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+86 13143444311
Email：sst@szsst88.com
Fax：+0086-755-27322278
Address：Building 9, First Industrial Zone, Tantou Industrial City, Songgang Street, Baoan District, Shenzhen 518105, Guangdong, China
Place of Origin:Guangdong,China(Mainland)
Brand Name:Shi Shi Tong
Model Number:sst 596
Materials:Stainless Steel
Surface treatment:Cleaned and passivated
Head Type:Pan Head
Drive:Phillips, cross recessed
Thread:M4 self tapping thread
Point:Sharp point
Forming:Cold headed; washer fitted; thread rolled over it
Size:M4*17
We make stainless steel combo screws with a pan head, a Phillips cross and a captive washer fitted before the self tapping thread is rolled, so one part carries the clamp and cuts its own thread. The washer spreads the load on thin sheet and plastic while the coarse thread grips, so the washer diameter, the pitch and the pilot hole are specified together.
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ISO 9001:2015 · Manufacturing since 1998 · 8,678+ customers · 89 countries and regions
Made to order – there is no stock catalogue. Each part is produced to your drawing, and production starts after drawing review and first-article approval.
A stainless steel combo screw is a screw supplied with its washer already on it, so the assembly uses one part instead of two. This one is a pan head Phillips screw, M4 by 17 mm, with a free captive washer under the head and a coarse self tapping thread with a sharp point, in stainless steel.
Pan head. A low domed head with a flat bearing rim, wider than the drive it carries.
Captive washer. A flat washer under the head that turns but does not come off.
Phillips cross. A cross recess that centres the driver and slips before it can over-drive.
Self tapping thread with a sharp point. A coarse thread that starts in a plain hole or in soft material.
They are used where a screw clamps thin sheet or plastic and a separate washer would be one more part to lose: enclosure covers, appliance panels, lighting backs, brackets and trim, where the joint has to hold without a nut behind it.
On a stainless steel combo screw the washer and the thread are doing different work, and it helps to keep them apart. The washer carries the clamp into the panel; the coarse thread forms its own thread in the hole. One solves bearing, the other solves grip, and a joint fails when either of them is asked to do the other’s job.
The bearing side is simple. A pan head on its own has a small bearing ring, and on 1 mm sheet or a plastic boss that ring dishes the material around the hole. Adding a washer of a few millimetres more diameter spreads the same load over a larger area and the panel stays flat. That is why the washer is there, and why its diameter is a functional number rather than a washer-size convenience.
The grip side is the coarse thread. A self tapping thread with a sharp point starts in a plain hole and displaces material as it goes, so no tapping operation is needed before assembly. The pitch is coarse relative to a machine thread because it has to move material rather than cut it, and that pitch sets the hole diameter the joint needs.
The two interact at the hole. If the hole is too large, the thread engages fewer turns and strips at a lower torque; the washer will still spread the clamp, so the panel looks fine while the thread is on the edge of failure. If the hole is too small, the driving torque rises and the Phillips cross cam-outs before the head seats, and the joint is left loose with a washer that is doing nothing.
The washer has a second, quieter function: it covers the hole. On a painted or coated panel the washer hides the burr and the chipped edge around the hole, which on a visible cover matters as much as the load path. That is also why a washer that is too small looks wrong even when the joint is sound.
The Phillips cross stays in the design for the same reason it is used on assembly lines everywhere. It centres the bit, it transmits enough torque for a self tapping thread, and it cams out at a set level so a driver set too high damages the bit instead of the panel. On a combo screw that last property is useful, because the panel is usually the softer of the two materials.
So the part is three functions on one screw – a bearing face, a captive washer and a thread that makes its own hole – and the drawing has to give a number to each of them.
A stainless steel combo screw is quoted by head diameter and height, recess size, washer diameter and thickness, thread diameter and pitch, length, point form, material and finish. Custom lengths up to 2,000 mm, subject to DFM review after drawing submission. The sample is M4 by 17 mm, model sst 596, with a pan head and a captive washer.
| Item | How it is defined | What we control |
|---|---|---|
| Head | Pan head | Diameter, height and bearing rim |
| Recess | Phillips cross | Size, depth and centring |
| Washer | Flat, captive under the head | Diameter, thickness and freedom |
| Thread | M4 self tapping | Diameter, pitch and length |
| Point | Sharp point | Tip angle and thread start |
| Finish | Cleaned stainless | Surface and freedom from burr |
The washer is written with its diameter and thickness, because the first sets the bearing area and the second takes length away from the thread. On a 17 mm screw a washer of a few tenths is a small share, but it is measured, because the same screw with a thicker washer is a different part.
The thread is written with its pitch and with the hole it is meant to run in. A self tapping thread is only defined together with its hole, and a combo screw that strips in service usually had a hole that was a little too large for its pitch.
The point is written by type. A sharp point starts in a plain hole and can also start in a moulded boss; a blunt point needs a chamfer. The difference is visible on the line as a screw that walks off the hole rather than one that starts straight.
Four properties decide this part, and each carries a cost:
| Property | What it gives | What it costs |
|---|---|---|
| Captive washer | Bearing area without a second part | A washer that has to stay free |
| Self tapping thread | No tapping or nut behind the panel | Hole size becomes part of the drawing |
| Sharp point | Starts in a plain or moulded hole | Walk-off if the hole is oversize |
| Phillips cross | A driver that limits its own torque | Cam-out with a worn bit or recess |
The value is a joint that needs one part, one driver and one hole, and holds a thin panel flat while it does it.
The cost is that the hole, the pitch and the washer all have to agree. A combo screw bought on length and diameter alone is a screw that either dishes the panel or strips in it.
The recorded finish is cleaned stainless steel. There is no coating over the thread or under the washer, so the washer keeps its freedom and the thread keeps the size it was rolled to.
| Option | How it protects | Effect on washer and thread | Typical use |
|---|---|---|---|
| Cleaned and passivated | Restores the oxide layer | None; washer stays free | General stainless assemblies |
| Bead blasted | Even matte surface | Softens edges; washer re-checked | Visible covers and panels |
| Electropolished | Bright, smoothed surface | Removes microns; thread re-checked | Hygienic and decorative |
| Passivated and lubricated | Oxide layer plus dry film | Reduces galling; no build | Joints opened for service |
| Black oxide on stainless | Dark conversion layer | No build; colour varies | Dark visible hardware |
A coating is a poor fit for this part for a practical reason: anything that builds under the washer takes away its freedom, and a washer that cannot turn slides on the panel and marks it. That is why combo screws in stainless are normally supplied bare and passivated rather than plated.
Where a lubricated finish is used the aim is to stop stainless galling on stainless, and the film is applied thin enough not to close the washer clearance. A dry film on the thread also makes the driving torque more consistent, which matters on a self tapping thread where the torque curve climbs as the thread forms.
Bead blasting and electropolishing are appearance finishes on this part. Both change the head edge slightly, and both require the washer freedom and the recess to be checked afterwards, because a washer that turns before the finish can be tight after it.
Three products where the screw has to hold a thin panel flat:
| Where it sits | The screw closes a cover onto a folded steel edge with no nut or tapped insert behind it. |
| When it goes wrong | The cover dishes around the fixing, or the thread strips before the cover seats. |
| Why it happens | A pan head with no washer, or a hole that is too large for the thread pitch. |
| What we control | Washer diameter and hole size agreed; sample fitted to the cover. |
| Where it sits | The screw fixes a panel to a moulded boss where the head is seen and the plastic is soft. |
| When it goes wrong | The boss creeps, or a stress crack runs out from the hole. |
| Why it happens | Too much torque on a self tapping thread, with the washer too small to spread it. |
| What we control | Torque and washer size proved on the housing; sample clamped and inspected. |
| Where it sits | The screw holds a back plate or a bracket that is serviced from time to time. |
| When it goes wrong | The joint loosens, or the Phillips cross wears after a few services. |
| Why it happens | A shallow recess in a small head, and a bit that is used past its life. |
| What we control | Recess depth and point form controlled; sample driven and re-driven. |
Four points that decide whether a combo screw holds a thin panel without dishing it:
|
Washer and hole written with the thread
The washer diameter, the thread pitch and the pilot hole are specified together, because the joint needs all three. Basis: Hole and washer specified. |
Washer fitted before the thread is rolled
The thread is rolled after the washer is in place, so the washer is held on by the thread and still turns. Basis: Washer captured by the thread. |
|
Driven into the panel
The first article is driven into the customer panel and the panel checked for dishing around the washer. Basis: Panel proved under clamp. |
Same washer, same pitch
Washer size, pitch and point are recorded with the drawing revision, so a repeat lot drives the same. Basis: Same feel across lots. |
A combo screw fails in the panel, not on the screw: the hole strips or the sheet dishes. These four points test the panel, and they show on the first article.
These screws are cold headed from stainless wire, the pan head and the Phillips cross are formed, the washer is placed on the shank, the self tapping thread is rolled over it to trap the washer, and the point is formed. The first piece of every run is measured on head, recess, washer, thread and point before cleaning and again after it.
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The washer goes on before the thread, and the thread roll is what keeps it there. The rolled thread leaves a shoulder just larger than the washer inside diameter, so the washer can turn but cannot pass over it. Doing it the other way round – thread first, washer forced over it – damages the washer and opens its inside diameter, which is exactly what makes a washer drag on a panel.
The point is formed after the thread so that the first thread and the point agree. On a self tapping screw the point is where the thread starts in the hole, and a point that is off-centre will walk on a small hole and cross-thread on a large one. The point is checked by driving, not only by measuring the tip angle.
The pan head and the cross are formed in one blow, and the recess depth is set against the head height. On a pan head that is thinner than a truss head the recess has less room, so the striking is controlled and the underside of the head checked for any bulge left by a deep strike.
Cleaning and passivation come last. No coating is applied, because any film under the washer would take away the freedom the part needs, and the parts are checked for burr at the washer and at the point before packing.
What decides whether a batch of stainless steel combo screws passes is the washer that spreads the load and still turns, the recess that takes the bit and the point that starts the thread. The inspection agreement covers all three.
Head, recess and washer on the projector |
Thread checked with gauges |
Wire and part checked |
Salt-spray testing where required |
The washer is checked in the hand: it must turn and it must not come off. Its diameter and thickness are measured on the projector, because a washer that is a little thick takes length from the thread and a washer that is a little small dishes the panel.
The recess is measured for depth and driven with the production bit at the line torque. A Phillips cross that is correct in size but shallow will cam out early, and the joint is left loose while still looking assembled – which is the failure this part is most often accused of.
The point and the thread are checked by driving the first article into the customer’s hole. A batch that starts well and strips at the last turn is a pitch or hole question, and it is caught by driving rather than by gauging the thread alone.
Where salt-spray hours are specified, a sample is run and the result recorded with the alloy and the passivation. Stainless is not automatically immune, and a part with a captive washer has more surface to keep clean than a plain screw.
Every batch of stainless steel combo screws is traceable to the wire it was headed from and to the washer lot. The material certificate states the grade and the heat number, and the inspection record carries the head, recess, washer, thread and point against the drawing.
ISO 9001:2015 |
RoHS test report |
Registered trademark |
Patent: security screw and driver |
With each shipment we supply the material certificate and the inspection record for the head, washer, thread and point. The records carry the batch number, so a repeat order of stainless steel combo screws can be compared with the first on the two numbers that decide the joint: washer diameter and thread pitch.
More than 8,678 customers in 89 countries and regions have ordered from Shi Shi Tong; 87% of orders come from repeat customers and 56% through referrals. Most projects follow the same route: drawing review, first-article approval, then batches checked against that sample. On a stainless steel combo screw the first article is driven into the customer panel, because the washer and the hole decide the joint together. Buyers visit during the process to review samples.
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8,678+
customers served
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87%
of orders from repeat customers
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56%
of orders through referrals
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89
countries and regions shipped to
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Buyers and quality engineers visit for sample review, process discussion and supplier audits; the visit is arranged around production so the heading cells and the finishing line can be seen on the day the parts run.
Customer group visit – sample review |
Customer group visit – process discussion |
Customer group visit – factory tour |
Customer visit – project kick-off |
Danish customer – drawing review |
Industry exhibition – meeting customers |
A screw supplied with its washer already assembled, so one part does the work of a screw and a washer.
No. The self tapping thread forms its own thread in the hole, which is why it is used on sheet and plastic.
It depends on the material and the pitch. The hole is agreed with the screw on the first article, because a little oversize is where the thread strips.
It is available everywhere and it limits its own torque, which suits a self tapping screw in a soft panel. A Torx version holds more torque and lasts longer.
No. It is trapped by the rolled thread, and its size is part of the drawing. A different washer is a different part number.
Head diameter and height, recess, washer diameter and thickness, thread and pitch, length, point, alloy and finish, plus the hole it is driven into.
How a stainless steel combo screw project runs from drawing to repeat delivery:
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1
Send the drawing and the panel it clamps
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→ |
2
Washer, thread and hole agreed
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→ |
3
First article driven into the panel
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→ |
4
Batch production and repeat orders
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Each step is confirmed with you before the next one starts; the drawing revision used for the first article is what repeat orders are checked against.
For a quotation, send the drawing with the head and recess, the washer diameter and thickness, the thread and pitch, the length, the point, the alloy and the hole it goes into.
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Head
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Recess
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Washer
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Thread
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Point
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Hole
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Sending a drawing? The form below does not take attachments. Email the PDF, DXF or STEP file to sst@szsst88.com with the part name and quantity in the subject line – the drawing is reviewed and a quotation follows.
+86 13143444311
sst@szsst88.com
