Machine for binding reinforcement bars

1. A machine for forming a two-stage wire tie around a pair of bars to bind the bars together, said machine comprising an arrangement for passing said wire in a loop around the bars and a twisting head for twisting the ends of said loop together, said twisting head comprising an arrangement for gripping the ends of the wire and a tie plate through which the ends of the wire pass, and said head being resiliently mounted relative to the bars so as to allow the tie plate to move through a predetermined amount of travel towards the bars, to thereby define the length of a first stage of the tie, the machine being adapted thereafter to release the ends of the wire and twist these to form a second stage of the tie, wherein the centre of the tie plate comprises one or more protrusions.

2. A machine as claimed in claim 1 wherein the centre of the tie plate is flat.

3. A machine as claimed in claim 1 comprising a latch to hold the tie plate at the end of its travel.

4. A machine for forming a two-stage wire tie around a pair of bars to bind the bars together, said machine comprising an arrangement for passing said wire in a loop around the bars and a twisting head for twisting the ends of said loop together, said twisting head comprising an arrangement for gripping the ends of the wire and a tie plate through which the ends of the wire pass, and said head being resiliently mounted relative to the bars so as to allow the tie plate to move through a predetermined amount of travel towards the bars, to thereby define the length of a first stage of the tie, the machine being adapted thereafter to release the ends of the wire and twist these to form a second stage of the tie, further comprising a latch to hold the tie plate at the end of its travel, wherein said latch is provided by a ratchet mechanism throughout some or all of the travel.

5. A machine as claimed in claim 4 comprising an arrangement for releasing the latch automatically after the tie is finished.

6. A machine as claimed in claim 1 wherein the arrangement for gripping the ends of the wire is adapted to clamp the wire securely.

7. A machine as claimed in claim 1 wherein the tie plate moves towards the bars by approximately 1 mm for each full rotation of the twisting head.

8. A machine as claimed in claim 1 configured to be placed on a reinforcement bar with the tie plate initially less than 20 mm away from the bar.

9. A machine as claimed in claim 1 configured to be placed on a reinforcement bar with the tie plate initially less than 15 mm away from the bar.

10. A machine as claimed in claim 1 wherein the twisting head comprises at least one clamping member.

11. A machine as claimed in claim 1 wherein the twisting head comprises two clamping members, one for each end of the wire.

12. A machine as claimed in claim 11 wherein the or each clamping member can apply a clamping force to clamp the wire securely when the tension in the wire is at least between 250 and 350 Newtons.

13. A machine as claimed in claim 1 adapted to detect when the tie plate has reached the end of its travel.

14. A machine for forming a two-stage wire tie around a pair of bars to bind the bars together, said machine comprising an arrangement for passing said wire in a loop around the bars and a twisting head for twisting the ends of said loop together, said twisting head comprising an arrangement for gripping the ends of the wire and a tie plate through which the ends of the wire pass, and said head being resiliently mounted relative to the bars so as to allow the tie plate to move through a predetermined amount of travel towards the bars to thereby define the length of a first stage of the tie, the machine being adapted thereafter to release the ends of the wire and twist these to form a second stage of the tie, wherein the second stage of the tie is configured to wrap around outside of the first stage of the tie.

15. A machine as claimed in claim 1 , wherein the second stage of the tie is configured to form above the first stage of the tie.