Need help understanding tap & die sizes

[Nittersing]

I have a piece of acrylic that is 5/16" (?) thick.

I want to tap four holes for screws with 1/4-20 thread

I believe the drill size I need to use is a #7.

But I can't figure out the tap size. I don't see any numbers that reference 1/4-20.

I have both metric and sae...

What am I not understanding?

[amerikat]

1. 1/4-20 tap must be missing from your set.

Available at any hardware store.

[Nittersing]

6. OK - found it... nt

[ret5hd]

2. yes, a number 7 drill is correct, BUT...

if i may make a suggestion:

if drilling by hand (as opposed to using a drill press or some other method that keeps your drill exactly perpendicular to the acrylic sheet)

use a #8 or even a #10 drill.

the acrylic sheet is “soft” enough…compared to steel etc…and using a hand drill…wobbly…the slightly smaller drill bit (#8 - #10) will give you a finished hole that will work perfectly.

don’t expect the finished tapped holes to be particularly strong though…acrylic is brittle and your sheet isn’t particularly thick.

[Nittersing]

7. Good tip about a smaller drill hole. Thanks. nt

[Nittersing]

9. Do you think a drill guide is worth it? Or is it mostly hype?

https://www.amazon.com/DEWALT-DWAIRDG-IR-Drive-Guide/dp/B01GLYTDRE/ref=sr_1_4

[ret5hd]

10. i won't speak to that...

i have decades of machine shop experience…i wouldn’t buy one for one little job, but i know how to “adjust” things and methods to make sure i get an acceptable result (like the drill bit advice).

do what feels right…is this job a “one-and-done”? are you intending to start or add-to a tool collection for future projects? do you want to become a tool addict and spend hundreds or even thousands of dollars on tools that you now have to invent projects for? will your spouse/partner now expect you to remodel the entire house?

welcome to my world.

[ret5hd]

11. and that item IS NOT what you need...

that item is used to hold screwdriver bits in a power drill…not what you might need.

i don’t know why they would call that a drill guide.

[usaf-vet]

3. You are on the right track. 1. look closer at the tap. 2. look for an accompaning chart that details the layout of the..

.... taps and dies in the kit they came in.

Finally, take a known 1/4-20 bolt and physically put the bolt alongside the tap. Tip to tip (one is up, the other down), and match the bolt thread to the tap. It SHOULD be an exact match. They should fit together perfectly.

If you are going to do a lot of tapping, buy a plastic guide that includes all of the tap sizes. Also, buy a small can of CUTTING OIL to use when tapping metal.

And one more tip: if you tap metal, take a turn or two into the hole, THEN take a half turn back, which breaks the cut material free. Now continue 2 turns in 1/2 turn back.

Hope this helps!

[Nittersing]

8. This is how I finally found it... matching up the threads

Just going thru some acrylic... but thanks for the metal tips as well.

[essaynnc]

4. Do. NOT.

Use alcohol on the tapped hole to clean it. Tha acrylic will crack and craze and be ruined.

I speak by experience.

[usonian]

5. I was surprised to see that there's a choice of tap drill sizes for 50% and 75% thread engagement.

But I'm not a machinist, so what the heck.

https://www.littlemachineshop.com/images/Gallery/PDF/TapDrillSizes.pdf

So #7 is what you want for plastic.

1/4-20 thread is so common that it's easy to lose the tap.

Universal tripod thread for cameras.