Troubleshooting guide

Double Ended CNC Grinder: How to Fix the Workpiece and Hold It

A double ended CNC grinder grinds both faces of a part in one cycle, so anything loose in the fixture shows up twice. The rule of thumb is simple: the workpiece must be located on the surface that controls the dimension, and clamped hard enough to resist the tangential cut, not so hard that it springs. This page covers the 7 symptoms we see most often, what causes each, and how to set the fixture so the size holds.

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double ended CNC grinder fixture holding custom auto spare parts for opposed face grinding

On this page

1
Double ended CNC grinder fault table
2
Why the double ended CNC grinder holds or loses a part
3
Fixture choices for a double ended CNC grinder
4
Wheel, coolant and infeed settings that keep the part still
5
What the double ended CNC grinder can and cannot hold
6
Step by step: fixing the workpiece on a double ended CNC grinder

Symptom to fix

Double ended CNC grinder fault table

Read the left column first. If two symptoms match, fix the one that repeats on every part before touching the wheel.

Symptom	Likely cause	What to do
Taper across the face	Workpiece not square to the wheel axis	Re-seat on the locating face, indicate to 0.01 mm
Chatter marks, even pitch	Clamp force too high, part springs	Drop clamp pressure, add a support pad
Size drifts over a run	Thermal growth in the fixture or bed	Let the machine idle 20-30 min, re-zero
One face shiny, one dull	Wheel wear differs side to side	Dress both wheels, re-check the gap
Part shifts mid-cycle	Tangential force exceeds friction	Add a positive stop, not more clamp pressure
Burr on the exit edge	Wheel too coarse or infeed too fast	Reduce infeed to 0.01-0.02 mm/pass
Parallelism out of spec	Fixture reference worn or dirty	Clean and inspect the locating pads

The verdict on fixing a workpiece

If the part has two parallel faces and a thickness callout, a double ended CNC grinder will hold it to +/-0.005 mm, but only when the fixture is indicated and the clamp is light. If the datum is rough or the part is bent, no amount of clamp pressure will fix it.

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Fundamentals

Why the double ended CNC grinder holds or loses a part

A double ended CNC grinder uses two opposed wheels that feed into the part at the same time. The part sits between them, usually on a locating face, a mandrel or a set of pads. Because both wheels cut at once, the forces cancel in the axial direction but add in the tangential direction. That single fact explains most fixture failures. If the clamp is the only thing resisting the tangential load, the part will creep.

The workpiece is not held by the wheel. It is held by the fixture, and the fixture is held by the machine bed. Every error in that chain shows up on the ground face. A locating pad that is worn by 0.02 mm puts that error straight into the parallelism reading. This is why we indicate the fixture, not just the part, before a run.

People often ask whether a double ended CNC grinder can be used to fix the workpiece. It can, but you have to define what fix means. The grinder removes material from two faces and sets the thickness. Position and orientation come from the fixture. If the part is not located on a ground surface, the machine will grind two parallel faces on a part that is still crooked.

1
Locate on a ground faceNever locate on a raw casting or a saw-cut edge.
2
Support the overhangAny unsupported length over 3x thickness will vibrate.
3
Clamp below yieldAluminium 6061 can mark above roughly 2 kN on a small pad.

Fixture design

Fixture choices for a double ended CNC grinder

There are three common ways to hold work on a double ended CNC grinder: a locating plate with pads, a mandrel or arbor, and a magnetic chuck. The choice depends on the part geometry and the tolerance you need. For a flat plate with a ground back face, a pad fixture is cheapest and easiest to indicate. For a ring or a bushing, a mandrel controls the bore and the face in one setup.

Magnetic chucks work well on ferrous parts with a large flat face. They hold the full area, so distortion is low and loading is fast. They do nothing for non-ferrous parts and they will not resist a heavy tangential cut on a small footprint. If the part is stainless 304 or aluminium, plan for mechanical clamping.

Clamp pressure is the parameter most people get wrong. More pressure does not mean more grip. On a thin disc, high pressure bows the part, the wheels grind it flat while it is bowed, and it springs back when released. The face is then concave. We keep clamp pressure low and add a positive stop instead, so the part cannot slide even if friction drops.

For small runs, a soft jaw machined in place is often enough. Machine the jaw on the machine, not on a mill, so the jaw matches the actual spindle axis. Then indicate the locating face to 0.01 mm. This takes 20 minutes and saves hours of size chasing later.

1
Pad fixtureBest for flat plates with a ground back face.
2
MandrelBest for rings and bushings where the bore is the datum.
3
Magnetic chuckFast for ferrous flat parts, no grip on aluminium.

Process settings

Wheel, coolant and infeed settings that keep the part still

Wheel selection sets the force level. A coarse wheel cuts cooler but pushes harder. On a double ended CNC grinder, that extra push goes straight into the fixture. For thin parts, use a finer grain and a softer grade so the wheel stays free-cutting. A loaded wheel rubs instead of cutting, and rubbing is what pulls the part.

Dressing matters more than most operators expect. If the two wheels are not dressed to the same sharpness, one face cuts more than the other. The part then sees an unbalanced axial load and tips. Dress both wheels in the same cycle, and check the gap with a gauge or a test part before running production.

Coolant should flood both faces, not just the top. A dry lower face heats unevenly, and the part grows on one side. That shows up as a taper that gets worse through the shift. Aim the nozzles at the contact zone and keep flow steady. If the coolant is dirty, the wheel loads faster and the force rises.

Infeed per pass should stay in the 0.01-0.02 mm range for finishing. Faster infeed raises the tangential force and the risk of slip. If you need cycle time, take a rough pass at a higher rate and leave 0.05 mm for the finish. Do not try to finish in one heavy pass and then blame the fixture when the part moves.

1
Free-cutting wheelFiner grain, softer grade for thin parts.
2
Dress both wheels togetherUnequal sharpness tips the part.
3
Finish infeed 0.01-0.02 mmHeavier passes raise slip risk.

Tolerances

What the double ended CNC grinder can and cannot hold

On a well-set fixture, a double ended CNC grinder holds thickness to about +/-0.005 mm and parallelism to the same order. Surface finish lands in the Ra 0.2-0.8 um range with a fine wheel and a steady infeed. These numbers assume the fixture is indicated and the wheels are dressed. Skip either step and the same machine will produce parts 0.05 mm out.

The machine cannot fix a part that is located wrong. If the datum face is rough or the part is bowed before grinding, the grinder will produce two flat faces on a bent part. Thickness will look good and the part will still fail on flatness. Check flatness before grinding, not only after.

Hard materials behave differently. Tool steel and 17-4PH push the force up, so clamp pressure and support need more attention than on aluminium 6061. Titanium TC4 grinds hot and can smear if the wheel is loaded. Inconel needs a softer wheel and a lighter pass. The fixture rules stay the same; the numbers move.

Where the grinder is a poor fit: very long shafts, parts with a single face to grind, and parts where the opposite face is a datum that must not be touched. In those cases a surface grinder or a mill-turn setup is a better choice. A double ended CNC grinder earns its place on parts with two parallel faces and a thickness callout.

1
Good fitTwo parallel faces, thickness callout, medium to high volume.
2
Poor fitLong shafts, single-face work, datum that must stay untouched.

Setup sequence

Step by step: fixing the workpiece on a double ended CNC grinder

Follow the order. Skipping step 2 is the most common reason a size drifts all shift.

1
Clean and inspect the locating faceWipe the pads and the part. A chip of 0.02 mm under the pad becomes 0.02 mm of taper. Check pad wear with a straight edge.
2
Indicate the fixture, not only the partPut a dial indicator on the locating face and rotate or slide the fixture. Aim for 0.01 mm total. Adjust shims before you touch the clamp.
3
Set clamp pressure low and add a positive stopStart at the lowest pressure that holds the part by hand. Add a stop pin so tangential force is carried by steel, not friction.
4
Dress both wheels in one cycleUse the same dress depth and feed on both sides. Check the gap with a gauge or a scrap part before production.
5
Warm up the machine 20-30 minRun the spindles and coolant at working speed. Then re-zero the infeed. Cold starts drift 0.02-0.03 mm in the first hour.
6
Run one test part and measure all facesCheck thickness, parallelism and flatness. If flatness is out but thickness is good, the clamp is too high or the support is missing.
7
Re-check size every 20-30 partsLog the reading. A slow drift means thermal growth or wheel wear, not a fixture problem. Dress and re-zero.

FAQs

Questions engineers ask about double ended CNC grinder fixturing

Can a double ended CNC grinder fix a workpiece that is already bent?

No. The grinder removes material from two faces and sets the thickness. It does not straighten a bent part. If the part is bowed before grinding, it will be bowed after grinding, with two flat faces on it.

Check flatness on a surface plate before you load the fixture. If the bow is over the flatness callout, straighten or re-machine the part first.

Why does the size drift during a long run?

The usual cause is heat. The bed, the fixture and the part all grow as the machine runs. A cold start can be 0.02-0.03 mm off within the first hour.

Run a 20-30 minute warm-up, then re-zero. After that, check size every 20-30 parts and dress the wheels on a fixed interval.

How much clamp pressure should I use on a thin disc?

Use the lowest pressure that stops the part from moving by hand, then add a positive stop. On a thin aluminium disc, anything above roughly 2 kN on a small pad will mark or bow the part.

If the part still slips, the problem is the stop or the wheel force, not the clamp. More pressure will only add distortion.

What surface finish can I expect?

With a fine wheel and a steady infeed, the ground faces land in the Ra 0.2-0.8 um range. A normal production setup runs around Ra 0.8-1.6 um.

Finish depends on wheel grade, dress quality and infeed per pass. A loaded wheel will rub and leave a worse finish than a sharp coarse wheel.

When should I not use a double ended CNC grinder?

Skip it for long shafts, for parts with only one face to grind, and for parts where the opposite face is a datum that must not be touched. Those jobs suit a surface grinder or a mill-turn setup.

The double ended CNC grinder earns its place when the part has two parallel faces and a thickness callout.

What tolerance can the process hold on hard materials?

Thickness and parallelism of about +/-0.005 mm are achievable on tool steel, 17-4PH and TC4 when the fixture is indicated and the wheels are dressed. Expect to reduce infeed and clamp pressure compared with aluminium.

For Inconel, use a softer wheel and lighter passes. The fixture rules do not change; the force level does.