View Full Version : SFM for Nylon?

07-14-2009, 08:33 AM
I must admit I've always set the speeds for cutting various materials on my lathe using the ol' Kentucky windage method. So far I've been lucky I guess. Recently, I ran across this HF contact tach/SFM meter for $29.99 and bought one. http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=66400 It has worked well for measuring all the speeds on my lathe and X3 mill. The lathe's speed chart was made useless when the original motor died and was replaced with a bigger one with different speed specs and a different pulley. The X3's speed indicator marks are a joke.

I have a small project to do for a friend and the part is nylon (which I have no experience with) so I figured I'd use my new toy. I've read that I gotta hone my HSS tool to a razor edge and the speed has to be slow enough to prevent melting. What would be a good SFM starting point? The HF tool has a contact wheel that you simply apply directly to the work as it's spinning and read out the SFM on a LCD display. Simple enough for me even!;)

07-14-2009, 09:01 AM
The basic rule for both turning and especially milling plastics in general is to keep the speed way down and the chip load as high as the work will tolerate in terms of work holding or clamping. This is most applicable to non-brittle plastics such as Nylon, acetal and polycarbonate. The actual SFM depends on the type of tool, the length of the cut (heating of tool) and the type of plastic and even the color and shelf age as well as the humidity in the case of Nylon.

The usual limit in turning is the ability to securely hold the work. A 4 jaw chuck is much better than a three jaw and a six jaw much better than a 4 jaw. If taking a heavy chip load in Nylon with a 3 jaw your first indication that you are taking too much is that the work will disappear for a moment as it bounces off the wall, ceiling etc. A three jaw chuck allows for too much flex of the work in the jaws.

07-14-2009, 09:48 AM
Thanks for the tips Evan. Sounds like low speed and high feed it is.

This job is just a quick .030" facing off the back side of a 3" Carl Goldberg spinner (I'm sure you remember those from your r/c days) so I guess my question was sorta silly. It'd be pretty hard to screw it up since I'm turning a mandrel to hold it securely and the cut will be very short. I thought I'd try to make the cut at the "proper" SFM since I have an easy method to check it now.

07-14-2009, 10:00 AM
With something like that take pretty light cuts. The work itself isn't very rigid. You also have to take that into account because if you take too deep a cut the work will deform and the cutter will hog into the work.

07-14-2009, 10:58 AM
Can't you put a 30 thou washer under the back face of the prop??

Regards Ian.

07-14-2009, 11:07 AM
Yeah...I would be tempted to go the fitted washer route if that doesn't run you out of crankshaft. Nylon props on model airplanes (my experience is 30 years old) already offered issues in that nylon has some water content. As they age, they get more brittle and can come apart. Some guys stored them in water to help minimize this.

Granted, a spinner is not subject to as much force given its much smaller diameter, but I would still hate to see it come apart at between 5 and 10 thousand RPM, or break when someone shoves an electric starter on it. What percentage of the spinner's backplate thickness is .030" (in other words, how much are you thinning it?


07-14-2009, 11:48 AM
Just a beginner here but have been working a bit with Nylon. The low speed and high feed mentioned has worked very nicely. I've had good results with a gently rounded tool without any particular extra sharpness.


07-14-2009, 02:09 PM
Thanks all; I'm sure there's enough prop stud left to fit a .030" shim but the friend is a u-control aerobatics fanatic. I'm sure there's a very good reason why we need to trim the back of the spinner instead of putting on a shim.;) These guys are *very* finicky.

You should see the plane; it's absolutely gorgeous. It's an Al Rabe Mustang with the coupled rudder (more outward movement on outsides than insides) and an immaculate middle 50's era Israeli Air Force paint scheme. I can only dream of such beautiful craftsmanship.

Oh, there's no danger of weakening the spinner backplate; the area I'm removing is on the back side and doesn't really do anything. Also, no danger of damaging it with an electric starter. These fellows ALWAYS hand-start them.

07-14-2009, 03:44 PM
A guy in our local club-- the Peoria Area Wireflyers--(this would have been late 70's, early 80's) used to judge control-line aerobatics at the Nat's back then. My Dad and I went a couple of years and served as a recorder for this guy one year (1980 as I recall-- they were in Seguin, TX that year...in August, no less).

I have been out of the hobby for decades, but somewhere, my Dad still has several of my planes and engines and other junk in boxes. I occasionally wonder what ever happend to say Bobby Hunt and the other guys who were top in the sport back then. I remember getting to meet Duke Fox and talk with him a bit (maker of Fox engines). Edited for content since I cannot find any web info that supports my last statement.

07-14-2009, 04:17 PM
Bob(by) Hunt, if the same fellow is still one of the very best and was until recently, the editor of the AMA magazine Model Aviation. Duke Fox passed away some years ago and the company is still going. http://www.foxmanufacturing.com/

07-14-2009, 06:06 PM
I run most all plastics on the slow side so I can pull the chips by hand and yank the last ribbon away from the machine and chip pan as I stop the feed.

other wise at the end of a cut it will wind the whole chip pan load around the chuck and its not a damn bit funny. :eek:

Always clean you chip pan out before turning plastic for the reasons I just mentioned. if the chips wind up and you are running fast it will grab up all the steel drill chips and beat you bloody.:cool: