Quote: (Originally Posted by
Skipbreather)
I needed clearances in the 0.0005” range.
5 "10ths" is really impractical in thermoplastics. The coefficient of expansion will dictate that such tolerances cannot be practically achieved. Even measuring in that range requires controlled environs, standards, and good technique.
Quote: (Originally Posted by
Skipbreather)
That requires a very smooth surface. I got it by a combination of fine cuts and a (relatively) rounded radius- call it several mm- on the tip of the tool bit instead of the usual tip profile that looks like a sharp point to the casual eye. The tool bit must be very sharp, too. Plastics dull HSS bits quickly so you need to study the grinding instructions which quite possibly came with your machine. If you don’t have any handy, the article below was copied from the Sherline website. Playing w/ turning speeds also greatly affects finish. Remember, you’re thinking in feet per minute/ meters per minute past the tool bit, not just RPM. The diameter of your work piece directly affects this speed.
Softer materials will benefit from a sharp tool. Positive rake, no honed land. That will rule out many carbide inserts. At low surface speeds, HSS tools will last reasonably long.
Soft / flexly materials can make a shallow depth of cut impractical, the material can simply deflect and "flow" around the cutter. Leaving a real "bite" for finishing is often necessary. This works against being able to "sneak up" on the finished dimension. Another good argument for some extr material for test cuts.
Larger nose radius will improve surface finish, but I will caution that in the case of less rigid work holding, or less rigid work pieces, or just plane less rigid machine tools large nose radi will tend to increase chatter. If chatter is encountered and cannot be resolved via feeds and speeds
less tool contact length will often save the day.
Thermoplastics are good insulators, electrically and thermally. This fact is the single most important concern in machining thermoplastics. The work piece will heat up as the heat is not conducted away from the cutting zone
Quote: (Originally Posted by
Skipbreather)
In the end, you’ll find that experimentation with YOUR machine with various materials- metals AND plastics- will teach you much more than you’d expect. If you are in uncharted territory- for you- ALWAYS cut a test piece first to get the basic parameters down before committing you precious I-only-have-one-of-them parts to the machine.
Ken
This is very good advice, buy a little extra material to experiment with.
Tobin