Milling Advice Needed

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi All, I am a complete beginner to metalwork and have recently acquired a Bridgeport Series 1 mill. I want to fit levelling feet to the mill and this involves using the mill to cut through slots in 150 by 90 parallel channel steel. My problem is that the surface of the steel seems to be much harder than the inside material. I don’t know what grade the steel is but it comes with a fairly thick layer of mill scale on the outside so presumably these are hot-formed sections.

 I have managed to destroy 3 HSS cutters so far. Everything is fine until I get to about 90% depth of cut. I am taking fairly modest cuts each time of about 30 thou and feeding slowly – around 5 inches per minute. I have been using 6 mm 2-flute cutters at about 2000 rpm – all OK according to what I’ve read.  As soon as I get near the bottom of the slot the cutters stop producing chips and if I try to force a cut the cutter overheats becoming red hot – end of cutter!

 I then thought that perhaps I should be using solid carbide cutter but when I phoned the technical support people at a good tool suppliers, I was advised not to use solid carbide on a manual machine and they are only suitable for CNC machines.

 I realise that there are other ways to do what I want that don’t involve milling but I would like some advice specifically on the milling and any suggestions of how to cut through slots successfully by milling.

 

Thanks, Hugh

[Hugh GilhespieParticipant @hughgilhespie56163]

[David Clark 13Participant @davidclark13]

Hi There

The tool suppliers are talking rubbish. Carbide can be used in a manual mill.

The only thing I would say is either use enough coolant to flood the job or none at all.

However, drill holes at each end of the slot and possibly all the way along the slot.

Plunge a finished size cutter at each end of the slot.

Use an undersize fine rippa cutter along the slot.

Finish slot to size, a carbide cutter would be fine.

You don’t need a size cutter for this.

Just move over to correct width.

That is the idea of finishing ends to size first.

You don’t need to get involved with the radius at the ends after plunging it.

Stops running into the end and the subsequent deflection.

regards David

 

 

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi David,

 

Thanks for a very quick response. Basically I am doing as you suggested, plunging a finished size cutter – 16 mm – at each end of the slot then using an undersize cutter to mill out between the holes. Have destroyed one 16mm cutter so far so next I will use a 16 mm HSS drill bit – at least I can resharpen the drill bit after each hole.

 

Can you explain what a fine rippa cutter is? Sorry to be so ignorant but as I said, I am very new to all this with no background in metalwork at all. I was a bit suspicious about carbide cutters not being suitable but as they cost an arm and a leg I didn’t want to buy one just to see it disintegrate.

 

Thanks, Hugh

[David Clark 13Participant @davidclark13]

Hi Hugh

A rippa cutter looks like it has a fine thread down it.

Have a search on Ebay for Rippa.

Also, carbide cutters are available on Ebay for a fraction of the new price.

http://www.sgstool.com/products/endmills.asp#roughers

Scroll down to roughers.

 

Bit dificult to explain but when buying carbide cutters, go for a double grind cutter.

This means that rather than being taken to size with one pass of the grinding cutter, it has two passes with a different angle.

With two passes it is not ground so far into the shank making the cutter weaker.

 

Look at http://www.sgstool.com/products/endmills.asp

Scroll down to standard and long section, first 6 cutters are reasonable, 7 and 8 are very weak on the shank and also have to stick out a long way.

 

 

regards David

 

[KWILParticipant @kwil]

Hugh,

 

See your messages re image of mill base.

 

K

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi All

 

As per David’s excellent suggestions, I have ordered a carbide rippa from fleabay and hopefully that will solve my immediate problems. I am just surprised as I can cut this steel reasonably easily with a bandsaw so why is it so difficult to mill? Oh well, all part of the learning curve!

 

Hugh

[Peter TuckerParticipant @petertucker86088]

Hi Hugh,

 

I would say part of your problem may be the speed of your cutter.  To calculate an approximate HSS cutter speed for mild steel divide 10,000 by the cutter diameter in mm, there for 6mm cutter should be about 1,600 RPM.  If the steel is harder, as you suggest, then the speed should be reduced perhaps to 800 RPM.  Of cores when you use a carbide cutter the sky’s the limit for speed.

Hope this helps.

 

Peter.

[Ian S CParticipant @iansc]

In Arnold Throp’s book Vertical Milling In The Home Workshop, Argus Books workshop practice series no., 2. (all imperial), 1/4″ in steel, depth of cut 0.045″, speed 650 rpm., with slot drills. with 4 flute end mills he suggests 1/4 dia. These are the speeds I tend to use, a bit slow but the cutters seem to last quite well, I’m not running a production line, so speed is not a priority,and mistakes happen more slowly. Ian S C

Edited By Ian S C on 29/01/2010 09:29:52

[mgjParticipant @mgj]

TC in milling ops in the lathe gives all the speeds and feeds for HSS (to maintain an edge). In particular the tooth loads, which means that one ends up feeding quite rapidly. Increase the feed and you prevent work hardening or rubbing.

 

the other question is “What steel is it?”  You may be using some very tough high grade alloy – inadvertently possibly?

Edited By meyrick griffith-jones on 29/01/2010 17:38:23

[Hugh GilhespieParticipant @hughgilhespie56163]

Thanks to everyone who has replied. I have now got a 12 mm carbide ripper end mill and that seems to have solved the problems. It’s a bit bigger than I wanted, a 6mm would have done but I don’t mind making extra chips! I am running this at about 1100 rpm with a feed of 7 inches per minute and a cut depth between 15 thou and 40 thou. I use shallower cuts near the surface as I assume these are more likely to have hard bits.

 

I have downloaded a program called G-Wizard from bob@thewarfields.com

 that gives machining speeds. According to the program I could go a lot faster but I am happy enough with the cutting speed I’m acheiving and not destroying any more end mills.

 

One more question though. I am confused about whether to use coolant with carbide. I am actually using a few drops of neat cutting oil per pass and although it smokes a bit it does seem to cut down the vibrations just as effectively as spraying with suds. Over to the experts!

 

Regards

 

Hugh

[David Clark 13Participant @davidclark13]

Hi There

You should be using the rippa right through the material, not nibbling it.

Try full depth and turn the handle rather than using the feed.

Then you can feel it cutting and decide how fast to turn the handle.

regards David

 

[mgjParticipant @mgj]

You will have a recommended feed rate, or tooth load for that cutter quoted in thou/mm per rev. . That is your start. Up to a point the revs are less important than the tooth load.

 

It would be wise to know whether you are operating in the right regime. Too little feed causes rubbing and work hardening, too much busts tools.

 

Coolant – Well I always use a lot to clear the chips and keep the whole thing cool. you don’t need a deluge, but the cutting area is best properly submerged. I don’t think it makes much difference whether one uses straight cutting oil or soluble, (except in certain extreme pressure situations like high speed gear hobbing) but lots of coolant tends to be good – tools last and finish is (often much) better. Also one ought to remember that coolant doesn’t just cool – it does assist the cutting operation.

 

You can cut dry – of course. If you wish.

Edited By meyrick griffith-jones on 29/01/2010 21:49:12

[Ramon WilsonParticipant @ramonwilson3]

Hugh,

As you have succeded in getting your slots done this may be a little late. However as you also request help on milling  perhaps the following will be useful.

 

Cutting speeds.

All materials have a respective ‘cutting speed’ normally expressed in feet per min or metres per min varying from ‘high rates’ – soft material to ‘low rates’ – tough (note, not hard) materials. This speed also relates directly to the material being used for the cutting tool ie tools made of carbon steel, high speed steel, or carbide. (There are others but the average model engineer  is very unlikely to use them).

 

Rpm can be found by the following

 

      cutting speed(ft/min) X 12                or                      mtrs/min x 1000

      Pi x dia of cutter (in inches)                                   Pi x dia cutter (in mm)

 

For High Speed Steel tooling mild steel can be considered to have a cutting speed of  approx 100ft per min. This can be considered  a ‘ballpark’ figure with brass and ally faster and carbon steels much lower. This 100ft can vary slightly either way depending on the quality eg super free cutting MS can be treated higher say at 110-120 ft/min.

 

Cutting speeds do not vary with equipment ie like for like, the cutter used on a Bridgport for example needs to run at the same rpm as say a smaller bench mill. However the feeds, normally expressed as feed per tooth will depend greatly on the depth of cut, the amount of radial cut and most importantly the ‘ability’ of the machine itself ie it’s rigidity and or it’s power.

 

Taking your original query the fact that your cutter was red hot definitely means too high a cutting speed for the material. (This can even happen  under a flood of coolant – if you work at it!!) and this can be, as it would appear in your case, a tough skin – much like hitting a hard spot in a casting. 

 

Carbides as a rule of thumb are normally treated as having a cutting speed about  3 to 5 times faster than HSS again depending on materials. Solid carbide tooling (as opposed to tipped inserts) are not normally used on conventional machines.  The main reason for this is that solid carbides need to be held very rigidly and flooded with coolant to operate efficiently. They are very brittle, do not take interupted or shock loads well and are prone to snapping. They do not like to be subjected to thermal shock either. If you can’t flood the work it’s best to use them dry.

 

This is not to say they can’t be used conventionally just that that’s not what they are intended for. When I first used them on a Haas machining centre after years of conventional machines the speeds and feeds recommended seemed ridiculously and unbelievably excessive!!!

 

They are expensive items though and the average model engineers kit probably isn’t up to being able to operate them efficiently. Hard as they are they soon lose their edge if they are not ‘made to work’.

 

 

Hope I’ve not offended you by ‘teaching granny’ here Hugh – hope this of use to you

 

Best of luck with your milling however – however you apply it

 

Regards – Ramon

 

 

 

 

 

 

 

 

     

[mgjParticipant @mgj]

Good post Ramon, but I think we have for HSS tools a disagreement if I read it right..

 

 I agree a cutter should be run at the right speed for the material being cut. Equally if you are cutting at a tooth load of .002- 003 per tooth per rev or whatever is recommended for that size of cutter, if the machine won’t handle that feed rate (say 8-10  thou per rev for a 4 flute mill), one is far better off maintaining the tooth load and cutting revs than reducing the feed –  ESPECIALLY if the cutter is getting blunt or a touch worn..

 

That will reduce the absolute feed rate in inches per minute, of course, but the tooth load should be maintained.

 

Selecting revs- its easier to cancel out the pi.

 

Cutting speed in fpm x 4/cutter dia in inches. , 

 

Feed is:  Tooth load  in thou x cutter dia x number of flutes x RPM  = feed in IPM.

 

One can argue about rates for carbide, (of course you can cut faster and at a greater tooth load) but for the ordinary model engineer with normal kit, probably the rates for HSS would work well. 

[Ramon WilsonParticipant @ramonwilson3]

Well I think we will have to agree to disagree on this one Merrick.

 

If the cutter is a tad worn then yes it’s as well to drop the rpm a bit but to maintain the feed then puts a greater load on. Higher load, more friction, more heat – road to cutter failure.

 

The years have shown that its virtually always the determining of the cutter speed (rpm) that causes most cutter failure rather than feed rate, not just for the novice model engineer but sometimes for the ‘professional’ as well!! Of course if you have it correct and then ‘bang’ into it with unrealistic feed rates the same thing will happen. Likewise to ‘pick’ at it with low revs and slow feed will just have a wearing effect rapidly dulling the edge (particularly with carbides). And again all dependant on type of cutter and material being cut.

 

Most ME’s machines won’t have powerfeed any way so determining the feed rate isn’t possible – it just comes down to feel and practice – practice bringing experience of course. Ergo to my mind for optimun use of the cutter getting the speed right is the first step in the right direction.

 

I’m not advocating getting to the exact rpm here but getting it with reason. Having said that of course on the softer materials and the smaller cutters the ‘correct’ rpm is unlikely to be able to be obtained – it’s then a case of top speed of the machine with feed and cut to suit.

 

Your simplification of the formula is ideal – CS X 4 over D (inches) is easily remembered and near enough for most occasions the ME needs it. My little ‘black book’  has two lists written down – 1/2″ HSS cutter @100 ft/min 763 actual, 800 as above.

 

For interest it also has the approximate cutting speeds written as follows (all in feet/minute) You may disagree with these, but remember they are not cast in stone just a good starting place to begin with – (personally I have always found the ally figures too high) – I just copied them from somewhere far too long ago to remember but they have stood me in good stead over the years.

 

Alloy steels  40

High carbon steels 40-50

Med carbon steels 50-70

Mild steels 100-110

Cast Iron  50-70

Brasses 150-250

Bronzes 50-150

Alluminium 300-600

Tufnol 600

 

Hope this helps someone somewhere

 

Regards – Ramon

 

 

 

 

 

 

 

 

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