[RCaffin]

Counterweights work in one way, but they double the inertia.
My machine uses gas springs.

Cheers
Roger

[Azalin]

What I was thinking was something like this:

[RCaffin]

Me too. Double the inertia.

Cheers
Roger

[ianagos]

Is that really a big deal though? My mill has a counterweight and the head weighs a couple hundred pounds and it does 400ipm fine.

I feel like gas springs would not provide a very constant force of those distances.

[RCaffin]

Hi Ianagos

All true. I don't know the answer. Maybe there is no perfect solution - that would be normal!
The thing about hobby machining is that you can have lots of fun experimenting.

Cheers
Roger

[JoanTheSpark]

I thought that pneumatic cylinders with an additional reservoir would be the proper solution.
The cylinder volume that changes by moving the head from top to bottom position must just be smaller than the reservoir volume (maybe a 10th or less?), so that the pressure in the system doesn't change that much when the cylinder volume changes.
This then means that the force that the pneumatic cylinders provide to the head to counter some of it's weight doesn't change much either.
@linux_fan writes about it here: https://www.cnczone.com/forums/uncat...ml#post2060556

[handlewanker]

Gas struts are more compact....inertia with fast traversing upwards does not affect the cutting ability as the tool is not in contact with the job...…….inertia for fast feeding rates probably won't enter into the field of inertia problems as the rate of travel is not that fast anyway......never heard of anyone plunging a cutter into a block of metal at extreme fast feed rates...…...the purpose of a gas strut is to reduce the force on a stepper motor on the up travel when the head weight is at it's moistest.
Ian.

[RCaffin]

never heard of anyone plunging a cutter into a block of metal at extreme fast feed rates
Really????

Cheers
Roger

[handlewanker]

Well, I'm thinking in terms of a drilling op where pecking is probably the way to go so inertia isn't a factor to worry about......you wouldn't get any joy by short fast drilling feeds either manually or with a very fast feed rate.

It' s better to have a positive up compensation to the head by whatever means turns you on,,,,,gas struts are just a neater way to achieve it to ensure you're always pushing the head down against a force.

That means if the head weighs 100Kg you need approx 150kg up thrust at it's weakest position or the top of it's stroke, there being little variation by the time it's compressed to the bottom
.
Just as a matter of interest, when I drill a hole manually I do not lift the drill out of the hole to clear the swarf...…..that is a sure fire way to get a chip under the point of the drill and cause problems......I just pause at about 2D depth to let the long curly chip break off and go on further with the same method as the depth increases.

With free cutting or leaded steels that short chip readily the prevalence of a chip under the drill point is highly likely to happen if you lift the drill point.

You cannot sense when a chip is stuck under the drill point when the cycle is in a CNC pecking mode and the drill is lifted as opposed to paused periodically.

As far as constant or varying up thrust from a gas strut.....it doesn't matter if it reduces as the head rises because it's just pushing the head up to offset the force on the ball screw and stepper motor when the head is being lifted.

My opinion is it's nit picking to think that you need such a very constant pressure on the screw with a pneumatic cylinder and an additional reservoir tank.
Ian.

[skrubol]

never heard of anyone plunging a cutter into a block of metal at extreme fast feed rates
Really????

Cheers
Roger

Not intentionally anyway..

I'm not sure why you would want more counter-force than your head weighs. That means if your servo brake fails the whole head will crash into the top stop instead of the workpiece or table. I think I'd rather have it somewhere around neutral, so with friction it would just sit there, or come to rest near the middle of its travel.

[ianagos]

I think the weight is the easiest way to get an even counterbalance. The springs go bad and aren’t even. There’s a reason real machines don’t use them.

[handlewanker]

Well you will still get a force going up or down if you have a gas strut that doesn't 100% balance a head weight.

I have a gas strut on my Skyfire SVM-0 mill and at all positions the head remains static...…….by your reckoning the head should either run down or up at the end of the gas strut travel.....due to the pressure increasing or decreasing etc as it isn't a constant force.

As I said, a gas strut is a neater way to compensate for the heavy head weight to help the stepper motor and reduce ball screw wear...….inertia is a factor that only applies to the mass of the head casting when it moves either up or down...….some mills don't have any weight compensation at all.

Taken to the most ridiculous scenario...….if the head is all the way down and the gas strut is fully compressed, when the power is off the head should be pushed back up by the gas strut...….but that never happens due to friction etc.
Ian.

[handlewanker]

I think the weight is the easiest way to get an even counterbalance. The springs go bad and aren’t even. There’s a reason real machines don’t use them.

Hi, you will find that adding a weight compensator will be the most difficult method to apply and bulky too....…….a gas strut only requires two tapped screw holes to mount it.
Ian.

[Azalin]

Hi,

Sorry I couldn't write since my last post. And thank you for your comments.

I have weighed the stock head and all the unmachined parts of the new head. The new head is about 5kgs lighter than the stock head. So maybe no counter weight or gas spring will be needed.

Attached a render of unfinished design of the head.

[handlewanker]

Now, how much simpler and neater can you get......I take it that the spindle drive motor will be mounted upside down on the top of the fabrication.....are you going to use the motor out of the old head?
Ian.

[Azalin]

Now, how much simpler and neater can you get......I take it that the spindle drive motor will be mounted upside down on the top of the fabrication.....are you going to use the motor out of the old head?
Ian.

Hi Ian,

I'm going to try a 24v 3kw brushless motor. This motor is a little fast for the spindle but I'll use a 1:3 pulley setup. Max speed of the motor is 18k. With the pulleys spindle speed will l be around 6k.

I know 24v sounds way too weak but I want to try it. If it can't make the torque then I can replace it with a 220v induction or even better a servo.

[Azalin]

Now, how much simpler and neater can you get......I take it that the spindle drive motor will be mounted upside down on the top of the fabrication.....are you going to use the motor out of the old head?
Ian.

Hi Ian,

I'm going to try a 24v 3kw brushless motor. This motor is a little fast for the spindle but I'll use a 1:3 pulley setup. Max speed of the motor is 18k. With the pulleys spindle speed will l be around 6k.

I know 24v sounds way too weak but I want to try it. If it can't make the torque then I can replace it with a 220v induction or even better a servo.

[mjoconr]

Where did you get the 24v 3kw brushless motor ?

[RCaffin]

3,000 W at 24 V is 125 A. With all due respect, that is ... unusual.

I imagine you could find a truck starter motor with those specs, but please note they are rated for a 15 second duty cycle (or something like that). They would melt after 15 minutes running.

I run a 500 W DC brushed motor (Baldor) on my machine, and that is plenty of power for machining steel and titanium.

Cheers
Roger

[handlewanker]

Yes...…..a regular 3 phase 1 HP motor running at 3,000rpm speed controlled with a VFD would give you 6,000 rpm with a 1:1 pulley arrangement and speed down to 100rpm still with a lot of torque......a 3 phase motor with an iron body does weigh a bit though.

I may be wrong on this assumption but a 3 phase induction motor has a solid armature, no windings in it, so it " could" rotate at 6000 rpm top speed with a VFD which means you could drive the spindle with a pulley reduction of 1:1 to give you 6,000 top wack and down to 100rpm..

I have a 1hp brushless motor and a 2 pulley set., …...that is. 2 pulleys that can be changed over on the motor and a fixed pulley on the spindle...…..changing the motor pulley gets you a 1:1 or a 2:1 drive for whatever torque you need.....top speed of 7,000rpm etc.

Changing pulleys is quite easy on my mill as the motor is inside the head casting with the pulleys at the top.
Ian.