Harmonic Vibration in Hobbing Machine

[gearcutter]

Hello all,
I need your help. Our 900mm capacity vertical hobber is gradually developing, what I can only describe as, a kind of harmonic vibration when cutting 6 module or larger pitches on blanks larger than half the machine's capacity. The vibration results in chatter marks being left on the tooth form. When it begins to develop the vibration causes the whole machine to develop a resonance that can be fealt right through to the concrete slab you're standing on! To make an attempt to rectify the problem we began by looking at all the obvious things that might cause this eg; work holding, tool holding, tool cutting edge condition, type of material the blank was made from, adjusting gibbs on the head and saddle slide ways, saddle clamping, re tensioning of nuts and bolts located throughout the machine, tool spindle bearings/end & radial float, machine levelling, etc, etc. The only thing we feel that is left to do is determining the condition of the table area. We measured the circular backlash of the table's worm & gear to be around 0.05mm or .002". We were unable to get any radial movement of the table although we were able to get around 0.07mm of axial movement. We're leaving the table's worm shaft bearings until last, although feel that if they were damaged this would reflect in the table's circular backlash. The only way to reduce the vibration is to lower the cutting speed to the point we're its becoming uneconomical to do the job. If anyone has any useful suggestions please post them.
Many thanks, Ron.

 

[unclesyd]

Have you tried a different style cutter, i.e. more or less teeth?

 

[diamondjim]

Is the hob tight on its spindle?
Is there play in the hob?

 

[scarecrow55]

Do you know the frequency, if so try and find out what is running/cutting at that speed

 

[gearcutter]

Thanks so far for all your suggestions but we have already covered them. I tried something new over the weekend but it did'nt help; I thought that if the machine was developing harmonics then a dampner needs to be added some where. So I placed paper between the work support and blank as well as the table. This gave the blank a dull sound when tapping it rather than the "bell" type of ring it was giving before adding the paper. I'm sure some of you already know this trick. An interesting point to remember, I thought, if trying to hob a blank which has a poor set up/support arrangement. Someone was telling me another thing is to use cast iron, rather than steel, for a work support. Needless to say this particular set up, on the weekend, was supported 5mm short of the root diameter by a heavy walled steel tube 50mm wall thickness and the blank was direct clamped to the table.....the machine still vibrated. scarecrow55 I don't know how to measure the frequency all I can say is that it oscillates in time with the rotation of the hob. Perhaps I've just answered my own question!

 

[strokersix]

Can you vary the machine speed with different pulleys or a variable frequency drive to excite the machine outside of it's resonant frequency?

 

[gearcutter]

I can vary the spindle's rotation speed. It's motor is controlled with a VF Drive. Along with a pot. on the control panel of the machine this gives virtually infinite speed control. Increasing the speed increases the resonance and decreasing speed reduces the resonance. But as I said at the start, the speed needs to be reduced to the point where it becomes uneconomical.

 

[strokersix]

If the resonance remains when you change the input speed then it's probably not resonance. If it's not resonance then I think you should continue looking for inadequate stiffness in the work, tools, or fixtures, inadequate bearing preloads, etc. I don't know what your machine or work look like but perhaps you can change the cutting direction? If the work tends to be drawn into the cutter it can have a grabbing action and cause vibration. One example of this is climb milling versus conventional milling. I expect you already know this.

Variable speed drives are very useful in production milling operation. Often you can smooth out vibrations by tweaking the spindle speed. Unfortunately this doesn't seem to be working for you.

 

[scarecrow55]

if it vibrates at hob speed then it must be something on this line eg broken bearings, bent shaft or hob (unless there is another rotating body inside the machine at the same speed). If it was the hob teeth then it would vibrate at hob speed X number of gashes

 

[BobM3]

Oee inexpensive way to measure the frequency is to hook a microphone up to a computer and record the sound. Once in the computer you can either zero-in on the signal or download a freeware frequency analysis program.

 

[MachineryWatch]

gearcutter,

BobM3's recommendation for finding the frequency you are dealing with may help. Sometimes the airborne sound frequency is not the same as the vibration frequency, however, so you need to be careful using this method. A similar technique would be to use a piezo-electric speaker attached to the machine. It will work like a vibration sensor to generate a signal rather than it's intended application to generate a sound from a signal. The signal can be input to the PC in the same way the microphone signal can be.

Either of these approaches only gets you a part of the vibration picture, however. There are really 6 parts to using vibration to diagnose machinery problems.
1) Frequency
2) Amplitude
3) Phase
4) Measurement location
5) Axis (direction) of measurement
6) Time variation

While you may have some success troubleshooting some problems using a subset of these 6 listed, there are many problems that require all 6 parameters in order to solve the riddle. In this case you have several options and how much it is costing you to live with the problem usually determines the path you choose.

Many gear manufacturing facilities have equipment available for dynamic balancing. If your facility has this equipment and you have time to experiment you could teach yourself how to use the dynamic balancing equipment to look at the 6 parameters listed above. The dynamic balancing equipment should be able to give you most of the information you need unless the dynamic balance equipment is hopelessly integrated into a balance stand, rather than being portable, which is often the case. But it is worth checking into.

If this problem is starting to become expensive as it goes unresolved you may need to call in someone who has all the right diagnostic equipment and experience for solving vibration problems. While this can be expensive, too, the value of it simply depends on the urgency of the situation.

If you do need to get help from a consultant make sure you ask some questions to determine if he has the right equipment and experience to help you solve the problem. You do not want to go through the process of getting a consultant in to help solve a problem several times. Not all vibration consultants specialize in the same thing. Many specialize in Machinery Condition Monitoring using single channel portable vibration data collectors that may not be powerful enough to quickly get to the bottom of your problem. They usually provide a monthly monitoring program for facilities with rotating machinery to capture vibration data, identify trends in the data that indicate problems like bearing wear, and help the client determine the severity and a course of corrective action. This type of consultant may or may not have access to test equipment to help you solve your problem quickly.

A good question to ask is if the consultant has a multichannel vibration data acquisition system. While even a guy using 2 channel portable vibration data collector can answer yes to this question, it does help eliminate those who are limited to single channel data acquisition. The 2 channel portable data collectors are more and more common and they do offer considerably more diagnostic capability than the single channel portable vibration data collector if the user is skilled.

I hope this helps!

Skip Hartman

http://www.machinerywatch.com

 

[electricpete]

Personally I would pay more attention to the references and less attention to the equipment.

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