question on milling machines

[havoc]

I was at the machine shop last weekend looking at milling machines as that will be the next wallet drain.

Basicly, I saw 2 types: one with the round column that looks like a large pillar drill and then the ones with a rectangular column and a dovetail guide for the head assembly. I spoke to the guy in the shop and he said that the ones with the rectangular column are better for 2 reasons:
- the box section is stiffer
- the dovetail guide let you lower the head without readjustment

Somehow I have my doubts about those arguments. I don't know anything about milling, so if I'm wrong, please point me out why.

It depends on dimensions of course, but my idea was always that for similar sections a tube is stiffer than a box section. Certainly for torsion (like when surface milling) and for bending (like when drilling) because they put the smallest dimension of the box to counter the forces.

I can follow the argument about not having to readjust when lowering/raising the head. But I was always under the impression that you never moved that head when working on a piece. You use the movement of the spindle to lower or raise the working depth. So this would be of limited use.

After reading a bit of the specs of those machines, I found that -for a comparable machine in working size and power- the ones with the circular columns were always heavier. Now this can be due to a more archaic design, but I have a feeling that for metal working machines weight is important.

Another point is that I would like to go to a machine with a MC3 cone. The idea is that this is the same as the lathe, so the "grabbers that hold your cutter" (don't know how you call that in english) could also be used in the lathe to grab small rods. Does this makes sense?

[SteveW]

Havoc,

If I dare chip in... I've got a (cheap) mill/drill variety mill and it's got a round column and crank driven rack to raise/lower the head. This gives quite a useful height range. Even though the column is quite thick there is still quite a bit of spring. I've got to be careful not to lean on the head while doing a cut. As there is no fine azimuth adjust reseting a datum after a height change would be near impossible and yes I've needed to do it a couple of times, mostly for tool changes. I've found it best to keep the height as compact as possible to reduce spring in the column and avoid over extending the quill in order to minimise any movement.

As for your MC3 cone (taper) I don't know. My thing uses an R8 (I think Bridgeport type) fitting. This opens up a whole load of cheap tooling including a very useful fly cutter and indexable end mills.

I've pushed the boat out a bit an have an R8/Posi-lock for the screwed cutters (you get repeatable tool insertion) and an R8/ER32. The later type opens up the very cheap (re-cycled??) solid TC tooling. The ER32 collets also double for the lathe, I've got a Myford thread/ER32 chuck so can do from around 2mm to 20mm or so.

The dovetailed columns I've seen tend to be a cast (box??) section and could well be a bit stiffer than a steel column while also being more height repeatable. I once had the privilege of using a Shoblin (sp??) mill. This thing was very compact and had what I can only describe a massive shoulders and was a solid as a rock. At the owner's prompting I was ploughing a 50 thou furrow with an inch cutter without any effort. Back home my beast quickly reminded me that much above a 10thou cut was pushing it.

Hope this helps.

[mmaidnz]

Havoc,I don't know which size you're looking at,but I have the ZAY 45FG.This is a geared head mill/drill,so you change the speed simply with a lever,instead of fiddling with belt tensioners,etc.
The dovetail column of course means that your work stays in register,which can be useful if you need to change tooling in the middle of a job.It is also much heavier and therefore more stable than the equivalent round-column mill.
If you look at commercial mills,they are always dove-tail columns.
The round column types are really a drill press with a milling head put on.
So if you can afford to,take the dove-tail unit.
Also,these usually have the extra facility to turn the whole head up to 90 degrees left or right.
Mine has a 4Morse taper,but this is easily reduced to the more common 3 MT by a sleeve.I use ER32 collets(the "grabbers),and intend to make a suitable adapter,so i can use these in my lathe as well.

[spurley]

Hi Havoc

I think you are talking about an 'Autolock' chuck which is used to hold the cutters in collets suitable for the cutter size either metric or imperial. These are manufactured by various companies, the most common being Clarkson. (I have an Osbourne 'Titanic'.)
They have a drive taper suitable for the internal taper on the mill spindle and again they come in a variety of sizes. I suspect you might mean yours has a Morse 3 taper, same as the tailstock and mandrel of common lathes, more usually described as MT3. The mill spindles have several tapers depending on the manufacturer these can be; Morse, as already stated, Int, Rseries etc. so it is very important to know the right one for your machine.
Picture of one at www.excel-machine-tools.co.uk/accessories/14/collets-and-collet-chucks/78/excel-collet-chucks/971/index.html they seem to have a range of different chucks to suit each machine.

Cheers

Brian

[ilvaporista]

Go for one with a dovetail column. You very often need to move the head up and down. A simple operation like boring a hole is much easier if you can use a centre drill to start, open out progressively with twist drills, then use a boring head. These changes require the head to move up and down to accomodate the various tools. Plus if you have to measure or offer up a mating part you can have unrestricted access.

Don't forget to look at quill travel as well. Make sure it will cover the range you need.

[ron]

There was an article about a year ago in one of the model engineering mags by a chap who had fabricated a bolt on unit which allowed a round column milling machine head to be raised and lowered without adjustment, I've a cheap round column machine but I can't say I've enough use for up and down adjustment to be bothered making one.
I would agree with SteveW about keeping the head as low as possible and the quill retracted due to flexing, particularly with heavy cuts.
Ron

[chris vine]

Hi Havoc,

1. As said above, all industrial machines use a big solid, cast column with not a round one in sight. Don't forget the stiffness also depends on the thickness of the column, tube or whatever.

2. Try very hard to get a machine with an R8 spindle taper. The tooling is very cheap and the best thing is to use collets which pull into the nose with a drawbar. You only need a few to hold milling cutters direct in the collet with almost no overhang at all. This makes it much more rigid. Also anyone making a machine with an R8 taper might have put a bit more thought into its design!

3. Be very wary of gear head machines for use in a small home workshop. They can be very noisy especially on higher speeds. Also if something breaks inside you may find it difficult to repair or get spare parts. A belt drive is simplicity itself and quiet.

4. If you have space, a second hand but small industrial machine or something from a school or college might suit you better than something cheap and new.

Have fun,
Chris.

[havoc]

Thanks for the replies. So that settled it more or less for the box shaped one. Rests only to determine the size. I'll be working on gauge 1 locs (maybe a single 3.5").

I'll go for the MC cone, since that is the most available standard this side of the water. I'll see what size when I'm that far.

Collet was indeed the word I searched for, thanks for that as well.

It will have to be a small cheap machine. First of all, there is the budget. A second-hand machine is more expensive than a new one, and I could get a serious machine for the price of a SH one of very dubious provenance. Secondly, all the SH machines are far too big (*). I would not be able to get it inside (only way is through the house, passing several doors and having to make a few serious turns) or over the house. third, I'm not able to examine a machine (new or old), let alone getting it in order is something gets wrong. For a new machine at least I can get spares.

So it will be a cheap chinese thing like the lathe. I have a feeling that in my case, it is the man (idiot) behind the machine that is the limiting factor anyway...

(*) about the size of SH machines, I did look for a lathe as descibed. The smallest I found was about 160cm long, 800kg heavy (thats 6', 1600 pound for those still in the middle ages ) and I would have had to change the electricity to power it. Price: 2000 euro, spares: not even a brand on it, I could not lift the 4 jaw single handed. (anyone a lathe with 2 meters between centers?)

[Chris Kelland]

Hi Havoc & All,

No contest really. For the sort of work I do I found the dovetailed machine far, far better. The drawback is the increase cost. If you need to raise or lower the head will you need accurate realignment? Round column machines may well be OK when new but just a tiny amount of wear at the keyway will magnify at the cutting tool. As they say 'horses for courses' but I would not willingly go back to a round column machine.

Chris.