broacning press

[rabdouglas]

hi there
this is my first thread for me, i am building a small press with a pneumatic cylinder to use for broaching keyways in small bores, I must admit it is not so exciting as the other post on here but it is my first, I am going to try and upload some photos of the start of the project,

regards rab

 

[Brian Rupnow]

Bad idea, Rab. Air pressure will build up pressure, then let go with a bank and break your broach. That is a job for either hydraulics or manual pressure, not pneumatics.-----Brian

 

[rabdouglas]

hi brian
I am suppying the cylinder with about 10 bar with a needle valve in the line so that the preasure will build up, the piston dia in the cylinder is aprox about 3inch so there will be a fair amount of constant weight behind the piston, I have not worked out the exact weigh/mass as in tons/ square inch yet but the broaches manufacturer say it takes 55kg/cm. I have set the cylinder up that way and the piston rod moves very slowly and I have tried to hold it down and i cant, I am not dismissing you advise but what do you think of the sort of pressures that i intend to use with the dia of the piston

regard rab

 

[Tin Falcon]

rab: Brian has spent lots of years designing machinery. If he says it is a bad Idea listen.
I have had a fair amount experience as a machinist all broaching I have ever seen or done is with a arbor press or hydraulic machine. I have spend hours and hours up to my elbows in cutting oil making many pounds of chips in broaching machines. if you make one use hydraulics and the preferred method is pull the broach not push it.
there is a reason many systems use hydraulics instead of pneumatics.
Tin

 

[steamboatmodel]

I have seen a few air oil systems, but a straight hand powered hydraulic cylinder type is much simpler. With pneumatics you have a lot of stored energy like a coiled spring or stick of dynamite and when it goes it goes very quickly.
Regards,
Gerald.

 

[hopeless]

I tend to agree with the advise given already BUT progress was never made without trying other methods. Go for it but please be very aware it could go pear shaped very quickly so make your safety a priority
just my .0001c worth
Pete

 

[f350ca]

Rab try loading the cylinder then releasing the load quickly. When your broaching the load on the cylinder will increase till it begins to cut, at that point the load will instantly decrease as static friction is overcome, at that time the air will expand and drive the cylinder fast really fast. Air over hydraulic would give you beter control, if you choke the fluid you can control the speed when the load changes.
Greg

 

[Swifty]

Rab, make sure that you throttle the exhausting air, not the incoming air. This acts as a cushion to slow down the air cylinder.

Paul.

 

[rabdouglas]

morning everybody

first of all thanks for all the comments, i have took them all on board,the broaches i will be using are to cut 1/8 max down to 1/16, when i charge up the cylinder with a constant presure of approx 10 bar and as paul says throttle the exhaust port with the needle valve the piston rod moves very slowly and you can control the speed thro the valve on the exhaust, my way of thinking is and i may be wrong is once i put the cylinder under pressure to push the broach the pressure would build up untill the broach starts to move and cut, but the air cannot excede the rate of air that is allowed to escape thro the exhaust port where the needdle valve controls the rate of flow of air. at the end of the day i still need to make the Frame up and maybe if i have to put a hydraulic cylinder instead of a pneumatic then that will have to be, its just that i have been givin 3 pneumatic cylinders and i thought i would put one to use in this wee project

regards rab

 

[lordedmond]

thats a lot of energy in that system

you seem to be going ahead without much thought and calculations as to the forces involved


by all means use a air to oil pump or a hydro jack /porta pack


but direct air no way a quick and dirty calc gives me 1319 pounds on that piston and its stored energy to boot

thats why we do ( the one that use boilers ) test with water if it lets go there are no drama's just a little spurt and a puddle on the floor ( from the boiler ;-) )



anyway welcome to the forum , please keep safe in your endeavours

Stuart

 

[rabdouglas]

hi Stuart
I do appreciate your comments, but I can reduce the inlet pressure down to a few pounds / square inch I just gave the 10 bar as a max pressure, The manufacturer of the broaches say the cutting force for mild steel is 45kgs which is roughly about 100 lbs which is about 6 bar but your are right I have not sat down and done any proper calculations yet,

regards rab

 

[Brian Rupnow]

Go ahead and use air pressure, as you seem to have your heart set on it regardless of advice from older and wiser people. Count your fingers before using it. Count your fingers after using it. Let us know the difference!!!

 

[Tin Falcon]

the home shop is a place for learning new things by doing.and sometimes through falure, or a unexpected result.

but a carvat is in order NEVER sacrifice Safety Health or Environment.
HSS and tool steels do not store energy well. too much energy in and they shatter.
Air on the other hand is a very good medium to store energy.
my concern is that this could be a lethal combination if the tool jabs breaks and all that energy is released. I have heard stories where people have been killed or maimed when such a failure occurred.
be creative but be safe.
Tin

 

[Jasonb]

The problem is that once the initial force needed to start cutting is met the broack will move quickly as the compressed gas in the ctylinder expands. If you have liquid in teh cylinder it cannot expand at anything like the rate a gas can. So no amount of restriction on the inlet or outlet will affect the stored energy within the cylinder.

You also have to consider what happens at the end of the stroke as you get less and less teath in the cut until they all clear and you have zero resistance to the piston which will fire the broach to the ground.

Can't really see the point in making a press for those size broaches, I've pushed that size through using an old drill press with out issue.

J

 

[f350ca]

Rab
You could charge the bottom of the cylinder with oil, then throttle the escaping oil. That will give you a controlled speed,
I use the reverse on my hydraulic press. I used a 4 inch double acting hydraulic cylinder with a hand pump acting on the top side of the cylinder, its slow but controlled. I inject air on the bottom of the cylinder to return the piston, by opening the hydraulic needle valve I can control the return stroke and stop where I want it.

I now have a valve and air fitting on the bottom cylinder port, once charged with air I close the valve and trap the air.
Greg