Milling – difficult to gauge exact cut

[WolfieParticipant @wolfie]

OK on a couple of occasions I have had to cut a small slot in something quite narrow leaving a pair of thin arms either side of the slot.

 

Thing is on both occasions I have missed the exact centre thus leaving one arm slightly but noticably thinner than the other. Its difficult to gauge exactly where the milling cutter is going to cut as once its spinning you can’t see its edge

There must be a better way

[WolfieParticipant @wolfie]

[JonParticipant @jon]

By centring the cutter up on centreline and taking equal cuts either side.

Assuming the spindle dont veer to one side on cheap foreign imports.

[EtheAv8rParticipant @etheav8r]

I am a beginner too, but I think you would use an edge finder or wobbler to locate one edge of the workpiece, then move the table by the radius of the wobbler towards the spindle. The center of the spindle is now exactly over one edge of the workpiece.

 

Now wind the table half the workpiece width towards the spindle – the spindle center is now in the center of the work and ready to cut your slot.

[David LittlewoodParticipant @davidlittlewood51847]

Wolfie,

 

I agree with E, I would always use an edge finder (here , ref EF4, about 1/3 of the way down the page) – though I would normally do it from both sides and halve the difference. Do remember to allow for backlash though if you don’t have a DRO.

On that subject, I fitted a 3-axis DRO to my mill about 4 years ago, it was the biggest single improvement to my milling ever.

 

Also, in case you did not know this, use a slot drill (2 flutes) for this purpose, not a 4-flute end mill – the latter is inclined to give a wobbly slot, especially if you are feeding by hand.

 

David

Edited By David Littlewood on 28/01/2012 22:44:37

[NobbyParticipant @nobby]

Hi

You can also using a DTI & Clocking attachment . Clock spindle on c/l of job / workpeice Then take a cut , don’t forget to lock the Cross slideway . then with a anvil micrometer you can check the result . If you use a slighter smaller slot drill ( not end mill) and you can make adjustments to suit . Set y axis micrometer dial to zero before fist cut . I f Adjustment is required and you take another cut don’t climb mill on the way back

Regards Nobby

[Chris TriceParticipant @christrice43267]

There’s a tendency when vertical milling for the cutter to pull to one side. You may be on the centreline. As previously suggested, it may be better to use a slightly narrower cutter and open the slot up by taking a lighter cut down each side.

[David LittlewoodParticipant @davidlittlewood51847]

Chris,

 

That sideways cut is why you should not use an end mill for slot cutting. With a 2-flute slot drill it doesn’t happen.

 

David

[BERTOParticipant @berto]

Another thing is workpiece flex particularily if the piece being machined is thin or narrow- in that case lessen the depth of cut and if possible use a backing strip of heavier material behind the side that the cutter is rotating towards .

Look on youtube and you may find some footage of using a wiggler to find the edge of a worpiece .

 

[Ady1Participant @ady1]

It’s called getting some skills Wolfie

Welcome to the club

 

An interesting video here for those sturmey archer bike hubs, about the production processes and the workshop area which made those little gear hubs for the middle of the back wheel on a bike

 

This gives you some idea of the huge amount of effort required just to make one little bit of kit accurate and reliable for selling to the public

It’s quite a big file, 85 odd megs if memory serves, great viewing though, a real eye opener

Some of their machines were as big as a bungalow.

 

http://www.sturmey-archerheritage.com/videos/video-593.wmv

Edited By Ady1 on 29/01/2012 03:08:49

[Jim GreetheadParticipant @jimgreethead]

Hi Wolfie,

 

If you don’t have a DRO, you can make a little one from a digital tyre tread depth gauge as shown here:

http://start-model-engineering.co.uk/machines-and-tooling/minilathe/micro-dro/

 

You just need to be careful to keep it square to the direction of travel.

 

Jim

[TerrydParticipant @terryd72465]

Hi Wolfie,

 

You don’t need a DRO or any other piece of fancy (expensive) equipment. You should set the machine to centre before inserting the cutter – which should be a 2 flute slot drill as others have pointed out. To centre the cutter on your centre line of the slot simply use a ‘wiggler’ as a centre finder. These are great little devices. You can buy a set  cheaply which include a ‘wiggler’, a couple of edge finders and a dti holder. There is a video of how to use them here, the centre finder (wiggler) is the second item demonstrated.

 

You can make your own centre finder though with a lump of Blu Tak or plasticine and a pin or needle. With the slot drill in the machine, stick the Blu Tak on the end of it, stick the pin into the Blu Tak at any old angle but more or less vertical. Then run the mill spindle and by using a metal rod against the pin (as in the video) gently get the end of the pin to run smoothly. It will then be on centre and you can use the point of the pin, with the machine running to accurately set the milling cutter to your centre line. If you don’t beleive me try it, you will be amazed and fascinated. When I was shown that many years ago by an old boy I had to keep trying it and I couldn’t stop smiling for ages.

 

Sometimes the simple old fashioned ways are just as good as the hi tech modern ones.

 

Best Regards

 

Terry

Edited By Terryd on 29/01/2012 06:54:45

[David HaynesParticipant @davidhaynes53962]

When milling a flute in a coupling rod, for example of narrow arms on the sides of the slot, the flute is usually milled first in a piece of metal wider than the finished rod. Then the sides can be accurately offset, one at a time. That may be easier that milling the slot in a rod of the finished width and having to get both arms correctly located in one attempt.

 

Dave

[JasonBModerator @jasonb]

Even less high tec than Terry’s suggestion if you don’t have a wiggler or edge finder you can alway use the good old fag paper.

 

With the mill NOT running touch the shank of the tool above the cutting edges on one side of the work so it just traps a fag paper, make a note of the handwheel reading. Do the same on the other side and divide the readings by two making any allowance for backlash. When you get the hang of it you can use the cutting edge rather than the plain shank

 

J

[Anonymous]

If I don’t need precision I just line the tool up on an edge, using an eye loupe to gauge distance. If I need more accuracy I use a 1/2″ piece of silver steel in a collet directly in the spindle using a fag paper and a DRO to set zero. So, old and new, cheap and expensive; must mean I’m confused.

 

Personally I think the DRO was the best thing I ever bought.

 

Regards,

 

Andrew

[Tony Pratt 1Participant @tonypratt1]

Jason, does your method actually work? As backlash is difficult to judge accurately would it not be better to touch on one edge using a fag paper then move over half the cutter diameter plus the dimension required.Obviously if the cutter is not running true you will get an error which will be half the cutter eccentricity

Tony

[jason udallParticipant @jasonudall57142]

one additional thing..if making forks ..some times best to leave a frame at open end of part..then remove later..this will stiffen you part until milling finished… But if question is “why doesn’t my cut go where I setup..” well sorry , thats life, machines,cutters,and work all flex. All the above advice good for setup. Also lock any axis not travelling..nothing fixes backlash like a lock.

[David Clark 13Participant @davidclark13]

Hi There

Mill slot first trim sides afterwards.

Simples.

regards David

 

[TerrydParticipant @terryd72465]

Posted by JasonB on 29/01/2012 08:02:33:

Even less high tec than Terry’s suggestion if you don’t have a wiggler or edge finder you can alway use the good old fag paper.

 

With the mill NOT running touch the shank of the tool above the cutting edges on one side of the work so it just traps a fag paper, make a note of the handwheel reading. Do the same on the other side and divide the readings by two making any allowance for backlash. When you get the hang of it you can use the cutting edge rather than the plain shank

 

J

Hi Jason,

 

I also use that method, but since I gave up smoking many years ago I have to buy fag papers specially, I have also seen it done using strips cut from phone directories allowing for the thickness of the paper (about 4 thou’).

 

In a variation of your method I was taught to, stick the paper to the side of the job with good old fashioned spit, then with the cutter running, just very carefully move the job to the cutter until the paper is torn off the job by the running cutter. Never had a problem with that. The ‘sticky pin’ wiggler (plasticine and needle – very cheap) is also good for centering centre punch marks or cross hairs for drilling etc without the possibility of error with simple calculation error creeping in.

 

By the way slot drills (2 flute) rarely wander unlike end mills. 4 flute cutters cause an inbalance of forces to one side which tend to pull the cutter out of line, this is not the case with 2 flute slot drills. That is why you should use slot drills for slots and not end mills.

 

Best regards

 

Terry

[David LittlewoodParticipant @davidlittlewood51847]

David,

 

Your suggestion is all very well if you do not already have the part with sides at finished width, no use if you do. It also involves as much (or more) measurement and cut-and-try as the methods already suggested, particularly in view of the difficulty of measuring the width of the lands without first removing milling burrs.

 

David

[NobbyParticipant @nobby]

Hi

I agree with Terry in the home workshop the sticky pin is great . Not only finding an edge

but brilliant for picking up scribed lines .

Nobby

[JasonBModerator @jasonb]

Tony You can do it either way, one side or two, with the one sided method you need to take off the thickness of the fag paper (Terry I’ve never smoked) and also allow for the fact standard bright drawn material is often just under nominal.

 

I really only use the paper method for touching down now, for edges I use one of the electronic edge finders that light up on contact and the 1/2 function on the DRO.

 

Another way to avoid an overwidth slot is rather than mill it do it sideways with a slitting saw, again touch the saw down onto the work and calculate how far down to lower the blade.

 

J

[David Clark 13Participant @davidclark13]

Hi there

If you put a drill shank each end of the slt and drop the shanks on top of the vice jaws

you can skim one side and turn it over and skim the other side.

Measure overall width and rause table or drop quill by half the excess and it will be central and to size.

regards David

 

 

[NobbyParticipant @nobby]

Hi

I use a 5 m.m test peace for touching down . Using electronic hand wheel or handle Take a light touch Set Z Plus 5mm enter .

Nobby

[Richard ParsonsParticipant @richardparsons61721]

The core of the problem lies in the use of the vertical milling machines for cutting narrow slots. It is made easier by using ‘Slot Drills. These are (generally) two cutting faces on each cutter where the centres at the ends are not centralised.
The real problem is caused by the fact that one side of the cutter is performing a ‘Conventional Milling Cut’ and the opposite side is performing a ‘Climb Milling Cut’. This gives you three problems which are:-
· With a thin Slot drill the drill will be bent towards the ‘Conventional Milling Cut’ side of the slot.
· With a thicker slot drill the cutting forces will move the bearings (and the quill) towards the ‘Conventional Milling Cut’ side of the slot.
· In both cases there will be problems with any backlash or looseness in the moving slide.
You can prove this for yourself by taking two or three shallow cuts followed by a deeper one and inspecting the results by looking down the sides of the slot.
This is why if I have to machine a slot in something I try to use a slotting saw, a Woodruff cutter or something similar. It is one very good reason why I still keep my vertical slide in good condition and saw carrier for my lathe. It is also why I want to ‘drop my hooks’ on a small Horizontal mill, a Shaper or a Box plainer.
I have looked at my vertical mill many and oft to see if I could make a small horizontal converter. I can design a converter easily BUT the problem is fixing it to the machine so that it is always parallel to the movement of the table and deals with the difficulties of off centred cutting loads.
Rdgs
Dick

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