Recommendations for machining simple bores in cast acrylic

I have a job machining a 1.5” thick piece of 12”x12” cast acrylic. It is a demo piece for a company demonstrating a mobile imaging tool for ultrasound inspection of carbon fiber components.

This is a semi loose tolerance piece with a selection of flat bottom bores of various diameters and depths.

I would like to have a few recommendations on feeds, speeds, tooling etc for circular interpolated bores in cast acrylic ranging from 1/4” to 2” diameter and 1/4” to 1.25” deep. I am looking mainly for as clean and aesthetically,optically clear bore as possible.

I will also be doing a simple logo engraving. I have done simple engraving previously with a center drill or spot drill but will spend money on tooling to make a clean engraving. I generally give a light polish after engraving stainless or aluminum to deburr but I assume this will not be possible as the clear plastic will show all marks from polishing.

I have a VMC with 10k rpm spindle. Please tell me if I missed any important info.

I got very good results using a boring tool with diamond cutting tool. Tool must be super sharp and speed not too high as not to melt the acrylic. I guy I used to work with was using soap water as lubricant.
With a very rigid and accurate spindle it is possible to get optically clear finish with a diamond cutter. Otherwise the bore needs buffing with a very fine buffing compound on a felt buff. Optically clear finish can as well be achieved by Methylene chloride application, but it is both tricky and not user friendly.

You can also use heat to do the final polish on acrylic materials. Either a heat gun or a quick "swipe" with a propane torch.

Do a google search for Vapor polishing of acrylic. You will need a box or a fish tank big enough to fit the part and a lid. I did this years ago but don't recall the details. It worked out well for us and was easy and quick.

Vapor polishing - Wikipedia

I found this here on the sight. it might help.

https://www.practicalmachinist.com/vb/general/vapor-polishing-142058/


Make Chips Boys

Will Delrin feeds and speeds work for this material? New 2 flute or four? Sorry if this is a few basic questions.

Milling you can't really screw up if you are machining it wet, treat it like delrin but limit your exit and entries because it likes to chip on corners and break through areas. 2-3 flute is better 4 flute will work fine in reality just make sure whatever you use is near new/new.

Drilling I just stick with 100 sfm, 1/2D pecks 12 IPM if thru coolant isn't available.

If possible anneal the material before starting, we anneal all acrylic before machining and definitely notice a difference if we try and cheat to save time.
You will not get an optically clear bore milling, turning with a diamond as another poster mentioned is possible though.

Get yourself an O-Flute from Onsrud like this one.

62-726

The downflute spiral will help keep from chipping out.

AARONT said:

Get yourself an O-Flute from Onsrud like this one.

62-726

The downflute spiral will help keep from chipping out.

Click to expand...

+1 on this recommendation. I've always had good results with Onsrud tooling in plastic and wood. 1 flute will be plenty unless you have a machine capable of feeding extremely fast with corresponding high rpm and need that extra productivity. One thing to add is that some of their upcut O-flutes are ground for improved bottom finish. I've had good results in delrin with those, have not tried in acrylic. Check out page 51 in the catalog, they are called out in a footnote. https://www.onsrud.com/images/2017 LMT Onsrud Production Cutting Tools Catalog.pdf

they have speeds and feeds for the tools here: LMT Onsrud Cutting Data Recommendations