Depth of cut

I need to cut a trough in mild steel (EN1A leaded) with a 1/2" end mill. What depth of cut should I be aiming for.

A lot depends on the ridigity of your mill. On my Bridgeport I would try a slot drill at around 700 RPM and 2mm depth of cut. A lot comes down to feel and sound, you will know when you have found the sweet spot.

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

Like Chris I'd be at around 700rpm (assuming a HSS cutter) but I'd go a bit deeper, say 4mm on the cut.

Andrew

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

The figures published by the tool makers are all very well for a rigid powerful industrial machine but of little use to someone with a bench top hobby machine. With a new sharp 2 flute 12mm dia cutter about 2mm deep per pass is a kind cut for the mill to cope with. You will probably have to feed at about 2" per min which is enough to still make chips rather than rub the metal into a pile of iron filings. If cutting with the side of the cutter then using more of the length is a good thing but when using the full width it is too much to ask.

Can't remember the post I took this video to go with but that is about as heavy a cut as I would want to take with my X3 and would usually take a bit less.

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

Hmm, I suspect you'd be surprised what you can actually manage if you do a few tests. The worst that could happen is you might break a cutter or two and / or soil your pants - and where's the harm in that?

I did a test cut in a piece of hot rolled steel with a 12mm 2 flute HSSCo end mill and a 20mm HSS tee slot cutter some time back. Given that the tee slotter was obviously going to have to cut full width in one pass, it seemed reasonable to expect the end mill to do similar or arguably better. This is on a Bridgeport clone, which is hardly a super rigid machine.

2 passes of the end mill (10mm per pass) followed by a single pass of the tee slotter. Power feed (to ensure a steady feed rate) and coolant (keep the tool cool).

Murray

Edited By Muzzer on 19/02/2018 12:44:03

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

That's not really a rigidity issue, more one of fundamentally poor design.

I would expect that any properly designed machine tool would have a transmission path from motor to spindle that is designed to withstand the forces from a stalled motor, due to foul ups or over-ambitious cuts. Or at least have a defined failure mode, such as a shear pin.

I don't suppose many people here would be impressed with their car if they left the gears on the road just because they floored the throttle.

Andrew

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

If, on the other hand Eric has something like an X1 mill he might struggle with anything more than 1 mm depth.

Russell

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.

If I rightly remember, an industrial machine like the Colchester master, uses a geared head but the motor runs from 2 V belts to the gear counter shaft, meaning that the point where it would fail on a lock up would be the belt slipping out rather than overheating or potentially canning the motor.

Michael W

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

Of course none of the points raised above alter the fact that if a machine has a motor capable of x W output then it is illogical if the transmission to the spindle is unable to transmit that power (plus a safety margin) without going bang.

I agree with Muzzer. In my experience of stalling my lathe the headstock gears take it in their stride and the V-belts do not slip. Instead the motor stalls. in the longer term that will lead to overheating. But one can tell if the machine is getting overloaded, or you know a foul up is on it's way, but aren't quick enough to knock the power off first. Last time it happened to me the hydraulic copy unit decided to see how deep it could plunge the tool before stalling the motor. The answer was about ¼".

As for towing it is perfectly possble, if not legal, to tow a large trailer with a small car. Of course performance is poor and you may spend a lot of time in the lower gears but it can be done. I've not seen a glider trailer towed by a Smart car, but I have seen one towed by a 1L Metro. It caused a few raised eyebrows, as the driver had driven from Hampshire to Talgarth in the Brecon Beacons. The main issue with a small car is stability. If the car is too light there is a good chance that the trailer will take charge with spectacular results. In the 1950s people towed glider trailers with much lower power cars than today. Some cars had towbars fitted on the front, so that when going up a steep hill, like Sutton Bank, you could go up in reverse. The idea being that reverse is a lower gear ratio than first.

Andrew

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

Some industrial machines have a meter indicating how much power the spindle is using. A Plano Mill I used to repair had a 100hp motor on the spindle and that spat big hot chips everywhere with a 12" inserted tooth face cutter mounted up. The operator had steel chip guards to hide behind. Luckily the machine was in a pit and the table was about 5 feet below floor level so only the operator was in the firing line.

Mike

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

A useful addition Neil, can be handy to know how hard you are running a machine and keep it in its comfort zone.

Mike