Magnetic Polishing Machine: The Answer When a Loose Charge Cannot Reach the Feature

A magnetic polishing machine is the fourth route in mass finishing, and it is the only one whose working principle is fundamentally different: the abrasive is not a shaped body tumbling past the part — it is a mass of fine magnetic needles that flows into the part.

That difference decides everything about when it is the right machine. It is not a better bowl. It is the answer to a specific class of problem that no loose charge solves, and it is expensive to run, so buying one for a job a bowl can do is a waste of both the machine and the consumable.

The problem it exists to solve

In every other family — vibratory, centrifugal, barrel — the medium is a shaped body. It is made in a size, and that size is the hard limit on where it can go. A 6 mm ceramic cone cannot enter a 3 mm slot, and no grade, shape or cycle length changes that.

This produces a specific set of features that resist every loose charge:

A magnetic machine changes the physics rather than the media. The working body is a mass of very fine ferromagnetic needles — much smaller than any shaped media — and a rotating magnetic field drives them. The needles flow into the part's features because they are small enough to, and the field keeps them moving.

That is why the machine is bought: not for a better finish on an accessible face, but for a feature no other route can reach.

What the needle mass does, and what it cannot

It reaches. That is the whole point. The needle mass follows the field into cross-holes, thread roots and blind slots. On a part where the critical burr is internal, this is the difference between a finished part and an unfinished one.

It is a fine cut by nature. The needles are small and the force on each is small. The process removes burrs and polishes, and it does so gently: it is not a stock-removal process and it will not bring a milled face down. A part that needs 0.2 mm off a face before polishing still needs a cutting stage first.

It needs the part to be reachable by the field. A closed internal cavity with no opening cannot be treated — the same physical limit, expressed differently. A deep blind hole with a small opening is harder than the same depth at a wider opening.

It is a wet process, with a compound in the bath. That brings the usual downstream requirements: a rinse and a dry, and the part is going to be wet at the end of the cycle.

It is slower per part than a bowl for the same accessible area, because the working volume and the needle mass are smaller. On a job where the feature is accessible, a bowl is faster and cheaper; on a job where it is not, the bowl is not an option at all.

The six models we build, with the numbers

These are catalogue ratings — confirm against your own part before planning output. The usable load depends on part shape and how much of the working area the parts actually occupy, and on a magnetic machine the working area matters more than the kilogram figure.

How to read the table. The models come in three pairs by footprint:

• OBD-30 / OBD-35 share the same 1300×850×750 mm cabinet; the difference is the working area (1000×580 vs 1080×680 mm) and the power (4.0 vs 5.5 kW). Same floor space, more area and more power.
• OBD-60 / OBD-70 share a 2000×850×750 mm cabinet; again the difference is working area (1800×580 vs 1900×680) and power (6.0 vs 11.0 kW).
• OBD-90 / OBD-100 share a 2500×850×750 mm cabinet, with working areas of 2200×580 and 2300×680 mm and power of 12.0 and 16.5 kW.

The pattern that matters for a buyer: within each pair the machine is the same size on the floor, and the upgrade buys working area and power, not a bigger cabinet. That means the decision is not "which footprint do I have room for" but "how much part area do I need per cycle" — and the answer is settled by the parts, not by the room.

All six run at 0–1800 rpm. The speed is a process variable rather than a fixed value, which is what allows one machine to work a delicate part gently and a robust part harder.

Where a magnetic machine will not help

This section matters more than the table, because the machine is the most expensive route per part among the four families.

The burr is on an accessible external edge. A bowl reaches it, and costs a fraction. Buying a magnetic machine for an accessible edge is paying the reach premium for reach you do not need.

The requirement is stock removal. The needle mass is a fine cut. A face that needs material removed needs a cutting stage, whatever machine is downstream.

The part is large and the features are large. Magnetic working areas are measured in hundreds of millimetres, not metres. A big casting with a large accessible face is a bowl or a manual job.

The feature is a closed cavity. No opening means no field penetration and no needle mass. The limit is physical.

The volume is high and the feature is not critical. The process is slower per part and the consumable is more specialised. On a job where the edge tolerance is loose, that cost is not returned.

The part cannot be wetted. It is a wet process. If the material or the assembly cannot go into a bath, this route is closed.

How it fits with the other three routes

The four families are not ranked; each answers a different question.

The practical rule: if the critical feature is accessible, use one of the first three — they are faster and cheaper per part. If the critical feature is a cross-hole, a thread root or a blind slot, the first three cannot reach it and the magnetic route is the one that can.

That is also why a magnetic machine is often bought alongside a bowl rather than instead of one: the bowl does the batch work, and the magnetic machine handles the parts or the features that need reach.

What to send with an enquiry

1. The part and the critical feature, marked on a drawing — where the burr or the finish requirement is, and whether it is internal.
2. The feature's dimensions: the smallest opening and the depth. This decides whether the needle mass can enter at all.
3. The stock to be removed, in tenths of a millimetre. "Deburr" is not a number.
4. The material — magnetic finishing works on non-magnetic parts too (the needles are the magnetic element), but the part's material affects the compound and the cycle.
5. The load per cycle and the batch mix, because the working area is the constraint rather than the kilogram rating.
6. What happens downstream — a rinse and a dry are part of the route, and on steel the rust window after the bath is a real consideration.
7. The site: these models run on three-phase supply and are floor-standing units up to 2500 mm long; the footprint in the table is the installation requirement, not an approximation.

FAQ

What is a magnetic polishing machine used for? Finishing features that a shaped media cannot reach — cross-holes, thread roots, blind slots, internal radii. It works by driving a mass of fine magnetic needles through the part's features with a rotating field, so the "media" is small enough to enter where a ceramic or plastic body cannot.

How is it different from a vibratory finishing machine? The vibratory machine tumbles shaped media past the part, so its reach is limited by the media's size. The magnetic machine uses a needle mass that flows into the features. On accessible faces the vibratory machine is faster and cheaper; on internal features it cannot compete because it cannot reach.

Can it remove a heavy burr? It removes burrs and polishes with a fine cut. A burr with a large root, or a face that needs stock removed, needs a cutting stage first. The question is how much material has to come off, in tenths of a millimetre.

Does the part have to be magnetic? No. The needles are the magnetic element; the part does not need to be ferromagnetic. What the part's material affects is the compound and the cycle, not whether the process works.

How do I choose between the six models? Start from the working area the parts need per cycle, not from the kilogram rating. Within each pair in the table the cabinet is the same and the upgrade buys working area and power — so the decision is driven by part area and by how hard the cut needs to be.

Is it faster than a bowl? Per part, no — the needle mass and working volume are smaller, and the process is slower on the same accessible area. It is bought for the features a bowl cannot reach, not for speed.

Max Xu, Sales — Xiamen Jin Tai Jin Grinding Technology Co.,Ltd.

No. 31, Xinchang Road, Xinyang Industrial Zone, Haicang District, Xiamen

info@surface-polish.com

The model figures above are catalogue ratings for the six models in our range; they describe the machine, not the result on a specific part. A finishing process is set by the material, the stock to be removed and the finish required, so the configuration for a specific part is confirmed against a trial on that part. We manufacture finishing equipment and the media and compounds that run in it; we do not offer a job-shop finishing service.