Category: Screw Assembly
Deprag's New Adaptive Direct Fastening System is a revolution automatic flow form screwdriving.
The use of flow form screws into two non-drilled sheets requires high down force and high speed to heat and form the hole. It is a complex assembly because to achieve optimal cycle times and proper funnel and thread forming, the down force and feed speed needs adjusted during the various stages of the screw assembly so that the funnel and thread forming can be done reliably.
Traditional systems required numerous pre-tests to determine the timing of the piercing point. Switching parameters too early leads to improper funnel formation and longer assembly times, too late can lead to thread damage.
Deprag's Adaptive DFS is able to sense the exact piercing point as well as other key stages in the screw assembly and switches parameters accordingly assuring:
- automatic adaptation to variances in the screw and part
- the highest process reliability
- shortest possible cycle times
- optimised funnel forming
- reduced set up times
- a more flexible system
- minimise damaged threads
Other features of the Adaptive Direct Fastening System are:
- Active nosepiece jaws that hold the screw firmly until drilling has started
- Adjustable down holder force
- High down pressure applied directly in line with the screw
- Head first screw feeding to minimise damage to the screw’s tip
- Fast interchange of the mouthpiece for minimal downtime
- Lock stroke for underfloor applications
Max Speed: 8000rpm, freely programmable
Max torque: 15Nm, freely programmable
Feed stroke: upto 3000N, freely programmable force and distance
Max Downholder force: 1200N, freely adjustable
Assembly at any angle or upside down.
Complete with industrial PC to set parameters, control the screwdriving and to analyse trends
Innovative Sheet Metal Fasteners
Flow Drill Screws, Flow Form Screws
FDS® screws from EJOT and FFS® screws from Arnold have been around for a while. They offer the benefits of a secure and strong joint from one side between two sheet metals. A big advantage is that they can join dissimilar materials like aluminium, steel and plastics where spot welding is not possible (they are finding more and more applications in the automotive industry where aluminium and other materials need to be joined). They also allow the use of thinner sheets without the use of additional hardware to strengthen the joints (this is making them more popular in the whitegoods industries).
The screws form a hole by high pressure and the speed of the screw, the heated material is then threaded and cools around the screw. The joint is strong, and the unscrewing torque is high. The thread formed is upto 3 times thicker than the sheet metal.
- Heating the metal with high end load and high speed screwdriver
- Penetrating the material
- Forming of the hole
- Forming of the screw thread
- Engagement of the threads
- Final tightening into the cool thread
Advantages for joining sheet metal:
- No swarf or chips
- Stronger joint, higher screw torque possible
- Higher torque means a larger torque window and less chance of stripping
- Screws can be replaced with standard metric screws
- No additional hardware required to strengthen the joint
- High unscrewing torque and vibration resistance
- Possibility of joining thinner materials
- Possibility to join Aluminium
- Can be accessed from one side
- Possibility to join sheets without pre-drilling (in automated applications)
Dirt or dust particles can cause damage to some products or the system into which the sub-assembly will be integrated.
In these situations it is important to avoid abrasion, reduce abrasion or target its removal! These are the basic requirements for screwdriving assembly in clean rooms to ensure the quality of the components to be processed. The DEPRAG CleanFeed Concept provides an overall solution.
Advantage: Integrated design for technical cleanliness. Deprag’s complete program is coordinated for all required individual components, from the feeder to the screwdriver, semi automatic or fully automatic – all from one source.
Function: The screws are stopped in switch position 1 and the residual dirt is removed from the feed parts via vacuum suction. The removed dirt particles are then collected in a filter with transparent inspection window and a replaceable filter element. In switch position 2 the cleaned connection element is fed into the screwdriving module (inline version) or prepared for pick up (Pick&Place version).
The components below can be used to obtain an optimal result:
- DEPRAG HSF sword feeder – vibration free screw feeding – therefore no component friction and minimal abrasion
- DEPRAG Particle killer – dirt particles from the feeder are targeted and removed in-line
- DEPRAG Clean Pick-and Place unit – Screws are fed to to the pick point in the unit, which also has a vacuum to remove particles
- DEPRAG SFM-V vacuum screwdriving module, which fits onto the screwdriver – residual dirt is removed by suction via an additional vacuum source when positioning the screwdriver.
Torque controlled screw tightening has always been the most reliable and easily applied method to produce a quality screw joint. But is torque control really going to give the perfect assembly every time?
Firstly, what is a perfect screw joint? What do we want to achieve by assembling a screw in a product?
We normally need to join two parts together, but more than that we want to induce tension in the joint so that the parts do not come apart. Sometimes we also need to seal the parts together to prevent leakage or keep dust and water out.
So the aim for a screw joint is not simply to assemble the screw to a target torque within a set tolerance, but it is to induce a constant clamping or pre-load force.
When we apply torque to a screw joint we can calculate the amount of tension induced in the joint assuming that there is no friction in the joint. In a low friction assembly (eg machine thread screw into a pre-tapped hole) almost all of the torque will induce tension in the joint. If you know the pitch of the thread and the torque applied you can then calculate the tension.
Fluctuating screw-in torque influences pre-load force!
What about thread forming and self tapping screws? We know that a portion of the torque is lost to friction and by doing some analysis we can subtract the friction and figure out the average torque used to clamp the parts. In most cases this will be enough, but what if the friction is not the same in every assembly?
What if a screw with a thread diameter at the high end of it’s tolerance meets a part with a hole dia that is at the low end of the tolerance? In this case less torque will be used to clamp the joint and it may not be enough. At the other end of the spectrum, there could be too much torque which could strip the joint or damage the part.
The solution is Friction Tightening. The advent of EC and EC servo screwdrivers has paved the way to improving the quality of screw joints. Today we can assemble the screw, calculate the friction torque and then add the pre-determined tightening torque. So if the friction torque varies, the actual clamping torque and consequently the clamping force will still be constant on every assembly.
Using the friction torque measurement we cannot use the final torque for statistical analysis anymore. The final tightening torque is the sum of the friction torque and the clamping torque the final torque accuracy will depend on the quality of the joint and the screws. We now have to look at the clamping torque only for statistical analysis.
The friction torque method is already well established in the international automotive industry as well as the electronic, mobile technology, household goods and medical industries.
For more information or if you would like to discuss an application, please don’t hesitate to contact us.