Lathe: Internal Surface finish

[JSGAuto]

Hi Everyone,

I am currently working on a project that requires a decent internal surface finish. Nothing fancy, just a smooth to the touch finish.

It is roughly a 6 degree taper, that I am cutting with a boring bar. With the flex of the boring bar, I am unable to get smooth. I have tried various shapes with no luck. The hole is roughly 1" ID.

Is there a specific tool to get the finish I desire?

Being made on a 9" South Bend workshop lathe.

Thanks!
Jim

[lane]

How deep are you boring. How big is your bar and how sharp is your tool and is it on center ,high or low.What feed and speed are you running . Using tool steel are carbide . All of this matters.

[Virgil Johnson]

What type of material? How deep is the bore? Are you using the largest dia. bar you can?

[tattoomike68]

A trick called a mexican endmill...

A peice of 1/2 wooden dowel split with a saw for about an inch with emory cloth through the slot and wrapped around the dowel.

Turn the part kind of slow ,stick it in the hole and rub it till is nice and smooth with a fast in and out motion.

You can get a very nice finnish with that trick. Cost almost nothing.

[dp]

Hi Everyone,

I am currently working on a project that requires a decent internal surface finish. Nothing fancy, just a smooth to the touch finish.

After turning it try honing it with a brake cylinder hone.

[JSGAuto]

Wow you folks are fast!

I am cuting steel (exact kind, unknown, as I am modifying a production part). Using a Tool steel cuter. The cuter could be bigger. It is set up to cut slightly below center (is this correct? Thats how I was taught several years ago). Depth of bore is 1.75". Cuter is sharp, but seems to dull fast in this material.

Thanks!
Jim

[tattoomike68]

Using tool steel you might need to slow down the rpm and squirt lots of oil in the hole too. If you are taking light cuts the tool on center is fine.

[DickDastardly40]

I'd guess that you have reduced the boring bar overhang to the minumum for your cut. I have seen advice as to attaching a weight to the oposite end of the bar to prevent it resonating though have never tried it.

Are you cutting with a taper turning attachment or the compound? If the compound I've used in the past a 6" handwheel extension to allow me a very slow and even rotation to prevent any jerkiness handfeeding.

If you have a taper attachment experiment with speed and feed and DOC, if still no dice then maybe the profile of your tool maybe off. Try cutting in the opposite direction ie. from in to out, this may give your tool a lead-in.

Let us know how you get on

Al

[JSGAuto]

Would it be better to use a cabide tool for this? If this project is a sucess I plan on making a small production run. (50pcs or so)

Thanks!
Jim

[tattoomike68]

Would it be better to use a cabide tool for this? If this project is a sucess I plan on making a small production run. (50pcs or so)

Thanks!
Jim

With carbide you can up the RPM and the finish will be less of an issue.

[Thomas Staubo]

It is set up to cut slightly below center (is this correct?

I'm definitely not an expert but I think that I have read that it should be set up a little bit above center.
(if you're not running in reverse, or have the tool set up side down at the rear).

Isn't that right, folks?


Thomas

[mochinist]

on center or above center just slightly




Where is a split wooden dowel with sandpaper on it called a mexican endmill? clan meetings?

[DR]

Is the carriage locked down? Is the compound accurately adjusted for side play? Is the part rigidly held in a good solid chucking device?

IMO, compounds are hard to use on small lathes like yours. The crank handles are so small it's hard to get an even feed since you have to keep repositioning your hand on the crank. Those re-positioning pauses can cause tool marks with a less than rigid setup.

[Pete H]

Harmonic resonance

I don't know if it's still used, but years back, I was told to use "dum-dum" to kill harmonics. It was a sticky, green-black stuff, kind of like soft putty or modelling clay. If you were turning a long piece and started to get a chatter, you could wind a lump of this goop on the workpiece, down past the area you'd cut already. If it was a boring job, you could put it on the shank of the boring bar. The added mass seems to damp out the resonances. - Pete

[macona]

Carbide is not necessarily the best option here if you cant run it in is happy range.

Sounds like you have rigidity issues. How big of a bar are you using? Get a bar as big as you can. Carbide shank is even better. What kind of holder are you using? Hone you cutter as sharp as possible.

Watch out for resonance.

Try using some other steel other than mystery metal. Stressproof is nice to work with.