Milling narrow slot on Myford lathe

I am trying to make a couple of Clevis ends for use on a classic bike brake. I put a post on this forum and the advice received was to use a small diameter mill to make the slot. The slot needs to be around 0.210 width and 0.6 in deep, in 10mm square stainless steel. I have used a 3/16 mill held in a collet in my Myford, with the square bar stock held in a vice on the vertical slide. I have centred the bar end carefully but the cutter wanders, cutting the slot too wide, slightly slanted.and not 'flat' - wider in the middle, slightly narrower at each end. I suspect the problem is insufficient rigidity in the holding set-up and the cutter flexing. I have a 7/32 cutter which will cut an acceptable width slot which should be slightly more rigid. Before I use up my limited length bar stock, any suggestions/workarounds? Thanks, Andy

Edited By Andrew Phillips 4 on 08/02/2022 22:09:34

I think using a slitting-saw might be more appropriate.

MichaelG.

If it had been mild steel, or even tool steel, you would have got away with it. since it is Stainless, you probably would have had the same problem even if you had a strong milling machine. It's the cutter that's weak, and unless it's a nice thick side and face cutter, a slitting saw will be the same.

Use the time honoured technique of drilling with a nice fresh drill, then cutting away the worst of it with an HSS hacksaw. Then when you're done, finish it with the endmill, or a nice thick slitting saw (at least 3/32", especially if it can cut on the sides.

Given it would probably be a one sided finish cut after the hacksaw treatment, obviously plunge the slitting saw. You can still slice with a side/face cutter.

Be sure to use a two-flute slot drill, not a four-flute end mill cutter. Big difference.

And pics of your set up would help assess rigidity.

Stainless can be tricky to machine too. You have to keep the feed up to it. If you let the cutter rub on it, the stainless will work harden and the cutter will then not want to cut it and wander about a bit. Use mild steel if you possibly can.

As you are using a 3/16" cutter and want a 0.210" slot then I can't see a problem unless the slot is excessively over size. Run the 3/16" cutter through on ctr height to get a 0.188" nominal slot and then raise the work 0.011 to take a finish cut along one edge of the slot and then either lower by the same amount from ctr to do the other side or just flip the part over.

The other method would be as Andy suggests drill say a 4mm hole at the base of the slot and hacksaw out most of the waste and then set up to mill one side of the rough slot followed by the other. A 5mm cutter may be a little stiffer than a 3/16 and will still need a reset of the work to get the desired 0.210" slot.

Sorry … my first comment wasn’t explicit

Given the combination of Stainless Steel, a Myford, and a Vertical Slide … my biggest concern would be get the working faces of the clevis-slot smooth and parallel.

Two cuts with a thin slitting saw … followed by waste-removal by whatever means, seems [to me] the best approach.

Rather like hand cutting a box-joint in woodwork.

MichaelG.

I do such millling jobs on the Myford all the time. It's no problem as long as you observe the basics. Rigid set up is key. Locking all slides not in motion is key. Use the gib adjuster screws to lock if you don't have lock screws, including on the vertical slide.

Why a four flute end mill should not be used to cut a slot:

If the above four-flute end mill is cutting the slot as shown, revolving anti-clockwise, then cutting edge 1 is cutting in the direction shown by the small arrow next to it. This generates an opposite force in the somewhat flexible cutter, causing the cutting edge at 90 degrees to it, 2, to be pushed outwards in the direction of the small arrow there.This causes the cutter to wander and cut a slot wider than the diameter of the cutter. It can get worse as the cut goes on, and is at its worst as the leading cutting edge in position 1 breaks through the end of the metal at the end of the slot. It is not unheard of to snap an endmill while cutting a slot at this very point as the dig in can be harsh.

By using a two-flute slot drill, designed specifically for milling, hey, slots, this effect is eliminated. When the leading cutting edge 1 is trying to push the cutter sideways there is no cutting edge at position 2. The second cutting edge is out in thin air in the middle of the slot where it can do no harm. At the critical point as the leading cutting edge breaks through the end of the metal, same thing. So it cuts nice and straight and true to size.

I just cut a 1/8" slot in block of cast iron for a GH Thomas style rear toolpost for the bolt on the spring loaded detente pin to stick out through. No problems -- using a 1/8" two flute slot drill. Nice true straight dead on size slot. I have also made several four-way toolposts from 2-3/4" square steel blocks with the 13mm slot for the tools to sit in milled in the Myfrod, again using a 13mm slot drill.

See GH Thomas's Model Engineer's Workshop Manual for a full discussion on the subject of slot drills vs end mills for such work, pp 77-79. It's a very useful book. He was a cagey old fox who worked all his life on machine tools and then went home at the end of his shift and built models and Myford accessories for light relief. Well worth buying a copy. I was taught the same thing as an apprentice; slot drills for slots, end mills for milling but nobody could ever explain why. Now we know. Good old George.

I routinely slot 303 stainless steel .052" wide x 3/8" deep. Anywhere from 3" long to 13". I use a Noga mist sprayer with a drip oiler dripping cutting oil into the air stream. No mess except on the part, no smell in the shop. The air removes the chips also, which is vital.

Thanks for your advice. I see the need for a slot drill and will get one for my next job. In the meantime I wanted to finish the job and did as Andy suggested, drilling 3 x 3/16 in holes, sawing/filing out surplus metal and finishing to size with the endmill as I had it to hand. I also revised the way I clamped the square bar stock to improve rigidity. Much better than my earlier attempts! Cheers, Andrew

Hi All, back again with another question on slot cutting. I have now made 3 Clevis forks with the help of your suggestions (thanks very much!). One needs a 5/64in longitudinal slot to allow the inner brake cable, which has a mushroom end that fits in a bored recess in the clevis, to be fitted, then the outer is adjusted to stop the inner coming out. I can get a 1/16 in slot drill, or a 2mm (near enough 5/64). My question is: would I be better cutting a 1/16 slot then widening it (using a end mill once slot is cut?) or going for a full width 2mm slot? I will be using my Super 7 with milling slide, but my main worry is the lack of stiffness in slot drills that size. Thanks in anticipation, Andy

Edited By Andrew Phillips 4 on 26/05/2022 09:48:01

Unless I've missed something, it's a small slot in a bit of stainless ? A new 24 or32 tooth hacksaw blade in a decent frame or a 1mm slitting disc in an angle grinder why use a mill and take 3 times as long ? Noel.

I would only risk an end mill for the final tidy up

If memory serves there are reinforcing/support systems, like home made collets, which only have the very tip of the cutter peeking out which make the cutter stiffer for tiny jobs

Another tip for safer break-through is to back the work-piece with a scrap of similar material so the cutter is effectively still in the middle of a solid block. The backing piece of course needs be at least the cutter diameter wide.

(It's analogous to using backing material when sawing or drilling materials like plywood or thin sheet metal.)

I must admit though I usually cut into thin air but just take that last bit very gently... without stopping and letting the tool rub. On the mill I nip the table lock up a little.

I make mine with a slitting saw using a cross drilling jig that’s mounted on the crosslide, it only takes a few minutes and if you use an 1/8 saw you can go either side to get your 210.