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Approximate Stock Removal and Depth of Scratch for Abrasive Sanding

Average depth of scratch created by the following grit abrasive belt:

                           Grit                                        Depth of Scratch

                           36                                              .028 - .030

                           60                                              .025 - .022

                           80                                              .015 - .018

                           100                                            .010 - .012

                           120                                            .008 - .010

                           150                                            .005 - .006

                           180                                            .004 - .005

                           220                                            .003 - .004

Average abrasive belt stock removal maintaining average belt

life yet capable of removing previous grit scratch:

                           Grit                                         Depth of Cut

                           60                                            .035 - .045

                           80                                            .020 - .030

                           100                                          .015 - .020

                           120                                          .010 - .012

                           150                                          .006 - .008

                           180                                          .004 - .005

                           220                                          .003 - .004

 

 



 

FINISH DEFECTS

 

Surface marks running the full width of the work piece are usually referred to as chatter marks. There are three types of chatter marks produced in a wide belt sander. The first is what is referred to as “machine chatter”. This chatter mark is usually close in spacing (approximately 1/8’ – 1/4” apart). It is similar to what “moulder marks” look like. It is usually caused from something mechanical in the machine. The second is what is referred to as “abrasive belt chatter”. This chatter mark is usually 1/2” in spacing. Most likely this chatter mark is coming from your abrasive belt. The third is “feed chatter”. Feed chatter is not as common as machine chatter or abrasive belt chatter. This chatter mark is most likely coming from your feed drive system. Normal causes of these chatter marks are a few of the five Items listed below.

  1. Lack of sanding pressure.
  2. Vibration in machine.
  3. Uneven feed.
  4. Out-of-round or flat spot in contact roll.
  5. Abrasive belt splice.
  6. Feed hesitation and stock slippage.

Inadequate Sanding Pressure

If marks show up in spots, it is generally due to inadequate sanding pressure or unevenness of surface on product. What is happening is your abrasive belt seam is “slapping” the material being sanded causing it to create a “skip” mark on your material. You can try to close the machine opening to increase sanding pressure or adjust your sanding head accordingly to the correct stock removal capabilities if that abrasive belt. If marks run across the full width of your work piece and are evenly spaced over the full length of the material being sanded and you are achieving the correct stock removal capabilities of your abrasive belt, then you should inspect the abrasive belt seam. A quick test to see if the abrasive belt seam is causing the chatter mark is to take a lumber crayon (preferably a bright color) and smear the entire abrasive belt seam with the crayon. Sand the material and see if that crayon mark transfers to the material. This is most likely abrasive belt chatter. Keep in mind you will ALWAYS get abrasive belt chatter when sanding with a steel drum not matter what type of seam you are using on your abrasive belt. If the abrasive companies could produce a seamless abrasive belt, they would eliminate abrasive belt chatter all together.

Vibration

Vibration in machines generally results from out-of-balance idler or contact rolls, worn v-belts or out-of-balance motor pulley. . If this develops after use, check contact roll and idler roll bearings and motor bearings. Also check your drive belts for proper tension. If the contact drum or idler roll has been changed or contact roll dressed, check for balance and run out. Prior to installing new bearings, make sure your shafts are not undersized. A tight tolerance must be maintained, usually less than .001”. Be sure the bearings were properly installed and not “hammered” onto the shafts. Hammering on your bearings could cause the precision balls to “pit” or “nick”. Make sure the floor on which your machine is mounted is of a solid surface (concrete, ect.). Yes I have machines out there sitting on a wooden floor! Be sure to check to see your machine is level using a machinist level. Your machine should have solid four corner contact with the floor. One other area to look at is the abrasive belt tensioning or lift pin used to tension your abrasive belt. Inspect for air leaks, wear or excessive "play". You can try to increase or decrease the abrasive belt tensioning pressure and observe if there is a difference in the chatter marks occurs. Check to make sure your wide belt sanding heads are locked down properly. If not, they will “bounce” while sanding your material. In some cases I have seen loose bolts as well.

Uneven Feed

Run your material at any feed speed and measure the distance between the marks. Then change your feed speed and run a second piece and again measure the marks. If the distance between the marks remains the same as before, then the problem may be in your feed drive system (with variable feed speed only).

Check to make sure there is not excessive backlash or wear if equipped with a coupling or gear reducer if you suspect the chatter is caused by the feed drive system. Also check for conveyor feed belt "jerking" action.

Out-of-Round or Flat Spot in Contact Roll

Contact drums/rolls should be checked for roundness if it has been dressed or changed. Measure to outside diameter of the contact drum/roll using a micrometer. Removal of the contact drum/roll may be necessary to achieve an accurate reading. A dial indicator could be used on smooth contact drums/rolls. While manually rotating contact roll, observe any high spots. If roll is out-of-round, I would recommend having the contact drum/roll ground/turned/balanced and replace the bearings at the same time. It is very critical to have a contact drum/roll that spins at a high RPM balanced dynamically and accurately. An unbalanced roll will create a vibration which leads to bearing failure as well as other critical component failure.                              

Abrasive Belt Splice

The splice of an abrasive belt can create marks/chatter. The abrasive belt splice should be as close to the same thickness as the abrasive belt on either side of the splice. An open area (void of abrasive grit) at the splice can also create marks/chatter. A suggestion is a splice that is butted together, yet has no glue squeeze out and is equal in thickness as to the main area of the abrasive belt. Check with your abrasive belt manufacturer for recommendations. Abrasive belt seams of today are far different than ones used in past years. Abrasive belt manufactures are able to hold tighter tolerances on the seams as well as offering a superior joint that minimizes if not eliminates abrasive belt chatter.

Feed Hesitiation

Feed hesitation generally shows up as a “dip” or “wavy” surface on the sanded material. Do not get it confused with a “chatter mark”. The causes can be readily identified by their length and location. A “dip” is sometimes caused by the increased dwell time the material has underneath the sanding head, due to hesitation.

  1. A long “dip” or a “snipe” near the lead edge is most likely insufficient pressure on the infeed holddown rolls or shoes if equiped.
  1. A long “dip” or a “snipe” near the trailing edge is insufficient pressure on outfeed holddown rolls or shoes if equiped.
  1. Sporadic “waviness” or “dips” may be caused by poor conveyor belt conditions. A glazed, hard, or saturated conveyor belt that has lost its grip can cause hesitation. Most conveyor belts may be dressed to restore their driving friction. This can be accomplished by slowly bringing a running 60-80 grit abrasive belt into light contact with the running conveyor belt provided your contact drum is level and in good condition. It would help to make sure the bed plates are not showing wear as well. Allow thebelt to cycle a few passes before adjusting for more removal. I usually paint a bright stripe across the conveyor belt. This allows me to see where the belt is cleaning up and how much more material I need to remove. Once the paint is gone from the top surface all the way across, my conveyor belt has been successfully dressed. Do not get to aggressive with this process as heat is being generated. You are only removing a few thousandths of an inch per cycle and want to remove as little as possible from the conveyor belt but still achieve a uniform flat surface and restoring the original belt grip. (Note:Improper alignment of infeed/outfeed holddown rolls and/or shoes can cause hesitation).           

Recommended air pressure settings on a widebelt sander

The air pressure settings listed are an average and may vary slightly from manufacturer to manufacturer of various wood products.

Widebelt Sander: Abrasive Belt Tension

25 inch wide.....................40 PSI

37 inch wide..................... 60 PSI

52 inch wide..................... 80 PSI

Widebelt Sander: Abrasive Belt Tracking

25 inch wide.....................40 PSI

37 inch wide.....................60 PSI

52 inch wide.....................80 PSI

Conveyor Belt Tracking

20-30 PSI

Air Caliper Disc Brakes to Sanding Heads

60-80 PSI

Air Loaded Billy Rolls (Roll Feed Veneer Sander)

20-30 PSI

Relieving/Floating Conveyor Bed (Veneer Sander)

15-25 PSI

Relieving/Floating Conveyor Bed (Solid Stock Sanding)

30-40 PSI

Air Loaded Hold-Down Shoe(s) (Veneer Sanding)

20-30 PSI

Air Loaded Hold-Down Shoe(s) (Solid Stock Sanding)

10-15 PSI

Air Operated Segmented Veneer Polisher “Up” Pressure

8-10 PSI

Air Operated Segmented Veneer Polisher “Down” Pressure

10-14 PSI

Air Loaded Infeed Sectional Hold-Down Shoe to Cutterhead

3-5 PSI

Air Loaded Outfeed Hold-Down Shoe to Cutterhead

20-30 PSI
PROCEDURES FOR DRESSING RUBBER COVERED CONTACT DRUMS
Note: This procedure should only be performed by a qualified individual as more damage could occur if not done correctly.

1. Remove the abrasive belt from the head to be dressed. You may need to bypass the abrasive belt tension safety switch interlock if equipped.
2. Check to see if the face of the conveyor belt does not have a wear pattern or is damaged. If the conveyor belt is in good condition, level the left side and the right side of the rubber contact drum to the edges of the conveyor belt, holding your measuring device about 2” from each edge of the conveyor belt. Sanders Unlimited offers a measuring tool designed to assist you with this procedure. Please see our Products and Services page and look for the “Alignment Tool”. The rubber contact drum should be leveled to within .002-.003 side to side. Be sure to compress your measuring device to the rubber conveyor belt to compensate for slight curving on the edges of your conveyor belt and any compression in the rubber top of the conveyor belt. On used wide belt sanders that have a fair amount of run time on them, it is also recommended to inspect your bed plates (located under your conveyor belt) for wear. If there is more than .010 wear across your plates, you may have to have your bed plates ground prior to dressing your rubber contract drum. Otherwise this wear pattern may transfer through while trying to dress your rubber contact drum. If you suspect wear in your bed plates, the rubber covered drum will need to be removed from the machine and ground and balanced. Bearing replacement at this time is also recommended.
3. Obtain a 1" thick, flat MDF panel. Do not use plywood or particleboard; the panel MUST be flat with no defects in the panel otherwise the defects could transfer to your rubber contact drum. Measure the width of the rubber on your contact drum and add 1”. Also measure the distance between the rubber covered pinch rolls between the head you are dressing and add 12”. Example: rubber covered drum width is 52”; the pinch roll distance is 18”. Your panel size would be 52”x30”x1”
4. Using a 60 grit or 80 grit, open coat, silicon carbide (preferred) CLOTH abrasive belt, cut abrasive belt to fit the panel mentioned above. Be sure to eliminate the abrasive belt seam. Using spray adhesive, spray both the cloth backing and the panel, then attach abrasive belt to panel. Be sure abrasive sits flat on the panel with not ripples.
5. With the sanding head off and the conveyor bed open to at least 1-1/2", feed the 1" MDF board until it is under the rubber covered contact drum centered between at least two sets of pinch rolls.
6. If equipped, reset the brake function on your sanding head and slowly close the opening on your conveyor bed assembly until you feel a slight drag on your rubber covered contact drum. (You may have to disable any electrical interlocking switches to reset the brake on your rubber covered contact drum...door switches, abrasive belt mistrack switches, abrasive belt breakage switches).
7. Start your conveyor belt and allow 1" MDF board to exit machine. Without moving the position of your bed height, start the sanding head. Place 1" MDF board on conveyor belt and feed though machine, making several passes.
8. After closing bed opening .002-.003 repeat process making multiple passes with 1" MDF board. Repeat this process until the rubber covered contact drum is clean up across the entire rubber drum.

A few notes: Use a bright colored spray paint (yellow, white) and prior to dressing, paint the face of the rubber covered contact drum with a paint line all the way across the rubber. This will help you identify areas that have not been dressed out completely. Be sure to have your dust collection on while doing this process. DO NOT allow the 1" MDF board to stall underneath the rubber covered contact drum for a lengthy period of time. This will create a poor finish on the rubber and could create unwanted heat on your rubber covered contact drum. Take your time. you don't want to create an extreme amount of heat on your drum as this could cause the rubber to expand and you may get an uneven sand across the drum. If the rubber covered drum becomes to warm, pause 5 minutes with the rubber covered drum still running to allow it to cool down. Leaving it run with your dust collection on will create air movement across the rubber face and aid in cooling. You may need to readjust your rubber covered contact drum due to decreasing the diameter of the drum. An alignment of your pinch rolls, shoes (if equipped) and other sanding heads may be needed. If equipped, the thickness readout on your conveyor bed opening may also need to be recalibrated.
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