On Thursday, 24 January 2013 21:35:40 UTC, gareth wrote:
You will have to set the hob over at its lead angle and feed at the rate of 1
pitch of the hob/rack per turn of the hob, however the resulting rack will have
a scalloped profile when viewed from the end as you have no feed across the rack
You certainly have to do that. However, to get a full cut across the
rack you would need to do multiple passes, stepping a few thou across
the width of the rack with each pass.
An alternative method (although not using a hob) is to strap the rack
blank to the outside of a (large) cylinder & screw cut it...if the
cylinder diameter is large relative to the tooth pitch the helix
angle is so small that it doesn't matter. This is how the racks for
the Taig lathe are made...they have a cylindrical fixture that can
hold several racks (8? 12?) & they are all screw cit at the same time.
Nice to finally shake your hand at the Midlands recently.
Yes, I'd considered that and one would need to have an accurate
backwards movement if hobbing electronically.
Talking of which, does your Division Master work with an incoming
pulse train for gear hobbing synchronisation?
(Not for me, I'll generate my own from the SSI TTL left over from my
computer project (own instruction set) from 40 years ago)
(Rack is needed for a Ham Radio RX, for a "Catacomb" style bandswitching)
On Sat, 26 Jan 2013 10:04:17 -0000, "gareth"
Indeed - good to meet you after all this time.
You can drive the DivisionMaster in "slave mode" from an external step
signal - but of course, at that point, there is no advantage in using
the DM vs using a separate stepper drive, unless the DM is the only
spare stepper driver that hou have to hand. You can also signal to the
DM to tell it to step to its next division, but I'm not sure that
helps much in this case.
Had you ever considered driving the hob from a stepper as well (Possibly
with fly wheel
attached to reduce the jitter); in which case, sybchronising the two
steppers (hob, workpiece)
would be very easy by dividing down appropriately from a common master
Certainly considered it - and yes, you may well need to use a flywheel
of some kind, but more importantly, you would probably need to gear
the motor down a fair bit to get adequate torque to drive the hob. (Or
an erfing great stepper motor...). I bought a couple of toothed belt
pulleys a while back with the intent of experimenting along those
lines, but nothing concrete so far.
Dividing down from a common clock to 2 stepper drives is potentially
an interesting solution - there is a bit of mathematical juggling to
do to ensure that you handle the rounding errors properly, but that
isn't fundamentally any more difficult than the maths that I already
do in the DM to handle rounding errors. The alternative approach is to
attach an encoder with a suitable fine resolution to the hob spindle &
divide down from that - which is essentially what Brian Thompson did
for his hardware-only hobbing controller (published in MEW a few years
back) - make the pulse-per-rev count on the hobber high enough and you
just divide down to get the pulse traing for rotating the blank.
Problem with that route is that you need a high count encoder, or a
low count encoder plus a suitable gearing up to drive it faster than
the hob spindle.
A few thought experiments ...
But if you start with a high enough master clock, then there is no essential
difference between the two approaches (that and a fine resolution optical
... or a low-count encoder acting as the frequency reference for a Phase
Loop master oscillator.
But even so, in the case of rounding errors, surely they'd even out (or
slightly about a mean) in the case of simple |(ie, non computed) TTL
because unlike the assumed calculation in your DM, where you'd be working
the count to the next division, in the case of hobbing, all oscillators /
optical disks /
division are running continuously?
In respect of driving the hob, Giles Parkes has suggested (since I first
that the hob should be rotated at about 180 RPM, 3R Per Sec, which would be
manageable with a direct drive stepper, but geared down might result in a
the stepper stalling?
Other thought - the stepper is AIUI a synchronous motor that has been
for stepping, so driving with high frequency quadrature sine waves would
... and don't remember if I posted this here; I've no connection with these
and download at your peril; I downloaded the Beta 2.0 version for Windows
PS. As well as Brian Thompson, there was a similar article in last Sept's
... forgot to add; if you have, say, a 40:1 reduction on the workpiece
(which is a mechnical divide by 40), then in the drive to the hob, you
an electronic divide by 40, then, in the first instance, the hob and
will rotate at the same speed.
All that is then needed is a simple divide chain in the workpiece control to
set the number of teeth (which, ridiculously and mysteriously, would
also work for a two tooth gear :-) )
Thinking on my feet!!!! The way to go is now clear; a conventional motor
driving the hob (probably the vertical head on the mill), with an optical
having the same number of slots as the stepper motor has steps per
a phase locked loop synchronised to the optical disk, but at a rate 40 times
higher in frequency (assuming a 40:1 worm in the dividing head), then
divided down by a simple TTL divider chain to give the number of teeth.
I've been throwing around some of these ideas recently, but how often
the solution presents itself, when you explain your nascent ideas to
(A problem shared is a problem halved)
I am currently in Spain for two months but wish I was at home with the
lathe and mill !!!
If I was I would be trying the above problem on my lathe with ELS. The
hob in the chuck and a blank (to end up as a worm wheel) driven by a
stepper replacing the leadscrew stepper. I need to make a worm and wheel
for a dividing head and I think this may be an option. I was considering
using a 5/8th UNC tap as a hob. I know that Tony has an ELS, do you
consider this an option?
On Tue, 29 Jan 2013 08:49:30 +0100, Richard Edwards
It is certainly an option that would be worth trying. What isn't clear
to me is how well it would work in practice, given that ELS only
allows use of a 1 pulse per rev encoder - i.e., it is basically down
to how well ELS will track the spindle speed & adjust the workpiece
rotation accordingly. Which reminds me, I have an updated ELS ROM that
improves the speed tracking algorithm - I haven't yet checked out
whether it works...so many toys, so little time..
Yes I know that the software was changed due to the "long thread
problem". I have yet to update my pic.
I will certainly give hobbing a try when I get home. My gut feeling is
that a 3Nm stepper with a aluminium blank just bored and grub screwed to
the shaft will probably work for a first try. When screw cutting I have
never seen any speed change whilst cutting. As long as the hobbing cuts
are light I feel things should work ok.
On Wed, 30 Jan 2013 08:41:09 +0100, Richard Edwards
I certainly noticed some speed fluctuation when screwcutting on my
Myford - shoulda gone for the bigger motor when I fitted the VFD - but
nothing that couldn't be fixed by sticking it in back-gear.
Exactly so (see my answers to your previous post). And it could just
as easily do a 1-tooth wheel ;-)
That is basically the Brian Thompson solution, except that he ups the
encoder count by the factor of 40 so that you can use the pulse train
out of the encoder direct rather than having to set up a separate PLL.
John Stevenson has that setup - in order to use a (commercial) optical
encoder that he had to hand, he introduced a gear train between the
hob spindle and the encoder to increase the number of encoder puulses
per spindle rev by the appropriate ratio. I guess that is just a
mechanical PLL though ;-)
Not quite sure I follow this, but..
..if the problem is that the encoder only gives one pulse per
revolution, why not use one which gives more? They are very easily and
cheaply available in mice - the one I just took apart has 40 pulses per
rev, but ymmv.
-- Peter Fairbrother.
Another thing I have done is to use the led/transistor pair unit from a
mouse and then make my own wheels - useful if eg you need a wheel with a
40mm hole in the center to go on a lathe spindle, or whatever. Make the
lines and spaces about the same size as the lines in the mouse wheel -
this will mean a few hundred lines on a large wheel.
To make the wheels I use a vector drawing program (inkscape) and then
laser print them onto transparent film. Make VERY sure you use
laser-compatible film, damhikt. I have then had the film laminated for
extra strength, but that is not always necessary.
Mice can be very useful - I buy 3 at a time for 99p each delivered from
China (ebay), one will be put to work and two get used for parts. Three
small microswitches, three optical wheels/sensors, plus a USB cord and
some other bits per mouse - well worth it.
-- Peter F
Depends how the "running continuously" is achieved. If you are working
from a master oscillator that conveniently runs at the (micro)step
frequency needed to drive the stepper for the hob, then it is a simple
integer divide, with no rounding errors, to get to the step rate for
the motor driving the workpiece. If you have direct drive to both
axes, it is all very simple; divide the hob pulse rate by 2 and you
get 2 teeth, 3 gets 3 teeth...etc.
If you decide that you need a reduction drive to either axis, as long
as you choose the same ratio for each, it is still nice and simple.
The problems come when you have different drive ratios on each axis -
say, for example, you decide that you need a 7:1 reduction to get
enough "grunt" out of the hob stepper to stop it stalling, and you
strap your gear blank onto your 40:1 dividing head. Now, for each
pulse on the hobber motor, you need 40/(7*T) pulses on the dividing
head, which is not a whole number of pulses. So you have to make sure
that the rounding errors don't accumulate over time, otherwise all you
achieve is a complicated method for reducing the diameter of a gear
What I had in mind was using gearing to increase the torque at the hob
- i.e., small gear on stepper, large gear on hob spindle, which would
reduce the risk of stalling (but would also need the stepper to run
That is essentially what microstepping aims to achieve - the sequence
of phase current settings for each microstep approximates to a sine
wave. (Not being of the analogue circuitry persuasion, I tend to think
in terms of pulse trains rather than sine waves ;-) )
No, much simpler, assuming that both steppers are of the same resolution,
be it 200 steps or 48 steps or whatever, if one side has a mechanical
division by gears or by worm and wheel, then in the other side insert
a fixed electronic division of the same ratio.
What may make my whole proposal fall flat on its face is the problem
of steppers stalling at high pulse rates.
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