Troubleshooting Moulder Head Vibration

      Many things could potentially throw a moulder head out of balance. Here, pros discuss the possible causes of excessive vibration. August 23, 2005

Question
About three years ago we bought two new 9" heads for the Wadkin moulder we have. One was bought through Woodtech and has never given me a problem, the other was bought from what appeared to be a reliable source and is now causing me a big headache. The head in question has been running great but recently has developed some problems.

It's a four pocket head and both filler strips are balanced as well as the gibs and straight knives in it. When I put it on the spindle and start the machine it vibrates very badly, so I took it off and checked the belt (it was fine) and made sure the lock down for the top head was tightened (it was). So I decided to take all the knives and filler stock out and see what would happen and the it still vibrated.

My thought after that was that maybe the bearings had went bad. To test this theory I put my 6" head on and started it up and it ran perfect with no vibration at all. To take it a little further I put the suspect head on the last bottom head spindle (which I know is in good working order) to see what kind of results it would have there and it vibrated there also.

My question is this - is it possible that this 9" head that has run with no problems for three years is now all of a sudden out of balance? If so, how is this possible? I am getting ready to send it off to check the balance on it - is this necessary? Any help would be appreciated.

Forum Responses
(Solid Wood Machining Forum)
From contributor S:
First, there is no way that head can run for as long as it did unless someone balanced it to begin with. So how did it run so well for so long and now will not? The real surprise is that it does not run smoothly with the knives and gibs removed. If you had that head truly stripped and it is out of balance, then one would have to guess that the manufacturer of the head used the gibs as balancers.

I would suggest weighing each of them to see if there is any difference in them. Are they marked? Sometimes, and especially when using jointed heads, the knives and the gibs will be marked 1, 2, 3, 4 and the slots in the head will also be marked. If so, 1 always goes in slot 1, etc.
Send the head and all of the gibs to be balanced and that will end it.



From contributor C:
Balance can be affected by many factors; including sawdust, gib screws of different weights or other foreign matter. A weight difference of 1 gram is equivalent to 22 pounds of rotational force at 6000 rpms on your moulder. Did you hit anything on the prior run that could have damaged the head in any way?

We can certainly look at the head and inspect it, and re-balance if necessary. We dynamically balance all of our heads. There are some manufacturers that statically balance their heads. Heads really need to be dynamically balanced to run at higher rpms as on your moulder. You can call with any questions at 1-800-TOOLING [1-800-866-5464]. I have attached a link page with educational information concerning tool balance.

Here's a link about Tool Balance
Tool Balance



From Dave Rankin, forum technical advisor:
With what you have said about the problem, I would suspect this as the most common answer to your problem.
1. The head body was probably out of balance from the start.
2. The spindle bearings were new when the head was first used.
3. The dove tails of the machine were all tight when the head was first used.

With this said, I think that you were sold a lower quality head. I would check the gibs, gib screws, fillers and knives for balance. Another thing that can cause additional problems with vibration is the type of belt that you are using. Most tooling companies can balance the head body for you.

Some possible sources are:
Wisconsin Knife
Charles GG Schmidt
Centric
Moulder Services
Wood Tech
Dehart Tool
Forest City Tooling



From contributor J:
One other thing you might want to try before you give up is to drag a straight edge around the outside of the bore opening and see if there are any burrs. Sometimes they get dinged up. A small burr when clamped on the spindle can do a lot to a 9" head at 6000 rpms. If you find any, hone them off with a flat stone and give it a spin. Burrs on your spacers can also cause problems. Also, make sure they are all clean and smooth.


From the original questioner:
Concerning the balancing of the gibs, etc - I am fanatical when it comes to the balance of my heads. Every gib, gib screw, filler stock, and knife is balanced using a Harvard trip balance scale.

Dave - I think you hit the nail on the head with the three things you suggested. You’ve got my curiosity somewhat on the belt issue. I was under the assumption that as long as we had the belts made locally to the specifics of the original belts we would be fine. Evidently I am mistaken. What kind of belts should I be using and the name of a supplier? Is it possible that I could be tightening the belts to much? The book gives no parameters on this subject.



From contributor B:
No one has suggested this yet, but I am aware of at least one instance of a cutterhead fracturing, and flying apart while in operation. It was a four-knife head, and it was mis-used over time. Typically, two knife pockets were filled, and the other two were left completely empty. The fracture started at the bottom of one of the filled pockets, beneath the corrugations, and extended towards the screw bores of the pocket behind.

It was speculated at the time that the repeated stress applied to the working knife pockets, and the lack of gib bars and filler stock in the adjoining pockets allowed the body segments to flex to the point of fracture. Metalurgy also revealed a high sulfur content in the cutterhead body, which makes for weaker steel. I'm not saying this is your problem, but maybe it's worth a look.



From contributor C:
To the original questioner: What model number Wadkin moulder are you using? I should be able to offer you recommendations on proper belt tension. A seamless belt, such as a Panther, could be an answer. A seamless belt will run smoother, whereas, a seamed belt (in some situations) causes issues as the seam travels across the spindle pulley.

In any case, the first order of business would be to have the head checked, and go from there. By eliminating the variables one-by-one, a better chance of determining the root cause of the problem will come to the surface.


Here's a link to the Panther belts page
Drive Belts



From contributor C:
Contributor B brings up a very good point. Always keep knife pockets filled with filler steel when not being used for cutting. This will help maintain head integrity. We see a number of operators wrongly assuming that running empty pockets is ok and it is not. Stubby reminds us all of the unfortunate consequences.


From contributor E:
I have a question about filler stock in heads. My moulder operator insists that it's ok to run 4" knives in a 7" head, leaving 3" of the slots open. The other two are completely filled with balanced stock. Is it right to run a head leaving that much space empty?


From contributor C:
I agree with Contributor E. Keep the pockets full of profile knife or filler steel of like thickness. The gib wedges can be distorted by running a portion of the pocket empty. Knives also need to have good contact between screws to assure proper clamping by the wedge. Abide by the rules of maximum knife projection. Your operator risks throwing knives out of the head at some point by not following safe operating procedures, and gain, why take unnecessary risks. Below is a picture of maximum knife protection.


Click here for full size image



From Dave Rankin, forum technical advisor:
It is not necessary to use full filler stock in the unused pockets. When I use short filler stock, I make sure that it is held in place by at least 1/2 of the gib screws. If I am using a 7" head with 4" filler stock, I slide the filler to the center of the head. The small unused parts of the gib do not cause a problem. Fully tighten the gib screws that are pushing against the filler stock and snug the gib screws that are not pushing against filler. It is safe to run knives shorter than the head body as long as your knife catches at least half the gib screws with a minimum of two. The same holds true for the filler.


From the original questioner:
I have a Wadkin model GA 6 head jointed moulder. The book I have that comes with the moulder does say this for checking the belt tightness "average thumb pressure (5 to 7 pounds force)". I have no idea how you could use that as a unit of measure. It does go on to say this also "Apart from mechanical failure (breakage), the majority of mechanical faults can be attributed to incorrectly tensioned drive belts". Perhaps I am tightening the belts to much leading to premature bearing failure?


From contributor R:
Moulder Head Balance

Vibration can sometimes be difficult to track down especially if the tooling has performed well for a known period of time. It is virtually impossible for a cutterhead to go out of balance unless it has been damaged in some form. The most common being damaged on the profile grinder when the grinding wheel contacts the body of the cutterhead due to selecting an incorrect width of cutter to suit the depth of profile. If this is the case then the head needs to re-balanced dynamically.

The clue to the cause of your problem is that you removed all of the gibs, gib screws, cutters and filler stock, and the cutterhead body still vibrated. The likely cause is that there is some form of damage to the end faces of the cutterhead that contacts the shoulder of the spindle and is causing the spindle to be distorted off center, which creates an out of balance situation, even though the cutterhead is still perfectly balanced in itself.

For example, if you placed a postage stamp on the end face of a perfectly balanced cutterhead up against the spindle shoulder, when the spindle nut is tightened the center of the cutterhead spindle would be forced off center by approximately 0.012". This will cause the head to vibrate badly, because at 6000 rpm the head is substantially out of balance, and this is more than enough to cause excessive vibration.

Spindle Drive Belt Tension

Your description of the correct belt tension being average thumb pressure of 5 to 7 lbs force is in reference to spindles that are driven by V belts, and not flat belts. V belts should not be run extremely tight, whereas the flat belts that are used on later machines have to be tensioned to a precise amount in order to be able to transmit the horsepower from the motor to the cutterhead.
Different manufactures of flat belts will have their own tensioning recommendations, which are normally described as being a percentage of the length of the belt.

For example, if the correct flat belt tension is to be 1%, then a 900 mm (35.43") long belt has to be tensioned until the stretched length of the belt is 909 mm (35.79"). If your moulder has V belts driving the cutterhead spindles then the belts are correctly tensioned if they can be deflected no more than 1/2" by average thumb pressure.

Length of Cutters and Filler Stock

There are many rules of thumb with regard to preparing cutterheads, especially when only using two cutters in a four knife head, but at 6000 rpm there is no room for guesswork and errors.

Some of my recommendations for tooling preparation are:
• Balance all cutters and filler stock to better than 0.1 grams
• Do not use cutters shorter than the gib. The cutter needs to extend under every gib screw, but if the cutter has to be shorter than the gib, due to an inadequate selection of available cutterhead lengths, then filler stock of the same thickness should be used to make up the difference.
• Do not run cutterheads with empty cutter slots. This will cause the cutterhead to distort. If only two cutters are used in a four knife head, or three cutters in a six knife head etc., install balanced filler stock in the unused cutter slots, again making sure they extend under every gib screw.
• Make sure all 4 cutters in each cutterhead are positioned in the same serrations relative to each other, or in the case of only two cutters and two pieces of filler stock, opposite each other. If one cutter is one serration out to the other cutter(s) they will be significantly out of balance.



The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor M:
I would agree with checking clean alignment between the spindle shoulder and tool shoulder. This is the most likely cause of imbalance on a 9" long tool, which was previously balanced ok.
If you have just 0.002" of dirt, or burr, at the shoulder this will throw the end of a 9" tool out approx. 0.018" - now that will rumble!

I’m not sure I completely agree with Dave's explanation of how he locks partial filler stock for two reasons:

a.) The gib can be distorted if the locking screw is tightened too much/too far from the end of the filler stock.

b.) Never tighten the locking screw against the gib without either the filler stock or knife stock in place. This is because the screw will be extended so far through the tool body it can easily strip the few remaining threads - then you will have fun getting that screw back out.

Good, safe policy is to always have either the filler or knife under each screw - or 'bridge' the gib by having knife under one end (axial-constant of course) and same thickness filler under the other end, with at least 20mm (3/4") gap between to allow some minor compensation if any thickness variance.



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