Tapping Acrylic Blocks

[Edward Weingarden]

I just some acrylic blanks to turn bottle stoppers. Tapping them for mounting on the mandrel has been quite challenging. Any tips/hints as to how to facilitate the tapping process. Thanks.

[Marvin Hasenak]

I drill and tap the same as I do Aluminum, The only exception is I use automatic transmission fluid for my cutting agent. Once it starts to get resistance, it is best if you back it out and clean the chips out.

[Valk Wilkinson]

Careful what kind of oil you use for acrylic. Some will cause crazing later on down the line. Most important is to keep it cool. Once it gets warmer it tends to "stretch" around the tap instead of being cut by it. I've had to go up a size in the drill bit because of this. Any kind of cooking oil works good. Are you using a drill made for plastics? Again go slow and use lots of coolant. Even water depending on your setup. Are you using the lathe to keep the tap straight? What part of the process is giving ya the problem? Practicing on some scraps is always a good idea.

[Edward Weingarden]

I'm drilling with the bit I got in the Ruth Niles kit. The drill press speed is about 650 rpm. I've got the mandrel/tap in the head stock, but getting the threads started while holding/turning the blank was almost impossible. I tried putting the blank in a chuck and the mandrel in the tail stock, then turning the chuck by hand while cranking the mandrel forward. My concern with that method is not being able to turn the chuck and crank the mandrel in sync, thereby not cutting the threads properly.

[Dick Strauss]

First, make sure that you are using an appropriate speed for that diameter drill in acrylic. Don't force it through...let the bit do the work of cutting.

Chuck the tap in your drill chuck mounted in your tailstock. Do not tighten the tailstock to lathe bed clamp lever at all. Have the acrylic block mounted on the headstock. Rotate the headstock wheel by hand while pulling the tailstock into the blank. After a few turns, the tap should self-feed into the blank. Continue turning the handwheel until you have tapped to the depth you deisre (while also taking time to back things out a few times for the cleanout process. Blow the blank out once the tapping is complete.

Take care,
Dick

[Valk Wilkinson]

Go to about 3:13 here and it should look about like that:


https://www.youtube.com/watch?v=KWA8YqgRaAo

After drilling the acrylic check to see if the drill will go in the whole just by hand. If is it too tight that could be the problem. Looks like the kits come with an 11/32 drill bit. If thats correct you could go up to an S or a T drill bit or even a 23/64. Was just looking at the kits. Did it come with a tap or are you using the mandrel to tap it? If so that could be the problem. You might need to get a proper tap for getting that started you would want a taper tap. You can see the difference.
321518
321517
The stopper should be mounted in the chuck end of the lathe and the tap in the tail stock end.

[Edward Weingarden]

Valk:
The end of the mandrel is supposed to serve as the tap also. It is not a continuous thread, but has a "break" in the threads lengthwise as other taps do. I'll test fit the drill bit after drilling to see if I goes in and out of the hole easily.

[Shawn Pachlhofer]

you can use a standard tap w/handle to cut the threads to make it easier

I believe those are 3/8x16 threads

[Marvin Hasenak]

Here is my method. For acrylic or wood I drill with 9/32" bit, the drill and tap charts say to use a 5/16" but I drop down a drill bit size because the chart is for steel. I drill on my lathe, then I pop my chuck loose from the tailstock and insert my tap. I lock the chuck on the headstock with a breaker bar so that it stays solid. I move the tailstock up with the drill chuck in it, but NOT locked into the MT and start to twist the drill chuck. Once it gets hard to turn I back the tap out and clean the hole. Usually about 3 or 4 turns and I have to remove it. On acrylic I add ATF to keep the acrylic from sticking to the tap. I then start the tap again in the hole and make a about 2 or 3 turns and back it out again, again using a squirt of ATF.

After about 6 turns of the tap inside of the hole I remove the blank from the lathe and stick it in my vise. Then I use the either a tap T handle or the handle from my tap and die set to turn the tap. Again I turn about 2 or 3 times and remove and clean out the junk from the hole. Sometimes it binds at 1 turn, I back it out and clean and start over. I repeat until I have enough threads that the chuck and the bottle insert will fit properly.

When the tap starts to bind, stop and back the tap out, the acrylic "gunked" up on the tap can mess up the threads that are already there. It is worse on wood if you try to force the tap past a binding up situation.

On wood I do not use ATF, but I strengthen the threads with CA glue. Before I start to tap I drizzle CA in the hole and LET IT DRY, then start my tap using the same method as I do fro acrylic. When I have the home like I want it tapped I drizzle more CA in the hole. LET IT DRY and then re-tap the CA hole.

[Edward Weingarden]

I used the method described by Dick Strauss above, with a drop of honing oil on the tap. Much easier process now. Thanks for all the replies.