MIlling cutter pulling out of collet

This has happened a couple of times, but today ruined a part I was several hours into .

The part was a 75x52x22 (mm) mild steel block stood upright in the vice.

The task was to remove to a depth of 10mm except for a spigot which would ultimately become a 6mm dia boss.

The machine is a VMR32L, A beefy thing for a hobby machine with R8 and a 1500W motor. Rigid enough that i can seriously consider climb milling with it.

MIlling cutter is a 10mm carbide running at full pelt (2200 rpm) - the cutter is a re-ground 'pro' cutter - possibly a little undersize because of it.

So I set the depth to 10mm an started taking 1mm - 1.5mm cuts. within a few passes I noticed that it was digging deeper on each pass so I stopped, tightened up the drawbar pretty tight and tried again.

Much the same thing happened, though with less creep this time. I ended up about 1.5mm too deep, so I'm going to have to junk the part.

The quill DRO showed little movement (around 2 thou).

The mill, by the way is not struggling with the cut and will quite happily cut this and more when there is no material at the bottom.

Am I expecting too much from this configuration? Should I be tightening harder? Would using R8 Collets give a better grip? I don't think there was oil in the collet / mill assembly, but could that be a contributor?

Looking forward to your answers.

Iain

You say you "tightened the drawbar" and then you say "would using R8 collets give better grip".

What were you holding the cutter in as only an R8 collet would tighten up if drawbar tightened

I'd be happy with those sorts of cuts on my Bridgeport, and that's only 1.5hp. The method of holding the cutter isn't stated, but I'd place a bet that is where the problem lies. If ER collets are being used, and the cutter was reground on the shank as well as flutes, then all bets are off as ER collets have a relatively poor grip at anything other than nominal diameter.

With a 10mm cutter I use a sidelock holder with a 3/4" parallel shank in a R8 collet. Never had a cutter come loose, although I have had the holder walk out on the CNC mill a couple of times due to incorrect speeds 'n' feeds.

Andrew

Assuming the cutter is held in the correct size collet, it is held with more than 2/3 length of collet gripping shank you need to look at your tightening torques. According to Regofiix depending on nut size and type you could need up to 105Nm on the nut, and for R8 according to Tormach 25 to 35Nm.

We need more details, btw milling cutters are in my experience never reground on the shank absolutely no need but hey there’s a first time for everything

Tony

With the R8 collets, there is no leeway for size, the tool shank diameter must match the collet. Assuming you have that match, then degreasing the collet and cleaning the bore with a bottle brush might help. Then, as the shanks of carbide cutters are usually very smooth and slippery, you could rub the shank with a diamond lap, say 1000 grit to give a little grip. Only a tiny bit of roughness will help.

More people tend to use er collets , the er25 holds up to 16mm, or er 32 for bigger sizes.

The task was to remove to a depth of 10mm except for a spigot which would ultimately become a 6mm dia boss.....

.......

.......

MIlling cutter is a 10mm carbide....

So I set the depth to 10mm an started taking 1mm - 1.5mm cuts.

Maybe you will not cut to final depth in one go, next time? Take a wider cut at less depth?

You don’t say where the tooling came from. If cheap chinese, then - as Andrew says - all bets are off.

I think I might drill a hole and ‘implant’ a 6mm steel pin as the boss. So much easier?

I have a 12mm carbide cutter that tends to do the same thing when held in an R8 collet. The shank is very smooth and as mentioned above this may be the problem. I must try slightly roughing the surface and see if that helps. It is downright annoying when it happens and I now use one of Arc's indexable carbide shell mills on an R8 arbor when there is any danger of creep from an endmill. As an aside these shell mills are superb and give a very good surface finish.

Doug

The only time i had this happen was when I 1st started out & was by accident climb milling. Never had it happen with conventional.

Steve.

Many years ago I ruined a very expensive lump of cast iron that I had been machining (to make a replacement mill 'saddle' at night school. The 20mm end mill was mounted in a plain collet holder in the Bridgeport I was using and was pretty tight - but the tool was just pulled into the work as you describe - about 10mm over the length of cut. I don't know what the problem with your R8 set-up is but my solution was to use a Clarkson collet chuck whenever I have a suitable Clarkson cutter available for the job - just for peace of mind.

I certainly use ER collets (for non-Clarkson cutters) and have never had one 'pull out' when doing so - but once bitten, twice shy as they say.

Regards,

IanT

Edited By IanT on 03/05/2021 23:28:50

It looks like I measured once and cut twice with the text of my original post.

The boss is 16 mm not 6mm and I was (as some of you have realised) suggesting use an ER32 collet chuck rather than the R8 collets I was using here.

I've had a similar problem with trying to tap with this machine is well = the shank of the tap doesn't seem to be gripped well by an R8 collet.

I will try ER32 once I've found some stock and taken it back to size!

I will double check the dimensions of the mill tomorrow.

Thanks all.

Iain

I personally don’t expect Clarkson holders were designed with extra precautions, such as using threaded end mills, for no good reason. Nor do I expect Weldon holder end mills had a cut-away (rather than most or all of the shank flatted) for no good reason.

An ER holder can also allow slippage, if cleanliness and tightening torques are not applied correctly - or as Andrew suggested, anything less than optimal fitting. This includes re-using collets that have previously been ‘strained’ in any way.

Even at 16mm, I would still likely fit a dowel pin as a spigot.  Depends on the item, of course.  No finish problems, choice of material, less swarf, less cutter wear, less time, less effort and accurate spigot dimensions are likely most of the advantages of that approach.

Edited By not done it yet on 04/05/2021 06:19:45

What about sticking the block in the 4-jaw and turning the boss.

Back to the original question then an ER 32 should hold better than R8 if there is any doubt about the shank diameter of the cutter

.

Now that [hopefully] I understand your situation ... solid carbide tool in R8 collet ... May I suggest that you inspect both the cutter and the collet ?

If the collet is even slightly over-size, it will only contact at the front, and the cutter will wobble.

... the combination of insufficient grip and a propensity to wobble, is a recipe for the disaster you have suffered.

MichaelG.

Thanks again. In fact the collect is relatively new bought to replace one which was a tad oversized.

Jason - I'd actually planned to do that (cut the boss on the lathe) originally, but then thought that I would trim off the bulk of the material to minimise the interrupted cut. Now it's more a question of working out the right technique as a matter of principle.

Sadly, I am now in the day job, so will try and do some more tests tonight. I believe I no longer have a torque wrench, so I may need to leave that aspect until Mr Bezos can deliver something.

Iain

6 out of the 7 CNC milling machines at work have "bites" out of the tables due to milling cutters working out of ER collet chucks - and that is machining graphite, not metals & we have bench mounted ISO 40 toolholders to get a good hold on the tools to tighten the collet nuts with the correct spanners. The machines do have a bit more power available, but against that most CNC operators are animals when it comes to tightening things - if all else fails, use a 3 foot pipe on the spanner !

Don't seem to have pull out issues with sidelock or Clarkson holders.

Nigel B.

They sound like untrained 'animals'

Tony

My Bridgeport has suffered in the past from a previous "Sloppy Operator" Evidence of cutter pull down on the table and marks on the table from drill points. I have large and small Clarkson chucks and lots of threaded shank cutters to ensure that I do not add to the damage.

They sound like untrained 'animals'

Doesn't seem to matter where they were trained - UK, Poland, Czech Republic, Slovakia. International default for operators when it comes to tightening anything is to heave on it and, if in doubt, heave on it some more !

In general terms, though, the East European operators have had a better (i.e. more thorough) training than UK trained guys, starting younger in technical high schools.

Nigel B.

Always worth checking that the work piece is held firm too. The force pulling the cutter out of the collet is balanced by the same force pulling the work piece in the opposite direction. The cutter and work can both move! Check everything.

In connection with 'holding firm', the area gripped is important, as is the state of the material. Damaged and short cutter shanks won't grip properly, while a vice will struggle to hold blocks that aren't parallel to its jaws etc. Clean everything and make sure it really is holding. Try marking the work and shank to make it visually obvious what's moving.

I've read ER collets are ruined by holding an even slightly oversized diameter. The collapsing forces applied when a collet is tightened are enormous and I guess gripping oversize permanently distorts the collet rather than squeezing it down evenly.

Most of my collets are ER32, but I also have plain examples in smaller tools. None of them slip unless I forget to tighten them! However, they are all in good condition, which can't be certain of a collet once the shank has slipped. Industry change busy collets regularly, or as soon as there's a been a problem : collets aren't expected to last forever!

It's all too easy to spoil good tools. You can guess how I know, boo hoo.

Dave