Any ideas on what I'm doing wrong ?

[JimM]

Hi all, I've got some issues with my milling machine and am hoping that the expertise here can point me in the right direction.

Although I've owned my Elliott Omnimill for almost a year it's only recently that I've actually started to use it ! When I bought it I knew that it had seen some use (it's got to be 50 odd years old) but I'm now thinking that it's a bit further gone than I had hoped.

Last night I was attempting to cut a dovetail in an insert toolholder so that I could mount it directly to the toolpost (ie so that I didn't need to use a tollholder holder)  Using a 1/2" two flute cutter running at 350RPM I was struggling to take more than a 10thou depth of cut. Anything more than this and the whole table started to shake and the chatter was frightening.

I'm assuming a machine of this size (it's 5' tall and weighs in at 750Kg) should be able to handle a much greater cut than this but with so many variables I'm not entirely sure what is at fault. The only two ideas I had were that

1. I know the Y axis is worn but I've got the gib as tight as I can and still be able to move the table. If I grab the table at each end and try to rock it back and forth it all feels pretty solid.

2. The toolholder doesn't appear to be hardened - it cuts easily with a file and hacksaw. The end mill did  blunt quickly but I put this down to the chatter abuse it got.

I know it's hard to give a diagnosis without seeing it in action but from reading the above is there anything that jumps out at you guys as to what the issue might or am I expecting too much from an old machine.

Thanks

Jim

[AdeV]

What sort of cutter are we talking about? If it's a standard end mill, then I suspect your RPMs are too low, and maybe your feed rate is too high. If it's a dovetail cutter, then a 0.010" cut is probably as good as you'll manage in steel; but TBH I've never seen a 2-flute dovetail cutter, so I'm guessing you're using a regular end mill.

What material is the toolholder? If it's mild steel, you should be able to double those RPMs with a 1/2 cutter. It might also be worth looking to find a 4-flute cutter for this job, as that will run smoother than a 2-flute.

Of course, the other problem is, once the cutter is blunted, you're on a slippery slope anyway. Attempting to get more from it will end in a broken cutter and some frayed nerves...

[kwackers]

I have an omnimill, you should have not problems taking a cut much deeper than that - with a decent cutter...

Assuming your cutter is good I doubt play in the table is an issue, the table is heavy enough so that a 10 thou cut would work even if the gibs were slack.

Much more likely is the tool holding itself. Presumably your omnimill has a vertical head and that's what you're using?
Make sure the tool is a good fit in the collet (or whatever you're using). Make sure the collet chuck (you are using a collet/milling chuck to hold it?) is tight and the drawbar nicely tight too. There should be absolutely no detectable play in the whole tool/chuck/head assembly.

Assuming that's all ok then the head bearings could be worn (I've just replaced mine), try running it at highish speed for a couple of minutes with no load and see how warm it gets, worth pointing out that even if the bearings are done for you may not be able to see any obvious play in them. There may be a technique for checking involving dial gauges etc but I'm not aware of it. In my case the bearings just felt rough and ran hot when spun.

It's a good amateur machine - particularly with the vertical head. Unless it's completely done for might be worth getting the slides reground - alternatively a good man can do wonders with a scraper.

[JimM]

Hi guys

Thanks for the input

It was a standard end mill I was just atrying to take the meat out of the centre of the dovetail but gave up after only removing a depth of a couple of mm. To be honest it was a cutter I got as part of a job lot but the edges looked like new - I do have some 4 flute ones (from the same lot !) so I'll give one of those a go at higher RPMs

Kwackers - I was using a Clarkson type chuck and I'm pretty sure it was all tightened up as it should be but I'll double check. Thanks for the heads up on the bearings I'll give it a run with no load but even if these were loose would that explain why the table was juddering so much - you could see it vibrating and feel the feedback in the Y axis handwheel !  I have investigated getting the ways ground and to be honest it's pretty reasonable but I'd rather not throw good money at it if in the end I'm going to be better off with a new machine.

I've just remembered that my camera does video so I'll try again tonight and if the results aren't any better I'll post a short clip so that you can see/hear the problem - if nothing else it will give you a laugh

Cheers

Jim

[kwackers]

If there is play in the head it can allow the cutter to continually grab and release the work. Apart from the enormous amount of noise it will move the table and make it look as though the play is in there. 10 thou is a fairly small cut, you should have no issues making a cut like that given the mass of the table.
Most likely scenario is just a duff cutter, play in the head bearings wouldn't help though.
I've taken to spending decent amounts on my cutters, Dormer seem pretty good. I also have cheap Chinese cutters I use for cast iron and the like - the two are chalk 'n cheese.

If the head bearings do prove to be worn they can't be adjusted. There are 3 in there, two at the bottom which are a matched pair (these run to around £130) and a cheap axial load bearing at the top (£7).
My bearings died because the previous owner used an airline to clean out the MT3 taper which forces chaff up into them...
If they do need changing a press is handy as is the use of the kitchen oven...

[JimM]

Ok I'm making some progress but still suffering !

I run the mill with no load for a good 10 mins and could feel no heat in the spindle, so I then mounted the Clarkson chuck and with a DTI resting on the spindle tried pushing/pulling the chuck - I measured about 0.0025 total movement with the spindle locked, not perfct but probably not too bad considering the age of the machine - either ways I'm hoping the bearings are at fault here?

Next I double checked everything was locked up (it was) so I put in a 4 flute cutter, Clarkson brand again not new but looked perfect, upped the revs to 585RPM and tried again. It cut great, nice and smooth with no table shuddering   However after 3 slow passes at a DOC of 20thou, a total cut length of 60mm, things started to get hairy again, lots of noise and vibration  

I think therefore the problem is that the cutters are getting blunted very quickly. I didn't think the toolholder was made of hardened steel as it cut readily with a files and hacksaw but is this still a possibility?  I was using my DIY coolant setup (blasts compressed air at the work piece whilst coolant drips into the air flow) so not much more I an do there but is there anything else I can try.

As a last resort does anyone fancy having a go at cutting the dovetail for me (beer tokens provided!) it's a bit of a cop out I know but at least that way it would prove whether it is the material that's causing the problem or something I'm doing.

Thanks

Jim

 

[Bernd]

I'd try a cut of .125" to .250" deep. Sometimes to light a cut will cause chattering.

Bernd

[AdeV]

Next I double checked everything was locked up (it was) so I put in a 4 flute cutter, Clarkson brand again not new but looked perfect, upped the revs to 585RPM and tried again. It cut great, nice and smooth with no table shuddering   However after 3 slow passes at a DOC of 20thou, a total cut length of 60mm, things started to get hairy again, lots of noise and vibration  

Were you generating much smoke during the cut? It is possible you're overheating the cutter (causing dullness), or even causing localised hardening in the material. A very smokey cut would suggest you're running a bit hot. Having said that, the cure to heat (other than flood coolant) is less DOC, less feed, less speed.... none of which worked for you before & I have no reason to suspect they'll work now.

If you rub your thumb very lightly & carefully across the edge of the end mill (at 90 degrees to whichever cutting edge you're checking), does it feel rough or smooth? Compare with a section of the end mill you haven't used, any noticable difference? The smoother the blade feels, when you drag your thumb across it, the blunter it is.

It's also worth getting hold of some "Rippa" (or "ripper") cutters - I've banged on about these things before, they're absolutely brilliant for rapid metal removal. Using a 6mm 3-flute rippa, I can take a 0.100" deep full-width cut - in steel - at around 12"/min feed rate. Admittedly, that's on a Bridgeport, which is pretty heavy - not sure how the Bridgeport compares to the Elliot mind.

[kwackers]

The amount of movement your getting in the chuck doesn't sound too excessive, there's a fair bit of metal between the chuck and the mount for your DTI so I reckon I'd be happy with that.

From the rest of your description it sounds as if given a sharp cutter the mill is fine, definitely sounds as if the cutter is blunting almost certainly due to overheating.
Could be the type of metal - I've had metals in the past that don't cut well on the machine, they can seem to harden as you try to cut them (high carbon??). Coolant is your friend here, if the coolant pump isn't plumbed and working then a squeezy bottle gravity fed to feed the cutter will do wonders - you can just pull the drain off the table to a container.
Worth trying a piece of known quality steel and milling that (new cutter obviously) just to demonstrate to yourself that everything else is ok.

[JimM]

Just to update, I've also been posting on the HMEM site and the consensus is that the steel the toolholder is made from is the problem - others have tried similar projects and it looks like carbide cutters are the only things that will work.  Whilst standard carbide cutters probably would be useful I'm not going to shell out for a carbide dovetail cutter so think I'll call it a day on this plan and will look at other ways to get the insert tooling to fit my lathe.

At least I can sleep happy in that it's not me or the mill that's at fault

Thanks again to everyone for your input

Cheers

Jim

[Bernd]

Ah Jim, weren't we talking about using a 1/2" end mill to remove excess stock? 

How did a carbide dovetail cutter get into the conversation.? 

I'm a bit confused now.

Bernd

[JimM]

Hi Bernd

My ultimate aim was to cut a dovetail in the insert toolholder so that this could be mounted directly to the AXA toolpost.  The 1/2" cutter was used to remove the bulk of the material of the 'slot' of the dovetail but once done I would then need a dovetail cutter to cut the dovetail angle faces.

To go any further I'm going to need carbide cutters, I could live with paying out for these for the straight end mill types but I can't justify the price of a carbide dovetail cutter which I will also need.

Any clearer ??

Cheers

Jim

[Bernd]

Yes Jim, clear now.

I can't see any reason for having to use carbide for a dovetail cutter. I'm sure you know they make HSS dovetail cutters. I guess I'm saying that carbide is not going to fix your vibration problem. It may fix the cutter getting dull.

Is there a possibility of getting a picture of your mill or a link to what one looks like?

Regards,
Bernd

[JimM]

Hi Bernd

Due to the hardness of the steel I would have needed a carbide dovetail cutter after hacking out the bulk of the slot. I do have a couple of HSS dovetail cutters but if I had got as far as trying these,  they would have presumably suffered the same fate as the HSS end mills and blunted very quickly.

It was the cutter going blunt and grabbing the work that was causing the vibration.

Haven't got any pics to hand but there are few on Tony Griffiths site (I actually have the earlier Juniormil but for intents and purposes it's the same as the Omnimil)

http://www.lathes.co.uk/elliottmillers/page2.html

Cheers

Jim

[AdeV]

You can always cut the dovetails using a straight end-mill, if you don't mind losing a bit of material out of the back of the dovetail seat (is that the right term?) All you need to do is tilt either the head, or the workpiece, at the correct angle. The attached c-o-c should make what I mean clear.

[edited for spelin']

[Bernd]

Jim,

That's quite a good sized mill at 5 feet tall and weighing 1500lbs. (750kg). Almost equal to my Bridgeport.

When I looked at Tony's mill at the link you gave me a question came to mind. Does yours have the round ram that holds the cutting spindle and motor? How far out do you have that when cutting? Keeping that as short as possible would probably help.

Have you had any trouble cutting aluminum or any other soft materials such as brass? Trying to see if we can narrow this down to machine, type of material, cutters being used or speeds and feeds.

I know that's a lot of questions and if you want to let it go for a while I understand. 

Bernd

[JimM]

Bernd

More than happy to answer questions etc, I'm just grateful that you guys are willing to help out and that you haven't got bored yet

Yes, my machine does have the round ram, I've measured the centre of spindle to column distance and it's 6". The ram isn't as far back as it possibly could go but if I push it any furthe in then I'm going to have problems with the vice snagging the flexible column shields etc

Have you had any trouble cutting aluminum or any other soft materials such as brass? Trying to see if we can narrow this down to machine, type of material, cutters being used or speeds and feeds.

This is possibly the crux of the matter, my machine time is very minimal, I've had a bit of a play around with some ali and brass but nowhere near enough to really get a feel for what I'm doing - literally I'd be surprised if I've got more than two hours cutting time under my belt ! Having said that the small bits I did do seem to go OK !  At the moment everything I do is try it and see,  if anything I think I may be being too cautious - hence the low initial cutting speed, I didn't want to risk the cutter running too fast !  Feed was equally slow, only about 1" per min

To try and resolve this one way or another I've sent an email to ISCAR asking them what their toolholders are made of and if they are hardened. With any luck they'll come back to me shortly and at least we'll know if it is the material that is the problem. Not sure what I'll do if they say that they are straight EN1A though!

Thanks again

Jim









 I don't expect to be perfect straight off the bat and know there's going to be a learning curve but when I was cutting the toolholder things were so badly wrong that

[andyf]

Jim, if you think carbide would help, maybe you could  concoct one of these:
 http://mikesworkshop.weebly.com/dovetail-cutter.html
I've visited Mike in his shop, and he does get good results with it, though he has to take things slowly on his Sieg X1 micro-mill.

Andy

[Bernd]

Ah yes, understand the time constraint. Not a problem.

It can be a bit intimatating to get yourself to feed in faster when the machine shakes and rattles like that. But that could be one of the other problems is not a fast enough feed. Also sounds like you haven't had much time on the mill to feel comfortable with it.

Well take your time. One of these days you'll have it solved.

Keep us posted as to your progress.

Bernd

[JimM]

Well heard back from ISCAR this morning and apparently the toolholders are made of EN24, they gave no indication if it had been hardened but a quick google search brings up

'A nickel chromium molybdenum steel with high strength and toughness. Used for gears axles and high strength studs. Supplied as rolled, annealed and hardened and tempered. Supplied as black round or square bar and bright round or square, and hexagons'

It doesn't mean an awful lot to me but does the fact that it's 'high strength and toughness'  explain why I was having trouble with cutters going blunt very quickly?

Thanks

Jim

PS. As an aside, I have to say I've been impressed with ISCAR, this is the second time I've contacted them and both times they've repsonded very quickly

[AdeV]

Well heard back from ISCAR this morning and apparently the toolholders are made of EN24, they gave no indication if it had been hardened but a quick google search brings up

'A nickel chromium molybdenum steel with high strength and toughness. Used for gears axles and high strength studs. Supplied as rolled, annealed and hardened and tempered. Supplied as black round or square bar and bright round or square, and hexagons'

It doesn't mean an awful lot to me but does the fact that it's 'high strength and toughness'  explain why I was having trouble with cutters going blunt very quickly?

A quick google (for EN24 Machinability) threw this rather useful looking PDF out:

http://www.wessexmetalstock.co.uk/images/files/bwdDOC_46e0189c81f27.pdf

Basically, EN24 is not the worst metal for machining, but it's pretty close... EN1A-Pb is the most machinable, but as it has lead in it, I'm pretty sure it can't be hardened. EN32 looks like it might be a sensible compromise - it's got a decent machinability index according to that pdf, and it can be case hardened.

Disclaimer: I'm new to this stuff too.... so I might be talking out my hat....

Edit to add: I can't see any copyright on that PDF, so I've attached it to this post.

[Bluechip]

Jim

I had some discs of EN24T, not easy to turn, in fact I gave some to Stew, I believe he made a bit for his Rotary Table from one.

But he used coolant on it, IIRC, I just turned it dry with a TCT tool.

Character building to say the least 

Trawl back thro' his posts. Some time ago though.

The suffix 'T' is the 'state' it's sold in. Other suffixes indicate the degree of heat teatment it's had I think.

Some info on here, but you have to go to the home page for all the info.

http://www.roymech.co.uk/Useful_Tables/Matter/Strength_st.htm

I expect ISCAR would heat treat the part to a higher level of toughness/hardness after machining.

Dave BC

[Darren]

I find EN24 very nice to turn, yes it's tough stuff and yes it will eat your cutters. But it sure does cut smooth and finish nicely, almost like chrome.

I found it needs carbide tooling and very high turning speeds, 2 Inch diameter 1,200rpm and below an inch 2,500rpm. Any slower and it'll give you nightmares.
Oh and keep those feeds constant and not too slow.

No need for coolant, but be careful about the heat generated as it will throw off your measurements quite a bit. Best take the final cuts after cooling from the bulk removal.

The most important thing is speeds ...  

Edit:... oh I see you're milling the stuff. Forget HSS cutters, they just won't like it and mess up real easy. Tipped cutters are fine though, keep the speeds up as suggested with lathe work.

[RichardShute]

Hello Jim,
I've caught up with this rather late, but thought I'd throw my 2d's worth in.

En24 _is_ tough -T is tempered, but still tough compared to average steel. If it's been heat treated as very probably it has, then it will certainly be hard work for HSS. For reference, you can get a wide range of material specs (including BS970, ISO and many ASTM specs) and heat treatments from the West Yorks Steel site eg:
http://www.westyorkssteel.com/EN24.html

I have reduced holders for tipped tools and they are generally pretty tough and certainly carbide is better, but you might still get away with HSS. You need to have the cutter speed quite slow to prevent over heating the cutter tips, a good flood of suds helps. A steady squirt from a washing-up liquid bottle if you haven't got a suds pump. If 300 RPM was shagging your HSS cutter, slow it down some more, carbide doesn't really suffer overheating problems teh same way as HSS, but you may get chatter or other problems with over speed.
Different materials react differently to different machining operations. eg stainless saws OK with effort, files fine, but is a notorious sod to drill. Allowing the work to work-harden may also be a problem in your case and one way to help minimise that is to keep the feed quite high, even with a relatively slow cutter speed. Once hardened it can be very difficult to start to cut again.

I have a Speeds & Feeds list (attached below) which I have found generally acceptable. It looks like the data probably came from Alan Marshall's site, or possibly we both plagarised it from the same place, and I have added somewhat to it.

The work-piece material and cutter size results in a selected RPM, the type of cutter then implies the feed in thou/turn, multiply the two together and you get the feed in In. (or thou)/min. eg 2 thou/tooth, two teeth => 4 thou per turn, 300 RPM => 1200 thou or 1.2" per minute. The depth of cut is mostly down to how rigid the machine is and how brave you are, but 0.1" should be no bother for your machine.

Dovetail cutters are damned expensive to buy, but for a simple one, using a trianglar (TCMT) tip, it's not too difficult to make one. Google 'home made dovetail cutter' and there are a fair few examples.

One problem with milling cutters, unlike lathe tools, is that once they're toast, you can't go to the bench grinder and tickle them up again and have another go. It's try another cutter time and that can get expensive and disheartening. One reason I recently got a T&C grinder, but that's another story....

Cheers
Richard

[Lew_Merrick_PE]

Well heard back from ISCAR this morning and apparently the toolholders are made of EN24, they gave no indication if it had been hardened but a quick google search brings up

'A nickel chromium molybdenum steel with high strength and toughness. Used for gears axles and high strength studs. Supplied as rolled, annealed and hardened and tempered. Supplied as black round or square bar and bright round or square, and hexagons'

It doesn't mean an awful lot to me but does the fact that it's 'high strength and toughness'  explain why I was having trouble with cutters going blunt very quickly?

I am not familiar with the alloy numbering system you are using, but it sounds like a medium-alloy high fatigue strength steel is being discussed.  My experience is that having it heat treated to a medium hardness (say Rc-42/38) gains a lot on the machineability properties.  In the normalized (annealed) condition, it is "mushy" and rips up cutters right, left, and sideways.  In the medium hardness condition, the "mushy alloying agents" (nickel & molybdenum) are held in better condition in the crystal structure for cutting.  Additionally, many nickel steels have the tendency to work harden (more accurately defined as strain hardening) in light cuts.  An attempt to "sneak up" on a dimension in these alloys will cause you grief.

I do a fair amount of work on things such as rocket nozzles and highly-corrosive fluid pumping, so I am often working with incredibly obnoxious to machine materials.  Few "small" machine tools (i.e. things people would have in a home shop) can handle such materials.  Heavy, rigid machines are the order of the day when working in such materials.  To machine a 17-4PH/H900 stainless steel (about as obnoxious as it gets until you are dealing with "super-alloys") part that was ø2.500 X 6.00 inches, we ended up on a granite way (400 psi) air-bearing machine to gain the stiffness necessary to get the geometry required.  Turret mills need not apply for such work.