Re: [CAD_CAM_EDM_DRO] Machine mounted Tachometer with Surface Meter Per minuteThis

I would go for optical off of a white painted half of the rotor shaft tail end at the tail bearing. I don't think Hall is going to be easy to make work with the stray electrical noise at router speeds and I think the proximity required off of the nut would make bit changes challenging. Hall is great for slow speeds, because it does not require any speed at all, not like an inductive pickup where the parts have to be moving.

I'm not sure response speed rules out hall because I know automotive applicationa like Ford's EDIS, use a 40 tooth wheel and read fine to 6500RPM, so thats 260K CPM which would easily allow for a divide by 8 at 30K RPM, divide by 6 should allow plenty of overhead. But a chopper wheel on the shaft above the nut IMO would make it a heck of a lot easier to do bit changes- could also have a negative effect on cooling airflow though.

AFAIK taking an optical from the tail end of the shaft where you can simply drill the housing to expose the shaft and paint half while and point a laser tach at it with a $25 BOM sounds like it's worth a try- if it doesn't work all you have done is drilled a superflouous hole in the router case, and if it does work you have saved yourself a lot of hassle with bit changes and other potential issues.

My. 02

-Will





Danny Miller
To: CAD_CAM_EDM_DRO@yahoogroups.com
Sent: Saturday, September 11, 2010 5:01 AM
Subject: Re: [CAD_CAM_EDM_DRO] Machine mounted Tachometer with Surface Meter Per minuteThis



Oh yeah there's also gonna be a limit to a Hall/gear tooth sensor
response time. A decade counter to lower the pulse count won't fix
this. If it's too fast, the sensor won't have time to fall after one
tooth leaves before the next tooth rises and it can't find a pulse out
of that. 30K RPM * 6 teeth in a collet= 180KHz. Pretty fast to ask
for. Especially since the collet "teeth" aren't very deep so its signal
level might not drop that far between peaks to begin with even when
going slow.

Danny

On 9/11/2010 2:55 AM, Michael Fagan wrote:
> In order for this to work, you need a gear tooth sensor, which is a hall
> effect sensor with a built in biasing magnet. It will read High when there
> is ferrous metal 0.5-2mm from the sensor
>
> On Sat, Sep 11, 2010 at 03:05, Tony Smith<ajsmith@...> wrote:
>
>>
>>>> Most routers have a nut at the collet. If using non-ferrous metals
>>> (or
>>>> wood or plastic), a hall effect sensor can pick up the points or cuts
>>>> (for ER type collets.)
>>> Can you elaborate a little on how this would work since Hall effect
>>> sensors
>>> are only sensitive to change in magnetic field.
>>> Gaston
>> Google 'gear tooth sensor'. The datasheet for the hall effect sensors
>> usually have this as well.
>>
>> The sensor can pick up the difference between air& steel, if you're got a
>> gear with big enough teeth, it can pick when it's moving. Not sure it'd
>> work with the nut though.
>>
>> Tony
>>
>>
>>
>
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