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View Full Version : Tapping holes while material is still clamped to my mill table?



HankMcSpank
17-11-2009, 03:59 PM
ok, here's my conundrum...see if you can help (all suggestions welcome).

I'm milling very small bits of acrylic - which are just 4.8mm wide, with 6 x M4 threaded holes in them. So, with just 0.4mm material left either side of the threaded hole once the part is cut, what I've discovered is if I cut the part out, then try to tap the holes afterwards - .... SNAPSVILLE!

I'm therefore having to tap the holes while the material is still in one larger piece *and* while it's still clamped to the CNC table/bed (the acrylic is clamped to the table via 5mm MDF, so I drill through the acrylic & into the MDF a further 3mm to allow me to tap the hole hole in the material). This is a crock, as I have to shuttle the bed out to one side to tap the six holes...compounded by my present tap wrench having a standard T bar type handle, making it very difficult to use (the Z plane gets in the way of the handle while threading the holes !)

What I'd like to be able to do, is shuttle the material to one side still, but use my electric screwdriver to tap these small holes (they're only about 5mm deep & this is just acrylic, so the electric screwdriver should be fine at it's slowest speed).

But what adapter do I need to connect my electric screwdriver to an M4 tap?

My electric screwdriver has a bog standard hex receiving socket (I don't know what that measures though!), and I think(?) the square end on a standard tap is 1/4"?


I see it's possible to buy tap ratchet wrenches (with no T bar aspect), but they still look clumpy & I'd rather have the electric screwdriver do the turning!)

Any input most welcome here!

(by the way...in the tool management of my CNC software, I see there are 'tap' tools available as an option to select...what's going on there?!!! My spindle turns at 7000RPM...how on earth can you tap a hole that way!! :confused: )

ptjw7uk
17-11-2009, 04:35 PM
Have a look for the cheap socket sets usually in the 1 shops in most I have seen they have a hex to 1/4 square socket that you could then fit to a socket to fit the tap. Not sure a tap that small will be 1/4, I thought they were a bit smaller. Not sure if the setup will be in line enough to tap square though.
Have you tried tapping the thread holding the material in a vice to try and stop it splitting.

Peter

Robin Hewitt
17-11-2009, 04:40 PM
(by the way...in the tool management of my CNC software, I see there are 'tap' tools available as an option to select...what's going on there?!!! My spindle turns at 7000RPM...how on earth can you tap a hole that way!! :confused: )


You can fly cut a screwthread with cnc, spiralling dwn in to the hole but the tool needs to have a considerably smaller radius than the hole.

Have you tried a spiral flute taper tap? That woiuld destress you somewhat at the fracture point :beer:

HankMcSpank
17-11-2009, 04:45 PM
Have a look for the cheap socket sets usually in the 1 shops in most I have seen they have a hex to 1/4 square socket that you could then fit to a socket to fit the tap. Not sure a tap that small will be 1/4, I thought they were a bit smaller. Not sure if the setup will be in line enough to tap square though.
Have you tried tapping the thread holding the material in a vice to try and stop it splitting.

Peter

Some top tips there Peter ! But you know what - you're right, the square male end of a standard tap can't be 1/4", becuase the other night, I was trying to find a hex socket to kludge it into ...it transpired that something like a 4.5mm socket was too small & yet a 5mm socket was too large. (then Golidlocks dropped by & we talked about threading & screwing at length lol )

I'm assuming that male square end is an imperial dimension/fitting of some sort? Does anyone know what the square male end dimension is on a tap?

re tapping in a vice afterwards, you know what, sometimes you can get so close to a problem that the solution might be blissfully simple - no I haven't tried tapping in a vice afterwards...but I will! (though it'd still be nice to have the electric screwdriver do it, so my query still stands!)

Edit - Hi Robin, no I haven't got or tried a spiral taper - that sounds like danger to me...I think I'll stay all 'quaint & rural England' & stick with my hand threading!

ptjw7uk
17-11-2009, 05:19 PM
Hi HankMcSpank
I dont think the cnc cutting of threads is remotly like hand cutting a thread in that the cutter is much smaller than the diameter of the thread being cut plus the cutter does not have a spiral on it just the thread form at the correct pitch. The cutter is then advanced into the hole centre moved to the edge the required depth and then the cutter is traversed around the hole whilst advancing at the required pitch so the cutter can whizz round as fast as is needed.
Hope decription shows how it should work, I think you are getting confused with power tappers.
Peter

HankMcSpank
17-11-2009, 05:23 PM
Thanks Peter.

I've just had an Ebay seller who suggested something like this (a small chuck on a hex shaft)...

http://cgi.ebay.co.uk/CK-1-4-Hex-Shaft-Keyless-Chuck-Max-6mm-Drill-Bit-T2463_W0QQitemZ390095213356QQcmdZViewItemQQptZUK_H ome_Garden_PowerTools_SM?hash=item5ad37c932c

which while a little 'cluncky', I suppose will get me away!

Kn8
17-11-2009, 05:24 PM
Hi The square end on my M4 taps is 3.10mm the hex drive for most standard srewdriver bits is 1/4". I could if it would help spark a 3.1mm square hole in the end of an old driver bit if it would solve your problem.
Regards
Ivan.

HankMcSpank
17-11-2009, 07:20 PM
Hi The square end on my M4 taps is 3.10mm the hex drive for most standard srewdriver bits is 1/4". I could if it would help spark a 3.1mm square hole in the end of an old driver bit if it would solve your problem.
Regards
Ivan.


Ivan,

That's a very kind offer. I've got home & measured mine - it's 3.8mm. I've being a little bit slow here - ie taps have different sized ends!

Rather than put you out I think I'll go with that little hex shaft chuck I linked to ...I reckon It'll get some use around my workshop anyway. (that said, I'd love to know how you 'spark' a square hole in the end of a bit though!).

Many thanks.

Kn8
17-11-2009, 10:30 PM
Hi HankMcSpank

The spark erosion process is expland here; http://www.edm-products.com/Dielectrics/ifase/ifase_1.htm

Ivan.