SHARPENING TECHNIQUES

4.1 INTRODUCTION

The discussion of sharpening techniques in this section assumes that the grinder is equipped with a modern micrometer lathe in good accurate condition. Older grinders not equipped with micrometer lathes or grinders having micrometer lathes in poor condition will have to alter their sharpening methods to suit their equipment. We will attempt to indicate in the following discussion of sharpening technique how to alter the method for old and worn equipment.

Fig.17 Sharpening Lathe

The bond posts holding the abrasive grit in the stone are glass-like materials, which are easily fractured by the sharp teeth of the burr. It is this fracturing of the bond posts which removes the abrasive grit and causes a pattern to be developed in the stone surface. The primary aim of good stone sharpening is to accurately control the amount of bond post fracturing so as to produce a proper long lasting pattern.

4.2 TRUEING A NEW STONE

A new pulpstone, after being mounted on its shaft in the grinder, will not be perfectly concentric with the shaft. It must first be trued before a pattern can be put into the surface of the stone. The following instructions apply for trueing a new pulpstone.

1. Mount a new No. 12 Diamond Burr in the sharpening lathe. A No. 10 Diamond Burr can be used if the stone is considerably out of round and a large quantity of abrasive must be removed. It is not recommended that a burr coarser than a #10 diamond be used as these coarser burrs are too severe for the stone surface.
2. Select a low lathe traverse speed of between 15 and 20 seconds.
3. Find the highest spot on the stone by feeding the burr down to its spark point near each edge and at the center of the stone. The spark point is the point at which the teeth of the burr first touch the stone surface. Feed the burr very slowly towards the stone until the first sparks are observed. Note each of these readings from the micrometer dial on the lathe.

   
   Fig.18 Locating Highest Spot

4. Locate the burr over the edge of the stone with half the burr width on the stone. Start at the side of stone having the higher spot or larger diameter. Traverse stone to make certain that burr is only lightly touching stone surface.

   
   Fig.19 When Truing Traverse Stone In Both Directions

5. Relocate burr over edge of stone with one half of burr width on the stone. Set micrometer for .015"(0.38mm) past highest spot reading found in step 3. Penetrate burr into stone to this micrometer set point. Quickly lock Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Traverse burr 3 times across stone without retracting burr. Do not allow burr to run completely off edge of stone. Do not allow burr to stop or hesitate at edges of stone.
6. Advances the burr a further .010"(0.25mm) and traverse the stone 3 times without retracting burr. Do not allow burr to run completely off edge of stone.
7. Advances the burr a further .010"(0.25mm) and traverse the stone 3 times. At the end of this step replace burr with a new #12 Diamond Burr.
8. Find the spark point with a new #12 Diamond Burr in the sharpening lathe.
9. Continue steps 6 and 7 until the burr is making contact with the entire stone surface. Each diamond burr should be replaced after making 9 traverses of the stone surface. When the burr is making contact with the entire surface the stone is now in truth and this procedure can be stopped.
10. It is desirable to remove the diamond pattern left in the stone surface by the above procedure. To remove this diamond pattern mount a No 16 or finer diamond burr in the lathe. Find the spark point. Follow steps 6 and 7 and make 3 advances into the stone for a total of 9 traverses of the stone.

The truing of the stone may result in the joint material between the stone segments being higher than the abrasive surface. This happens because the burr will not cut or wear down the soft, resilient joint material in most pulpstones. Before proceeding any further, the joint material must be bricked until it is level with the stone surface. Normally bricking is done by hand using either a piece of stone segment or an abrasive block backed with a wood handle. The brick should be moved evenly and continuously across the rotating stone face.

4.3 ESTABLISHING THE INITIAL PATTERN

Before putting the initial pattern into the stone, adjust the lathe for the proper traverse speed for sharpening. The following formula should be used for determining this traverse speed:

BT = (60 x Ws) x 0.90/ (Ss x Wb)

N.B. Use either inches or millimetres throughout the formula. Do not mix inches and millimetres.

The speed found from the formula will provide for a 1" overlap of the burr for each revolution of the stone. Use a stopwatch to set lathe speed. In the case of hydraulic lathes operating off the grinder hydraulic system, make certain that lathe speed is uniform throughout the traverse and that grinder pocket cycling will not affect this speed of traverse. The traverse speed must be the same in both directions across the stone.

Select the proper sharpening burr for the pattern desired and proceed as follows:

1. Mount a new burr in the lathe. Move lathe to center of stone and advance burr to spark point. (See page 22 item 3 for definition of spark point. Note reading and retract burr.
2. Move lathe to either edge of stone and locate burr so that one half its width will come in contact with stone. Set micrometer for .020"(0.5mm) penetration past spark point. Feed burr into micrometer set point. Quickly lock the lathe Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Make one pass ending with running burr off opposite edge of stone.

   
   Fig.21 First Pass - 0.020"(0.5mm) Penetration

3. Remove used burr and discard. Mount a new burr of same specifications.
4. Find spark point as in step 1. Set micrometer for .030"(0.76mm) penetration past spark point. Move lathe to same side of stone as in step 2, feed burr into stone to micrometer set point. Quickly lock the lathe Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Make one pass in same direction as step 2 ending off the stone.

   
   Fig.22 Second Pass - 0.030"(0.76mm) Penetration

5. Remove used burr and discard. Mount a new burr of same specification.
6. Find spark point as in step 1. Set micrometer for .040"(1.0mm) past spark point. Move lathe to same side of stone as in step 2, feed burr into stone to micrometer set point. Quickly lock the lathe Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Make one pass in same direction as step 2 ending off the stone.

   
   Fig.23 Third Pass - 0.040"(1.0mm) Penetration.

7. Remove used burr and discard. Mount a new burr of same specification.
8. Find spark point as in step 1. Set micrometer for .045"(1.14mm) past spark point. Locate burr over opposite edge of stone from edge used for starting last 3 burrs. Feed burr into stone to micrometer set point and make one pass in opposite direction from step 6, ending off the stone edge.

   
   Fig.24 Fourth Pass - 0.045"(1.14mm) Penetration

9. Remove used burr and discard. Note in grinder log book on which side of stone last sharpening ended and total depth of sharpening.
   
   If a greater pattern depth is required, proceed as follows:
   
   
10. Mount a new burr of same specification.
11. Find spark point and set micrometer for .050"(1.27mm) past spark point. Locate burr over edge of stone where last sharpening ended. Feed burr into stone to micrometer set point and make one pass in opposite direction from step 8.
12. Remove used burr and discard. Mount a new burr of same specifications.
13. Find spark point and set micrometer for .060"(1.5mm) past spark point. Locate burr over edge of stone where sharpening in step 11 ended, feed burr into stone to micrometer set-point and make one pass in opposite direction from step 11.
14. Repeat steps 10-13 until desired pattern depth is reached. For burrs 10 pitch and higher do not penetrate more that .060"(1.5mm) past spark point. For burrs 6-9 pitch do not penetrate more than .080"(2.0mm) past spark point. For burrs 2-4 pitch do not penetrate more than .100"(2.5mm) past spark point. Note that the pattern in the stone will be about .005"(0.13mm) to .010"(0.25mm) less in depth than the burr penetration shown on the micrometer dial due to the wear on the burr teeth. Always note in the grinder logbook that side of stone on which last sharpening ended and the total depth of sharpening.

4.4 RE-SHARPENING THE STONE

1. Mount a new burr in the lathe of the same specifications as previously used.
2. Move lathe to center of stone and feed burr down to spark point. Note reading and retract burr.
3. Locate burr with 1/2 its width over edge of stone where last sharpening ended. Set micrometer a distance past spark point equals to the total groove depth required.
4. Feed burr into stone to micrometer set point. Quickly lock the lathe Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Make one pass across stone ending off the stone.
5. Note in grinder logbook on which side of stone this last sharpening ended and total depth of sharpening.

This completes re-sharpening of the stone. If the sharpening was not sufficient after several hours of operation, repeat steps 1-5 above but do not increase total groove depth.

4.5 LAND SURFACE RECONDITIONING

Sometimes after sharpening, no increase in freeness or production is experienced. Rather than sharpen harder or deeper it is better to attempt to recondition the land surface, to expose fresh abrasive on the tops of the lands. This same procedure can also be used to lengthen the periods between regular sharpenings. If this procedure is to be done immediately after a regular stone sharpening it is suggested that you wait 3-4 hours to see if the stone surface will open up on its own and become free cutting. The land surface reconditioning should be done with a new No 18, or 24 Diamond Burr. The following steps are used to recondition the land surface of the stone:

1. Mount a new No. 18, or 24 Diamond Burr in the lathe.
2. Locate the burr over the center of the stone and feed down to the spark point. Note the micrometer reading and retract the burr.
3. Set the micrometer for .002"(0.05mm) past the spark point.
4. Locate the burr with one half its width over the edge of the stone where the last sharpening ended.
5. Feed the burr into the stone to the micrometer set point. Quickly lock the lathe Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Traverse speed should be the same as sharpening traverse speed.

4.6 STONE SURFACE DULLING

Dulling of the land surface is recommended only in extreme cases or when making major grade changes. Frequent dulling indicates poor sharpening practice. A brick, worn #24 Diamond Burr, or smooth burr available from the burr manufacturer may be used to dull the stone surface. Bricking will produce the greatest degree of dulling followed in order of degrees of dulling by the worn 024 Diamond and finally the smooth burr. The procedure for using the worn diamond burr or smooth burr is as follows:

1. Mount burr in lathe.
2. Locate burr on center of stone and feed down to spark point Note micrometer reading and retract burr.
3. Locate burr with one half width over edge of stone where last sharpening ended.
4. Set micrometer for .002"(0.05mm) past spark point.
5. Feed burr down to micrometer set point and make two passes, without stopping the lathe, at the same setting ending at the same side as you started. Use the regular sharpening traverse speed.

The stone surface may be dulled by bricking, The brick may he a piece of soft fire brick or a piece of stone segment. 37C36-N7V is a good segment specification. It is suggested that the brick be backed with a wooden handle for safety and ease of use. Move the dulling brick by hand slowly back and forth over the stone surface.

4.7 PERIODIC PULPSTONE TRUING AND PATTERN REMOVAL

The sharpening burr always tries to track in the same grooves with every sharpening. As the stone wears smaller in diameter, the circumference of the stone becomes smaller. As the burr is trying to track the same number of grooves, the grooves become wider as the stone circumference becomes smaller. This narrows the lands causing them to eventually break down. It is recommended that on a regular basis, every three months, the pattern be removed and a new pattern be put into the stone surface. The following procedure is used for pattern removal:

1. Mount a new No. 12 Diamond Burr in the lathe and move the lathe to center of stone.
2. Find the spark point, note reading on micrometer and retract burr.
3. Move lathe to either side of stone with one half the burr width located on edge of stone and set micrometer for .010"(0.25mm) past spark point.
4. Advance burr to micrometer set point. Quickly lock the lathe Burr post and begin traverse. Do not allow burr to hesitate too long on edge of stone. Make three passes across stone without changing setting. Do not run burr off edge of stone. Lathe traverse speed should be 15-20 seconds.
5. Reset micrometer to .020"(0.5mm) past spark point and make three traverses of stone without changing setting. Do not run burr off edge of stone.
6. Repeat step 5 one more time increasing penetration by .010"(0.25mm) increments to a total of .030"(0.76mm) past spark point.
7. After penetrating the stone a total of .030"(0.76mm) or 3 separate burr advances into the stone replace the worn #12 diamond with a new #12 diamond burr.
8. Repeat steps 2-6 for three additional penetrations of .010"(0.25mm) each penetration. At this point you will have removed approximately .050"(1.27mm) of abrasive from the surface (total burr penetration less tooth wear).
9. Mount a new No.14 diamond burr in the lathe. Repeat steps 1-6 starting the burr on the side of the stone where the last pass of the burr ended. Make a total of 3 penetration advances of this burr.
10. Remove the worn burr and then brick the stone surface to wear the joint material down even with the abrasive. The stone is now ready for a new pattern.

4.8 BEVELLING EDGES OF STONE

Fig.25 Double Faced Bevelling Burr

The edges of a new pulpstone are beveled to prevent chipping of the stone edge. In time this bevel will wear away and require refurbishing. Chipping of the stone edge will cause the burr to vibrate, destroying the burr pattern. Stones should be beveled every three months after periodic trueing and pattern removal. Follow these procedures for stone bevelling.

1. Bevelling burrs are manufactured in two diameter sizes; 4.5"(114.0mm) and 6.0"(152.0mm). Select the largest size "Double Faced Bevelling Burr" which will fit into your lathe fork.
2. The bevelling burr is fitted with a bronze bushing having grease grooves in its inside diameter. Fill these grease grooves with heavy weight grease.
3. The pin upon which the bevelling burr rotates in the lathe fork should be designed especially for use with the bevelling burr. The diameter of the pin should be .003"(0.08mm) smaller than the bore of the bronze bushing in the bevelling burr. Grease the surface of this pin and install the pin and Bevelling Burr in the grinder lathe fork. If there is room in the fork a bronze or brass thrust washer may be installed between the burr and the side of the fork to prevent wear on the side of the fork. The burr must rotate freely.
4. Carefully locate the Double Faced Bevelling Burr on the edge of the stone so that the edge of the burr is just off the edge of he stone.
5. Feed the Bevelling Burr down onto the edge of the stone using firm pressure to produce a high rate of abrasive removal. This operation should be done quickly to prevent rapid wear of the Bevelling Burr teeth. Make certain that the lathe fork does not touch the stone. You should have a bevel about .875"(22mm) wide.
6. To bevel the other edge of the stone, raise the burr clear of the stone and move the lathe to the opposite edge. Repeat steps 4 & 5.

4.9 ADAPTING OLD GRINDERS

Many older grinders do not have micrometer lathes making it difficult to follow the preceding sharpening methods. If the Burr post feed screws of older lathes are in good condition an approximate calibration of lathe fork travel can be calculated making it possible to follow these sharpening instructions. Refer to Section 5.3 on sharpening lathe maintenance.

Lathe fork travel can be measured with the use of a dial indicator attached to the fork.

Fig.26 Measuring Lathe Fork Travel

With the stone stopped, feed the lathe fork towards the stone until the dial indicator touches the stone. Make one revolution of the lathe feed handle and read movement of lathe fork on dial indicator. Knowing this measurement you can now calculate what fraction of a revolution of the lathe feed handle is necessary for each penetration of the burr mentioned in the preceding sharpening procedures.

Many mills, because their equipment is in poor condition or through lack of proper training, have adopted a method of sharpening which depends upon the operators senses of touch and hearing to control the penetration of the burr into the pulpstone. If the stone sharpener is highly skilled and is sharpening stones often he may be able to do a good job of sharpening. The risk of sharpening too hard and damaging the stone surface is great with the sound and feels method of sharpening. With the trend in most mills towards many people doing stone sharpening, this method of sharpening becomes difficult to control and perform properly. The results tend to be higher than desired freeness after sharpening, short sharpening cycles and short stone life.