Eliminating Snipe on Mouldings
Looking for the cause of moulder snipe, an operator finds his solution in a custom-fabricated chipbreaker. March 13, 2007
Question
We run a fair amount of small mouldings, 1/2" x 3/4" x random length stock, on a Diehl 405. We have no finish problems other than an occasional snipe on the top horizontal head. I've noticed the snipe is approximately 2' from the trailing end of the moulding. From observation, the snipe occurs as the blanks are exiting the top horizontal chip breaker, which is as close to the cutterhead as I can get without causing a major wreck. The pressure plate is also as close to the cutterhead as I can get it. All of my blanks are cut very accurate on a gang rip. The snipe doesn't happen on every blank, but I have noticed under a florescent light that I can see where I've lost control of the wood after the horizontal chip breaker. Is there a known method of totally eliminating snipe on every strip? I'm thinking that some strips are naturally bowed, thus causing the wood to spring upward as it exits the chipbreaker. Any tips would be greatly appreciated.
Forum Responses
(Solid Wood Machining Forum)
From contributor S:
If you were running something wide and thick like a crown moulding, I would say that you have to accept the fact that occasionally there will be a warped piece stronger than your hold down system and a bite is not preventable. However, the strips you are running are so narrow they should be flexible enough to hold in place.
Trailing edge bites are caused by the hold down behind the head not being adjusted correctly. So the pressure bar is either worn, not made correctly or not adjusted correctly. As the trailing end leaves the chipbreaker, the pressure bar allows it to lift up. If the piece were 6" or 8" wide, oak, pecan or other mean to warp material, this will happen occasionally. But 1/2 by 3/4 should not be a troublemaker.
My comments assume there is no wear in the bed. Narrow strips tend to cause wear in the area next to the guide, making it difficult to get the hold down pressure correct. Be sure the bed is level right to left and end to end.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Check the archives for a thorough discussion of snipe.
From contributor C:
Yep, getting the last pressure shoe and bed plate leveled and positioned correctly is one of the keys, but I find that an even bigger one is making sure that the following piece is feeding rock solid tight behind the stock at the top head as it exits. On a through feed moulder, even with perfectly set pressure shoes and bed plates, I can still get a snipe if there is a small gap between them. On every profile that goes through, I have all the pressure shoes, chipbreakers and outside fences set as lightly as practical before the last top head and the fences just a little tighter after it. At the same time I have all the feed rollers set at regular height and air pressure before the last top head and barely touching with light air pressure after it, just to guide the stock out. I use the most aggressive style feed rollers I can get away with on the front end of the machine and as rubber rollers wear, and lose a little diameter, they go as the last two rollers after the top head in the maybe mistaken theory that the smaller diameter will have a slower rim speed and not pull the piece away from the one behind it. I can run thousands of feet like this without a single snipe, but you can bet that the last piece through with nothing behind it will have the snipe from hell at the end. If any of your pieces do have a bow to them, make sure that the concave side is down to the bed, too.
From contributor M:
I've found that lifting the leading edge as the piece exits helps a lot. Set up some sort of ramp on the outfeed table. Your pieces are too flexible to reliably stay on the bed as the cutterhead is trying to pull them up as it cuts.
From Dave Rankin, forum technical advisor:
When you have a snipe on the wood, the distance from the edge of the wood to the snipe location is valuable information. You stated that the snipe is about 2" from the end of the wood. This distance will be either of the following:
1. The distance from the center of the spindle to the point on the chipbreaker shoe that is pushing down on the wood.
2. The distance from the center of the spindle to the point on the pressure shoe that is pushing down on the wood.
A few things to look at:
1. Is the pressure shoe parallel to the tables below it?
2. Can the pressure shoe or chipbreaker shoe be replaced with phenolic, hardwood, or aluminum to allow for better hold down?
3. Is the wood being fed into the machine with the bow referenced up? This means that the two ends of the wood will touch the infeed table and the center of the wood will be above the table.
4. Are all of the tools sharp, especially the top and 1st bottom head?
It should be noted that an occasional snipe is not a major concern.
From the original questioner:
Thanks for the responses! We examined the bed plates and they showed no wear. We then made sure everything was level with a straight edge. I didn't notice any problems with the moulder. The profile is basically worn into the pressure shoe since we use Teflon for our hold down pressure. Sometimes we will purchase formed pressure shoes for profiles that don't provide a good flat surface for adequate hold down pressure. The pressure shoe is parallel to the bedplate.
I think contributor C has the right idea for eliminating snipe. If the pushing pressure of trailing piece of moulding is greater than the natural spring of the wood to lift as it exits the chipbreaker, then there should be no snipe at all regardless of which way the blanks are fed into the moulder. Of course, there is a drawback to too much roller pressure which will cause excessive wear in the bedplates. I'm sure there's a range between too much pressure and not enough that should work fine. We do use poly feed tires all the way through the moulder for this particular profile.
We've never had any complaints about the snipe. I just personally like to always strive for perfection even if it means trying something outside the realm of textbook guidelines. I have even considered an outfeed ramp as contributor M suggested.
From contributor J:
I'm not familiar with your moulder at all, but if you have removable bedplates, I would recommend taking some time to pull them off and clean underneath them. The same with your pressure shoe assembly. Although bolted or clamped in place tight, stuff builds up over time and can cause minor problems. This might not be your problem, but still something to look into for the future. As you know, the finer you can tune these machines in, the better and more flawless finish you will have.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Be careful of the effect of gravity on the outfeed. The outfeed table must be a few 1/1000 inch higher than the molder/planer to avoid pushing the end of the piece into the top head.
If gravity is indeed a problem (or unsupported outfeed ends), then you will note that short pieces do not have as much snipe as longer.
From the original questioner:
I'm now running 100% snipe-free mouldings! The problem was in the chip breaker. We were within 1/2" of the largest cutting radius but it was 1 1/2" from the minimum cutting radius of the knife. The cutterhead was basically lifting upward on the strips after the wood exited the chip breaker. We removed the stock chip breaker and replaced it with a longer reaching aluminum chip breaker which we fabricated ourselves. We noticed an improvement in our finish quality as well as snipe elimination. Thanks again for the suggestions.
From contributor J:
Thanks for posting your solution! A lot of people don't. It's nice to hear what fixed the problem once in a while. I think I might switch to a bigger chipbreaker and see what happens.
