The Home Machinist!

This is further to the leadscrew lubrication issue I had earlier and the reason why I investigated it to begin with.

I am getting a high frequency vibration on my "Y" axis intermittently which is annoying. It gets worse when cranking the handle faster. It's almost like it gets vibrating to such a high frequency that the handle becomes very easy to turn, sort of like there is no resistance all of a sudden.

These are the parameters:
- It appears in a specific spot
- With or without the endmill cutting, so even when just cranking the handle.
- Only during a specific operation using a 3/16" endmill to cut a slot.
- Only when cranking the table toward the column
- Only when the machine is running

Gibbs are snug, backlash is about .00015" Ways are well oiled as is the leadscrew. When I change to a different sized endmill the issue is gone, even running it over the same spot. I thought the fact I had grease instead of oil was causing this problem but after changing to oil the issue still exists. It doesn't really effect anything other than my sanity! See if you can figure it out, I sure can't.

I'd really appreciate some insight.

RG

These

- With or without the endmill cutting, so even when just cranking the handle.
- Only during a specific operation using a 3/16" endmill to cut a slot.

seem contradictory. Explain?

Your mill has found a "harmonic". The slide is resonating to the motor modulation and yes, the screw will turn easier. There could be a lot of things; unbalanced pulley, bad belt etc. You already distilled it down to the more esoteric reasons.

John Hasler wrote: ↑Mon Mar 30, 2020 10:11 am These

- With or without the endmill cutting, so even when just cranking the handle.
- Only during a specific operation using a 3/16" endmill to cut a slot.

seem contradictory. Explain?

Sorry,

- With or without the endmill cutting, so even when just cranking the handle.
Yes, sometimes it happens while the endmill is cutting a .050" slot and sometimes it will do it even while the endmill isn't cutting, same travel, only not cutting.


- Only during a specific operation using a 3/16" endmill to cut a slot.
While working on this particular project there are 28 pieces and 4 different operations. Only the operation consisting of using a 3/16" endmill to cut a slot is when this happens. I know it doesn't make sense that it does this even while the endmill isn't cutting but this is the only way I can replicate the problem. With all the other tools within the milling process, this problem does not exist.

spro wrote: ↑Mon Mar 30, 2020 11:08 am Your mill has found a "harmonic". The slide is resonating to the motor modulation and yes, the screw will turn easier. There could be a lot of things; unbalanced pulley, bad belt etc. You already distilled it down to the more esoteric reasons.

Very interesting Spro! I was wondering if that could be a problem but dismissed it. Come to think of it every time I'm running this part I have the RPMs always set at 3300. I think I might try a reduction or increase in speed tomorrow.

spro's got it. Is there any change in the sound of the mill when this happens or even just at that speed?

The fact that it affects only the Y axis at one spot while cranking in one direction leads me to suspect that the Y axis screw itself is vibrating. What are the dimensions of that screw? Does it have bearings at both ends? How many flutes does that endmill have?

John Hasler wrote: ↑Tue Mar 31, 2020 7:49 am The fact that it affects only the Y axis at one spot while cranking in one direction leads me to suspect that the Y axis screw itself is vibrating. What are the dimensions of that screw? Does it have bearings at both ends? How many flutes does that endmill have?

The screw dia is approx. 1.25" and is only connected to the crank end housed in a flange type bearing. The cutter is a two flute HSS tool intended for aluminum.

It was exactly as Spro mentioned. If you reached up under and touched the screw at the outer end (as I had a second person do while I was cutting) you could feel it resonate. Increasing the RPM even by only 300 changed the frequency and it went away. To validate this, while cutting another operation using a different endmill that wasn't giving me a problem before, I was able to get it to do the same thing by increasing or decreasing the RPM. So big thanks to Spro for figuring it out.

Additionally, upon inspection it makes sense why it only happens in the one spot, when cranking the table to the column from the very front, most of the screw is hanging out of the nut at that point. The nut pinches off the screw vibration as it gets closer to the end.

So my next question - is there anything I can do to change how the lead screw is attached and is it worth bothering, if I can just adjust the RPM a bit to correct it?

Thanks

I asked about the dimensions because I can calculate the length at which the screw will resonate at the tooth rate you get at 3300 rpm. The resonant frequency is inversely proportional to the square of the overhanging length. I get roughly 10" as the resonant length for 3300 rpm and a two flute cutter. This is very, very approximate, of course: I'm using a simple tuning-fork model.

You can't put the far end of the screw in a rigidly-mounted bearing because it has to be free to move about a bit. You only need to damp it (as you did by touching it) to solve the problem. Perhaps a bearing secured by an elastomer mount?

Thanks John,

Interestingly that's about the length that sticks out at the point it resonates. Knowing that RPMs cause this and being able to tweak them accordingly should suffice for now. I appreciate your and Spros help immensely.

Regards
Ron

Late to this, but yeah a specific set of circumstances that result in just the right natural frequency to create the problem just like Spro mentioned. If you do a search for natural frequency of vibration etc you'll quickly find out how lacking most peoples knowledge and understanding is about it. Mine sure is at least. It's an extremely complex problem. For most of us simply knowing about some of the causes is at least a big help in a home shop. As others have mentioned move even a bit outside the parameters that start and maintain that vibration and most times it goes away. I'd suspect just the right amount of wear at one point that causes just enough clearance to start the movement. But there's possibly something within the machine with barely enough imbalance at that rpm that's causing it. John's mention of a tuning fork is a perfect analogy. Somewhere between the floor the machine sits on and all the parts between it and the end of the spindle is the cause at that specific rpm. Since it also seems somewhat tied into that end mill it could even be sharpened non concentrically or bent by a few 10ths enough to where you see it causing vibration. There could also be dozens of combinations within the machine that might be part of it. One ft. lb. tighter or looser on the spindle bearing pre load might even cure it, who knows? My best guess is it might be just about impossible to pin point the exact reason with 100% certainty.

The source of the excitation is the cutter. There's nothing wrong with it: it's just that a tooth biting into the part 110 times per second inevitably causes vibration. This happens to be a resonant frequency of Ron's leadscrew when it's sticking out about 10".

John Hasler wrote: ↑Wed Apr 01, 2020 7:52 pm This happens to be a resonant frequency of Ron's leadscrew when it's sticking out about 10".

Yes, when the vibration is happening you can really feel the leadscrew vibrate at the end albeit microscopically (for lack of a better description). As soon as you touch it the vibration completely stops. Change the RPM by a about 300 revolutions and it's gone.

I'm just glad my friends on THM were able to point me in the right direction, and offer advice