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Tapping Mode on Mill

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David Noble10/08/2020 23:24:12
203 forum posts
8 photos

Have a seig SX2.7 mill. It has a tapping mode where, by pressing a button, the motor reverses. I've never used it for two reasons, the first is that I can usually tap perfectly successfully without (famous last words) and the second reason is that I am scared to death of it! As there is no feedback, how can it be used without snapping a tap? Or is there a technique that I don't know about?

Many Thanks, David

JasonB11/08/2020 06:59:23
18637 forum posts
2047 photos
1 articles

Faith is what you nee! Avoid old worn taps and preferably use machine taps particularly on anything thicker than 1 x diameter as they clear the swarf better and seem to put less load on the machine.

I would suggest staying between M3 and M10 or 1/8" to 3/8", and go as slow as you can run without stalling to start with as the coarser pitches will see the tap going in quite quickly.

Paul Lousick11/08/2020 07:22:25
1498 forum posts
570 photos

I originaly had an SX3 with the tapping feature and only used it on thin materials with a thru hole. Not on blind holes as too easy to bottom out and snap a tap and not on thick material.. Much prefer to do it by hand where you can feel any resistance and back off after half a turn to break the swarf. The reverse function on the mill is not that quick and will do a couple of revolutions before reversing.


Ron Laden11/08/2020 08:04:13
1985 forum posts
393 photos

Where possible I do most of my tapping on the mill, I don't have the built in tapping mode but I could see myself using it a lot.

I only have the smaller SX2P at the moment but is was very easy to fit a reverse switch. My range is up to M6 in steel and I can manage M8 in aluminium providing its not too deep.

I also machine tap most blind holes, I measure the hole depth, knock off 2mm and set the DRO with the tap contacting the start of the hole. Seems to work fine though it does concentrate the mind when close to the bottom of the hole, like Jason says you just need faith.

David Noble11/08/2020 08:40:58
203 forum posts
8 photos

Thank you all but it still seems like black magic to me. Thank you Jason for the suggestion of spiral taps though.

I might give it a go on something unimportant.


David George 111/08/2020 09:43:14
1303 forum posts
447 photos

Whilst at work we used tapping head adaptors to hold the tapps for machine tapping. These had a clutch built in and protect the tap from excessive force and you could drive the tap to the bottom of a hole without breakage.


Andrew Johnston11/08/2020 10:02:45
5635 forum posts
652 photos
Posted by David Noble on 11/08/2020 08:40:58:

Thank you Jason for the suggestion of spiral taps though.

There are two types of spiral tap, spiral point (aka gun tap) and spiral flute. They're for different applications. A spiral point tap pushes the swarf ahead, and is intended for thru holes. A spiral flute tap pushes the swarf back and out of the hole, and is intended for blind holes. Of course a spiral flute tap will work just as well on a thru hole, or by hand. Where possible I buy spiral flute taps - only one tap needed per thread size.

Generally I'm with DavidG. I prefer to use tapping heads; better torque and depth control, and much faster.


JasonB11/08/2020 10:31:17
18637 forum posts
2047 photos
1 articles

Oh you can go faster with the tapping mode but need reaction times to suit so only really to be used on through holes. The SX2.7 does limit you to 500rpm but as mentioned once you start to get to the larger sizes things start to happen a bit faster vertically particularly as rpm rises!

Also if you go for a slightly larger tapping drill size as Andrew tends to advocate you are less likely to snap a tap. All the videos I've posted have used the general rule of thumb for metric of dia less pitch for the tapping drill size.

Edited By JasonB on 11/08/2020 10:35:48

John Haine11/08/2020 10:35:41
3270 forum posts
175 photos

Have a look at Eccentric Engineering's Instagram postings. He has a very neat tapping machine, latest video shows it being used to thread a batch of toolholders for the Acute grinder.

I tap all the time on my mill, using the VFD running at 10 Hz or less (depending on the tap size and pulley in use). For blind holes I start the tap under power and finish usually off the machine by hand. The VFD is programmed for pretty fast stop/reverse

I haven't broken a tap yet (famous last words...) though once i forgot to reduce the speed - that was exciting!

Clive Foster11/08/2020 11:17:57
2319 forum posts
76 photos


Entirely agree about the "pucker factor" when first tapping using a mill.

Like David George and Andrew I'd always used tapping heads for power tapping. I have a full set of the Pollard / Etteco ones which work just fine on my Pollard 15AY drill and can be used on my Bridgeport if I feel upp to the knee cranking needed to make room. They are long, especially the big one.

I also have a simple spring loaded tap holder with a few small "sort of collets" that cover sizes up to 5 mm or so. That came in a freebee box of might be useful to you stuff. The spring travel is maybe 5/8" inch so its theoretically quite easy to keep the holder floating in the sprung range as the tap drives down. I've used it occasionally for smaller tapped holes but never been super confident.

Anyway came the day I needed an 8 mm through hole tapped in a job on the mill. Enthusiasm for knee cranking being at a low ebb I thought "Stuff it. Loads of folk on Practical Machinist say its easy peasy. Try it." and slapped a tap in the drill chuck, anointed the hole with Trefolex, dropped the Bridgeport into back gear, truned the motor switch to reverse and ... did it ....

Just like That

No fuss no drama, just a slight stretch to operate the quill feed with my left hand whilst hovering over the stop button with my right. I din't even mess with the Varispeed setting, odds are it was between 800 and 1,200 rpm in direct drive giving 100 to 150 in back gear as that suits most of what I do near enough.

Shoulda tried 15 years back. The tapping heads are starting to feel a bit neglected!

On the UK Bridgeports the knee mounted control box puts the stop button in a decent place to push with your knee if need be.

All it needs now is an axillary stop switch linked to the depth thingy so I can do blind holes.


PS does anyone know what size the collets are in the No 1 Pollard / Etteco head and where they can be got from.

David Noble11/08/2020 13:29:32
203 forum posts
8 photos

I can see that I'm going to have to 'Man up'

Many Thanks all, David

Gary Wooding11/08/2020 16:34:32
753 forum posts
194 photos

I fitted VFDs to my Centec 2B (X-axis and spindle) and made a small control box that's attached to the end of the table. When the workpiece can be put on the table I always tap under power. I don't have spiral taps. The slow speed and forward/reverse switch provide excellent control for power tapping. I use the quill digital depth gauge when tapping blind holes.


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