Knee mill conversion

[evo4wrx]

Need some advice on my knee mill conversion.

To fix up the poor repeatabilty issue by base flex im going to mount the lead screw and ball nut assembly onto the column.

However to take the weight of the table I need to add pneumatic setup to it.

Ive purchased a Grex100 BOB, I was just wondering what parts do I need to control the cylinders up and down movement.

Also what kind of cylinder do I need....Magnetic, Double action??? What electronics do I need...something Like a solenoid im gathering and what kind of solenoid. Ive read into some of the options like NC and NO....Any advice anyone can give would be fantastic.

[evo4wrx]

Just a thought???? Is it as simple as putting a cylinder in place and a pressure regulator to counter act the weight of the table.??

[chich2]

evo4wrx,
Yes sure is! Just make sure your regulator is a vented type. This means it will maintain a constant line pressure. As the cylinder is compressed by the lead screw going down for instance, the air pressure will rise. A vented regulator will exhaust this excess and maintain constant cylinder pressure and force therefore giving you a weightless knee. The servo and lead screw will have a much easier time positioning the knee.
You could also run a counterweight like a lot of cnc mill's also do.

Hope this help's
Chich

[Al_The_Man]

My Excello's have an air cylinder either side of the knee, they also use a air balancing constant pressure system, as mentioned in the previous post.
This is motorised for initial positioning only by using a ball screw jack, the driving motor has a DC brake fitted.
The cylinders are single acting and no solenoid is required with this setup.
Al.

[evo4wrx]

Okay just making sure I have this clear....
Air from compressor--> control valve --> controller---> some kind of pressure transducer--->(load, being cylinder). Sorry my back ground is electronics.....So gotta have magical smoke or it just doesnt feel right.

[Al_The_Man]

Line air in > Manual Pressure reducer > Automatic Pneumatic Constant pressure Unit> Cylinders.
The standard manual pressure reducer is used to set the required balance pressure, this is set so the raise device has the same load in both up and down direction.
Al.

[evo4wrx]

Wouldnt happen to have an example part number or link to the products....
Just been checking out the cost of the electronics path and its about $1800 which is about half the price that I paid for the mill. ....Kind of seems a waste. So back to the manual method.
Just picked up a Norgren Double action , Magnetic , 100mm bore , 350mm stroke.
Im fairly sure I dont need the double action. So I just leave one of the ports open????

Whats the point of the magnetic piston????

[Al_The_Man]

I will see if I can round up the part numbers, the magnetic piston is to use external magnet sensor to indicate the piston position.
Al.

[evo4wrx]

Cheers thanks for your help.
Looking around for Pulleys and belts now....
Im thinking 4:1 ratio for X,y,z...im not after super fast speeds and this mill is a bit heavier. My motors are 4600 Rpm Constant torque of 1.2Nm 90Vdc.

The gecko 80vdc will bring that back to about 4200 RPM. The screws have 4 turns to 1/4 inch. So that should give me plenty of resolution with the 1000 CPR encoders. I did actualy work that out somewhere exact a long time ago. Ims ure its more accurate then the chinese machine could ever hope to repeat.

The question is how big do I make the pulleys and what pitch. Try as I may I cant find load ratings anywhere for the different kind of timing belts.

Ill begin the build pictures Tuesday next week when everything turns up.

Anyone have any supplier suggestions for pulleys....already seen PIES and RS Australia.

[evo4wrx]

For the next person trying to find power ratings of belts YAY.
http://docs-asia.electrocomponents.c...6b803c5d9d.pdf

[Al_The_Man]

For charts and sizing see the Sdp-si web site.
Also for availability, Misumi of Japan are world wide and have an excellent selection of hardware, the free catalogue is worth it alone.
Al.

[chich2]

evo4wrz,
On my mill I used XL series belts and pulleys 4:1 from Associated World Bearings (AWB) http://www.awbbearings.com.au/ They are pretty much straight out of the Gates catalogue and overnight delivery.
The vented regulator I used is from SMC and the part No is IR2020-02 it has 1/4" port's and works very well. My cylinder is a 4" double acting tie rod aluminium cylinder. I leave the top port open to atmosphere and connect the precision regulator to the bottom port with 1/2" copper pipe. You will not need a magnetic switch for the cyilinder in this application as all the cylinder is doing is constantly taking weight of the knee so the servo and screw can moove the knee without having the weight of the knee to contend with. Think of it a bit like the power steering in your car - You are still making the wheel turn, the system is just assisting you.

Chich