Choice of spindle for drilling on CNC.
Hi,
I have a CNC milling machine with a three-phase chinese 2kW / 24000rpm spindle controlled by a VFD.
It is fine for milling wood/ali/steel .
I don't do much work on steel because the chassis lacks sufficient ridigity but it will cut mild steel plate with light cuts.
My problem is with drilling. With small drills where I can run around 4500 rpm it's fine on ali or steel. However if I try 8mm drills it always ends up snagging and stalling the spindle. Basically, despite the 2kW power rating, it does not have enough torque at low speeds.
What is the problem here?
Is it , one spindle cannot do all jobs and I need a second spindle for drilling ?
Did I buy the wrong kind of spindle or driver?
I want the machine to be as polyvalent as possible and that obviously means compromise.
I would like to keep high rpm for wood routing but mostly I work ali or steel. I'm beefing up the frame to increase rigidity.
If I need a larger hole I usually drill to 5 or 6 mm then increase with an endmill but Ideally I'd like to be able to drill at 8mm or 10mm and I clearly need more torque than I have with the current set up.
What do I need to improve this situation?
Thanks for any tips or explanations.
Re: Choice of spindle for drilling on CNC.
Hi Reg - high speed spindles as you have found have no torque at low speeds. Drilling requires low speed and high torque. My solution was to make an extra mount for a normal hand drill for when I want to do lots of drilling. Peter
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Re: Choice of spindle for drilling on CNC.
Hi,
yep, the problem is that highspeed/low torque spindles are a very poor choice for steel or indeed any material or process that requires high torque.
I have two spindles, a little German made 800W 24000rpm asynchronous spindle.....and this I use day in and day out and have done for ten years....simply brilliant. I might have
thought it expensive when I bought it but it has repaid me many, many times over.
It hopeless for steel though. To that end I built another spindle based on a second hand 1.8kW (6.1Nm cont, 18Nm overload, 3500rpm) Allen Bradley servo mated to an ER25 toolholder
in angular contact bearings. It has proven very useful for steel, stainless etc...even if it sees only a small fraction of the use that my little highspeed spindle gets. I've been using it for about
four-five years. Here comes the tragic part, I had it sitting in the bottom of my machine when not in use but it was getting doused in coolant.....and like a f******g idiot I let the coolant
get into and wreck the optical encoder....extremely disappointed with myself. I have found another....but just have not been able to save up the funds to get it....about $600NZD.
Lesson: low speed/high torque spindles are like gold......treat them with respect.
Craig
Re: Choice of spindle for drilling on CNC.
Quote:
Originally Posted by
joeavaerage
Hi,
yep, the problem is that highspeed/low torque spindles are a very poor choice for steel or indeed any material or process that requires high torque.
I have two spindles, a little German made 800W 24000rpm asynchronous spindle.....and this I use day in and day out and have done for ten years....simply brilliant. I might have
thought it expensive when I bought it but it has repaid me many, many times over.
It hopeless for steel though. To that end I built another spindle based on a second hand 1.8kW (6.1Nm cont, 18Nm overload, 3500rpm) Allen Bradley servo mated to an ER25 toolholder
in angular contact bearings. It has proven very useful for steel, stainless etc...even if it sees only a small fraction of the use that my little highspeed spindle gets. I've been using it for about
four-five years. Here comes the tragic part, I had it sitting in the bottom of my machine when not in use but it was getting doused in coolant.....and like a f******g idiot I let the coolant
get into and wreck the optical encoder....extremely disappointed with myself. I have found another....but just have not been able to save up the funds to get it....about $600NZD.
Lesson: low speed/high torque spindles are like gold......treat them with respect.
Craig
Thanks. So what do I need to look for in terms of spec. What kind of motor construction and motor driver ? I could lash up a hand drill as Peter suggested but they usually have ridiculous amounts of radial play.
Is the servo drive the key to low torque or the motor construction?
BTW , sorry to hear about your loss. Can't you repair/replace the encoder for less than 600NZ ?
Re: Choice of spindle for drilling on CNC.
Re: Choice of spindle for drilling on CNC.
Hi,
Quote:
Can't you repair/replace the encoder for less than 600NZ ?
I can't find any new encoders for sale. These are really high quality units and were expensive back in the late 90's and early 2000's when they were made. Even if I could get a new one
I'd be surprised if I could get it for under $600NZD. I can buy a new old stock servo complete for between $2800USD and $3100USD, so to get a second hand servo complete for $200GBP, plus freight and
tax to New Zealand is about $600NZD, which about fair.
When I spend a fair sum of money like this I consider the risk that what I have purchased may not work or work well. I am taking a bit of a gamble on an old servo, as nice as they are, they are still fifteen years old.
I'm doing so to take advantage of the fact that I already have a working drive. If there were an any larger question about the risk, then I'd rather buy a new 2kW Delta servo. I already use Delta servos and I am
very confident that a new 2kW servo would be reliable and good performer for me.
https://www.fasttobuy.com/delta-2kw-...nc_p33936.html
The failure of my original servo happened about eight weeks ago. Since then I have renewed both my Fusion 360 (Basic) subscription for $786NZD and then just a few weeks later the Fusion Machining Extensions for another $2312NZD,
not counting some smaller expenses with my ongoing construction of a trunnion fifth axis. So, it's not like I have not saved up and paid for things, its just that I cannot stretch my budget any further without leaving important
bills unpaid. I just have to be patient.
I do not use the spindle a lot, but I sure do miss not having it.
As to your questions it all really rather depends on budget. There are spindles out there that can not only rev to 12000 or 15000 or 18000 but also have good torque down low for drilling......but they cost a bloody
fortune. Thats what you buy when you buy a good industrial machine. I can't afford one and my machine is not big enough to handle a 300kg spindle even if I could afford one!
A budget friendly option is something like a belt driven spindle matched with an induction motor or an AC servo.
https://www.ebay.com/itm/26572765950...3ABFBMgqOS07hi
A regular three phase induction motor and a VFD would make a nice combination. AC servos are even better again. They are very torque dense and are smaller than an induction motor of the same power.
They are also position capable and depending on your controller can be used for rigid tapping. I could do that with my servo driven spindle...I didn't use it a lot, but I miss not having it.
An AC servo and matching drive is quite costly. The example above is $895USD (plus shipping for servo/drive/cables) at 2kW. I think you'd get a 2kW induction motor and VFD for about half that.
The servo is smaller and lighter and with a little imagination can be made to do rigid tapping.....so it comes down to budget.
Craig
