Tapping

[UKRobotics]

Just wondering if somone could explain about tapping on a CNC mill and if its feasible to do on a home brew system.

If I understand correctly with most industrial CNC milling machines and machining centres you can tap a thread and using the relevant software it will drill the hole and then after a tool change tap the hole moving all the axes and spindle in the relevant preportions to do the job correctly.

Now with a typical retrofit, you have your rutex drives on the axis motors, a cnc4pc/soundlogic spindle board and a VFD going to the spindle motor and mach 3 to control it all.

Now from what I have read, there seems to nothing written in any of the manuals or datasheets for provisions of connecting an encoder on the spindle back into the VFD or interface board, but without servo style feedback from the spindle motor I dont see how tapping a thread is possible.

As you have probably guessed by the vaugeness of my question, I really dont have much of a clue of what i'm talking about here ,so any tips to point me in the right direction would be a huge help.

[Rekd]

The amount of accuracy required to sync the spindle rotation with the head movement would probably be more than could be achieved (economically at least) on a retrofit.

An option would be to use a Tapmatic floating head attachment.

[MarkT]

Tapping sequence on a CNC machineing center;

1.) preexisting hole
2.) tool change for tap
3.) Positioning move to cordinates of tap + safety plane above the work
4.) Tapping move ( similar to G01 in that it is a cutting move but uses a different , ussually mfg. specific G-code which allows controller to "sync" spindle revolutions with feed rate to allow the pitch <linear distance between two threads> to be followed on its constant lead
5.) Tapping moves can be self contained in one line of code or can be contained in multiple lines of code ( the intent here being as the tap feeds in on specified lead, the spindle must stop <inconjunction with the relative axis stopping>, reverse direction and the relevant axis must reverse also.
6.) return safely to rapid plane above the work.

That said, I believe you could do so on a home made machine as long as -

1.) Your spindle will operate effectively and with good torgue at low rpms
2.) Your spindle will maintain constant rpm
3.) Your axis drives allow for acceleration and decceleration adjustment

You would also have to understand that not all tapping situations may be able to be performed. Through hole scenarios being the most easy.

Linear slip style holders could be beneficial.

Hope this helps.

Mark T.

[UKRobotics]

I think the problem is not so much with the axis drives but with the spindle drive. In order to be able to sync the spindle with the axis you would need an encoder on the spindle, but where would this encoder data be interpreted and processed? Ovbiously the mach software would deal with it eventually, but it would have to be connected though somthing first.. Would the encoder go into the VFD ? or perhaps into the soundlogic vfd interface or what ?