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Chief_Wiggum
10-14-2003, 11:22 AM
Hi all,

I have a Taig micro lathe and just bought some cheap indexable toolbits from Grizzly. They use 21.51 size inserts. Yesterday I tried machining some 6061 3/4" rod and had a hard time with chatter.

If I'm facing say the outer 1/2 of the rod, I have bad chatter. But as the bit passes into the inner 1/2 of the face, I get a nice smooth finish???

Same problem while turning. I've tried faster spindle speeds and faster feeds, but to no avail. Still chatter.

Switching back to my HSS bits yield a good finish that I'm used to.

Any ideas? I'd really like to use these bits http://bbs.homeshopmachinist.net//smile.gif Thanks!

DR
10-14-2003, 11:38 AM
Chief,

You may have negative rake inserts or at least inserts that don't have a high positive cutting edge like your HSS.

Check with HB Rouse 1-815-943-4426. They rep a line of very high positive inserts that cut like butter, sharper than you can ever grind yor HSS. They may have something to fit your holders, but it's more likely you may have to get new holders.

Beware of buying bargain indexable insert holders, in the long run they aren't A bargain.

debequem
10-14-2003, 11:43 AM
I wonder if the speed is too high since you get a better cut as you approach the center of the work. Only things that change are the speed of the work against the tool and the force distribution on the work as the tool cuts.

Unless something is loose, tool misaligned, tool edge angle is wrong, or cutting tip has improper rake, etc, are these correct?

spope14
10-14-2003, 04:20 PM
Too high of a RPM for the outer diameter - stated above well, and probably a bit of "shallow cut" in the process. Those 21.51 inserts have an aprox 1/64 Nose radius, and shallow cuts tend to rub on the nose radius.

Just an idea.

Thrud
10-14-2003, 11:10 PM
Chief_Wiggum:

Your machine is not rigid enough or your belts might be slipping due to excessive horsepower requirements at high speed.

For a small machine such as yours you want to make sure the inserts have small nose radius and are positive geometry.

nheng
10-14-2003, 11:49 PM
Chief_Wiggum: You may want to try slower rpm or feed it a little harder. Also, if it sticks out too far it will probably chatter on any machine due to the stock flexing. I had the Grizzly 1/2" set and got it to cut but never had a very good surface finish. I suspect that the quality of the inserts or their finish (or mounting hardware) was the culprit here but never followed up on it. Got excellent surface finish from a 3/8" (actually 10mm) set from lathemaster (the one with 55 and 80 degree diamonds, threading and cutoff tools). They were around $69 but just one of them could have cost that much if brand name. Hardware and inserts seem decent.

Den

Chief_Wiggum
10-15-2003, 09:19 AM
Thanks for all the replies. I'm going to goof around with them some more, but I suspect it's a combination of the geometry/rake and the lack of rigidity of the lathe. Thanks for the tips, I'll try some different things.

spope14
10-15-2003, 05:15 PM
One hint.. Tried a bit of "testing" on a machine of mine, low HP, same insert. I determined the following fer ya.....

1. As little as possible sticking out of the work holder does wonders.

2. Check your compound rest. Do you have "overhang' of the compound from the base?

3. "Dampen" the tool. Maybe a small piece of copper sheet shim under the tool between shank and holder might help if this does not set you high on center. Maybe even some of that "plastic shim" stock - Just .010 did it for me - used both. This isolated the tool from the holder and metal a bit, thus "breaking up" the harmonics with a fine "vibration dampener".

4. Centering. Carbide tools with positive rake are extremely sensitive to center height.

5. Is the insert seated in right? Take it out, blow out the pocket, clean extremely well, and seat it right. May seem right to you, just do a double check.

6. Mini machine? Check your tool holder area to see all is properly secured. Small ammts of chatter on little machines compound through the entire machine, but start with a single part. Work down the line - tool holder, compound, cross slide, carriage, even the head and spindle.

All of my testing was done on an EMCO CNC 5 machine, which as i see is about the same size as a Taig micro lathe. I have also used previous experience from Spectralight 200 machines (sheerline lathes).

Just some observations. I have used the "Dampening" while turning bigger stuff that seems to chatter while facing or turning many a time.

[This message has been edited by spope14 (edited 10-15-2003).]

Chief_Wiggum
10-16-2003, 06:31 PM
spope,

You really hit it right on.

I spent last night trying some various things you suggested.

Turns out I had a few problems.

- the inserts bolts were loose on all my tools. I had assumed they would be tight. I was wrong http://bbs.homeshopmachinist.net//smile.gif

- I had been turning a piece of 3/4" aluminum 3" long with the first 3/4" held in the chuck. Cutting the rod in half and working with a shorter piece was _much_ better

- I discovered that I have to have my QC post bolt tighted down _really_ tight.

- Upped the spindle speed to about 1300 rpm from 825 rpm. Should I be turning it even faster?

Making these changes has vastly improved the cutting action. No more chatter and I'm getting some pretty good finishes.

Thanks for the help everyone!

spope14
10-16-2003, 09:51 PM
Your kind comments are greatly appreciated. I look to this board now and then to help me create some experiments to "trouble shoot", and I truly enjoy sharing my results.

BTW, learned the dampening thing from finding a few old tool bits in an old tool box in my shop. They had copper welded on the bottom of the bits for this reason - to dampen vibration.

Do your insert screws have a "bottom access" to the screw? Many have a hex or torx on the top of the screw for tightening, but many more on the bottom of the same screw have a smaller hex for a wrench on the bottom of the screw (underside of the holder, the hole goes thru, a hex on the bottom of the screw).

I often tighten the top, then the bottom very snugly to help in seating the inserts properly. Not tight enopuh to break the screw or strip the hex / torx hole, but - like tightening a jacobs chuck on all three holes, enough to create enough positive torque from dual direction to keep it good. Doing this on "dual direction" screws also prevents stripping out the screw, as you snug one way, then the opposite way.

Justan additional idea.