Building a CNC router

[steveald]

Hi Jeff,

Thanks for the reply.

Are you referring to 250 - 500 line encoders? The encoders on the motors I'm looking at are 500 line, quadrature 2000 count. Doesn't that mean that 35kHz pulse rate will move the servos max 1050rpm? (35,000 steps/sec * 60 sec)/2000 count = 1050

My mechanics work out to 2.286 revolutions of the motor per inch of travel, so 1050rpm will give me a max rapid of 459 ipm, which is what I'm looking for. Most of my work would be done with the motors spinning at 200rpm or so, and this is where I will need to have torque, am I missing something in my calcs, and are the speed/torque curves for servos basically flat from lets say 50rpm up to max rpm?

Sorry if I'm flogging a dead horse here, but I'm really new to servos and want to get this figured out before sinking another wheelbarrow of cash into it I really do appreciate the help.

Thanks,

Steve

[HomeCNC]

You just answered your own question from before. You will not be able to get 1750 RPM. So you don't need to have a better power supply than your 77 VDC.

You must have a better PC than I have. I can't get 35kHz rate. I think I can get 25kHz.

You will have all the torque of the servo motors if you have enough amps in the pool of power we talked about.

You are fine with your thinking. Build away

[barbwirebi]

Jeff

Thanks again for the power supply info.
As you stated in another post 36vdc @ 20 amps will be fine
for four 360 oz/in servos.

Your 4th axis looks great (what do you draw in?)
Are you building the bearing block/ holder & shaft to the cog gear?
Whats your planned ratio?

Thanks
Bill

[steveald]

Thanks Jeff,

I guess what I was trying to figure out was if the lower voltage would affect available torque, and apparently it won't, so I feel a lot more confident about purchasing some of the parts. The PC I use is just a P3 - 866, but I have a Flashcut signal generator that takes the output from the Flashcut software and uses its own hardware to generate the step pulse stream. I really like it, but I think I'll try Mach2 on my new machine before investing in another Flashcut system.

Thanks again,

Steve

[HomeCNC]

Your 4th axis looks great (what do you draw in?)

I use SolidWorks 2004

Are you building the bearing block/ holder & shaft to the cog gear?

If you look way back in this post to the very first one. You will see my CAD drawing of this router. Look at the spindle. At one time I was wanting to use a spindle and a DC motor so I could hold my R8 tooling. Well the spindle did not work out, so I'm using the bearing body for the 4th axis. I just need to make a new shaft that has the metric thread to accept my 9X20 chucks. The cog gear will bolt to the end of the shaft and squeeze the tapered bearings to hold everything together.

Whats your planned ratio?

It is 4:1 ratio.

[daytrader]

Nice machine Jeff. Did you scratch build your table top, or did you find a source for T-slotted aluminum of that size? If so, what was your source?

What repeatability are you getting? How are you measuring it? Is there any racking of your gantry, especially when the router is not in its center?

I have read on McMaster's ball screw page that using ball screws requires a brake, in order to hold a position. Have you noticed any problems holding position?

Thanks.

[HomeCNC]

Nice machine Jeff. Did you scratch build your table top

I did make the T slot table top from scratch. It was just aluminum bar stock that I milled a rabbit (woodworking term) on each edge. When two are bolted next to each other it forms a T slot.

What repeatability are you getting? How are you measuring it? Is there any racking of your gantry, especially when the router is not in its center?

I am getting repeatability as good as the rolled ball screw stock will give me. The spec is .004 per foot. That is much more than I need for cutting wood projects. There is no racking that I have noticed. I am glad that I did not make the gantry any higher than I did. I can tell on a deeper cut while doing the roughing that the router is flexing in the X direction just a small bit because the surface floor will show small height changes when passing back and forth.

I have read on McMaster's ball screw page that using ball screws requires a brake, in order to hold a position. Have you noticed any problems holding position?

I think what they are talking about is the Z axis might have a problem falling when the power is turned off of the servos or steppers. This can be a problem because a ball screw turns very easy and tends to have smaller TPI. I don't have this problem because of my 2:1 ratio. I have just enough mechanical advantage to hold my Z axis when the power is turned off from the servos.

[xairflyer]

A RABBIT ???? is that another US term for a Rebate !!

[Hobbiest]

Xairflyer,
A "rabbit" is a cut, usually made in wood joinery, that is like an upside down V. The router bit display at your hardware store would have them, no doubt. I think another term is dovetail, but not positive. Edit: come to think of it, I believe that a rabbit is simply a dado cut into the side of the wood, with a tenon running the full length of the piece to join it to. Look for a dovetail bit.

[ballendo]

Originally posted by xairflyer A RABBIT ???? is that another US term for a Rebate !!

Yes. Typically spelled Rabbet...

Hope this helps,

Ballendo

You say Cramps, we say clamps (and for us a rebate means money back!)

[ballendo]

Originally posted by Hobbiest Xairflyer, A "rabbit" is a cut, usually made in wood joinery, that is like an upside down V. The router bit display at your hardware store would have them, no doubt. I think another term is dovetail, but not positive. Edit: come to think of it, I believe that a rabbit is simply a dado cut into the side of the wood, with a tenon running the full length of the piece to join it to. Look for a dovetail bit.

Hello,

A rabbet/rebate is usually 90 degrees, and is at the edge of the piece (has a bottom, and one "wall"). If it has two "walls", it's called a DADO, or PLough, or plow(latter two are older terms)

FWIW, you CAN have a rabbet with an angled "wall", made with a dovetail bit. This is used in cheap(or small) drawer construction sometimes. NOT to be confused with sliding dovetails, which have two "walls", OR with dovetails of the "pin and tails" variety...

Ballendo

[xairflyer]

Like to see my rebate (rabbet) wood plane cut a V !!!!

A rebate for us is also money back, Cramps are something you get in your stomach after too much curry !!!!!