Cut a slot in round stock

Can anyone advise me how to do this accurately? Fourth attempt coming up! I need a through slot 19mm X 9mm in 14 mm mild steel bar 27mm long.

Have tried a slot drill which just made a mess. Tried enlarging holes I thought to be accurately placed on the axis then cutting out the 'bridge' with a slot drill. Latter made a further mess and although I did get the slot I wanted by filing it was dangerously over to one side to the extent the part is unusable.

Guidance much appreciated, and many thanks, as always, in advance.

If your mill is not very rigid, either take much smaller cuts and/or use an undersize slot drill, then skim to size.

Neil

Hi Martyn

You could drill 8mm through at each end, (centre drill 4.5mm from end of the slot) so when drilled there will be .50mm of material to remove at each end with your slot drill, also there will be .50mm each side of the drilled holes if the drill didn't wander.
The holes provide a space to lower the tool in before each pass along the centtre of the slot, DOC per pass will depend on the mill you have but with an 8mm cutter start at 2mm and see how it goes and adjust DOC and feed accordingly.
When the middle part is taken out you may be able to keep full depth and with light cuts increase the width on both sides to that required, make sure you are not climb milling if there is any backlash in the feedscrews.

Emgee

Seems to be one dimension too many to me? or is your bit of bar only 27mm long? If it is then drill out say 6mm dia at each end and again in the middle, open out the two end holes by plunging with an 8mm cutter then change to 9mm dia and mill out the slot 19mm O/A length

Edited By JasonB on 31/01/2019 18:24:29

Are you doing this with a milling machine or a lathe ? Maybe a photo of your set up will give us some clues as to what is going awry .

Cut most of it out with an angle grinder and finish off with a slot drill

You must be hogging out 90% plus of the material here, there aint much left, 2.5mm each side, is that right?

Filing off might be the better option

Edited By Ady1 on 31/01/2019 19:02:00

Holding a short piece of bar only 27mm long and trying to mill a 19mm slot is difficult. Better to use a longer bar, securely clamped. (both ends if possible) and cut to length after machining the slot.

Paul.

Edited By Paul Lousick on 31/01/2019 21:43:00

What exactly is the problem you are having? Rough finish? Cutter digging in? Not cutting straight to the line? Pics of the problem would help too. "Made a mess" is a bit vague to pinpoint the possible cause.

Edited By Hopper on 31/01/2019 23:08:32

By dint of washing it with coolant, or simply blowing compressed air at the cutter, make sure that you are not trapping and re-cutting the chips. Otherwise this is about stiffness/rigidity of the set-up and/or sharpness of the cutter, which could well lead to a conversation about climb milling.

If you can first drill a hole right through with your slot drill and then wash the chips down the hole it gives them somewhere to go.

Quite a difficult thing to do actually, so some photo's of the set up would help to show what is going on.

HTH Simon

Hi Ady1 An angle grinder ?

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Glad you got the job done at a useable level. That's the main thing.

It sounds like you may have got more rigidity by holding the job in a vice with suitable packing etc.

A good slot drill should plunge straight down. But it depends on if its properly sharpened etc. Drilling a smaller hole then dropping the slot drill down should work ok, having provided clearance for the critical centre part of the cutting edges.

The method of taking say 2mm deep cuts along the length is probably a good idea on smaller less rigid milling machines (eg home hobby machines smaller than a Bridgeport) particularly on the roughing out stage. By using a slot drill smaller diameter than the finished slot width, you culd then take a light full-depth cut along each side of the slot.

What make and model milling machine are you using?

Hi Martyn, you could try milling a flat on the bar the length of the slot with a 7 or 8mm cutter first before you drill any holes, this should stop the drill wandering off course. Another thing you can do to help stop the collet block from tilting, is to use a hold down clamp on the top of it. Your mill should be plenty capable to do this job.

Hopper, he is using a Warco Economy mill, which is a smaller version of their Major mill.

Regards Nick.

P.S. The Economy mill was last advertised in their issue 12 catalogue and was one down from their Minor mill.

Edited By Nicholas Farr on 02/02/2019 10:12:31

I'm having trouble visualising the object Martyn - is it like this?

If not, a photo of the finished item would help. I promise not to laugh if it's a shameful botch - living in a glasshouse myself I know better than to throw stones!

From the description, and as I've drawn it, this is a difficult object to hold rigid during cuts, and the problem gets much worse as metal is removed.

I suspect that most of your problems are due to the work twisting under heavy cuts resulting in gouging, dig-ins, and a bent workpiece. One solution is to improve the holding AND to reduce the forces on the work by taking lighter cuts at high speed with a small diameter cutter.

Assuming the object is as pictured, I'd firmly clamp a long rod direct on the table, perhaps making a shallow V block to add longitudinal support. Then I'd rough out about a third of the slot with a large slot drill before switching to a smaller slot drill for the next third, and finally finishing off with a small diameter slot-drill shallow cutting at high speed. Only after the slot is cut would I shorten the rod to size.

Dave

Rigidity is always an issue! But that ain't the whole story.

That bit of mainshaft may not be hard, but I bet it's tough as old boots. Cutting a deep slot like this is difficult territory, without trying it out on some difficult material. For curiosity, try the same procedure on something more forgiving and see how much better a result you can achieve.

Your slot drill, if we mean by that something with a cutting edge which meets the centre of rotation of the end face, should plunge perfectly happily. But, again, trying to do that in the material you are using may be easier said than done. If it doesn't plunge as easily as it would if you were just using a standard twist drill then there is something wrong with the cutter geometry, though a tough material won't help things. Drilling a pilot hole is a good work-round, but you shouldn't need to.

The other thing we've not mentioned is the actual cutting procedure. In principle you can cut a slot by going to and fro, but you maybe running into problems with cutter deflection so try cutting in one direction, then returning to the start before feeding down for the next cut. IIRR Jason's series of articles in MEW on milling for beginners include some comments on milling about the geometry of cutting edges and how the number of flutes can affect the accuracy of the finished item. Make sure you are not re-cutting the swarf, that will deflect the cutter erratically and frustrate your best endeavours.

But good to hear you got something functional, if not pretty. Your mill is plenty capable of doing pretty but it may need a bit of fiddling with the cutter geometry and sharpness also the technique to get there.

But the lesson my milling machine has taught me is that it always needs a rigid set up, and I have learned that time making sure the work is held securely is always well invested.

Best rgds Simon

This photo shows a supported and clamped overhanging workpiece which is the way to do what you wanted. A hard collet block held by hard vice jaws gives poor grip as you found. A piece of paper between the block and the moving jaw may have helped. Putting milling cutters in lathe chucks can have similar issues.

The other thing that is a problem is the lack of material at the sides of the slot. If you think of a twist drill as a twisted ribbon you can understand hiw easily it can deflect. In this case as soom as one side is weakened enough the drill will be pushed that way by the forces acting on it. A stiff stub drill or spotting drill would have helped but a 3 or 4 flute milling cutter would be stiffer so better.

Regarding plunge cutting with milling cutters even centre cutting ones do not go in as easily as a drill. One way to improve on this is to ramp the tool into the workpiece. This is easily done with CNC but is possible when manually operating a mill. It requires lateral movement of the workpiece as the tool is lowered into it. Since you needed a slot it was possible in this case.

Martin C

Edited By Martin Connelly on 02/02/2019 15:39:12

Swinging vee centering gadgets are best avoided. The right way to centre the cutter over the shaft is to use an edge finder.

Thanks everybody, I think we can make progress here!

Rigidity, rigidity, rigidity is a good axiom I feel.

Maybe turn down the centre of the stock shy of diameter first. Clamp the collet block vertically certainly, but with a long piece of stock there's also the opportunity to hold the other end rigidly.

Not working to the finished diameter would strengthen the sides of the slot during the cutting process and once it was done an interrupted cut on the lathe would take it down to size before finally cutting off the part at each end.

Take the point about the material. That's the trouble with non-virginal material isn't it? You don't know the composition. I'll buy new steel.

I'll forget slot drills. Cut a flat with an end mill I think. Use twist drills at each extremity then gently take out the balance with end mills of increasing size.

I appreciated the drawing. That's not quite it. The bar, 27mm long at the finish remains intact at 14mm diameter. The slot - 9mm wide x 19mm long - goes through it. Thus you can't take out a complete portion as in the drawing but are leaving about 2.5mm on each side which is what makes it tricky. The slot needs to be straight, consistent and in the middle so as not to weaken one side or the other. Nice artwork though!

Thanks again everybody

Martyn

You can always fill the slot, for final machining, with a softer material - say Al - or glue in a piece of material which can easily be removed after machining to size.

The job in hand, to be cut from the bar after milling . I will stick with the method I described earlier.