan chopped off, and a rubberflex power
cable with 4-pin Amphenol screw connector fitted....
...the round mounting flange has been squared up (and repainted)....
... two 80x80mm 24V box fans have been mounted for cooling, using
a 12x12mm RHS aluminium profile attached to the motor's T-slots.
The thin pigtail cable is the 24V supply to the 24V fans wired in parallel.
The Base Plate
The base plate is fabricated, simply because I happened to find a suitable 8mm thick Aluminium
plate of this size in my scrap metal box. Had I found a larger plate, I would maybe have made it
one piece. So I simply added a 4mm plate at the front and riveted all together and gave it
straight milled edges all around. Sometimes a little more work seems easier than going out and
order a plate of the right size and then find its bent or twisted...
Overall size is 297mm wide and 300mm deep, rectangular cutout around the spindle top
bearing plate, round cutout for the motor, M8x35 studs to mount the motor by its flange....
The screw at the back left is to pre-tension the spring for the belt tensioner... I use the already
existing holes in the head casting, plus drill two more at the front.... the slot milled into the bore
for the motor is to tighten the lower pulley grubscrew... the two side plates are 110x50x12
Aluminium, as rear plate I use a steel plate 1.5mm from the scrap box...
...motor mounted...
... motor pulley mounted.... the large pulley just lays there loose to see how it all will come
together.... I use a 5M690 belt (690mm long), so the calculated distance between motor center
line and spindle center line is 205mm – and in practice too, because as it turns out at final
assembly, all holes are drilled spot-on.
The Belt Tensioner
That is the next job in line. I decided to use the existing hole and ball bearings of the existing
first gear shaft (input to the gearbox) as the pivot for my new belt tensioner. A new longer shaft
needs to be made though.
The idler roller is made of stainless steel with two 6000Z ball bearings pressed into (and a
spacer in between). Mild Steel would certainly do as well, but I had a chunk of S.S. The right
size in the scrap box, why not use it....
The two plates were milled out and drilled together from anodised 4mm Aluminium plate (from
the scrap box, where else). At the top one can see the 8x8mm steel keyway, the excenter lever
from the handle will push onto this via a small ball bearing to release or apply belt tension.
Two grubscrews will attach the whole tensioner unit to the pivot shaft, whilst providing precise
high adjustment during assembly
Detail view of the idler wheel, it is stainless steel, 50mm diameter at the top and bottom flanges,
and overall 21mm thick.
The hour-glass shaped steel spacer at the rear, that is where the tensioner spring will attach
Sandwiched & assembled, using medium strength Loctite on all screws
Why so many pics for a simple tensioner you may ask? If not well finished and perfectly parallel
to the motor and spndle axes, it can cause nasty problems like the belt flipping over (inside out),
it can cause premature belt failure, and it can be the source of vibrations if it begins “dancing” in
an oscillating motion. A perfect tensioner is the key to a well functioning Polybelt drive. By the
way, the original Gates 5M690 Polybelts can be bought on eBay from a very nice fellow called
1petspalace for $US13.99 each (his main business is pets, but he obviously has a 920 lathe..)
and that price is half what most bearing dealers would like to charge.
Removing the Old Components
Notice that so far I still did all the milling in the X3 with its stock motor and gear drive – I do not
have a second mill available, so all parts needing milling have to finished before I can convert
the X3 head to the new design. But now it is time to clean out all the obsolote components.
These are the original X3 drive components from the X3 mill head that I remove and discard...
......leaving the head emty.....
......like this....
....and these items too can all now be removed and discarded or sold... I put the lot on eBay
and got AU310 for it, or about 2x what I would have expected.
We are now ready for a completely new start, continuation in Part 2 of this document