.030 taper while using a boring bar (?) you get what you pay for, the HF dial indicator stand makes a better frisbee than it does a tool, etc.

[STEMtheMachining]

Been awhile everyone. Life, and some learning got in the way. As did the realization that I needed some parts for my Unimat to make it a usable piece of equipment.

Anyways, I finally finished my first project: facing, turning OD, drilling, boring, and parting acetate to replace some missing parts. Having done that, I have some thoughts, and a couple of questions.

First off, whoever designed the HF dial indicator stand should be beaten with one. Because that's about all it's good for. It doesn't stay in place, the magnet isn't very good, and it doesn't articulate. So lesson learned, and now I'll buy a good one.

The same can be said for my Chinesium 4 jaw independent chuck, which stripped the threads out of one of the jaws the 2nd time I used it. On acetate. Plastic. And it stripped. Of course, since I have a Unimat SL, finding a 4 jaw chuck of quality is no easy feat. But again - lesson learned.

Now my questions. And there are a couple of them.

First off, my order of operations:

1) center drill 1 inch acetate bar stock for live center

2) Turn down OD to .950

3) Cut down into 2 inch sections

4) center drill, and then drill to .500

5) use boring bar to increase ID to .540

6)use parting tool so that finished piece is .195 thick

That's it. A straight forward job. Even on a micro lathe like the Unimat.

But I noticed two things that occurred that surprised me.

1) I had trouble keeping the acetate securely clamped (I was using a 3 jaw scroll chuck, with bully bars) and sometimes as I was boring, it would back out of the chuck

2) a couple of time when I was boring (using a HSS boring bar, 1/4 inch square shank), I noticed that the snap gage would differ by as much as .030 between the top of the blind hole, and the bottom before the drill cone. I was cutting a taper with a boring bar, which I definitely wasn't trying to do. (QCTP was verified as being parallel to the piece using a reference block, boring bar was centerline to the part and the bar itself showed no damage. Cuts were never deeper than 0.010, verified by dial indicator referenced against the QCTP holder or the compound slide.)

So tl;dr , here are my questions:


- What is the best way to clamp acetate or other plastics that will keep them secure?

- What, other than a bent/deflecting boring bar will cause a taper to be cut? Does acetate deflect that much internally?

 

[682bear]

Any pictures of your setup?

I'm having a problem picturing your setup... you are boring a ½" hole with a square ¼" piece of HSS? Or is it a square shank with the end ground smaller to fit in the hole?

You have drilled a blind hole... how deep? If I'm understanding correctly, the workpiece is 2" long, correct?

Acetate... is that the same as Acetal... also known as Delrin? If it is, I've never had any issues with it moving in the chuck, and it always machines accurately.

We need pictures, I think...

-Bear

 

[jrkorman]

You did verify that that there was no taper in the headstock!? Just asking as you mention other items and didn't address this.

 

[rabler]

The cutting point of a boring bar moves parallel to the ways with the carriage, even if bent or mounted non-square. Flex in the bar is the only reason to deviate. Flex can be caused by a noncutting part of the bar contacting the material. Otherwise your headstock axis is not parallel to the ways, or your material is moving.

A common issue is not getting the boring bar twisted correctly in the mount so that it actually cuts instead of just rubbing. This would also explain why the acetal is shifting in the chuck.

 

[RJSakowski]

Flex in the boring bar is the same at the beginning and the end of the bore.

I suspect that either a dull boring bar or it is angled wrong so that there is no relief from the cutting edge. Regrinding the bar to gain relief or changing the angle of the bar can correct. I mostly use brazed carbide boring bars but I regrind them for more relief. For very small holes, I make boring bars from old taps.

Another issue that occurs with small holes it the back side of the bar hits the hole, Again, this problem can usually be corrected by adjusting the bar. Another option is to use a smaller bar so it will clear the hole at its deepest depth.

 

[STEMtheMachining]

Any pictures of your setup?

I'm having a problem picturing your setup... you are boring a ½" hole with a square ¼" piece of HSS? Or is it a square shank with the end ground smaller to fit in the hole?

You have drilled a blind hole... how deep? If I'm understanding correctly, the workpiece is 2" long, correct?

Acetate... is that the same as Acetal... also known as Delrin? If it is, I've never had any issues with it moving in the chuck, and it always machines accurately.

We need pictures, I think...

-Bear

Unfortunately, I work out of one bay of a 2 car garage, so I had to tear apart my lathe and put it away when I finished. So no pictures. But I'll try to address your comments.

I drilled a blind hole up to 0.500" that was 1.5" deep.

I then used that hole as clearance for my boring bar to increase the ID of that hole an additional .050 to reach my final ID of 0.550" , and verified using telescoping gages and a micrometer.

The boring bar is from a pre-ground HSS kit, using 1/4 inch shank M2, and because of the machine limitations (and my inexperience) no pass was deeper than .010" .

Concentricity at the headstock was verified using a dial indicator in a crappy base. (see above rant).

And yes, acetate = acetyl = Delrin. And I had heard the same thing, that's why I was surprised by the difficulties I had clamping it.

No noticeable wear on my 3 jaw chuck that I can see.

 

[STEMtheMachining]

Double post.

 

[STEMtheMachining]

Flex in the boring bar is the same at the beginning and the end of the bore.

I suspect that either a dull boring bar or it is angled wrong so that there is no relief from the cutting edge. Regrinding the bar to gain relief or changing the angle of the bar can correct. I mostly use brazed carbide boring bars but I regrind them for more relief. For very small holes, I make boring bars from old taps.

Another issue that occurs with small holes it the back side of the bar hits the hole, Again, this problem can usually be corrected by adjusting the bar. Another option is to use a smaller bar so it will clear the hole at its deepest depth.

The bar was oriented so that the QCTP was perpendicular to the ways, and then the bar was oriented to be parallel to the ways.

This is the boring bar I used. Should I have raked it slightly, was that my problem?

 

[STEMtheMachining]

A common issue is not getting the boring bar twisted correctly in the mount so that it actually cuts instead of just rubbing. This would also explain why the acetal is shifting in the chuck.

"Twisted"? I don't understand. Can you elaborate?

 

[RJSakowski]

The bar was oriented so that the QCTP was perpendicular to the ways, and then the bar was oriented to be parallel to the ways.

This is the boring bar I used. Should I have raked it slightly, was that my problem?

The back clearance looks aggressive enough. You may be hitting the bore with the bottom of the bar. I use round shank bars and rotate them slightly to adjust the rake, I also grind an arc on the bottom edge so there is adequate clearance.

Delrin/acetal cuts very easily if the tool is sharp. Very little cutting pressure is required. Your work shouldn't be moving in the chuck because of cutting force.