Re: Re: CNC'ing a Mini Mill

Message: 21
Date: Mon, 17 Nov 2003 23:41:17 -0600
From: Jon Elson <elson@...>
Subject: Re: CNC'ing a Mini Mill

>>>Jon, thanks for the good and direct feedback, much appreciated.


>Am converting a Chinese MiniMill circa 250KG, with round column on Z. Plan is to convert the X (stroke 405mm) and Y (Stroke 200mm) first. End use would be on plastic, wood, al alloy, brass and mild steel. My "max cutting conditions" would be 2 flute HSS end mill 25mm dia at 750mm per min with a 2mm depth of cut, what would be the load generated?
>
>

You are kidding yourself! I think a Bridgeport Series II would be
humming VERY loudly and maybe dancing across the shop floor at this load. My calculator indicates this will
absorb 4 Hp in mildest steel, and almost a full HP in aluminum. You don't indicate your RPM for this, so I can't calculate chip load or linear force generated, but I'd guess
it would approach half a ton. I can tell you with absolute certainty that I've never gotten within 10% of this cutting rate (2.4 In^3 / min) on my Bridgeport.

>>>>What would be a more realistic rate, based on your experience? Most of my work will be in wood or al alloy, with the occasional bit of mild steel for brackets, and would need to keep the feed rate up as much as possible for the wood to stop the burning. What would be the typical cutting forces (thrust) that would be seen on such a mini mill?

>Have some new bipolar stepper motors Sanyo Denki 103 H8223-5011, Nema 34, 200 steps, 7.2 V, 2 A/ph, 3.6 ohms/ph, inductance 32.5mH/ph, holding torque 6.85 Nm, which hopefully i can use. With the lead screws being T24-2.5mm and the motor hooked direct (1:1) the theoritical resolution will be 0.0125mm / step.
>
>

You'll never handle your desired cutting forces with these motors, but
then, your machine can't handle that, anyway.

>My targets are cutting at 750mm/min which when hooked up directly (1:1) would be 1000pps with a torque value of 4.6 Nm from data sheet and for rapids 2250mm/min equates to 3000pps and torque value of 1.4Nm.
>
>Am a little worried that the high inductance will slow the reaction of the control process.
>

Reaction of the control process? There is no "control process" used with most stepper motors, as they are used "open loop", without position feedback. 7.2 V,
2 A/phase motors do sound like they have high inductance, but at only 1000 steps/sec, that shouldn't be a big problem.

>>>Agree "control process" is maybe a incorrect terminology, but does high inductance equal slower

reaction of the motor to change in speed or rotation of the motor? As the rapids are usually linear, then can i assume that even at 3000steps/sec i ignore this issue?


TIA

John