Need help with acrylic rod milling

mollon · Nov 2, 2018

I have a solid cast acrylic rod. It is transparent and it needs to maintain most of the transparency after the machining.

The rod is 80mm long and 16mm in diameter. I want to make a hole in the middle. The inner diameter will be 13mm. I don't want the hazy finishing. I need the machined part to be mostly transparent. Flame polishing isn't really an option.

How can I achieve this on my own? What type of machining can achieve the required finishing? What machine should I use? What drill, cutting speed, and feed rate should I use? I have seen some related forums on this topic but none of them are from this decade.

Also, are there any shops in the USA that can do this?

[edit] The hole doesn't go all the way through the bottom. It stops 1.5mm above the bottom. The bottom of the hole has to be flat.

Bluejeep · Nov 2, 2018

drill with drill with a 90 deg. tip grind. then ream. then it could be vapor polished after machining. that's going to be some thin wall though for acrylic.

mollon · Nov 2, 2018

Thanks Bluejeep. I didn't know about vapor polishing. That really helps.

adama · Nov 2, 2018

Honestly you can buy 16mm tube with a 1.5mm wall, it is out there, its a lot less effort than DIY to a optical finish too.

rcoope · Nov 2, 2018

Adama is right you should just buy tubing and ideally adjust your design to whatever nominal size you can get. I see McMaster has 3/4" in 1/16" wall or 5/8" in 1/4" wall. You'd be close with 5/8" in 1/16" if you could get it somewhere. If you have to machine it we have had good success with diamond tools specifically for leaving a really good finish on plastic. Google for more. I'd get 1/2" ID which is 12.7mm and turn the outside down if you had to.

By the way I had an engineering student in my office earlier this week and talked him out of needing to look at his sample just because it would make him feel good. It was a matter of him staying with thin wall stainless tube rather than going to glass which was going to cause all sorts of other problems. In that case he was measuring displacement of the stuff in the tube and force at the end of the tube so there really wasn't a reason to look at it. In the present case, you should ask yourself the same question: Do I really need this tube to be clear? Maybe this isn't for science at all but we encounter this question all the time.

mollon · Nov 2, 2018

adama and rcoope thanks for responding to my thread. I understand you can buy acrylic tubes. I made the mistake of not stating that the hole doesn't go all the way through. The length of the hole is 78.5mm. The shape is a cylinder with a hole in the middle. So the tube option doesn't work for me. Any other thoughts/suggestions.

mollon · Nov 2, 2018

rcoope said:
Adama is right you should just buy tubing and ideally adjust your design to whatever nominal size you can get. I see McMaster has 3/4" in 1/16" wall or 5/8" in 1/4" wall. You'd be close with 5/8" in 1/16" if you could get it somewhere. If you have to machine it we have had good success with diamond tools specifically for leaving a really good finish on plastic. Google for more. I'd get 1/2" ID which is 12.7mm and turn the outside down if you had to.

By the way I had an engineering student in my office earlier this week and talked him out of needing to look at his sample just because it would make him feel good. It was a matter of him staying with thin wall stainless tube rather than going to glass which was going to cause all sorts of other problems. In that case he was measuring displacement of the stuff in the tube and force at the end of the tube so there really wasn't a reason to look at it. In the present case, you should ask yourself the same question: Do I really need this tube to be clear? Maybe this isn't for science at all but we encounter this question all the time.

Thanks for your response. It's not a tube because the hole doesn't go all the way through. Please see my other response.

Can you specify the diamond tools you used to get successful results? What brand? What speed? Did you need to use coolant?

FredC · Nov 2, 2018

Your plastic supply places will have a water soluable paste that could be applied to a small felt on a Dremel tool. Good cleaning would remove the abrasive after you finish.

rcoope · Nov 2, 2018

The relevant guy who knows about the diamond tooling has gone home but I know he did it based on a video. I think it was this one that gives you some idea:

With a closed tube I would machine little end caps and solvent weld them in with methylene chloride, which works super well in acrylic if the tolerances are close. If you really need it clear it's much better to have a transparent inside and then machine the outside if you have to because then you can use the diamond tool and then hit it with Brasso or equivalent polishing compound if necessary and get it really clear. On reflection I would also point out that acrylic is the most transparent solid you normally encounter in the world, much clearer than regular glass, so why not get some 1/2" x 3x4" nominal and glue the end caps in. The extra thickness won't matter from a transparency point of view.

Can you explain the application more and in particular how much design flexibility is available? I feel there are a lot of solutions in this area such as test tubes in glass and quartz in all sorts of sizes.

adama · Nov 2, 2018

Solvent weld the plugs in with the liquid acrylic glue, forget its number, but google tensol (sp) theres a thick snotty kind and then a runnier than water liquid, thats the one you want, done right its as good as invisible make plugs a light push fit, use something like a hypodermic to let the solvent just capillary into the joint, will be just as strong as solid and still beautifully clear. Full strength takes a couple of hours don't sniff the glue its toxic as hell!

mollon · Nov 3, 2018

rcoope said:
The relevant guy who knows about the diamond tooling has gone home but I know he did it based on a video. I think it was this one that gives you some idea:

With a closed tube I would machine little end caps and solvent weld them in with methylene chloride, which works super well in acrylic if the tolerances are close. If you really need it clear it's much better to have a transparent inside and then machine the outside if you have to because then you can use the diamond tool and then hit it with Brasso or equivalent polishing compound if necessary and get it really clear. On reflection I would also point out that acrylic is the most transparent solid you normally encounter in the world, much clearer than regular glass, so why not get some 1/2" x 3x4" nominal and glue the end caps in. The extra thickness won't matter from a transparency point of view.

Can you explain the application more and in particular how much design flexibility is available? I feel there are a lot of solutions in this area such as test tubes in glass and quartz in all sorts of sizes.

It is a liquid container. I initially considered glass and quartz, especially quartz. They are costly to manufacture and machine in large quantities. And I require pretty tight tolerance levels (+/- 0.05mm). I think my only options are acrylic and polycarbonate.

I have already seen the YouTube video you linked. That type of finishing would be incredible. I am not sure if it is possible to machine like that inside tight space. Inner diameter is only 13mm.

mollon · Nov 3, 2018

adama said:
Solvent weld the plugs in with the liquid acrylic glue, forget its number, but google tensol (sp) theres a thick snotty kind and then a runnier than water liquid, thats the one you want, done right its as good as invisible make plugs a light push fit, use something like a hypodermic to let the solvent just capillary into the joint, will be just as strong as solid and still beautifully clear. Full strength takes a couple of hours don't sniff the glue its toxic as hell!

Unfortunately, it isn't possible to use plugs or tubes. It has to be made from acrylic or polycarbonate rod. There is more to it than just the hole in the middle. The other aspects of the design are easy to manufacture and don't require finishing.

Making the hole in the middle and achieving transparency is the difficult aspect of this part. At 1.5mm thickness, I am not even sure if acrylic won't crack. Polycarbonate probably is better option.

mollon · Nov 3, 2018

I am going to give these techniques a try and see how it goes. All of the replies are greatly appreciated. This website is incredible.

adama · Nov 3, 2018

Don't know what your putting in it, but you need to also consider compatibility with both these plastics, acetone and more than a few other solvents will destroy both of them in short order.

Other thing, if its a liquid your putting in them you don't need to sweat the finish as much as you possibly think, a cleanly drilled hole will go nearly totally transparent when wet in acrylic.

Have you considered casting it too? Theres lots of similar resins that will give you very compatible properties to these plastics and casting them to a clear finish in silicone rubber molds is easy, certainly way less effort than polishing on mass.

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