How to Mill a Round End on a Square Bar Safely?

Another bad day in my workshop!

I'm trying to make this item (part of a jig) out of 18x18mm mild-steel bar:

I attempted to make the item by offering the end of the bar up to a 20mm 4 flute cutter. Once aligned with the cutter, I pivoted the bar on a 4mm bolt through the hole. The bolt went through a similar bar clamped in a mill vice with a nut underneath.

I did half the radius with no problem but doing the other side required climb milling. Almost immediately I had a violent dig-in that damaged the work, bent the pivot, and moved the vice.

What's the best way of making a part like this? My method was too exciting!

Thanks,

Dave

Rotary table..... ( more expense...)

Piece securely clamped...works fine.

I'm sure there are other methods to be proffered....

Edited By John Rudd on 28/11/2016 18:36:06

Yep, clamped down well to a rotary table is the safest way.

You should not have needed to climb mill half of it. Start the cut at one edge and move the work through a 180deg arc cutting conventionally all the way.

If you have a DRO then teh ARC milling facility can be used

As John has said, rotary table would be safer than pivoting on a bolt, and give a better finish. But either way you should start at one end of the arc, rather than in the middle, and move in the correct direction to avoid climb milling.

Mike

In my opinion, I would clamp it down onto a Rotary Table.

This would give full control of the Feed Rate and be able to reduce the initial 2 sharp edges progressively.

What method of 'Feed' were you applying in the first attempt ?.

Edit- Seem's I was too slow.......see as above

Edited By Richard S2 on 28/11/2016 18:53:02

+1 for John's suggestion.

This is how I rounded the end of connecting rods. The rod is nutted up to a threaded mandrel held in the chuck - the round bit you can see is a sacrificial washer:

+ 1 for the rotary table , i have a MT 2 blank drilled through the centre then reamed to 8 mm so i can fit a stepped pin into it and drop it into the morse taper bore of my rotary table to locate the eye of the rod accurately and it only takes a few minutes to turn up what ever size pin i need ( one end is of course 8mm ). It also makes locating the rotary table on the mill much easier as i can clamp the end of the pin in the mill spindle then lower the spindle and jiggle the rotary table around until the pin drops into the reamed hole in the MT blank that is fitted to bore of the rotary table then clamp the rotary table down to the mill.

I would be checking the tram of the mill head after something like that accident and you really are lucky to not have seriously injured yourself !

Gosh that was one dodgy set up. With the absence of a rotary table you could mark out and freehand walk the cutter to the line using X/Y handles then file to tidy up. Easier than doing the laundry after a scare like that !

One of the first machining lessons I was given was that the work and the tool must be held rigidly in the machine. If not you are likely to be on a hiding to nothing.

I don't like the idea of pivoting the bar on a pin because of the lack of rigidity. Likewise I have doubts about some lathe ball end turning devices.

JA

Gosh I'm embarrassed! Now I'm educated it's obvious that this is a job for a rotary table. Another lesson learned.

Thanks for being gentle with me,

Ta

Dave

Another way if you have a DRO **LINK** or **LINK**

Bob

Don't be, that's the way LBSC always said it should be done, but best to have a long workpiece (more leverage) and always move it against the cutter (and light cuts).

RT is much better and safer.

Neil

Scrap plate mounted in chuck on a rotary table (or directly to the RT). Job held with bolt and clamp.

As you probably don't have a rotary table (or you would have used it) and as the first half of your job worked just fine until you had to climb mill for some reason, I would repeat the process but do as Jason suggested and start at one end of the radius and rotate steadily around to the other, conventional milling against the cutter rotation the whole way.

You might try using a high tensile pivot bolt for more strength and clamp a longer piece of bar on to the job to make a handle giving both more leverage and finer control over movement.

This method is fairly commonly used to round ends without problems so you should be able to do it as long as you set up to avoid climb milling. Rotary tables are a relatively new arrival in home machine shops (and many commercial shops) and ends were rounded for many years before that.

Edited By Hopper on 29/11/2016 02:26:19

Edited By Hopper on 29/11/2016 02:26:45

In the absence of equipment you could fabricate: mill out the channelled part, cut off part of the end (vertically), cut a slot (horizontally), insert and solder in place a small cylinder. Depends upon the application I suppose.

How about simple plunge cuts to remove bulk material. That way the part can be clamped on its pivot for each cut. Need multiple small increments to get the part to shape and finishing would need another approach, but safer than cutting anything that is not fixed down securely. I simply avoided climb milling (on most materials) with my old mill. No need for rotary table nor sophisticated dro.

And there's always filing buttons

And a 4 flute end mill and not a 2 flute slot cutter

Yes, without a rotary table and if the part was 'awkward' in some way - then 'clamp & cut' is one (slow but effective) way to do this kind of thing - I probably wouldn't plunge cut (as NDIY suggests) but take tangential cuts myself if the work was not too thick, so that a number of small facets are cut. In this case, the work would still move around the pivot point but be clamped well down before each pass. A similar approach can be used with a shaper to produce part-rounded parts. Finish depends on the number of facets and perhaps a touch with a file. Slow but above all safe to do, especially for hard-to-hold parts on an older mill with a good deal of backlash in the table that might catch you unawares.

In this case though, it sounds like the first part of the operation went OK, so the simple solution would seem to be to just start at one end and go all the way around in one go.

Regards,

IanT

"As you probably don't have a rotary table (or you would have used it) " - but I do have a rotary table! Oh, the shame, the humiliation...

I'm going to try the rotary table and 'no climb milling' methods after lunch. With care and as advised the pivot method should work and it's quick to set up. Thinking about it last night I realised that a 4mm bolt in a 4 mm hole has a lot of slack: if a dig-in starts it's likely to grab, much like loose gibs make parting off iffy. There was more to doing the job than I thought, hence the boob.

What I like about the Rotary Table approach is that, once set up, it should be much more rigid and idiot proof. I want to prove to myself that pivottng works if climb milling is avoided, but thereafter I shall use the Rotary Table. The amount of force generated by a dig-in is surprising and it could be hurtful. As I'm a little prone to carelessness I shall feel more confident with the work well clamped down.

Thanks again,

Dave