Tapping Aluminum Cross hole seizing up

[kkip8686]

Hello,
We are using a STI 1/4-28 tap to tap a cross hole on some aluminum. The issue we are having is the go gage will go on both sides of the hole (Thru hole not a blind hole), but when it gets to the middle of the hole the gage seizes up and will not go any deeper. Checked the hole before tapping and I am within .001" of being on center. Hand tapped outside of machine and got same results. I've never seen one do this before? Anyone have any experience with this happening and have a work around? Don't have much exp with STI threads.

Machine- Tsugami MU-38 with 18i-TB Fanuc Controller
Tap- Helicoil 1/4-28 STI TAP Bottoming With .1" lead (Going deep enough to almost have tap completely come out on negative side of stock, but not quite)
Drill-. 261"

Tried 50,100,300,500,and 1000 RPM all with same result.

Thanks!

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[MCImes]

Does retapping help or hurt?

I mean, for example, command
G84 Z1. F.1
Z1.
the 2nd Z1. will repeat the tapping cycle to the same depth. Give that a try and tell me, Does that bell mouth the threads at all? or remove the sticky spot in the middle?

Or peck tapping...?
G84 Z.4 F.1
Z.7
Z1.

Also, I just thought of this: Is your part banana'd? Find the best fit pin for the thread minor (only insert it .125" to find the best fit size). Then, does the pin drop through the part freely? or does it hang up? you can also roll the part on a surface plate. Does it wobble? or put it in a concentricity checker. Roll the bart and put an indicator on the OD. Does the reading seem high?

[kkip8686]

Thank you for the reply. I tried doing 2 passes and got the same result. Also tried peck tapping and had the same result. Still tight in the middle but good on each end of hole.

[kkip8686]

FYI, tried these yesterday already which is why I replied so quickly

[Superman]

Tap- Helicoil 1/4-28 STI TAP Bottoming With .1" lead (Going deep enough to almost have tap completely come out on negative side of stock, but not quite)

What do you mean by 0.1" lead ? .... 28 TPI is a 0.0357" lead ( lead is the same as pitch for a single start thread )

Are you tapping from both sides ? ........... threads won't line up with each other
Is the thread crossing a hole ? ..... are there burrs ? .... or bad deburring ?

"neck" the tap to allow it to cut thru in the one position, don't come in from the other side


Do a hole on some flat Al plate or even steel...... do you get the same result

Is the gauge correct ? or worn ?
( check OD & Pitch diameter on the full length )

[kkip8686]

On a swiss lathe so cannot load plate stock. However, we are not tapping on 2 sides, just coming in on one side and going all the way thru as trying to do half on one side then rotating to the other side would never work No other holes are being crossed. My initial thought was burrs as well, but it wouldn't make sense as to why there would be burrs in the middle but not on each end. Especially since I can hand tap outside of machine to where I can take tap in and out with just finger strength (No tap chuck needed) and it still gets seized up right in the middle. I know the tool/gage work together because we had used it on a face hole on another machine center and gage functioned just fine (Aluminum as well). I am also only drilling from one side and going all the way thru.

[Superman]

On a swiss lathe so cannot load plate stock.

I meant drill & tap manually on a pedestal drill, using same thickness as the section on the lathe....what size is the spigot you are tapping ?
( could the part design be that 2 screws are used on assembly....meaning you could possibly tap from both sides...and install 2 inserts )

What grade of Alum. ? 2000 & 5000 grades can be "sticky"
Is the tap sharp ?
I assume you are using some sort of cutting fluid, any difference if a tapping compound is used ?

When you hand tap, is it removing any material ? , if it does, then the tap in the machine may not be working properly
Does the "G" letter drill pass thru after tapping ?

[chunkymonkey]

Kyle,
Have you tried using form taps? Chips are not an issue, and soooo much nicer. Give me a call if you can't get this. I'm assuming you are Kyle....heehee