Ok, I decided that I wanted C5 or C7 accuracy class, I have a servo motor that can produce 4 Newton meters of torque at 1350 RPM. I am aiming for 4000 Newtons of machine force available on the axis. The servo drives I have purchased can measure rotation to 1/2000 of a revolution. Which ballscrew will give me the desired performance for my RF45?

 

Again, trade offs;

1: Spend more, get more ballscrew accuracy. Spend less on ballscrews, have money left over for better bearings.

2: Gear down the motor to ballscrew shaft with timing belts and pulleys to gain more force, but lose speed and accuracy due to belt slop, or, go direct drive and purchase a good quality shaft coupling.

 

Buying from the internet Vs buying from a dealer:

I would encourage anyone buying something as complex as ballscrews to contact a local dealer, talk to them face to face and see what they have to offer. I got really good advice which saved me lots of time and money. There were several ballscrew manufacturers I could contact via the web. I ended up going with Isel automationís ballscrews because there was a local distributor who were really helpful, and prices were pretty good. I could have saved maybe $50 on my $500 purchase if I went through the internet stores, but I would have missed out on the information, good practical advice and after sales support.

 

Isel ballscrews were available in the following pitches; 2.5mm, 4mm,5mm, 10mm and 20mm. Shaft diameters were available in 16mm and 25mm.

 

I looked at load ratings for various ball nut types, and concluded that for my needs the 5mm pitch 16mm diameter ballscrews were going to be a very good compromise after having done the torque / machine force calculations, and top speed calculations. The ballnuts had a dynamic load rating of 4600 Newtons, and a static load rating of 7200 Newtons. This was a little less than the 5000 Newtons of linear force that (theoretically) the screw and motor can produce. I was informed by the manufacturer that approximately 90% of the torque is actually converted into linear force after friction losses etc, so I guess that after all this is a pretty good match.

 

 

Buying ballscrews.

Grae-Tech†

Ball nuts

Ball nut clamping blocks.

I decided that I would make my own end supports because that gave me the flexibility to make dimensions that were going to be convenient for me, and I could place bolt holes where I wanted rather than having to make adaptor plates to fit someone elseís design. After disassembling and measuring the mill, I designed bearing blocks, motor mounting brackets and had decided that I needed to find out about shaft couplings to connect the servo motors to the ballscrew shaft.

 

I ordered two ballscrew shafts, Rolled C7 grade, 16mm shaft width 5mm pitch. One for the X axis and one for the Y axis.

 

The mill with the table and covers removed.

The saddle. Showing the Y axis brass nut to be replaced with a ballnut.

X axis brass leadscrew nut.

Measuring clearances around the saddle.