Process How-To

How to Cut the Sharpening Bar of a CNC Surface Grinding Machine

A practical guide for machinists and toolroom engineers who need a straight, well-dressed bar without burning the wheel or losing geometry. It covers wheel selection, cut parameters, coolant, and the checks that tell you to stop. Read it before you touch the feed override on a cnc surface grinding job.

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Wheel: 46–60 gritDepth: 0.005–0.02 mmCoolant onDress after 3–5 passes

How to cut the sharpening bar of a CNC surface grinding machine

On this page

1
Key takeaways
2
What the sharpening bar does on a cnc surface grinding machine
3
Wheel and fixture setup for cnc surface grinding
4
Choosing cut parameters for cnc surface grinding
5
Common mistakes and how to avoid them
6
Step by step: cutting the sharpening bar
7
Which wheel to use for bar material and finish target

Quick Answer

Key takeaways

Cut, then dressRough the bar to near size, then dress the wheel and take a light finishing pass. Skipping the dress pass is the most common cause of taper.

Depth per pass 0.005–0.02 mmAnything deeper on a 46-grit wheel loads the wheel and pushes the bar out of flat.

Table speed 10–18 m/minToo slow burns the edge; too fast leaves chatter marks along the length.

Coolant alwaysDry cutting on hardened bar stock draws temper and cracks the wheel edge.

Check flatness with a dial indicatorAnything over 0.01 mm over 100 mm means the wheel needs re-dressing, not more passes.

Before you start

What the sharpening bar does on a cnc surface grinding machine

The sharpening bar is the wear part that rides against the grinding wheel and holds the cutting edge true. On a cnc surface grinding machine it also sets the reference the machine uses when it compensates for wheel wear. If the bar is not cut parallel to the table travel, every part that follows inherits that error.

Most bars are made from hardened tool steel or 4140 in the 45–55 HRC range. Some shops use carbide inserts for long runs. The material choice decides your wheel grade more than the machine does. Hard bar stock calls for a softer wheel; soft bar stock calls for a harder one.

The job itself is simple: bring the bar to length and section, keep the working face parallel, then dress the wheel that cut it. The difficulty is holding the last 0.01 mm without burning the edge or loading the wheel. That is what the steps below address.

1
Bar materialHardened tool steel, 4140, or carbide for long-run fixtures.
2
Typical tolerance±0.005 mm on the working face is achievable on a rigid machine.
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Surface finish targetRa 0.8–1.6 μm on the ground face is normal for a dressed wheel.

Equipment

Wheel and fixture setup for cnc surface grinding

For general bar work, an aluminum oxide wheel in 46 to 60 grit with an H to K grade covers most jobs. A 46-grit wheel removes stock faster but leaves a coarser finish. A 60-grit wheel gives you Ra 0.8–1.6 μm more easily but cuts slower and glazes sooner if you push it.

Balance the wheel before mounting. An unbalanced wheel at 3,000 rpm shows up as a wave pattern on the bar rather than as a vibration you can feel by hand. Most CNC surface grinders have a balance arbor or an in-machine balancer; use it.

For the fixture, clamp the bar on a magnetic chuck with a parallel under it, or hold it in a vise with soft jaws machined to the bar section. The bar must not move when you reverse table direction. Any movement shows up as a step in the middle of the cut, not at the end.

1
Wheel speedKeep at the machine-recommended surface speed; do not lower it to reduce burning.
2
Fixture checkDial the bar top face within 0.01 mm before cutting.
3
Wheel balanceRebalance after every wheel change, not just when finish drifts.

Parameters

Choosing cut parameters for cnc surface grinding

The three numbers that matter are depth of cut, table speed, and cross feed. Depth controls how much heat goes into the bar. Table speed controls how long the wheel stays in contact with any one point. Cross feed controls how much of the wheel edge does the work.

On hardened bar stock above 45 HRC, keep depth at 0.005–0.01 mm for finishing. On softer 4140 in the low 30s HRC, you can take 0.02 mm roughing passes without trouble. The limit is not the machine; it is the wheel's ability to clear chips.

If you see burn marks, the fix is not always slower table speed. Check coolant flow first. A blocked nozzle starves one side of the wheel and produces a burn stripe that runs the full length of the bar.

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Roughing depth0.01–0.02 mm per pass on 46-grit aluminum oxide.
2
Finishing depth0.005–0.01 mm per pass, plus one spark-out pass.
3
Cross feed50–70% of wheel width for roughing, 30–50% for finishing.
4
CoolantFlood both sides of the wheel. Check nozzle alignment before every job.

Problems

Common mistakes and how to avoid them

The most common mistake is cutting the full length in one direction. On a long bar, the wheel wears more at the entry end than at the exit. The result is a taper of 0.01–0.02 mm that you cannot see until you measure. Alternate the table direction on every pass, or index the cross feed so the wear evens out.

The second mistake is skipping the dress between roughing and finishing. A loaded wheel rubs rather than cuts. You feel it as heat and see it as a shiny, smeared finish. The fix is one dress pass at 0.01 mm, then a fresh finishing cut.

The third mistake is measuring the bar while it is still hot. A bar that reads on size at 60 °C will be undersize when it cools. Let it sit on the surface plate for a few minutes before the final check.

1
Burn stripes on one sideCoolant nozzle misaligned or blocked. Realign and re-dress.
2
Chatter marksTable speed too high or wheel out of balance. Rebalance and slow down.
3
Taper along the lengthWheel wear on one side. Alternate direction or index cross feed.
4
Size drifts after coolingMeasured hot. Let the bar cool before final inspection.

Procedure

Step by step: cutting the sharpening bar

Follow the order. Rushing step 4 is what ruins most bars.

1
1. Inspect and measure the barCheck for cracks, bending, or previous grinding burn. Measure length, width and thickness with a micrometer. Note the stock you need to remove. If the bar is bent more than 0.05 mm over its length, straighten or replace it before grinding.
2
2. Mount and indicateClamp on the magnetic chuck or in soft jaws. Run a dial indicator along the top face and the side face. Adjust until both read within 0.01 mm. Re-tighten, then re-indicate; clamping often moves the bar.
3
3. Rough grind with a 46-grit wheelSet depth of cut at 0.01–0.02 mm per pass. Table speed 10–18 m/min. Cross feed 50–70% of wheel width. Leave 0.05 mm stock for the finishing pass. Listen for a steady hiss; a change in pitch means the wheel is loading.
4
4. Dress the wheelStop after every 3 to 5 roughing passes and dress with a single-point diamond at 0.01–0.02 mm per pass. A glazed wheel rubs instead of cutting and pushes the bar out of flat. Dressing takes 30 seconds and saves the bar.
5
5. Finish grind to sizeSwitch to a 60-grit wheel or keep the dressed 46-grit wheel. Depth of cut 0.005–0.01 mm. Table speed 12–15 m/min. Take a spark-out pass at zero infeed until sparks stop. This is where you hold ±0.005 mm.
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6. Check flatness and finishUse a dial indicator on a surface plate. Flatness should read within 0.01 mm over 100 mm. Check surface finish against your target. If either is off, re-dress and take one more light pass rather than adding depth.
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7. Deburr and cleanStone the edges lightly by hand. Clean the bar with solvent and wipe the machine table. Grinding dust left on the chuck will throw off the next setup.

Decision Guide

Which wheel to use for bar material and finish target

Pick the row that matches your bar hardness and finish callout.

Bar material / hardness	Wheel grit and grade	Depth per pass	Expected finish
4140, 28–35 HRC	46 grit, J grade	0.015–0.02 mm	Ra 1.6–3.2 μm
Tool steel, 45–50 HRC	46 grit, H grade	0.01–0.015 mm	Ra 0.8–1.6 μm
Tool steel, 50–55 HRC	60 grit, H grade	0.005–0.01 mm	Ra 0.8–1.6 μm
Carbide insert bar	Diamond wheel, 100–150 grit	0.005 mm	Ra 0.2–0.8 μm
Soft steel, under 28 HRC	46 grit, K grade	0.02 mm	Ra 1.6–3.2 μm
Thin bar under 6 mm	60 grit, J grade	0.005 mm	Ra 0.8–1.6 μm

FAQs

Frequently asked questions

How often should I dress the wheel when cutting a sharpening bar?

Every 3 to 5 roughing passes is a safe interval for hardened bar stock. On softer material you can stretch it to 6 or 8 passes.

If the finish starts to look shiny or smeared, dress immediately. A glazed wheel stops cutting and starts rubbing, which pushes the bar out of flat.

Can I cut a sharpening bar dry on a cnc surface grinding machine?

No, not on hardened bar stock. Dry grinding concentrates heat at the contact point, which can draw temper and crack the wheel edge.

If coolant is not available, take very light passes at 0.002–0.005 mm and let the bar cool between passes. Expect a shorter wheel life and a rougher finish.

What flatness can I realistically hold?

On a rigid machine with a balanced wheel and a properly dressed face, 0.005–0.01 mm over 100 mm is realistic.

Tighter than that needs a temperature-controlled room and a fixture that does not move when you reverse the table.

How do I know the bar is clamped properly?

Indicate the top and side faces after clamping. If either moves more than 0.01 mm when you push the bar by hand, the clamp is not holding.

Re-check after the first pass. Grinding forces can shift a bar that felt tight before the wheel touched it.

Should I use a 46 or 60 grit wheel for the finishing pass?

A freshly dressed 46-grit wheel can hold Ra 0.8–1.6 μm on most bar materials. Switching to 60 grit helps when the finish callout is tight or the bar is thin.

The trade-off is speed. A 60-grit wheel cuts slower and glazes sooner, so dress it more often.

What causes a step in the middle of the bar?

A step usually means the bar moved during the cut, or the cross feed stopped and restarted mid-pass.

Check the clamp first, then the cross-feed drive. Re-dress the wheel and take a light pass over the full length to blend the step out.