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F.A.Q. - Frequently Asked Questions

1. What is the reason to apply the flow drilling process and what are its advantages:

The flow drilling process is a hot forming process for creating holes in thin-walled metal components and profiles. These holes can be used for bushings, including bearing bushings, soldering holes and thread forming.

The most important benefits:

  • No more wobble, loosening or twist,
  • This solution eliminates the need for welded -, rivet -, blind rivet - and piercing nuts,
  • Production of the bushes with threads in only two operations,
  • Increase in the drawing forces of threads (thread forming),
  • 100% centric position of the thread,
  • High precision and reproducibility,
  • The process can be automated,
  • High speed process, production costs can be saved per hole and thread,
  • Low investment costs for this flow-punch-forming process.

2. Machines and tools for centerdrill flow flow-drilling process:

For trouble-free flow forming is the availability of a drilling or milling machine with a motor that has the necessary kilowatt output and spindle speed. Normally, this means a column drilling or milling machine or NC or CNC machines with a minimum speed of approx.. 2100 rpm and an output of 1,5 kW, what requires the production of a M 8 thread in a 2 mm thick metal sheet or profile. Further information process data.

Depending on the machine equipment, the following collet chucks should be used: MC 2, MC3, SK 30 or 40, HSK and BT. The collet chucks should be equipped with a alu-cooling ring, which ensures, that the heat, which is generated during this process, will be deflected.

The centerdrill flow-punch-former should be fixed in collet and the collet in the collet chuck.

The centerdrill parting paste, which is water soluble and does not contain any oil, prevents metal from building up on the centerdrill tool or from baking on it.

3. Can we also apply this process with a portable hand drilling machine?

It is not recommended to apply this process free hand without a rig. As mentioned above, a minimum speed, kilowatt output and axial forces are required, there is a great risk to tremble and wobble and to brake the Centerdrill.

If the hand drilling machine with sufficient power and rpm is spanned in rig, the Centerdrill process can be applied.

4. Can we also work with a three jaw chuck?

No, because the risk is very high, that the centerdrill brakes and that the spindle becomes overheated. We do not give any warranty, if a three jaw chuck is used.

5. Is it necessary to apply a lubricant?

To increase the lifetime of the centerdrill, a parting paste must be used. The Centerdrill parting paste prevents metal from building up on the Centerdrill or from baking onto it. Depending on the type and thickness of the material, it should be applied in small quantities every 5 to 10 drillings. Too much paste can cool the former down and thus adversely affect the quality of the formed hole and the collar.

6. What metals can be processed with the flow forming process?

Virtually all thin-walled metals 0,75 – 12 mm (except tin and zinc), in other words, all:

  • Welding steels
  • Stainless steels,
  • Aluminum
  • Copper
  • Brass
  • Bronze
  • Magnetic materials
  • Special alloys.

7. Can we process zinc-plated materials?

Only in some cases. As zinc has a different melting point than standard steel, this has a very negative effect on the quality of the flow-punch formed hole and the collar. Depending on the thickness of the zinc, this effect is even more pronounced.

8. What process sequence do you recommend to produce a flow-punch formed hole and a thread in zinc-plated material?

For the reasons explained above, it is generally better to zinc-plate the material after flow punch forming. If this is not possible the zinc layer, if it is too thick and uneven, should be removed before flow punch forming. If the work part is zinc-plated after the thread forming, the threads must be cut afterward if it wasn't closed with a plug beforehand.

9. What is the maximum thickness that can be flow-punch formed?

There are known applications with a wall thickness of 12 mm in which flow punch forming was used. In our experience most applications involve a material thickness from 1 - 3 mm. Thinner material can also be processed, but an underlayment beneath the work part is required because of the risk of deflection. Flow punch forming in solid material is not possible. See table "Maximum Wall Thicknesses".

10. Should I use a short or a long flow punch former?

The length of the flow punch former depends on the thickness of the sheet steel, the desired core hole, the type of metal, and the desired surface (with or without collar). Refer to the table "Maximum Material Thickness". For pipe profiles the working length of the flow punch former must not exceed the inside width of the profile.

Every former tip consists of a cylindrical and a conical part. The cylindrical part is responsible for forming the core hole. If a thread is formed afterward, we recommend leaving the core hole slightly tapered at the end so that the thread is well formed. However, if the core hole is fully formed because it functions as a through-hole, the cylindrical part must have a corresponding length.

11. Examples of a former selection:

  • A core hole for an M8 is to be flow-punch formed in 2 mm thick sheet metal made of S235JR/ST37: required is a machine with a speed of 2100 rpm and an output of at least 1.5 KW. Recommended is a short flow former Ø 7.3 mm; alternatively, if the surface should be smooth, a short/flat flow former Ø 7,3 mm
  • The same core hole as above in 4 mm thick sheet metal.: in this case the long or long/flat version Ø 7.3 mm should be used. If problems occur during thread forming that result in the thread former "squeaking" and wearing excessively, the cylindrical part should be extended. That means that a special flow punch former must be fabricated.
  • The same core hole as above in 2 mm thick sheet metal made of stainless steel: in this case we recommend the above mentioned flow punch former, but with a 0.1 mm larger diameter, i.e., Ø 7.4 mm.

12. The collar formed by material that is displaced upward, is a problem. How can I achieve a smooth surface?

For this we recommend the flat version centerdrill. With this model the collar is removed in the last part of the operation. Of course, this results in a smooth surface only for flat sheet metal. With round pipes leftover metal remains on two sides and must be removed mechanically.

13. Is the thread formed in the same operating step?

No, if the thread were produced in the same operating step it would be destroyed again when the larger-diameter flow punch former is extracted.

14. The flow punch former gets dark red during forming? Is that dangerous?

No. Usually, the flow punch former develops a temperature of up to 600° and begins to glow dark red. If the color changes to bright red or yellow, that means that the flow punch former is too hot. This reduces the tool life and adversely affects the quality of the core hole.

15. How can I reduce the material that runs inward?

The best way to achieve this is predrill a hole before beginning the standard flow punch forming process. With the predrilled hole a reduction of the bushing toward the inside and smoother edges of the bushing can be achieved. However, this also reduces the number of possible thread turns.

16. The bushing that emerges toward the inside is too long or torn.

Predrilling of an appropriate hole will reduce the length of the bushing and prevent tearing on the edges of the bushing.


Centerdrill GmbH
Valterweg 19
D-65817 Eppstein

Tel. +49 (0) 6198/5858-97
Fax. +49 (0) 6198/5858-99


Tel. +49 (0) 6198/5858-97


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