120mm diameter Delrin machining problem.

Hi All,

Have just been given a new lump of 120mm diameter by 110mm long Black Delrin by my neighbour which is too wide for the external jaws of my ML7 chuck.

He wants me to make a sleeve for his spinnaker pole on his boat which is basically a ring with a small flange one end, seems pretty simple if I can only get it mounted.

There is plenty of spare material for chucking if I can get a bit of the diameter down to suit the ML7 chuck jaws. What is the best way of doing this please? I am thinking of somehow mounting it to the mill table, (not sure it will fit in my vice but may be able to rearrange the jaws to grip it).

I have a centre finder dial gauge which I have yet to use! Seems a good time to try it out.

I have no idea of the best way to mount this softish material safely to the table nor any idea of the best way to mill the shoulder for the lathe chuck. No idea of cutters or correct speeds etc.

Presumably It will be OK to use a boring bar in the lathe after turning the correct outside OD?

Any help and advice will be most welcome.

Regards,

Martin

Hi Martin,

I would drill and tap 3 holes in the back and bolt the delrin to a faceplate then machine as normal fast and big cuts with a ultra sharp tool.

Merry Christmas

Andy

Drill a hole through the Delrin and clamp it between two nuts and washers/spring washers on a bit of threaded bar with centres in both ends. Mount the whole lot between centres in the lathe and finish the outside profile. Remove and mont in chuck to do the bore.

regards Martin (I'm a K too, or should that be a K 3)

If you have enough material then you can use a hacksaw to cut three sections out at 120° intervals around one end to leave a triangular end for gripping in the chuck. Turn the other end down to a diameter to suit the chuck then reverse and machine where the three cuts were made. If you are going to machine away a lot of one end then you are only cutting away material that would become swarf anyway. Without knowing exactly what the finished item will look like its hard to know how good any of these suggestions are but hope these two ideas (mine and Andy's) help.

Martin

If the end is reasonably flat you could possibly use a spade bit to drill 3 pockets at 120 degrees apart on the same radius from the centre, and use a 3 jaw chuck in these. I have done this to hold a large aluminium pulley blank, cnc milling the pockets. A spade bit should work in delrin I guess, or maybe a Forstner bit as used for setting hinges in chipboard.

Push it againts the faceplate with a rotating centre and then turn it to a diameter that fits the chuck. Niko.

Thanks a lot guys!

Clearly more than one way to skin this particular cat.

I quite like the idea of doing it between centres as I have not done this before.

What would be the best drill speeds for this, say a hole 12mm or so please? I have some all thread this size which would do. Is there any danger of 'melting' the material?

Regards,

Martin

Your biggest trouble drilling Delrin is the drill snatching, keep the tailstock lock on a bit to prevent the drill pulling in.

Good luck with getting it to size, I find that I can measure the stuff correctly and hours later the sizes are all over the place.

I second that method.

As long as one face is somewhere near flat, put double sided stickytape or any thick tape on the faceplate and press the Acetal against it with the tailstock centre. As long as you get it somewhere near concentric you wont even need to drill a conical centre hole as the point will make its own dimple. You should be able to face (almost to the centreline) and reduce the OD easily. (sharp tools are best)

Ian P

One small point watch out is that Delrin will explode on you if gets too hot by you taking a too big a cut and/or feeding the cutter to fast. Speak from experience.

Regards

Luigi

It is easy to melt when drilling. The other problem I have had is that the hole closes down on the drill after the point has passed. Even with a new drill the material moves out of the way rather getting cut by the sides of the drill. The deeper you go the greater this drag on the drill gets and the more the heat builds up. Take the drill out frequently and check it's temperature, cool it with suds as well. Low speed high torque is best for larger sizes.

I found drilling partway with small drills then opening out that hole with larger drills up to the required size before drilling further using the same process kept the friction under control.

Martin

I've never had any problems turning, drilling or milling Delrin using conventional tooling. It's definitely one of the nicer plastics to machine. As a default I use high rake polished inserts intended for aluminium when turning. For drilling HSS jobbers drills are fine and for milling either HSS or uncoated carbide. It is ok for holding tolerances of a couple of thou or so; nothing like PTFE! It does close up a bit on drilled, and tapped, holes.

However, like most plastics Delrin will melt if the machining parameters are wrong. The key is lowish spindle speeds and high feedrates. For turning 8-12 thou/rev is fine.

Andrew