Is my mill past it?

[JimM]

Evening 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.

Tonight 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" 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

 

[Tin Falcon]

Jim some important facts left out here
material? number of flutes on the cutter?
a bit of guessing assuming steel plugging numbers in my handy free download speed and feed calculator assuming a HSS steel cuter I come up with 750 rpm for free machining feeding at 3.5 inches a minute steel and 500 rpm for High carbon steel feeding at 2.5 inches a minute.
Sounds like your speed is to low and your feed too high.
I suggest you either get used to looking up speeds and feeds in a machinist reference of down load a machinist calculator.
I was taught always use proper speeds and feeds for any machine . The bigger and heavier the machine the bigger the depth of cut per pass you can take.
The program I have is part of this one http://www.betatechnical.com/autonc.htm
Tin

 

[JimM]

Hi Tin

Thanks for replying

The toolholder is an unknown steel but I would guess it's a high carbon, as said it doesn't appear to be hardened

I was using a 2 flute cutter as effectvely the first cut at each depth was a slot and I made sure to use conventional milling when widening this slot to the width of the dovetail.

For the speed I assumed the toolholder to be HRS so a surface speed of 70 sfm

4 x 70 / 0.5 = 560 rpm

Presumably I should of used the next speed up on my mill of 585rpm but I thought that going slower was OK - I take it there comes a point which is too slow?

For the feed I tried to cut at approx 2" per min but this was somewhat determined by what the mill let me go at, in reality it was probably slower than this !

I only managed to cut to a depth of a couple of mill before I gave up

Cheers

Jim

 

[Omnimill]

Jim, for what it's worth my old Omnimill was badly worn with lots of backlash etc. For the most part I still managed to get good work from it although I was often only cutting Aluminium Alloy. When things did get a bit "rattleley" I locked up as many axis as I could when making a cut and that normally did the trick.

Vic.

 

[JimM]

Hi all

I've made some progress on this, somone on another forum suggested that it might be the spindle bearings but I think I've ruled that out. Running for 10mins plus and I can't fel any heat in the spindle and pushing/pulling a chuck mounted in the spindle only registered 0.0025 movsment on the spindle - not perfect but hopefully not the problem.

I tried another cut tonight, used a 4 flute cutter (Clarkson brnd not new but looked perfect) and run it at the 585RPM, this cut really nicely at first but after 3 slow passes, a total cut length of 60mm, things started to get hairy again, lots of noise and vibration I had upped the DOC to 20thou though

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 can 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.

Cheers

Jim

 

[Omnimill]

Re-reading your original post Jim, I remember someone cut a toolholder at work recently and it was commented that it was made of "very tough stuff". I expect this may well be part of your problem.

Vic.

 

[mocaquita]

It may be worthwhile to check that you are not accidentally running the spindle backwards.

Dave

 

[JimM]

Thanks again for the input

Vic - is there any way to determine how hard the steel is, I can't get my head round why a hacksaw seems to breeze through it but the cutter struggles. Obviously the hacksaw is much thinner but I thought hardened\tough steel would present just as much a challenge to the blade ?

Dave - thanks for the suggestion but I've already double checked that one (would be like me to do something like that though!)

If this is a particularly tough steel would a shaper be more appropriate to the job or is this going to suffer a similar problem ?

Thanks

Jim

 

[steamer]

Hi Jim,

Without being there it is tough to tell, but It does sound as though your cutters are going away.

I didn't quite follow your speed calculation...your using a 1/2" diameter two flute endmill? Additionally, are you sure it's HRS?.....if your not sure of the material, then cut your speed and feed in half with a new cutter and try again. A piece of 304 stainless steel would behave like this as it work hardens....miserable stuff to work.

Always lock any slide that is not moving.

Use a cutting lubricant

Make sure the part is locked down tight to the table

Send us a picture of the part, tool and setup. That would probably help the most.

Video with sound would be wonderful........

Hope that helps...

Dave

PS

"Vic - is there any way to determine how hard the steel is, I can't get my head round why a hacksaw seems to breeze through it but the cutter struggles. Obviously the hacksaw is much thinner but I thought hardened\tough steel would present just as much a challenge to the blade ?"

A hacksaw probably isn't running at the same speed as your cutter...Its probably slower....which is another pointer towards this NOT being HRS....but 304 doesn't like anyone, including hacksaws...so ,,,,need pictures.

 

[Omnimill]

I'm not an experienced machinist Jim so I would normally expect, like you, that if it can be cut with a hacksaw then it should be machinable? My own other test would be drilling, if a lump of unknown stock resists a standard HSS drill enough then I won't waste milling cutters on it.

Vic.

 

[steamer]

Jim,

Reread the post....it's not HRS....More like hardened and tempered tool steel in the mid to upper 30's Rc..They draw the temper back to make the tool holder tough...so it will bend but not break...I too have cut this stuff before....you will probably need carbide.

A new part from a more manageable steel would be my choice,

Dave

 

[JimM]

Thanks Dave, that's what I wanted to hear (sort of !)

At least I know it's probably not me that's the issue - for once

Downside is I've got to work out another way of getting these things to fit on my lathe - thinking I was being clever I bought 3 of them plus inserts.

May try carbide cutter and notching the holder to make it thinner to fit in an AXA toolholder holder

Cheers

Jim

 

[Niels Abildgaard]

Please let us see some pictures to enjoy the luxury of knowing better.
To me it sounds like a quater of an hour on a wire spark maschine.

 

[steamer]

Well...if you've got a wire edm...make it out of carbide! :big:

Dave

 

[JimM]

Well I emailed ISCAR yesterday asking them what they made their toolholders from and got a reply this morning, apparently they use EN24 but they didn't mention if they harden them or not.

Did a quick google search and EN24 would appear to be pretty tough stuff so think that probably confirms the suspicions that the material was blunting the HSS toolbit and causing my issues !

Cheers

Jim

 

[steamer]

I found this...

http://www.nhtc.ie/EN_Steels.html


Equivalent to AISI 4340 or perhaps 4340 HT.

.... tougher than woodpecker lips!.... ;D

I wouldn't touch it without carbide...then it shouldn't be a problem.

Dave

 

[Tin Falcon]

.... tougher than woodpecker lips!...

I like that very descriptive yet family friendly phrase.
Tin