Reground Planer Knives Trapping Chips

Woodworkers help figure out why chips won't clear out from the workpiece after a set of planer knives comes back from the grinder. March 14, 2006

I just changed my planer knives and now I am having problems with chips getting in-between the wood and knives. It is leaving indentations on the wood. It looks like I need to change out the vacuum bags and it isn't taking away the chips fast enough. This is the original set of knives. I just had them sharpened by my usual sharpening company. This is the first time they have been changed/sharpened since new.

I've tried adjusting everything - speed, chip breaker, in/out wheel tension/pressure, cleaned the vacuum and it’s still the same. I've tried light and heavy cuts. I don't have a problem with extremely light cuts (1/128") but this is worthless. Does anyone have any clues or anything else I might try? I'm going to call up the sharpening company and ask if they might have changed the angle of the bevel, but that's all I can think of that they could have messed up. I’ve never had this problem unless the vacuum was stuffed.

Forum Responses
(Solid Wood Machining Forum)
From contributor J:
Did you adjust your knives out to compensate for the amount that was removed during sharpening?

From the original questioner:
Yes - I used the gauge that came with the machine to adjust the newly sharpened knives. I also went one step farther and re-adjusted them a second time bringing them out a bit further (1/2 turn of the setting screw), and this had no effect and it did exactly the same thing. I just can't imagine what the deal is. The only thing that has changed was the knives have been sharpened. The machine just sat there for a week while this was happening and no mysterious adjustments could have happened. It must be the knife angle or something.

From contributor R:
In my opinion you’re right on target with the vacuum bag theory. I think you don’t have enough cfm from the dust collector to get the chips out of the way fast enough. I have seen this problem many times; trust in your first thoughts, clean the system and you will see a better product.

From the original questioner:
To contributor R: That was the first thing I did. My vacuum was full and I emptied it. It didn't change anything. I have been using the same system for years now and this is the first time I've encountered this problem with my vacuum operating with an empty bag.

From contributor R:
To the original questioner: If the bags were full could there be a chance that there is an obstruction in the piping slowing down the cfm?

From contributor M:
To the original questioner: Can you tell me more about your planer, size, brand, number of knives and size of dust port? What is your dust collection system and what is the duck size from your planer to the collector? I’m very convinced it has nothing to do with you knives’ angle.

From the original questioner:
I have a Planer Jet 15" JWP-15CS, 3 Knives (1 x 15 x 1/8), 4" dust port, Penn State 1 1/2HP DC, 15' of 4" flex hose. I've never had this problem unless the DC is full. I've been using this system combination for quite a while now and the only problem I have as a little snipe. The only thing that changed was the knives were sharpened. Before that everything was fine. And the knives were beat and I wasn't having problems.

From contributor F:
It sounds like inadequate dust collection to me as well. You know, when the cuttings get slammed back into the freshly cut surface by the cutterhead. Since you said it was "full" and then you emptied it, is there a chance that part of your system is still plugged? If not, test the performance of you dust system because maybe as a coincidence, it is starting to malfunction at the same moment you changed the planer knives.

From contributor F:
As a way to narrow it down to a cause, try removing the dust collection from the planer all together. I ran my planer/planers for years before I started dust collection and the cuttings just shoot out the front of the machine and land on the board and floor and cause no surface defect to the board being planed.

From the original questioner:
To contributor F: That is the next thing I did after cleaning out the vacuum. The bag was full not stuffed. That’s a great idea Contributor R. I jumped right on it only to find all my lines are clear.

When I removed the top plate and didn't use any vacuum it did the same thing, I even took my air nozzle and blew the chips to the side so there wasn't a chance that they could get back underneath.

I was looking at the knife holding clamp. What I saw was small chips getting stuck between the knife and the holder. Maybe this is where my trouble is coming from. And yes, they were fully cleaned of all pitch before I put them back. I have been doing this for 20 years now. In most of the shops I worked at I was the guy who took care of the machines, changing cutters and maintenance, so this isn't something new to me.

From contributor F:
I discovered when I first began to hand grind my own molder knives that there is a relationship between the knife projection and the feed roller height. Also, if the knives did not project the correct amount from the cutterhead then cuttings would become jammed between the knife and its gib and that would cause a surface defect on the molding being run.

You said you moved the adjusting screw and I am wondering if that could be the problem. You see the relationship between the cutting edge and the cutterhead should remain the same no matter how many times the knives are ground/sharpened.

The only difference is that as the knives are ground away there is less material being held by the knife clamping mechanism, the amount of projection should remain as what the makers of the planer set it at unless there is a change to the feed rollers height.

From contributor D:
Your problem is familiar to me, but I never found a reliable solution or could be sure of the cause. Our problem was with the top head on a molder as the worst culprit. We ended up calling this "chipbeat" for lack of a better term. The chips ride around the spinning head and get compressed between the bevel of the knife and the wood.

I even attended several tooling seminars with industrial heavyweights like Weinig engineers and university people. I showed them the samples and told them what we tried. Everybody agreed it was a problem, and if we had tried the fixes I mentioned, should have seen improvement.

The dust collector people showed us how to measure water column, and then got us to go away by quoting a whole new dust collection system. They supplied the one we were using, and installed it a mere 2 years earlier, and said it was working properly.

We even tried augmenting the dust collection with compressed air nozzles built into the hood and got some relief. The only factor we could attribute to this better or worse was relative humidity. The higher the humidity the more likely there is chipbeat. When we graded the severity of the problem (hourly, on a scale of 1 to 10 for a month), we correlated it directly to the relative humidity in the shop. Winter came, and it went away, along with the humidity. We still see it on things every now and then, in a different shop with different equipment and products.

From contributor S:
Some of the imported planers of that design have a chip deflector (maybe plastic) above the cutterhead which must be adjusted up close to the knives. Is it possible this was not readjusted after the knives were replaced?

From the original questioner:
Contributor S - that is one of the last things I did. I had it moved away from the cutterhead (1/4") when the old knives were in there because the planer really roars when it is close. I moved it within a 1/16" of the knife and it cleared up most of my problems. I am still having a problem with soft maple, the wood I am currently working with on a kitchen project. I get clean cuts with poplar, pine and even soft curly maple, but not plain soft maple. Still getting chips under the cutters which leave little scrape marks. I am going to adjust the plastic chip deflector even closer tomorrow and we’ll see what happens.

From the original questioner:
I changed the cutter to a new angle - 35º instead of 40º and it helped a little. I cleaned my vacuum system and bags and made all adjustments to make the machine operate smoother. I moved the plastic chip catcher as close as possible to the rotating cutterhead.

I get good results on pine, poplar, and soft curly maple. The normal soft maple I am using still has a problem with the chip dragging. When I take a light 1/64" pass I don't get the problem. The last pass I need to take is 1/64" to get a perfect surface which means two extra cleanup passes on wood for about 30 doors. It’s better than sanding it down with my 5" DA sander.

From contributor F:
I am still concerned about the chips that are lodging at the base of the knives at the gib or knife holder. That should not happen and will leave marks on your work. For what it is worth, when it happened on my planer style molder, the cause was that the knife projection from the cutterhead was incorrect – it was too small.

From the original questioner:
To contributor F: I used the gauge that was supplied with the machine when I bought it. It isn't an aftermarket gauge; it was made specifically for this machine. When I checked the thickness, it matched the gauge perfectly. This proves to me that I have the knives at the factory set height.

From contributor B:
To the original questioner: What is the factory set height of the knives? On my Powermatic 100 it's 1/8" projection above the surface of the cutterhead. It it's more like 1/16" on your Jet planer then that will contribute to this problem. New properly sharpened knives might create problems where none existed before.

From the original questioner:
Yes, it is just about 1/16" above the cutter head. I did raise them another 1/32" but it didn't do anything that I would call better. So I put the newly sharpened (2nd set at 35º) knives back to the factory setting. I get good results in most of my standard woods (pine, poplar, soft curly maple). Just not in the normal soft maple. If I keep having problems I will try that.

From contributor G:
When my old 1 1/2 HP dust collector threw in the towel last year, I ended up purchasing a 2HP unit (220v) to replace it because the old one could only keep up with the planer under ideal circumstances. What a difference! That 1/2 horse upgrade seems to make for a twice as powerful collector. I have a 5" hose about 5 feet long connecting to the planer and my life is a heck of a lot easier.

From contributor E:
I guess I am not the only one who has had this trouble. You say the problem is new since you had the knives sharpened. I would look very carefully at the edge, on the side opposite the bevel. Check for a minute burr. This can hang up a chip and cause it to ride around and impact the wood on the out feed side.

I suspect you have all the air suction your machine needs. The fact is, no matter how much volume of air your collector can move, it is going to be limited by the air gap around the cutterhead and chipbreaker. Have you tuned your gibbs so they have a smooth clean surface and edge where they touch the knife - no gaps to trap a chip?

From the original questioner:
To contributor E: This is one of the problems that has occurred with my machine, the trapped chips. The edge that touches the knives has the corners relieved and the chips get trapped in there. I can't see how to overcome this problem without having the holder ground down until the rounding over is a nice sharp square corner. I didn't have this problem before I changed my knives. I removed the knives several times and re-inserted them with the same results. I am using the gauge that came with the planer to set the knives with. How would I eliminate this gap where the chips seem to get trapped? There is no burr on the knives or on the knife holding unit, just the rounded over corner that seems to be enough to let the chips get trapped.

From contributor E:
To the original questioner: The knife clamps have not changed. So either you have them in backwards (did you pull all the bolts out and mistakenly re-insert them on the wrong side)? There also could be another problem here. Normally the intersection of the knife and clamp is either square, or the clamp bar has a radius to curl the chip away from the knife. Having a groove there that the chip can impact into and get stuck is definitely incorrect.

From contributor F:
I had this problem on my single knife molder. It was when I first began to hand grind my own molding knives. On my first couple of knives, I noticed the "rub marks" on the finished molding and traced the problem to the chips that were jamming between the knife and gib.

I bought the machine used and it came with only one molding knife. I measured from the base of that knife to the deepest part of the profile grind (which is the highest point on the molding it makes) and found it to measure 3/4" approximately. On my next knife I followed that 3/4" to the deepest part of grind guideline and after grinding 35 or so more profiles I never again had that problem.

How can this help me you may ask? Well, of course the 3/4" guideline and so on are of zero help but the major lesson learned by me was that there is a minimum distance that the cutting edge can be from the top of the gib (knife holder) and not have chips get jammed in there.

What you can try is to first, devise some method of recording the current factory setting of you planers knife setting device so that you can set it that way again if all else fails. Then, you could reset it if possible or get one that can be adjusted to a new setting that brings the tip of the knife, lets say, 1/16" further out of the cutter head and further away from the tops of the gibs (knife holders). Try a pass or two with the new setting and see if that cures the chip jam problem.

The only ill effect that I can think of may be that the feed rollers will not contact the boards firmly enough. If it cures the chip jam trouble, there should be a way to adjust the feed rollers lower.

I posted before along these same lines and you told me you were using factory specs. Stranger things have happened than sticking the knives out a bit more and having it work even though it is different than the original setting.

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

Comment from contributor N:
I am a timber machinist by trade. I have had this problem with planers at times and it is usually related to the flow of extraction air around the cutter head. I would suggest that you cut yourself a piece of wood that will fit tightly into the extraction hood and therefore reduce the suction area you may need to experiment with the placement to find the position that creates the desired airflow.

The reason that it is probably noticeable now is that the shavings/chips will be larger with sharper knives. With really dull knives the waste tends to become crumbly almost dust like. To clarify that it is chip marks you have, they will appear as elongated dents in the surface and there would be a furry effect with some fibers lifting which would indicate the rear edge of the knife rubbing. This is caused by an incorrect grinding angle. The cutting angle is not variable as it is set into the head.

Comment from contributor L:
I now have a new vacuum system, a 2 1/2 HP cyclone system. The planer is piped with 7' of 7", 15' of 6", and 10' of 5" flex which is terminated with a 5" to 4" reducer at the dust port of the planer. I am getting about 650 CFM at the port - 150 CFM more than recommended by Jet. However I still have the problem.

After determining the vacuum system wasn't the culprit I went back to my original suspicion - the knives. I talked with the Jet technician and he said that the knives need to be ground with a double bevel. The main bevel needs to be 40º and the micro bevel needs to be 42º and 1 mm wide. The shop that ground my knives only ground a single bevel on them. I have had many sharpenings since then and the problem still persists. I have instructed the sharpening company of the Jet specifications for the grinding of the knives and will receive the knives soon. Soon I’ll see if I was correct in my assumption all those years ago.

Comment from contributor A:
This is a common problem. Chip bruising can be caused by multiple reasons. Since the problem only came about after grinding the knives I doubt it is the gib shape. I like a right angle at the gib to break the chip. I encountered this with LV knives with a half round gullet spitting chip back into the cut. Projection should not really matter as some moulders have an adjustable plate which is adjusted as close to the cutterhead as you dare and this helps the extraction draw on the right side of the cutterhead. Material removed should not exceed 0.5mm and multiple heads breaking down really helps. Lastly moisture content, green requires more velocity for extraction.