Drilled and Tapped but Way too Loose

[bigal2749]

Need to drill and tap holes in a new 4 jaw chuck.
Gauge shows a M-8 with 1.25 pitch when held up to socket screw that holds the chuck to threaded ring mount on lathe.
In a trial piece I used a .290 drill and tapped with 8 x 1.25 tap but socket screw seems very loose. Retains screw but seems to have excessive lateral movement. I've seen reference to 75% thread and seems like I got 20%. Trial holes were done in 1/4 alumium.
Any ideas what I may be doing wrong.

 

[Davo J]

My tapping chart tells me a 6.8mm drill which converted is 0.2677 inch for a metric 8 x 125mm thread.Dave J

 

[Jasonb]

For metric threads the easiest way to determine the tapping drill is to subtract the pitch from the OD. So 8.00-1.25 = 6.75mm Soy you can either go 6.8mm if you have metric drills or 17/64 is 6.747mm if you have an imperial set.

If its a far eastern chuck the screws can be a bit iffy as well, so worth changine for decent ones.

Jason

 

[bigal2749]

Went back to my charts that I had printed for tap drill sizes and saw that chart title is Tap Drill Sizes for Forming Taps. I had downloaded several charts from a couple sites (another chart does list 6.8mm).
Really don't know what forming taps is all about but have now filed that chart in the circular file.
Thanks for setting me straight.

 

[Jasonb]

Forming taps look like a normal tap but without the flutes, they are used on thin and/or soft metals and work by "forming" the metal to shape. As no metal is removed the hole needs to be bigger as whats pushed out of the high spots fills the low spots.

Jason

 

[mklotz]

tap diameter = 0.3150 in
pitch = 20.32 tpi

dot = 75 % => tap drill diameter = 0.2671 in (H = 0.2660 77 % actual dot)
dot = 70 % => tap drill diameter = 0.2702 in (H = 0.2660 77 % actual dot)
dot = 65 % => tap drill diameter = 0.2734 in (I = 0.2720 67 % actual dot)
dot = 60 % => tap drill diameter = 0.2766 in (J = 0.2770 59 % actual dot)
dot = 55 % => tap drill diameter = 0.2798 in (J = 0.2770 59 % actual dot)
dot = 50 % => tap drill diameter = 0.2830 in (9/32 = 0.2812 53 % actual dot)