We are all hobbyists (mostly) so go gently, and if it all starts vibrating and smoke or blue chips appear then the cut was too deep and/or fast!

Depends on the machine size and set-up, but as a start Eric try 300 rpm, 0.040" (1mm) depth and gradually increase the speed of advance until you get small chips rather than dust. Oh, and put some cutting oil on it from a spray or drip bottle.

Norm

My handy dandy slide rule "window" calculator fro Osborn tools says 764 rpm(!) for a 1/2" cutter in tough or mild steel.

Says:-

End mills are for profiling so cut depths up to the diameter of the cutter, 1/2 " or 12 mm, and cut widths up to 1/4 the diameter.

Slot drills, aka centre cutting end mills these days, are for for slotting so cut width is diameter of cutter and depth up to 1/2 diameter.

Feeds given as 6 & 11/16" (!) or 170 mm per minute for a 4 flute end mill and 4 & 1/4" or 108 mm per minute for a two flute slot drill. Feeds top be halved for long series cutters.

Leaving aside the rather theoretical precision of some figures its important that you give the cutter something to bite on. The action of cutting tends to stabilise things, too shallow a cut promotes rubbing which is not good. Folk tend to feed too slowly. Also important to get rid of the chips. Recutting chips is seriously bad news.

Very shallow cuts merely reduce the life of the end mill. To a first approximation it can only make so many passes before its blunt so taking 1/4 of the depth of cut effectively means a quarter as many jobs before its done. You have paid for the sharp flutes all the way up the side so you might as well use as much of the length as possible.

Clive

I'm in agreement with the other advice, but experience matters. As a beginner I made excessively tentative cuts, now I'm far more aggressive. But another common beginner mistake is expecting far too much. Actually what's best is a balance between several factors such as:

• What the workpiece is made of
• Cutter type and condition
• Coolant
• Swarf removal arrangements
• Motor power
• Rigidity of machine
• Rigidity of work and it's fixings to the machine
• Operator skill

What I do, and I know I'm still learning, is to calculate the rpm and depth of cut roughly as described in other posts. I set the work up and the machine as rigidly as possible. If a lot of metal is to be removed I flood cool. Then I apply the cutter to the work at about 20% less than theoretical. Depending on what happens, I either reduce or increase the attack until the mill sounds as if it's working hard without being distressed. I look for the maximum rate of metal removal that can be achieved without abusing the machine. You shouldn't pussyfoot, nor be a gorilla. What that means comes with practice and isn't easily described in words.

Dave

A Bridgeport may be light by industrial standards but is considerably more rigid than say a X2 benchtop mill that will just spit out its plastic teeth if those sort of cuts were taken

The gear breaking IS the defined failure mode for the X2 and mini-lathes. Metal replacement gears are available but many sellers try to persuade people not to use them as instead of losing a cheap gear something more expensive breaks.

Bear in mind they are used by beginners and there's a difference between overloading a cut leading to a stall and running a big cutter flat out into a heavy cut.

FWIW the gears on my X2 lasted a good ten years or more and I abuse it horribly.

Neil

Andrew, I expect no body would be surprised if they burnt out the clutch trying to tow a bus up hill, but if they stuck to towing a similar sized car then the clutch would survive. If used within it's work envelope then the gears would last but ham fisted use will as Neil says strip the gears which are the intended weakest link in the drive train.

Agree it is not a rigidity issue but is an indication that the small benchtop mills just can't perform in the same way as an industrial machine in the same way you would not use a smart car to tow a large glider trailer or trailer with a traction engine on it.

I think Eric has a WM14 which is about the same size as an X2.

Vee belts are not designed to slip. The whole point about the vee concept is that the belt tightens into it as the load increases. If you are seeing slip, you have a loose, worn or oily belt. Motor protection is provided by the motor starter thermal overload trip (in old money) or in the VFD, sometimes with a motor thermistor on a big (ie expensive) machine.

Similarly, car clutches won't slip unless you depress the clutch – or the clutch is in need of replacement. If you try to pull a large trailer with a small car, you would tend to end up moving very slowly in low gears rather than making the clutch slip. Of course, you can toast a clutch trying to set off up a hill if you are unrealistically ambitious or lack any obvious skill.

Cars are typically designed to be able to set off on a 20% gradient when fully laden, so are capable of pulling significant loads on normal roads. The combined weight of the car and trailer are the "train weight" and are often specified, eg a 3.5t vehicle may have a train weight of 6t, allowing a 2.5t trailer to be pulled all day long. Trailers above 500kg have to have self braking, so the limitation is generally the powertrain rating ie engine power and gear ratios.

Murray

Yes but if you crash the cutter and it stops dead from 2000 rpm, there's a lot more power than the rate motor power going though the spindle due to flywheel effects. It's those shock loadings that kill plastic gears, not chronic overloading – that overheats motors!

Neil

Not quite 100hp, but the inverter on my mini-lathe can monitor power use, so I have an analogue meter on the front that shows percentage rated power (misleadingly I have it set so that 0-1 is 0-200%).

<edit> as you can see spinning a 4" chuck, unloaded, at 242 rpm (on tacho) takes 2.18 amps (on inverter) and is about 40% of output power (on meter readout).

Neil

Edited By Neil Wyatt on 21/02/2018 07:59:41

Re Slitting Saws, hope that no one fits a key when using one! That is courting trouble. I once saw a slitting saw shatter because it had been keyed to the spindle. Fortunately, I had just walked past, so the debris flew behind me.

With friction drive, if it jams, it slips, but does not shatter. You back off, and return to the cut with a little less gusto.

Howard