Hi all. I'm finishing up my Bridgeport look alike knee mill conversion. I'm wondering what the ratio should be on the Z drive.
One revolution of the Z axis will move the tool 3+ inches. I want the mill to be able to do metal, but I'm targeting wood
engravings as the main product. It seems that the higher the resolution the better the detail would be. On another forum one gentleman
suggested to use a 2:1 on the Z and 1:1 on the X & Y.
The motor is a 740 oz in Rex Step w/ Gecko 201 drive. Any thoughts or ideas would be appreciated to help a newbie.
UPDATE - I moved the machine under power today. Very exciting! Now I really have to learn lots!
Thanks in advance for any advice!
olf20

Hi olf20

I'm sure there are folks here that know a lot more than me but as it's quiet I'll pitch in. There are basically two things to consider when it comes to the gearing. One is resolution and the other is mechanical advantage.

with 1:1, your 3inch lead, 200 step/rev motors and the G201's 10 microstep capability, the max resolution you'll get is 3/2000 or 1 and a half thou. More than fine enough for wood! and up to you for metal. However bear in mind that this is theoretical. The quality of your mechanics with regard to screw pitch accuaracy and backlash etc. will diminish this from the theoretical. Also the microsteps are not guaranteed to be exactly a tenth of a step, all that is for sure is that every ten microsteps will equate to 1 full step. so your true accuracy will be again somewhat less than the theoretical.

with ref to mechanical advantage I don't know the weight and drag of your Z axis but I'd have thought your motors should be more than OK at 1:1 as long as your power supply is giving them plenty of VA to work with. I'm running 260oz/in models at a lead of 20mm on XY and 5mm on Z and nothing will stop them - they snap G-clamps like toffee.

Thanks Stirling for the reply! I forgot to mention that I'm moving the spindle not the knee.
Also my power supply is 72 volts at 25 amps. The x & y motors are 1250 oz in.
I guess I will start with the 2:1 ratio. I can always change it later if needed.
Do you do any wood engravings, or just metal?
Thanks again, I appreciate you help.
olf20

you've got me intrigued now regards your power supply. I looked up your motors and arn't they 2 and 2.5 Volts? if so that would mean that you're way over the max 20 times rated voltage recommended by Gecko or have I got your motors wrong?

I cut wood exclusively - mainly electric guitars but trying to get a small business running, offering anything I can handle on the machine.

I got my guide lines from Bob Cambell.
Here is the link to the article that I followed.
http://www.campbelldesigns.com/files/power-supply-part-1.pdf
Also several other web sites and individuals told me the same thing.
I was told also that the higher the voltage, up to 80V the faster the
motors would move, but at the risk of getting warmer. The spec on my x & y motors are;
2.7V * 25 = 67.5V
I can reduce the voltage as I wound the transformer myself.
I sure don't want to fry anything.
Again thanks for the reply. All feed back is welcome.
What router motor do you use?
olf20

(with respect of course) I’d either disagree with the max 80V bit or suggest that maybe you’ve misunderstood. The 80V limit is recommended by Gecko as the max for their drives. They and Bob Campbell along with many others recommend that your motor voltage should be somewhere between 4 and 20 to 25 times the motors rated voltage. Your motors (at least your X and Y) you say are 2.7V so your power supply of 72V is 72/2.7 or 26.7 times, i.e. in excess of their recommendations. Maybe not by much but certainly on the high side. Also if I’m right in thinking your Z axis is a 2V motor then you’re at 36 times, well outside their recommendations.

Regarding "the higher the voltage, the faster the motors will go" - well... yes... in a way. What I think they meant is that the faster stepper motors go the less torque they produce. In order to keep the torque at a usable level at the highest speed you need, you usually need to up the voltage.

Sure, they're going to get slightly warm and I use that as my key to setting my voltages. My rule of thumb would be firstly set the current limiting resistors as advised by Gecko, then up the voltage until the motors run just warm over 15 minutes or so under load. If there's enough torque for the job at the max speed needed and V is below the 20 to 25 times the rated voltage then pay dirt. Otherwise the motors aren’t up to the job. I've read several times where it's said that they should be run hot, as in really hot. This I don't understand. To me, a hot motor is an inefficient and underpowered motor and wont last very long.

Oh yeah, my spindles (I have two rigs) are little Kress electronic 850s. Small I'm sure compared to some of the beasts out there but have proved big enough (just) for what I do at the moment.

Cheers

Stirling

Thanks Stirling for the comments. This is how we learn and transfer knowledge. I agree that the voltage for the two 1200 oz in
( I mis posted earlier they are 1200 oz in) x & y axis is a little high. Like I said I can adjust this and will.
The voltage on the 740 oz in is 2.0V.
I do understand that the 80V max is the spec for the Gecko drive and not the guide line for the motors.
I will run a test of my x & y axis after I adjust the voltage and see how warm they get.
Like I said I'm a newbie and have lots to learn!!
I just recieved this from homeshopcnc
Bob,

Run both of them between 60 and 68 volts from the same supply. The 740 will run warmer than the 1200 but they both run
well in that range.

Regards,

Rick LaLonde
www.homeshopcnc.com

Thanks olf20

No problem - hope all goes well - like you say - swapping info is how we all get by. I got my motors from homeshopcnc too and would recommend them, Rick LaLonde was the guy, great to deal with, friendly, helpfull and fast. In fact I'm over here in the UK and without wanting to knock Old Blighty too much, I'm very jealous of you guys in the US when it comes to sourcing all the kit needed to build these things - you've got so much more choice. I got the motors from homeshop because even after import duty, UK taxes and shipping they were cheaper than buying over here. In fact Rick apologised once when he took 24 hours to reply to an email - Ha! over here I can wait three weeks for somone to respond - if they bother at all. I also import the Geckos, either from homeshop or from Gecko direct depending on whether I'm getting motors as well. I wanted to get the screws from homeshop but of course over here we work in metric so I got them locally - and that was like pulling teeth! Anyway - whinge over - sorry.

Speaking of spindles, what do you use? - again, you seem to have way more coice.

cheers Stirling.

You are right, Rick has been great to deal with!
I'm not sure which router motor I'm going to use yet. On my big Atlas Knee Mill I'm going to mount a
a router motor with a R8 collet and some sort of bracket so I can easily and accruately
change from woodworking and metal. No need to run the 3hp 3 phase motor and home made phase
converter on wood.
If I find something cheap I will start with that and work my way thru all this.
As you already know there is sooooooooo much to learn.
As soon as my Z drive and motor gets here I'm going to clean up my electrical box / transformer.
Just in the testing stages, but things are really going well at this point.
Do you use any particular software? I've been looking at Vcarve which seems very impressive at
first look.
Thanks again!!!
olf20
 

I use Rhino3d, RhinoCAM and RhinoART cos they were doing a reasonable deal last year. Havn't used VCarve but as you say it looks pretty good. I must get into this sign carving and such cos I need to make some money from this lark. It's getting V expensive!