Acorn Encoder Question - Max bandwidth? (Resolved)

[gMike]

Hi All,

I have been dabbling in cnc for sometime but only just started getting serious about it recently, I am coming from Mach3 and looking to finalise my centroid purchase. More or less every question I have slowly googled my way to victory but I ran out of steam on this one and need some help.

I am running a sherline mill with a high speed pulley set which gives me a sindle speed of 10,000rpm, can the acorn board handle the bandwidth of the 40,000 pulse per turn encoder going that fast? Id like to achieve
- Rigid tapping
- 10krpm with constant MPM (or SFM)
- I need to mill with small bits less than a 1mm

So in a nut shell, can the acorn board handle 400mhz on the encoder input? Should I go for the 8,000 pulse per turn instead?

Thanks...Mike

[martyscncgarage]

Hi All,

I have been dabbling in cnc for sometime but only just started getting serious about it recently, I am coming from Mach3 and looking to finalise my centroid purchase. More or less every question I have slowly googled my way to victory but I ran out of steam on this one and need some help.

I am running a sherline mill with a high speed pulley set which gives me a sindle speed of 10,000rpm, can the acorn board handle the bandwidth of the 40,000 pulse per turn encoder going that fast? Id like to achieve
- Rigid tapping
- 10krpm with constant MPM (or SFM)
- I need to mill with small bits less than a 1mm

So in a nut shell, can the acorn board handle 400mhz on the encoder input? Should I go for the 8,000 pulse per turn instead?

Thanks...Mike

For a spindle encoder a 1000ppr is good enough. However, you need to look at the encoder itself to see if IT can work at 10K rpm, both mechanically (does't blow itself apart) and electrically. I think you will find those encoders quite expensive..
You won't be rigid tapping at 10K rpm...

[gMike]

haha no 10krpm rigid tapping agreed! I did want to keep the constant SFM while machining at those speeds though.

[Muzzer]

There was a query about this recently and one of the Centroid guys answered. IIRC they actually measure the time between pulses rather than count pulses per time. Might be wrong there but either way, 400MHz sounds a bit "optimistic".

To simply achieve constant SFM you could get away with 1 PPR (single tooth pickup) and I've even seen an example of rigid tapping with 1 PPR but the way they've done it, Acorn seems to require 1000PPR minimum to achieve rigid tapping. This also prevents the use of a gear tooth pickup (my bull gear is something like 40t and would have been a very neat solution).

A common solution seems to be the bootleg Omron clones from China https://www.ebay.co.uk/itm/1x-OMRON-200 ... SwJc9aYr5t. I've used mine up to 7krpm but there's no telling what these are actually good for. Having said that, they are a glass disk on the end of a short, thin shaft, so you'd think they would be able to spin at a decent rate easily enough.

[tblough]

...so you'd think they would be able to spin at a decent rate easily enough.

Mechanically, assuming the encoder bearings can handle that speed, and their lubricant doesn't sling onto the encoder disk, and that they are balanced good enough.

Electrically, it depends if the internal electronics can process the grating interference fast enough.

If it's a cheap encoder, try it. Worst that can happen is it will fail.

[cncsnw]

I did want to keep the constant SFM while machining at those speeds though.

This is a common misconception.

Surface speed (SFM) is a matter of spindle RPM control. Centroid does not use a spindle encoder for RPM control, whether you have one or not.

On a mill (you did say you had a mill, right?) a spindle encoder is used only for rigid tapping, and for display of RPM in the status window.

[gMike]

"Surface speed (SFM) is a matter of spindle RPM control. Centroid does not use a spindle encoder for RPM control, whether you have one or not."

Ah! makes perfect sense now. I must say these forums are really great, a big thankyou to everyone.

Cheers...mike