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rocketsled
06-01-2012, 10:37 AM
I'm productionising a part I made that sets bushings in a bore, a precise distance from the face of the cylinder. It's a low volume type project where I figure I'll be lucky to make 10-15 of them. (Call it beer money)

One of the things I've always had difficulty with is setting and reproducing a diameter. Usually I'll set some outside calipers to it, make slight cuts on a sacrificial end of the part, then creep up on the final diameter a little bit at a time, then cut the important section of the part...which is fine if it's a single item. It's kinda time consuming if you're doing it to 20 of them, with three diameters and three depths. There's also the issue that the calipers aren't very stiff, it's easy to measure the dimension, then either find it's too thick with the digital caliper, or you walked a gnats-ass too far and you've made scrap.

So, it's easy to get reproducible cuts along the bed direction, and recommendations (beyond 'record the dial, don't change the cutting tool for the length of the run')...or is that the answer? Or...is this why a good machinist used to be a desired quantity, they've gotta be fiddly with each piece? ...a problem that goes away with CNC machining centers.

firbikrhd1
06-01-2012, 12:42 PM
If your machine is in fair condition and well aligned you should be able to do as you've mentioned, "sneak up" on the proper diameter for the first one, note the reading on the cross feed dial and use it as an approximation for the reminder of the pieces. Tool wear, rigidity of your setup and machinability of the material used will cause variations so you'll still need to measure accurately when you get close to the desired dimension. Even temperature variations can have effects. Drawings for parts have design tolerances so as long as you stay within them the job gets easier, providing the tolerances aren't .0001".

lakeside53
06-01-2012, 12:44 PM
CNC doesn't do much more then what you can do. it "knows" where it is which is like you measuring.

Yes, you need to believe the dials and make cuts of a reasonable depth. I stop roughing say 10-15 thou out, make a critical measurement, divide it in two and make two finish cuts. How far out will depend on the metal and your tooling. For multiple parts, I still stop at least one finish cut out and check that the dials still correspond to my record settings.

What are you tying to "fit" - bearings, or something less critical? What's your tolerance? What material? Surface finish?

If you want to get it to a gnat's whatever, emery paper for the last few 10th's may be the best choice.

And... use a Micrometer; calipers can only take you some of the way...

Arthur.Marks
06-01-2012, 12:53 PM
Agreed. Always use a micrometer for determining the finish cuts if you want to be anywhere below a couple thou- tolerance. Also, I recommend being aware of heat in the workpiece if you are within a single thou- tolerance. Dimensional change is real, especially if you are feeding slow or taking too many cuts to edge up on your final diameter.

rocketsled
06-01-2012, 01:11 PM
This is all good stuff (I'd neglected to account for thermal changes..that's important)..I also want to minimise removal and re-chucking of the parts being built.

In this particular case, it's duplicating an existing tool that seats a bushing in a bore. If you don't get the bushing deep enough in, it leaks, if it's too far in, it leaks. I don't think the dimensions are all that critical, beyond fully supporting the bushing and making sure it's 1.6" down the hole. ;)

The material being used is Delrin, both to make the tool cheaper (in many cases, they'll be a one time use tool), and to ease machining, wear, and tool service life. I dropped the prototype (7075) and it left a nice gouge in it that an end user probably couldn't file off...they could 'fix' the tool with a hobby knife if it's made out of Delrin.

lwalker
06-01-2012, 02:16 PM
What's the tolerance on the part?

Jaakko Fagerlund
06-01-2012, 02:20 PM
Usual method for turning just about anything to a specific diameter:

1. Take a small truing cut from the workpiece surface, note the dial reading and measure this diameter. Now you know to adjust your DRO or dial to the measured diameter.

2. Rough out all the material so that you have around 1 mm left in diameter. Cool off the workpiece, take a measurement and move your tool half of this measurement. Change feed/speed settings for a finish cut.

3. Take a cut and check your measurement. In theory it should be the same that you just turned off, but in reality there is a small variation every time. Turn this much to your dial more and take the finishing pass.

4. Verify that your diameter is as per drawing and if it is close, use emery or take a new cut if your tool can do that.

rocketsled
06-01-2012, 02:46 PM
What's the tolerance on the part?

That's the sticky wicket. I dunno. I'm going from reference measurements from a tool. No telling what the +/- is, so I'd best just keep it to a couple thou.

(It's not like a piston to bore clearance, I'm pretty sure 2 thou would be fine.)

rocketsled
06-02-2012, 12:22 AM
Thanks guys. I dunno why now was when the penny dropped (I think it was the comment above about half the desired distance), but I was pretty easily able to get to a desired diameter by referencing on a known surface, doing a little math, and using the dial on the cross slide.

http://farm9.staticflickr.com/8162/7318031842_701ab611b0_z.jpg (http://www.flickr.com/photos/33743995@N00/7318031842/)
DSC_5233 (http://www.flickr.com/photos/33743995@N00/7318031842/) by Matey-O (http://www.flickr.com/people/33743995@N00/), on Flickr