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pntrbl
04-05-2007, 11:27 PM
I've been having trouble with what I think is such a basic lathe operation that I can't believe I can't figure it out for myself. :mad: Wits end at this point tho so it's time to ask!

I need to part what are basically washers off at .180 from 7/8" 303 stock that I center, drill, and ream to .375 first. My problem is I can't control the .180 to better than plus or minus .010. .170 to .190 is the best I can do with the occasional .180 dropping into my lap by dumb luck.

Compound at 90. Insert style carbide cutter. Carriage locked before and after movement. I have extra gib screws in my compound and lately I've been using one of those to tie that down. To control how far I've tried gage blocks against a carriage stop and also my new carriage mounted dial indicator that just arrived today. That dial mount was kinda my last hope. While waiting for that I even tried using the compound for the dimension. Unlock the gib, advance it, lock back down. Same results no matter what I do.

My touching off skills are certainly suspect but once I've cut one I got a zero.

Here's typical on all of the scenarios listed above. .180 dimension plus .089 for the cutter should be a .269 movement. Get a .190 part. Try .259 to lose the extra .010. Get .170. Believe it or not, I get the most .180's when I move it .265!

My latest brainwave is maybe the insert style cutter is walking back and forth in the holder somehow. Is that possible?

SP

Herm Williams
04-05-2007, 11:40 PM
The only time I have had consistant luck in making a true hole in this type of operation is to drill a pilot hole then use a boreing bar to finish to correct diameter. Sometimes a ball end mill will work but a boreing bar is the most reliable for me.
re

Rustybolt
04-05-2007, 11:54 PM
Yeah. Insert type parting tools do flex. Check and see if your washers are dished. Try a wider tool. Also. If your stock is hanging out there in order to get more than one piece per chucking, choke up on your stock.

lane
04-06-2007, 12:03 AM
Make shure the part off blade is square to the work . and truely vertical not slanted eather way any. let only enouth stick out of holder to do the job .Dont hang an inch out if you are only cutting 1/4 inch deep. If you are using a carbide insert blade speed lathe up about 600 rpm pluss run coolant they dont like to run dry . if no collant use cutting oil or something. use a steady feed eather by hand or under power. should work.

motomoron
04-06-2007, 12:07 AM
Unless you're going 1000 mph, I'd hide the carbide insert business and use a nice, stiff 3/32 or 1/8" HSS cut of blade thats way sharp, and get it as square to the workpiece as possible. Lube from a little squeeze bottle, and use a fairly heavy feed. You shoud get "washers" of the desired thickness.

Jim Caudill
04-06-2007, 12:18 AM
Getting accuracy along this axis can be difficult. Your parting tool can be either RH,LH, or Neutral. As the tool engages the metal it may be "pulled" slightly until it reaches an elastic limit. This is one type of error. Secondly, the gear train may have some slop such that .260" on the carriage dial may be producing a different amount of movement (your dial indicator should have helped you here). Hardinge used to have a technical article on the difficulty in holdinh tight tolerance along this axis, and is why they developed "dead length" collet systems. When I need something like what you are describing (that must be accurate), I part-off oversize and place the washer in a specially machined emergency collet with a flat back. I face one side of each washer, producing the quantity I need plus a few extra, and then turn them around and do the final facing cut. I'm not sure that I would trust a parting operation to produce a "flat" surface with a high degree of precision.

BadDog
04-06-2007, 12:39 AM
Yeah, I would expect that with a parting tool, unless you go with a wide/rigid blade that is VERY square and flat on the end as well as exceedingly sharp.

I'm a newbie, but I wouldn't try to part off to a close tolerance length. If it were me, I would turn a shallow step, maybe 0.120 or so, in the front of a set of soft jaws. Part off at +0.030 or so, something I was confident I could hit without effort with no chance of going under final size. Zero on that face in the soft jaws, and turn one about 0.010 over size, then mic. Mic probably won't be +0.010, so adjust for mic'd size and finish turn. Set compound to zero and face the next one. Mic that one too so you can see if you are where you think you are (may be slightly out of tolerance due to different DOC depending on several things). Adjust as needed so the next and subsequent should be almost dead on as long as the jaw step is sharp cornered and has a good finish.

Edit: Oh, and when parting off, you are almost always going to get one side with a good finish, the other not so good with a final bur. Obviously the "good" side goes next to the chuck face. And since you were trying to do the whole operation parting, I'm assuming the parting finish is acceptable as final finish?

pntrbl
04-06-2007, 01:30 AM
Well apparently this is more difficult than I thought. I feel better already! LOL!

My cutoff is a 1/2" integral shank holder type, not one of the double ended jobs, so perpendicular to the work is pretty much a function of the the toolholder and post. Got it from McMaster. Let's see if this link works ....

javascript:TargetLink('TR781','tr','PT7811','3212A 26');

I've been indexing by eyeballing the edge of the shank parallel to the cross slide but I have used an indicator in the past to zero it with similar dismal results.

I did not realize until now I had bought a RH 8* insert. The McMaster catalog page says that's better for tubing and with a .375 hole it looked like a tube to me! I wonder if a neutral cutter might run a little straighter. That'd be cheap enough to buy and find out.

Looked at T-shaped carbide at one point but couldn't fit it up to my current tooling or something. Can't remember.

But bottom line this doesn't sound like the right way to do business. I've been beating a dead horse.

Thanx guys. You da best!

SP

pntrbl
04-06-2007, 01:37 AM
No link! Well I tried ....

Back at McMaster I realized why I didn't get a T-shaped bit. They only sell it in C-2 which they claim is general purpose non-ferrous.

SP

Evan
04-06-2007, 02:10 AM
I also would not depend on a parting cut to hold close tolerance in the Z axis. It's possible on top of the line production equipment but not easy on the average machines found in a home shop. Facing the washers after parting is the way to do it.

darryl
04-06-2007, 02:10 AM
I don't suppose you'd want to part them all off slightly thick, them face them all to size in a second op- seems like what I'd do. I'd be facing the stock, then parting off, face, part, etc. Then re-chuck to face the 'bad' side. Use a block of some kind behind the washer and inside the jaws to get them spaced the same and parallel in the jaws. You might want to make this block before you get going on the parting and subsequent ops. You could pretty much lock the carriage and dial in the compound to arrive at a dimension which should be the same for each washer done this way. I seldom get a decent result simply by parting off- the surface is never very good or flat.

JCHannum
04-06-2007, 09:59 AM
What Lane says.

Your cutoff holder is an integral shank, so set it as close as possible to the end of the toolholder with no overhang. I use a parallel or straight edge on the side of the cutoff tool and physically bring it into contact with the chuck to set the tool at 90* to the work.

I question how tight the insert is held in the holder, and whether it can move in the cut, but a neutral insert will minimize any tendency to wander or force the cut in one direction.

The other area for movement is flex in the part itself. If a long extension is used to permit multiple cuts, a live center will help stabilize the part. It is to be used with caution, and should be backed off just before the cut is finished to prevent jamming, but with practice it can help.

A totally different approach is to reverse the process and install a fixed stop on the toolpost or compound 0.180" from the cutoff tool. In use, the carriage is locked to the bed, and the work advanced in the chuck or collet to contact the stop for each cut.

Carld
04-06-2007, 10:20 AM
In my experience parting tools never cut a perfectly striaght side. Even a neutral tip will wear and cut to one side more than the other.

The way I do it is to face the work and then set the parting tool and part the work about .010-.020" oversize. Face and part all the washers for the job then move to the surface grinder and lay the parts machined side down, parted side up and grind them clean. Flip them over and grind to finish size. That is the only way you will get them to a given size consistantly.

jimsehr
04-06-2007, 12:34 PM
Easy way to hold close length is to time stop to cut off tool. Touch back side of bar stock with stop with machine spindle off. With cross slide move stop back off of stock, turn on spindle cut off. Repeat. Should hold a couple thousands easy.









http://i86.photobucket.com/albums/k106/jims_03/IMG_0005-2.jpg

pntrbl
04-07-2007, 02:33 AM
Thanx for all the input guys.

I'm drilling and setting up to cut 3 at a shot which is about an inch of overhang on 7/8 stock. Didn't I read something about some sort of "rule of thumb" on this. No more than 2 times the dia? Was it 3? Can't remember! An inch on 7/8 should be OK, shouldn't it?

I'm getting a fine finish on the parting cut with this 303 by hosing it with a can of WD40 to keep it cool, so I might as well continue trying.

First up I'll get a neutral insert. It's too cheap not to give it a shot and I won't know if I don't try.

Next I'll try a .120 T-shaped and it might even be HSS like motomoron suggested. I did try an HSS cutter sometime back when I boinged my 1st insert and wrecked the holder, but never thought to reduce the feed and speed from the carbide settings. :eek: This was a mistake. LOL! I thought it was gonna start a fire! Burnt that tool down in no time.

But with the WD "flood coolant" heat isn't an issue anymore. Then again I already know what feed and speed works nice for carbide so I should probably just stick with that.

If I still can't get these things consistently between .179 and .181 then I'd better learn how to machine an emergency collet and face 'em to dimension.

At one point I did try to face a .190 down to .180 by backing it up in the collet with a piece of the stock, but never did get it straight. Face cuts don't lie. You could tell right away.

Thanx again for the extremely useful conversation guys. I'll just keep fumbling along and we'll see what happens. I'm just glad to find out this is not as easy as I thought.

SP

Evan
04-07-2007, 04:18 AM
Make an aluminum collet like this but add a face cut to hold the washers. This one is for a 4 jaw but if using a 3 jaw make three slits instead. The slits don't have to be perfect, a hacksaw cut will do fine. 4 jaw is easier as it can be cut on a bandsaw.

http://vts.bc.ca/pics/collet1.jpg

JCHannum
04-07-2007, 08:40 AM
There is no need to go to heroic efforts or second and third operations for an operation like this. Washers like this are made all day every day by the millions on worn out screw and turret machines in one pass.

While ordering the inserts, also order a gallon of water soluble cutting oil. WD40 is good for aluminum, but no help on stainless and other materials. McMaster Carr #1020K11 is probably a good, inexpensive coolant, or #11365K61. You dilute either about 30/1 and apply with a brush or squirt bottle. Plain old dark sulfur cutting oil is just as good for that matter. A couple of drops for each cut will be plenty.

Take some time and make a dedicated set up. jimsehr's suggestion is the simplest I can imagine, and will work fine once the proper cutoff tool has been found. You don't mention what kind of toolholders you are using, but I recommend the Aloris style from the start.

I am partial to HSS, and seldom use carbide inserts. In a case like this, they can add to the confusion. A rigid HSS cutter with minimal extension should do the job as well as the carbide inserts.

pntrbl
04-07-2007, 11:42 AM
Continued Thanx.

Using a fairly massive KDK toolpost but I don't have a toolholder to mount up a blade type cutter for it. Hence the integral shank. All of the shank is in the the toolholder BTW and I've even backed the thing up to where a quarter inch or so of the toolholder sits directly on the compound. Luckily that's right where the bit height is centered on the work.

When I try a blade type cutter I'll have to step back to my Phase II. I dunno if that'll make a difference or not. It might. The Phase II is nice for general work tho because it doesn't take up near as much space as that whompin' big KDK.

We'll see.

I'm losing this weekend cuz my nephews getting married in Vegas. I wouldn't go but my kids have bribed me into attending with a chopper ride thru the Grand Canyon! Now that I'm down for ......

SP

winchman
04-07-2007, 01:09 PM
If you don't have to make a whole bunch of them, here's how I did it.

Chuck up the stock with about an inch sticking out. Mark the stock around one of the chuck jaws, so you can take it out and put it back in exactly the same orientation to eliminate chucking error.

Drill and bore the ID to to size. Turn the end flat, and the OD to size.

Remove the piece and cut off the piece with your saw. Set the piece aside until you have them all done.

Return the stock to the chuck, being careful to get it in the same place as before. True up the end, and then cut off another piece with the saw.

When you've got enough partially-finished pieces, make a collet to hold them on the ID or OD with the finished side against the collet. Face them to length using your carriage stop to get them all the same thickness.

I did all the sets of washers in this project like that.
http://img.photobucket.com/albums/1003/winchman/Projects/Tire_Mold_1.jpg

Roger