How Can I Adjust The Chip Size On A Drum-style Tree Chipper?

Drum chippers are workhorses on jobsites,turning branches,limbs,and whole trees into uniform chips for mulch,biomass,or disposal.One of the most common questions operators ask is:"How do I change the size of the chips?"On a drum-style chipper,chip size isn't controlled by a single dial-it's the result of several interrelated settings.Understanding those variables and knowing how to adjust them safely will help you get the chip size your customer(or your boiler,or your mulch pile)needs.
Below is a practical,step-by-step guide aimed at owners,operators,and fleet mechanics.
1.What Actually Determines Chip Size?
On a drum chipper,chips are formed when a rotating drum-mounted knife sweeps past a stationary bed knife(anvil)and shears off a section of wood delivered by the feed rollers.The main factors are:
Factor How it affects chip size
Knife-to-bed-knife clearance Smaller clearance=thinner chips;larger clearance=thicker chips.This is the primary adjustment point.
Knife extension(radial position)Pushing the knife farther outward from the drum increases the bite;retracting it reduces chip thickness.
Feed roller speed vs.drum speed Slower feed relative to drum RPM=shorter chips;faster feed=longer chips.Many machines have a variable-speed feed controller.
Number of knives on the drum More knives per revolution=shorter chips(each knife takes a smaller bite).
Wood species&moisture Hard,dry wood tends to produce cleaner,more consistent chips;wet or twisted grain can vary size even with good settings.
Most day-to-day adjustments focus on clearance and knife position,with feed speed as a secondary lever.
2.Before You Touch Anything:Safety First
A drum chipper is unforgiving.Always follow lockout/tagout:
1.Shut down the engine and remove the key.
2.Engage the emergency stop if equipped.
3.Relieve hydraulic pressure on the feed system(cycle the feed lever a few times with the pump off,or follow your manual's bleed procedure).
4.Put the machine in"maintenance mode"/insert the lockout tag on the battery isolator and fuel solenoid.
5.Wait for the drum to come to a complete stop-never rely on the brake alone;physically check by trying to turn the drum by hand(after loosening a belt guard if accessible).
⚠️Never reach into the chute or near the bed knife while the drum can still rotate.Even a single stored-energy revolution can cause serious injury.
3.Locate the Adjustment Points
Every manufacturer(Bandit,Vermeer,Morbark,Peterson,etc.)has its own layout,but the principles are similar:
•Bed-knife adjustment bolts–Usually two or three bolts behind the anvil that move the entire bed knife forward/backward relative to the drum.
•Knife-mount shims–The drum knives are mounted on blocks or pockets with shims underneath.Adding shims pushes the knife outward;removing them pulls it back.
•Feed-roller speed control–Often a dial or digital setting on the control panel(varies hydraulic flow to the feed motor).
Check your operator's manual for the exact locations and torque specs-they matter.
4.Step-by-Step:Adjusting Chip Size(Primary Method–Knife/Bed Clearance)
Step 1:Measure the current clearance
Use a feeler gauge at the center of the knife(not the tip,not the heel).Typical clearance on many drum chippers is 0.010″–0.030″(0.25–0.75 mm),but check your manual.Record the number so you know what you started with.
Step 2:Decide which direction
•Thicker chips?Increase clearance(move bed knife away or retract knife slightly).
•Thinner chips?Decrease clearance(move bed knife closer or extend knife outward via shims).
On most machines the easier route is adjusting the bed knife,because you don't have to unbolt the drum knives.
Step 3:Adjust the bed knife
•Loosen the locking nut(s)on the bed-knife adjustment bolt(s).
•Turn the bolt:in to move the anvil closer to the drum(thinner chips),out to move it away(thicker chips).
•A small turn goes a long way-typically¼turn changes clearance by~0.005–0.010″.
•Re-tighten the lock nuts and re-check with the feeler gauge at multiple points along the knife(drums can bow slightly;you want consistent clearance across the width).
Step 4:Alternative–Shim the drum knives
If your machine uses knife pockets with shims:
•Unbolt the knife bar(you'll need a torque wrench rated for the knife bolts-often 150–250 ft-lb).
•Add or remove stainless shims under the knife mount.One shim typically=~0.015–0.020″of radial movement.
•Re-torque to spec.Never guess on torque-under-torqued knife bolts can throw a knife;over-torqued can crack the drum pocket.
Step 5:Adjust feed speed(secondary)
If you want shorter chips without touching the mechanical clearance,slow the feed rollers.If you want longer chips,speed them up.This won't change thickness,only length-but customers often say"chip size"meaning both dimensions,so it's worth mentioning.
5.Test and Verify
1.Start the machine(following normal startup,not in lockout anymore).
2.Feed a few test branches of known diameter.
3.Catch a sample handful and measure:
•Thickness(radial dimension,set by clearance)
•Length(axial dimension,set by feed speed vs.drum speed)
4.Most mulch markets want½″–1″thickness and 1″–2″length,but biomass boilers may want 2″–3″uniform chips.Adjust accordingly.
If chips are ragged or tearing rather than cleanly sheared,your knives are likely dull-sharpen before chasing clearance further.
6.Common Pitfalls
•Chasing size with clearance alone when knives are dull.Dull knives bend the wood instead of shearing,producing inconsistent chips regardless of setting.
•Uneven clearance across the drum width.Check at least three points along the knife.Unevenness causes vibration and premature bearing wear.
•Forgetting to re-torque after shimming.A loose knife is a thrown-knife incident waiting to happen.
•Ignoring feed roller pressure.If the top feed roller isn't pressing hard enough,the wood can"skate,"making chips longer and more variable.Most machines have a spring-tension or hydraulic-pressure adjuster for this.
7.Maintenance Tips to Keep Size Consistent
•Rotate/spare knives regularly so you always have at least one sharp set in the drum.
•Check bed-knife wear plates-when they're gouged,effective clearance changes even if the bolt setting hasn't moved.
•Keep the drum bearings in good shape;a wobbling drum alters clearance dynamically.
•Log your clearance settings per job.If you switch from mulch chips(thin)to biomass(thick)often,having a"recipe"saves setup time.
Adjusting chip size on a drum chipper comes down to clearance between drum knife and bed knife(primary)and feed roller speed(secondary).With the machine locked out,measure your starting clearance,tweak the bed-knife bolts or knife shims in small increments,re-torque everything to spec,and test.Do that routinely and keep your knives sharp,and you'll hold chip size consistently job after job.
📌Always consult your specific model's service manual before making adjustments-bolt torque values,clearance ranges,and adjustment mechanism details vary by manufacturer.