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 Centreless Grinding

Below, are listed problems, which may be encountered whilst grinding. Under each are listed probable causes, and under each cause, possible corrections. Possible corrections are used advisedly here. It is practically impossible to establish hard and fast rules for turning out perfect grinding. There are many variables, which must be correctly attuned, and variables which must be correctly attuned, and an apparently irrelevant one may prove to be cause of trouble experienced. Also, the same cause may produce a varied number of grinding problems.

SCRATCH MARKS ON WORK

 

Improper work support blades
Change blade to a softer material; if tungsten carbide was used,
try steel, cast iron or aluminium bronze.

Wide irregular marks, varying in depth, from too soft a wheel
Use harder grade wheel.

Widely spaced spots on work piece due to oil or glazed spots on wheel
True spots out of wheel, balance and redress.
Avoid getting oil on wheel.

Even fine spiral diamond lines
Dress at slower traverse rate. 
Turn diamond frequently. 
Diamond cracked or broken; replace. 
Turn diamond frequently. 
Diamond cracked or broken; replace.
Diamond holder loose; tighten clamping screw.
Try lighter diamond cuts.
Make final dressing in opposite direction to initial runs.

Even spiral lines on work piece whose lead corresponds to feed rate.
Dress wheel face parallel to work piece to prevent leading or trailing edge from digging in.
Use crowned cam when dressing grinding wheel.
Cut back front of face of wheel approximately half of stock removal over 1" distance for a six-inch wheel (relatively more on wider wheels)
Increase or decrease successive traverse rates to break pattern of diamond lines. When through feed grinding, make certain guides on regulating wheel side are parallel and even with wheel face.

Uneven fine lines on work piece due to faulty dressing
Do not allow diamond to dwell on wheel. Barely contact wheel face at high spot—dress across face only, starting at an edge, do not start on face. Maintain diamond traversing at an even rate.

Uneven, or evenly spaced lines on work piece due to extraneous vibrations
If impossible to change machine location, mount grinder on some vibration insulating material. Be certain insulation is thick enough to be effective.
If it is not sufficient, the natural frequency of grinder normally damped out by rigid construction may be emphasised and combine with external vibration to make conditions worse.

Wavy traverse lines due to faulty dressing, leaving ragged wheel edges.
Round off wheel edges nicely—chamfering or dressing back is not sufficient.

Irregular grit marks as result of wheel bond’s disintegrating. ("Fish Tails".)
Try a different coolant, or if soluble oil used, cut down soda content of coolant. High soda concentrations attack resinoid and shellac bonds. Wheel too soft—use harder grade.

Coarse grains or foreign matter in wheel face.
Dress these out of wheel.
Determine whether caused by type of wheel, dull diamond, dirty or too strong a coolant.

Irregular scratches of various lengths and sizes due to dirty coolant. ("Fish tails.")
Empty and clean tank, lines, guards, etc. Flush guards and wheel after each truing. Use coolant filter for fine finishes.

Too great a difference in grain size between roughing and finishing wheels.
Dress roughing wheel at slower traverse rate. 
If this doesn’t help, try a finer cut with roughing wheel.

Deep irregular marks caused by loose wheel.
Use one standard paper washer (blotting paper if other not available) under flanges and tighten down.

Isolated deep marks due to improper dressing.
Rotate or change diamond to—get sharper dressing. Wash wheel thoroughly with coolant after dressing. Reset gib on truing device.

Grain marks due to too soft or coarse a wheel.
Substitute harder grade or finer grained wheel

Rough uneven finish.
Diamond cracked or broken; replace.

Regulating wheel too hard.
If regulating wheel too hard, work will spin and score on blade but it would be practically impossible to grind.

Wheel too smooth.
Reset diamond or use new one. Use coarser or softer wheel.

Roughing wheel breaks down too fast.
Rough grind with more and smaller cuts, or use harder wheel for roughing.

   
VIBRATION  

Loose levelling screws.
Set all screws so they bear evenly on floor.

Grinding wheel out of balance.
Inspect spindle and collet—remove burrs if present. Use only one standard or blotting paper gasket under each flange.
Make sure balancing stand is level both ways before attempting to balance wheel. Always balance wheel in following sequence:

  1. True wheel.
  2. Run wheel long enough to throw off coolant.
  3. Balance wheel on same mount as used for grinder.
  4. True wheel after mounting on spindle

Motor bearing defective or armature out of balance.
Renew motor bearings or balance armature.

   
MARKS ON WORK  

Work too high above centre.
Lower work support blade.

Too great an angle on workrest blade
A 3O° angle should be a good average starting angle for most work. Long blades will cause chatter unless blade angle is reduced. Also for small diameter work it will generally be necessary to reduce angle.

Work support blade too thin
Use a heavier blade. Blade length must be slightly longer than wheel width. Blade thickness should be under work diameter just enough so that it doesn’t cause wheel interference.

Work support blade loose.
Tighten clamping screws.

Too heavy stock removal - particularly on in feed work.
Try lighter cuts and more passes.

Wheel out of round.
Make certain cap screws are tight in mount. Use only one gasket under each flange. When two are used constant soaking and drying out may eventually cause loosening of cap screws.

Type of wheel incorrect for particular job.
Consult your grinding wheel manufacturer’s representative for specific particulars.

Loose Wheel Mount.
Tighten spindle nut or clamping screws through collet.

Extraneous vibrations set up by nearby machinery.
If impossible to change machine location, mount grinder on some vibration insulating material. Be certain insulation is thick enough to be effective.
If it is not sufficient, the natural. frequency of grinder normally damped out by rigid construction may be emphasised and combine with external vibration to make conditions worse.

Loose spindle bearing
Adjust spindle bearings. If old type machine with plain bearings (not Filmatic) it may be necessary to rescrape bearings.

Loose diamond holder ; loose or cracked diamond.
Tighten diamond holder clamping screw; reset or replace diamond.

When shoulder grinding if regulating wheel is forward of grinding wheel, it prevents grinding to shoulder and causes work to pick up or chatter.
Remove or change spacer behind regulating wheel so side of regulating wheel is back of grinding wheel.

Work rest blade bowed so work not supported for full length.
True edge of blade. Make certain support slot free of dirt before inserting blade.

   
LOW ENDS ON WORK CHAMFERED (THROUGHFEED)   Work guides are deflected toward regulating wheel.
Set guides parallel to wheel face. If ends still small, with only a few pieces to grind, decrease angle of regulating wheel housing by small amounts (1/8°)until work right. Do not dress wheel or change angle of truing attachment.
However, increasing truing angle and re truing wheel without changing housing angle can attain same result. If a large number of pieces are to be ground, and it will be necessary to true wheel at times, then decrease angle of regulating wheel truing attachment by small amounts(1/8°)until work is right. Redress wheel. Do not change angle of housing.
   
WORK NOT STRAIGHTENED BY THROUGH FEED GRINDING  

Work not straightened sufficiently before grinding
Make certain it is straightened on presses before grinding. A Centreless grinder is not designed as a straightening machine although it will remove slight kinks.

Insufficient stock left on piece for grinding.
Allow grinding stock equal to approximately three times amount of run out.

Stock removal on first pass too great.
First pass should be a light cut with high spindle housing angle but slow regulating wheel speed. This gives maximum straightening effect on grinder. Use as wide a grinding wheel as possible and cut full width of face.

Bowed short piece, about twice length of wheel face.
Straighten in press before grinding.

 

   
WORK OUT OF ROUND (THROUGH FEED)  

Work centreline too low.
Set work centreline ½ of its diameter above wheel centreline, but on work 1" in diameter or above never exceed ½".

Stock removal too heavy on roughing cuts.
Make first cut light at high traverse rate to attain maximum straightening effect. Then use heavier cuts and finish with a light cut.

Insufficient passes on hardened work.
Increase number of passes.

Wheel grade too hard.
Increase traverse or in feed rate to make wheel act softer. Or use a softer grade wheel—consult your grinding wheel manufacturer’s representative for specific particulars.

Insufficient coolant when grinding tubing or other hollow work such as pistons, etc
Increase flow of coolant at point of contact.

Grinding pressures too high. Especially on tubing, pistons, etc.
Dress wheel more open with a sharp diamond and more rapid traverse.

Regulating wheel speed too slow.
Increase regulating wheel speed.

Regulating wheel loose on spindle or mount.
Tighten spindle nut and collet screws

Loose spindle or regulating wheel bearings.
Adjust bearings.

Angle on work blade too low
Gradually increase blade angle until right

Loose or tight regulating wheel chain will cause periodic or intermittent flats on work piece.
Remove or add links from or to chain until setting is correct.

Irregular flats on in feed work pieces when work left between wheels too long.
Eject work pieces as soon after sizing as possible.

Flats on in feed work caused by ejector ‘cocking work into grinding wheel.
Eliminate cause of side strain on ejector mechanism.

Flats on through feed work caused when it stops rotating between wheels.
Dress wheels correctly, and, if necessary, increase inclination angle of regulating wheel spindle so work feeds all the way out.

   
FAULTY SIZING - IN FEED  

Insufficient lubrication at lower slides causes slides to hang and jump.
Lubricate per manufacturers "Lubrication Instructions and Specifications". Be certain to adjust size again.

Worn spiral gear on slide adjusting screw.
Move spiral gear on screw to new contact face and lap gears together.

Slide adjusting screw thrust bearings loose.
Tighten adjusting screw.

Worn gears in hand feed unit.
Replace with new gears.

Worn or loose lower slide gib allows side movement of slide, cocking regulating wheel.
Reset gib

Loose work support blade allows deflection.
Tighten blade in body and tighten body to lower slide.

     
FAULTY SIZING - THROUGH FEED  

Work piece over size (generally), or under size a few tenths of a thousandth.

In a majority of through feed jobs, grinding should be done with ‘upper slide clamped, lower slide unclamped, and adjusting screw set in middle of backlash so there is no strain on screw. If in feed screw is under strain either way any considerable jar on machine, such as from a passing industrial truck, nearby trip hammer or a light blow to machine, etc., may cause a slide movement of a tenth of a thousandth. Also unclamping or clamping either slide might cause a slight movement.

To adjust slide either way a few tenths of thousandths do so with a 0.0001" dial indicator mounted against slide.

Loose regulating wheel spindle bearings.
Adjust bearings. If sizing still out, after checking every other possible source of trouble, scrape bearings.




WHEEL LOADING - METAL LODGED ON GRAINS OR IN PORES  

Grinding wheel too hard for particular work.
Use a coarser grained wheel with more open bond to allow greater chip clearance. Increase coolant flow.

Scale from hardened work, or dirty (oil, solder, welding flux, etc.)
Clean work before grinding. Dress wheel more often.

Incorrect dressing of grinding wheel
Turn diamond or replace with new one. Increase rate when dressing.

Stock removal too fast.
Check set-up. Take lighter cuts or in feed at slower rate. Increase regulating wheel (work speed) to make wheel act softer.

Coolant incorrect or dirty.
Change coolant. If using soluble oil, use thinner coolant, add soda, or if necessary filter coolant.

 

   
WHEEL ACQUIRES SLICK, SHINY APPEARANCE - GLAZED  

Incorrect wheel for particular job.
Consult your grinding wheel manufacturer’s representative for specific particulars.

Scale from hardened work dulls wheel grains.
Clean work before grinding. Dress wheel more often.

Incorrect dressing of grinding wheel
Turn diamond to avoid fiat spot or replace with new one. Increase penetration of dressing diamond up to 0.001". Check slides on truing attachment; they may be worn low in centre. Truing attachment gibs may be loose enough to allow diamond to back off instead of cutting.

Stock removal too slow when in feed grinding.
Increase in feed rate.

Coolant incorrect or dirty.
Change coolant. Decrease concentration of soluble oil. Increase flow of coolant. Clean coolant tank and machine.

Coolant gummy.
Change coolant. If water hard, treat with a water softener before mixing with oil. Increase soda or decrease oil content of coolant when using soluble oil.

 




WHEEL ACTION - GRADING  

Wheel acts too hard indicated by:

  1. Loss of cutting action.
  2. Wheel loading.
  3. Work discoloured, burned
  4. Work chattering.
  5. Increase work speed (regulating wheel) and pressure.
  6. Dress with sharp diamond at increased traverse rate.
  7. Use thinner coolant; avoid gummy coolants. Increase work speed; use softer wheel.
  8. Use coarser grain size and softer grade.

Wheel acts too soft indicated by:

  1. Short wheel life.
  2. Wheel markings on work.
  3. Not holding cut.
  4. Decrease work speed (regulating wheel). Wheel too soft.
  5. Slow down traverse and work speeds, also work pressure.
  6. Dress wheel with slow traverse and light penetration. Take lighter cuts when grinding. Wheel too soft.

 

   
WHEEL BREAKAGE  

Radial Break, 3 or More Pieces.

  1. Flanges pulled up unevenly causing side strains on wheel.
  2. Wheel overheated.
  3. Flange screw loose.
  4. Work jammed into wheel causing excessive pressure.
  5. Use only standard paper or blotting paper gaskets. Be sure gaskets and flanges are clean. Pull up evenly on alternate bolts before cinching up.
  6. Use plenty of coolant for dressing and grinding. Do not force or jam work into wheel.
  7. Keep flange screws tight at all times.
  8. Prevent work jamming into wheel.

Irregular Break.

  1. Faulty wheel.
  2. Wheel dropped or subject to hard blow.
  3. Work jammed into wheel.
  4. Wheel forced on collet when mounting.
  5. Suspend wheel and tap lightly before mounting. If result is not a clear ping do not use wheel.
  6. Do not handle heavy wheels except with special lifting device. Do not subject wheels to hard blows.
  7. Prevent work jamming into wheel. Make certain none of the pieces are oversize, particularly on in feed.
  8. Wheel should be just a nice sliding fit (Maximum clearance of 0.003" on diameter) on collet and must be put on squarely. Have collet on bench. Start wheel on collet and tap lightly all around with a small piece of wood - do not let wheel drop on collet.
  
MAINTENANCE TIPS  

1. Keep machine clean. Be certain all stations are kept properly lubricated. Particularly when doing in feed work, be sure to lubricate all slides in accordance with instructions.

2. Immediately upon receipt, all grinding wheels should be closely inspected for possible damage in transit. Inspect for cracks by tapping gently (while suspended) with a light implement, such as screwdriver handle. Wheels must be dry and free of sawdust when applying this test. If they do not emit a clear, ringing sound, they are cracked and should not be used. Regardless, new wheels should be run at full operating speed for at least a minute before grinding any work. During this Interval, the operator should stand to one side.

3. Store grinding and control wheels in a dry place.

4. When through-feed grinding relatively short work, without using a loading device, do not use fingers to push work between wheels. Use a piece of wood dowelling or other suitable strip.

When in feed grinding, without using an automatic loading device, construct a sheet metal guard on workrest so operator’s fingers are protected from wheels.

Before stopping machine run grinding wheel without coolant long enough to throw all water out of wheel. Otherwise moisture concentrated at bottom will cause unbalance.