[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 !!!!!

[daytrader]

Originally posted by HomeCNC
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.

Just because the "accuracy" of the lead screws is stated as .004in/foot does not mean that is what the "precision" or repeatability is. I think many people here do not know the distinction of the two terms.

Accuracy basically means how close the machine gets to the actual amount when it is instructed to move. For instance, if you tell the machine to move 1 inch, and it actually moves .990 inches, the machine's accuracy would be .010 per inch. Now, if you repeated this test a second time, and you got 1.010, you would average the .990 and 1.010 together, and get 1.000. If you did this 10 times and the average of all the numbers was 1.000, then the accuracy of the machine would be perfect for a 1 inch length.

Precision is different than accuracy. It is a measure of a machines ability to repeat to a location or a set distance. For instance, you tell the machine to move 1 inch and it moves 1.250 inches. However, you try this 10 times and each time it moved exactly 1.250 inches, it would have excellent precision.

The best machines will have excellent accuracy and precision. However, precision is the better measure of the quality of the machine. If the machine is precise, but not accurate, you can make adjustments through the electronics to adjust the machine. If the machine is not precise, that means the improvement must be made to the structure of the machine and its design.

Another way to visualize it is with a dart board. If one player aims for the bulls-eye and makes a perfect circle of 5 inches around it with 10 darts, his accuracy would be excellent, because the average of all 10 darts is the exact bulls-eye. However, his precision would be poor, because the darts were 5 inches from the target. If on the other hand, the player aims for the bulls-eye and hits a spot 10 inches from the target, but all 10 darts hit the exact same spot, his precision would be excellent, and his accuracy poor. Finally, if all 10 darts hit the exact bulls-eye, both his precision and accuracy would be excellent.

I think the only way to reliably measure the precision of a machine is using indicators. One thing you can do is mount an indicator in the spindle, and then zero it against a 123 block (or something similar) clamped to the table. Also zero the x, y, z from the controller. Then, move the spindle to several arbitrary places along the table. Finally, return the spindle to 0,0,0 and see if the indicator also reads 0. Repeat this several times, and approach the origin from several directions, to determine how precise the machine actually is.

To measure the accuracy, you will need a precision ground block that you know the length of (a 123 block is fine). Pick up the edge of the block using an indicator mounted in the spindle, then instruct the machine to move the length of the block, pick up the other edge of the block with the indicator, and see if the indicator still zero's out. If it does, that means the machine moved the exact amount. If it does not, the amount it is off is the accuracy of the machine for that length.

One way to measure the racking of the gantry might be to place a dial indicator on each side of the machine's frame, so that the plungers touch each side of the gantry. Zero out the indicators. Then tell the machine to hold its current position and turn on the router. Feed a piece of wood freehand (carefully of course) under the spinning cutter, at a similar rate that the machine moves at. This should simulate the cutting forces the machine normally experiences. Have someone else look at the two indicators to see how much they moved while you were feeding the wood. Repeat the same test with the router in the middle of the gantry, and at different places along its length. I suspect the racking would probably be least when the router is in the center. You can also repeat the tests feeding the wood from different angles and from different sides of the cutter. You can also mount the indicators so that they show the relative movement between the z axis and the gantry.

I realize for woodworking these numbers aren't that important. But as your machine seemed to be one of the more sturdy ones, I was curious as to its actual measured performance.

Thanks

[turmite]

"Another way to visualize it is with a dart board. If one player aims for the bulls-eye and makes a perfect circle of 5 inches around it with 10 darts, his accuracy would be excellent, because the average of all 10 darts is the exact bulls-eye. However, his precision would be poor, because the darts were 5 inches from the target. If on the other hand, the player aims for the bulls-eye and hits a spot 10 inches from the target, but all 10 darts hit the exact same spot, his precision would be excellent, and his accuracy poor. Finally, if all 10 darts hit the exact bulls-eye, both his precision and accuracy would be excellent"

Daytrader I can tell already that you don't shoot competition benchrest rifles cause if you did and you shot those kind of groups it typically is referred to as "******" well being that I don't use course language you will have to fill in the blanks!

Seriously though I may be one of those that thought repeatability meant something else. In my machine which was the first "kit" cnc machine offered for sale in the US repeatability meant being able to make the same part for a 1000 parts and they would all fit. My machine uses rack and pinion which most people seem to frown on but I can drill hole anywhere on the table which is 60"X120" run the machine all day and go back the next day run the same program to drill that hole and it won't even fuzz the edges. On the other hand since this machine is so large and made of aluminum rectangular tubing it flexes and vibrates like crazy even when making shallow cuts.

I am sincere when I ask this. Is that repeatability or accuracy?

There are advantages to working with wood.

Mike

[turmite]

Oh yes, Jeff I am in the middle of rebuilding one of the 3 1/4 hp Porter Cable routers I have and had to take the threaded end off of the shaft to get to the bearing. Well I found it wont be much of a problem to make a new nut with the capacity for one of your neat toolholders for 1/2" bit. I have to make sure I get the angle right then I will send you the cad file if you want it.

Mike

[Mr.Chips]

Daytrader.

Good example...

For thoes of us with movable tables, mounting an indicator on an non CNC machine surface and indicate each side of the gantry then run the machine in the "X" axis directions. This would indicate how much gantry flex there was. Naturally this would vary with speed, depth of cut, tool condition. . .

I would like to see a standard test method written for testing and comparing machines. Whats good for one person may not be so good for another.

Thanks
Hager

[daytrader]

Mike,

If your machine can make many parts that come out the same, then it is a precise or repeatable machine. You can use those two terms interchangeably. Comparing parts is probably a good way to measure precision because it takes into account any flexing of the machine only experienced while cutting. It also does not require a lot of indicators.

Now, if your holes were programed to be 10 inches apart, and you measured the actual holes to be 10 inches apart, your machine would also be accurate.

The reason I brought all this up is because the machine I am going to build will be used for inlay cutting in pool cues. This will require a precision of .001 inch I estimate, so that there will be no visible gaps between the inlays and the surrounding structure.

[turmite]

daytrader I hope you didn't think I was trying to flame you. I really have been calling it repeatability and just wanted to know if I had been calling it the right term.

I don't know if you make cues yet or not but there is another sure way to get good inlays and especially with a cnc. When doing your cut in the wood and the inlay material use a tapered cutter on both but cut trhe inlay from the back side. This is a spin of the way old time gunsmiths did their intricate brass work on the old flint and percussion black powder rifles. Some of those things are pure art. Basically they put a draft on the pieces and the wood.

What kind of controller are you going to use?

Mike

[daytrader]

Turmite,

I didn't think you weren't flaming me, and found your post amusing.

I have been doing cue repair work for about 5 years, but have yet to build one from scratch, mainly because I can't cut the "curved taper" required for the front half of the cue (the shaft) without making a curved taper bar for my lathe. The problem with that is that it would require a different bar for each taper I desired. I figured by building a cnc machine, I could make any taper I desired, plus do the inlay work.

I just got all the linear bearings and rails from ebay and will design the machine around them, and hopefully start building in about one month. I'm going to initially try EMC as the software. If I can't get it to work (I'm not a linux user) I will try Mach 2.

[daytrader]

Oh, and the beveled edge is a great tip. Thanks. The only thing that worries me about it is the possibility of a gap on the bottom side of the inlay.