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Member postings for John Stevenson

Here is a list of all the postings John Stevenson has made in our forums. Click on a thread name to jump to the thread.

Thread: Milling Machine Low Speed Torque
01/08/2009 23:43:00
After fitting well over 300 plus inverters over the years and working closely with IMO, Telemechanique and Yaskawa technical let me explain a few truths about how these work and how the ratings are arrived at.
First how they work, they take AC in, turn it to DC and chop it into pieces, organise it then change it back to AC but 3 phase. Its how the chopped up bits are organised that's the magic bit .
Now the BS part. the all singing all dancing feature is called Vector drive, where this differs from the old type of variable frequency drive is that it monitors the load and when it drops it bungs a bit more down the line to play catch up then drops back as it does.
The next bit they all push is Constant Torque and we need to explain a bit more about these chopped up bits. Technically at 50 Hz [ in the UK ] 240 volt single phase in with spit 240 v out 3 phase but as you drop frequency it drops voltage so at something like 15 Hz it probably putting out 60  to 70 volts AC three phase.
Now they say it will produce constant torque at all speeds, what they NEVER tell you is that torque is based on output voltage so at 50 Hz you get full torque of whatever the motor is rated at.
At 15 Hz you get full torque based on 60 to 70 volts, so a 3.3 actual reduction but in theory Constant Torque .
Most machine tools get torque by gearing, a 2:1 reduction doubling the torque but this cannot happen electronically, only mechanically .
What happens in practice and I can prove this as I have a CNC  X3 here that has a two speed direct belt drive and inverter, no names but well know torque vector all singing all dancing. At low speed 20 Hz in the high range which equates to about 1,000 rpm when the cutter enters the work it slows as the Vector drive part kicks in and speeds the spindle up to compensate for load.
However the feed isn't linked to the vector drive and that slowing down doesn't take into account the feed has remained constant with the result it's chipped, rubbed the cutting edge and we are now playing with half a deck.
It can be over come but it's acase of working on available power and gear ratio's, I have even got the technical side agreeing with my findings.
Whilst a inverter drive is nice and to be honest I wouldn't be without them it's not the answer to a straight swap on something like the small Chinese millers with DC drives which have inherently more torque at low revs anyway .
 The above is a very advanced technically description of how these things work, for a more laid back description you need a Heavy Goods license.
John S.
Thread: spin indexers
30/07/2009 19:28:55
Just to point out that the Spin Indexer as sold by Arceurotrade, is to a unique design that can hold ER32 and 5C collets on the same tool with no modifications.
Given that the 5C series works in fixed sizes and the ER system works over a 1mm or 40 thou range per collet with no gaps this gives you two bites of the cherry depending what collet system you run.
Another plus is the ER series is the only work holding and tool holding system readily available so it has a spin off for the people who share collets between a mill, lathe and attachments.
John S.
Thread: collet thread
24/07/2009 23:17:34

John S.
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