Tool creep?

[trlvn]

I'm still a newbie in so many ways and I managed to blow past a dimension while milling a piece of continuous cast ductile iron (Dura-Bar). I think it was tool creep but I wanted to check with the brain trust here.

I needed to make a short piece about .400 thick by .750 wide and 1.600 long. I have some 1.86 diameter stock and so I could cut off a slice about .4 thick (plus a little machining allowance) and then machine off enough to leave the .750. See below:

I decided to use an auction-acquired 5/8" cutter that I hadn't used before. I have a few such cutters with a threaded end and basically haven't used any of them. But my understanding is that R8 collets mostly clamp near the end (away from the threads) and should therefore hold the cutter OK. The shank on this cutter is a few tenths under nominal (maybe 0.6243?)--it slides into the collet quite easily and has a tiny bit of play. I believe it has been resharpened in the past. I was running the cutter at 440 RPM. A calculator says it could have been run at up to 550 RPM.

After surfacing to .400 thick, I clamped the 'coin' in the vise and cut down one side. I needed to remove .555 inches from each side to leave .750. I touched off and took 2 passes of .200 and another pass of .150. That should have left me with 1.310 but it measured 1.306. At the time I thought that was close enough so I put it in the vise with the newly cut side down. The 1.306 measurement meant I would need to remove .556 to hit my size.

I touched off on a 0.002 feeler gauge and again, I took 2 passes of .200 and another of .150. This should have left the part 8 thous oversize. Instead, it measures .743 on one end and .745 on the other. So I was out by 13 to 15 thous.

I think the most likely answer is tool creep? The .200 passes were more aggressive than I normally take but it did not feel like the mill was straining. The cutter is quite sharp and the chips were nice clean tight curls.

A possibly related question: I seldom work with larger end mills. Should I torque the draw bar more for a .625 collet as compared to a .375 collet?

The other possibility is that my new DRO is not measuring right. I really haven't done much with it since I got it replaced before Christmas. But tool creep seems far more likely to me.

Thanks,

Craig

 

[Tom Kitta]

I generally touch off on metal iteself. Take rough passes to clean both sides and then measure. Without touching anything I then take final passes.

.743 and .745 is a lot of difference on such tiny part. Your mill may also not be trimmed right or its tool creep. I generally take many light passes - not even close to your .2

Heck with a 12mm bit I usually don't exceed .1

I think your tool pulled out under heavy load.

I took a side cut with dull 12mm carbide, speed 1450, feed like 3.5ipm depth .75in and step over 0.03 The difference in 14 in of metal was 0.001 so I guess my mill is well set.

 

[DavidR8]

It's possible that it pulled out 2 thou over that short distance.
How's your tram?

 

[jgimp69]

End mills can easily walk out of R8 collets especially with heavy cuts. The flutes "pull them out" of the collet, and can actually pull the workpeice up out of the vise as well, as has happened to me plenty of times. There are threaded endmills that lock into specific holders (weldon shank) and there are proper end mill holders that lock in with a set screw. I switched over to ER collets as they grip the entire length of the specific collet, better than R8.
This is probably what happened given the description you gave.
I would also check the vise as well. Most people only indicate along the x axis for squareness to the table, but i also check height as sometimes you can get swarf underneath you don't notice when bolting it down. The error grows depending on which way you flip that work piece to machine each side.
I would not put a ton of torque on the drawbars as I've seen them warp from overtightening. They shouldn't be red faced tight.

 

[PeterT]

All good points, Other things not mentioned
- if you somehow introduced backlash with the progressive DOC settings? (always remove backlash then dial in, stopping at setting, as opposed to just pinching in a setting from last DOC. At least on final pass hitting target dimension.
- positional lock. If you have a stout machine its often possible to leave that axis un-locked. But its also a good habit to lock it because it movement is stabilized & also takes cutting load off the wear items which are leadscrew & nut. Now a machine that moves BECAUSE of tightening the lock needs to be examined. Ideally you have DRO on the axis & its a dead giveaway
- gripping odd shaped things can distort as a function of material removal but looks like your bar must have had a parallel & good vise contact, so I'd be betting on swarf or edge burr if flipping the part without filing the corner edge clean

 

[trlvn]

The flutes "pull them out" of the collet, and can actually pull the workpeice up out of the vise as well,

I never thought of that! I did deburr the stock each time before putting it in the vise. I *think* I had the vise clamped to normal tightness but maybe there was some creep there due to the heavy DOC.

Craig

 

[trlvn]

Re Tram, I surfaced the .400 face with a fly cutter just earlier. I got the same pattern of fly cutter marks going both L->R and R->L so I think the spindle is perpendicular to the X axis.

Craig

 

[lucsimoneau]

Had this problem (but much worse) the other day when a 1/2 inch endmill was pulled out of my TTS style tool holder and mangled my vise jaw. The tool you have is for a Clarkson holder and could very possibly be pulled out from a collet. ER40 collets tend to have more holding strength then other types. Backlash can also be a problem in your Z axis. Good luck!

 

[Susquatch]

@trlvn - others have provided good input. Here are a few other thoughts.

I try to never make my last pass much different than the previous pass. I do that on both the lathe and the mill. Tool pressure often pulls the tool, moves the part, stresses the vise, etc etc. I'll often take a light pass much earlier so I can have 2 or 3 passes left that are optimum cuts. With carbide, stiff enough to load everything without straining anything and designed to leave a good finish. With HSS, several light passes for good finish on dimension. Then I divide the remainder in two (or 3) and take my next passes as though they were final passes. I measure the first one vs what it should have done and then adjust the last pass based on that input/output to nail what I'm after. That way, the tool always does what I wanted it to do. Doing 3 or 4 similar passes also allows me to check, adjust, and finalize the finish I want.

I think big cutters need more torque on the drawbar. As others have said, big cutters often pull out a bit under heavy loads. I'm not suggesting you go crazy, but a drop of oil (not a squirt) on the collet threads helps increase the tool clamping without pulling a shoulder out. I don't use an automatic drawbar and never will. I'm big enough to reach easily and I don't trust those things to be as good as I am at adjusting the torque as needed. You should try marking the tool and making some heavy cuts with the usual torque and then checking to see if it moves.

Good luck.

 

[Martin w]

Craig, sorry about your part. I hope it is salvageable. Keep your eyes peeled on Kijiji for a Clarkson Autolock holder. That style of end mill will never pull out from one of those.
Martin

 

[jgimp69]

I never thought of that! I did deburr the stock each time before putting it in the vise. I *think* I had the vise clamped to normal tightness but maybe there was some creep there due to the heavy DOC.

Craig

I'm also talking the vise itself. I usually lightly stone both the vise bottom and table, wipe clean and lightly oil. I then clamp the vise down and tram the edge so it's square, but also the bottom of the vise jaws to make sure it's not off kilter. Off by a few thou or two and errors can compound quickly.
Doesn't take much under the vise to toss it off.

 

[trlvn]

Craig, sorry about your part. I hope it is salvageable. Keep your eyes peeled on Kijiji for a Clarkson Autolock holder. That style of end mill will never pull out from one of those.
Martin

Woohoo! More tools I can buy!

Joking aside, I actually have several keyway cutters with threaded shanks. (Another auction score.) I guess I'll keep my eye open for a 1/2" shank Clarkson holder.

Craig

 

[trlvn]

I'm also talking the vise itself. I usually lightly stone both the vise bottom and table, wipe clean and lightly oil. I then clamp the vise down and tram the edge so it's square, but also the bottom of the vise jaws to make sure it's not off kilter. Off by a few thou or two and errors can compound quickly.
Doesn't take much under the vise to toss it off.

I do the same when putting the vise on the table. It hasn't been moved in quite some time.

I did a quick check of my DRO/tram... I locked a 1-2-3 block in the vise and put another block on top of it. Then I put a hardened gauge pin in a collet and touched it to the top of the 1-2-3 blocks. After zeroing the Z axis, I could run the spindle up and down (this is not a knee mill) and the DRO returned to the same value (within 0.0005 normally). I ran the table back and forth and got the same zero on all corners. I removed the loose block and touched the top of the clamped block. I got the expected -2.000 reading on each corner of that block (again within a few tenths).

I think this shows me that the mill is not grossly out of tram (at least across 3 inches of X travel) and that the DRO measures accurately and with good repeatability.

Bottom line: I'll avoid threaded shank tools when possible. If I have to use them, I'll be sure to add a little extra gronk when tightening the draw bar. And I won't take heavy cuts.

Craig
(Or is it too late to make New Years Resolutions? )