Machine tool question

[enginesrus]

This is more for someone involved with design of machining centers or large boring mills etc.
Any idea what the highest axis load would be in ton's or pound of force that an axis will exert on the spindle when using something like a large drill or spot facing tool, or pushing a large cutter or facemill at a fairly high feed rate?
I know not many folks would do it but in a pinch for a very high accuracy press such a machine would be useful especially on a light interference fit and small item like 1/4 inch dowel pin etc. Then of course there are the rare but sometimes inevitable crashes.

 

[SwinnyGG]

Forces are not that high - maybe a few tons. Most CNC machines are working harder to move the bed during part repositioning than they are during cutting, just because it's ideal to accelerate and decelerate the bed very quickly.

What you're really asking is unclear - are you contemplating using a CNC machine as a press? If so this is not a very good idea. Presses are dirt cheap.

 

[tygerdawg]

Perhaps you need to get a book on machining technology and check the chapter that discusses chip formation theory. Of course I don't have those equations off the top of my head, but I recall that chip formation theory analyzes forces applied to the cutting tool by the mechanical action of shearing chips off of the workpiece. Those forces are transmitted through the machine tool structure.

The real issue is not so much the machining forces, but the vibrations produced by the rapid cyclic forces generated by each tooth of the cutting tool. Those vibes cause cutting defects (chatter) in the surface of the workpiece if not attenuated.

TygerDawg
Blue Technik LLC
Manufacturing Engineering Consulting

http://www.bluetechnik.com

 

[mfgenggear]

answer is no

 

[BrianE22]

Interesting application - for what it is worth, my 20"x40" VMC has no trouble breaking 1/2" end mills with the x or y axis. It does a lot of damage also when the vertical spindle crashes into the part.

 

[enginesrus]

Funny about a book on machining. I have many years of machine shop experience. And as one of the last of the true manual machinists in aerospace industry (the big heavy iron Cincinnati and K&T Mills early 70's), we did back then what CNC does now, there where only a hand full of tape machines when I started, digital read out was a dream then. Trig and sine bars where king. Manual calculation of compound angles, and dividing head gear sets, you know all the real machinist stuff. Computer?
Yes we had them a nice Marchant rotary calculator in the layout area.
Okay back to the loads on recirculating ball lead screws, lets just forget about the cutting forces, even though I ran a cnc machining center that we loaded the z axis to the point of over loading and alarming the machine when doing a 3 or 4 inch spot face had to baby it. Lets talk about the almost instant axis movements from 0 to 400 or 500 inches per minute rapid traverse moves. I'd just like to know the approx tons of force applied to that screw and nut, while doing such movements with maybe a wild guess of 5000 lbs(could be more) of structure being moved, I know the axis servos are ramped but still it appears to be quite rapid, so the ramping is milli seconds.
What maybe on the order of 20k pounds constantly all day long ? Or more? And what I am really asking is what load on those axis drive systems is it designed for? And what load will the head and spindle safely take?
Just figure a 630 size horizontal machining center. Any Matsuura, Mori or Hyundai engineers here?

 

[John Donlan]

I would like to put my two cents in. I understand that you may need to improvise a makeshift press sometimes (if one is not available) and a machining center spindle looks like it might fit the bill. I don't think it would work well and this is why...

Yes, a machining center spindle is able to exert a kinetic force of some amount... But I doubt it will be enough to affect a press fit situation. For one thing, machining center spindles are extremely accurate, and using them as a punch press may damage their accuracy.

As was mentioned in an earlier response, arbor presses a relatively cheap and easy to obtain. Take the hint.

 

[enginesrus]

There is a huge difference between a "Punch Press" and an arbor or low tonnage hydraulic press. These machines are crashed every day, spindle into the work, and usually survive that just fine. I see no difference between that and pressing a light press of say a 1/4 inch diameter pin, as long as the spindle is not supported by ceramic bearings. There is a lot of force on a part and the machine when making very heavy cuts and utilizing the maximum HP capacity of the machine in the cut. That is why in the old days of manual machining if the part wasn't clamped and blocked you could slide it on the table.
On some parts there is no way to fit it into a press or hold it in the correct position to do the operation.
Not sure if I mentioned it but I have used them to pull dowel pins from holes using a proper size collet, and watched the Z axis load at the same time, it was almost nothing, I had no other tool to accomplish that job at the time. I may have used them to start a pin in the hole as well its been some time now since doing all that.

 

[SwinnyGG]

I have many years of machine shop experience

These machines are crashed every day, spindle into the work

hmm.

Anyway... The force on the spindle parallel to the ways/bed is not very high at all. If you're using the full horsepower of the spindle, that power is being used to supply torque to the tool - not linear force to the movement of the part or head.

You can validate this a couple of ways. One, next time you jog a CNC machine in rapid at 100%, listen to how loud the motors whine when they're moving the mass of the bed at full speed. Then listen to the volume of those Motors while you're making a cut.. they will be very quiet, because they aren't working very hard.

Spindle motors are sized based on how large tools need to be and at what RPM they need to cut during the actual machine work. Bed axis motors are sized based on how fast the bed needs to move between operations. They're putting out way more power between cuts than during them.

The other way to validate this is simple - go run a Bridgeport for 2 minutes. Notice that the X and Y manual axes have a relatively small gear reduction between the hand wheel and the movement of the bed. In other words, you can move the bed pretty quickly with the manual wheels on a Bridgeport.

When you make a cut on that same Bridgeport, the effort to turn the wheels goes up a little. But only a little. Unless you're spinning your tool super slow or they are very small, the machine will eat material about as fast as you can manage to turn the hand wheels.

If huge cutting forces were required, the manual hand wheels on the classic knee mill we've all used would have huge reductions and jogging the bed by hand would be SLOW. It's not.