press fit woes

[kendo]

I'm trying to install bushings into the aluminum table on my Haas gantry router to use for part alignment and fixture location
the bushings are supposed to be press fit into the table and slip fit a 1/2" dowel pin
the OD is .7518
the ID is .5005
now Im milling holes in the table for these bushings to a diameter of .7518
and pounding them in by hand with a hammer and soft block of aluminum

the problem is, the 1/2" dowels will only slip into 1 out of 10 when Im done, the rest are too tight
all these bushing slip over the pin fine prior to installation
the pins measure .500

what am I doing wrong?
what would you guys recomend for the ones too tight?
pull them out?
open them up somehow?
is the .5005 ID bushing too tight?
should I find something .501?

I tried a reamer unsuccessfully, and we don't have a tool to extract the bushings (yet)

thanks in advance,
Kenny

[Geof]

You say the bushings are 0.7518 and your hole is 0.7518 so they should still have the .5005 ID after inserting them in the table.

Are you using something like Never Seize on the bushings and the hole in the table? Many times when pushing steel into aluminum it will gall up and this squeezes the bushings undersize.

Bashing them in is maybe not the best practise; it is very easy to get the bushing going in at a slight angle which increases the chance of galling.

I suggest tapping the bottom of the holes 1/2-NF then screw in a stud and use this to pull the bushings in with a washer and nut. This holds the bushing in alignment better and pulls it in evenly.

Are these bushings hardened? You will never ream or bore them to size if they are, and you are going to have a bit of fun getting them out.

To get them out if they are hard you can try "hydraulic extraction"; provided you have not bored all the way through the table.

Make a collar piece that is maybe 3" diameter, about 3/4" thick with a hole the same size as the bushing ID.

Make a drift that is a slip fit in the bushing and collar piece with a larger head; the length of the slip fit size should be the length of the bushing plus the collar thickness, the head can be about 1" long.

You fill the bushing almost to the top with grease.

Put the drift through the collar and down into the top of the grease filled bushing.

Give the head of the drift a mighty blow with a good big hammer.

If everything goes correctly the hydraulic pressure created by the hammer blow on the drift forces the bushing up; you may need to refill it with grease for a second or third shot.

The collar is to hold the drift true and it also protects the table and prevent a squirt of grease coming out the side of the pin and maybe hitting you in the eye.

[kendo]

yes, the bushings are hard
but what about maybe using a grinding compound and "drill" them with one of the 1/2" hardened dowels?
wishful thinking?

I like your idea of pulling the bushing in with a stud, nut and washer,
wish I'd have done that first

and I tried a less involved version of the hydraulic trick, but couldn't give it a very mighty blow without going the extra mile like you describe.
the extractor tool I found is a couple hundred $$ so I'll be making a guide collar to try first if it comes to it

[Geof]

Originally Posted by kendo ....and I tried a less involved version of the hydraulic trick, but couldn't give it a very mighty blow without going the extra mile like you describe....

I learnt that the 'mighty blow' can cause a lot of collateral damage when you don't go the extra mile.

If you want to try lapping them with grinding paste do not use the hardened dowels use a piece of brass. Machine the brass a few thou oversize for a length of about .2" then relieve it to a bit less than 1/2" for a total length that is longer than the bushing; then split the end crossways.

The split allows the brass 'hone' to spring into the hole and the relief means it will not bellmouth the hole. Being brass it is softer so the grinding paste embeds and grinds the bushing.

[kendo]

HUGE thanks Geof!

thats some ingenious stuff there!

I wish I worked around people like you, I'm 2 years into machining, self-taught, and I'm the answer guy at my shop
Nobody here could help me on this

thanks again,
Kenny

[cam1]

Drop the bushings into the freezer for a bit (or into a container of dry ice), it will make assembly much easier,in fact the bushings will probably drop right in.

regards

[Geof]

Originally Posted by cam1 Drop the bushings into the freezer for a bit (or into a container of dry ice), it will make assembly much easier,in fact the bushings will probably drop right in. regards

This is a good idea if you have a convenient source of dry ice. A dry ice alcohol bath, isopropanol, aka isopropyl alcohol that you can get from Home Depot will give a temperature close to minus 80 Celsius. You have to be careful doing this because they will freeze to your skin.

This procedure is used to shrink valve seats into aluminum cylinder heads.

[mc-motorsports]

Originally Posted by cam1 Drop the bushings into the freezer for a bit (or into a container of dry ice), it will make assembly much easier,in fact the bushings will probably drop right in. regards

+1

Heat the hole if possible and shrink the bushing. I'm 100% against hammers for precision fitting. And use a heat gun, not a torch (and yea, it might take a while).

Good luck

[Geof]

Originally Posted by mc-motorsports +1 Heat the hole if possible and shrink the bushing. ....

Heating the hole may be a bit tricky, he is dealing with a 4' by 8' piece of 1" 6061.

Yea it may take a while.

[mc-motorsports]

Originally Posted by Geof Heating the hole may be a bit tricky, he is dealing with a 4' by 8' piece of 1" 6061. Yea it may take a while.

Actually, he may not be able to heat it. You have to get the material around the hole to expand. It would work if you put the whole thing in an oven, been there, tried that. 12" X 8" X 16" solid steel spindle housing, had a 6" hole going thru, it was an old school lathe spindle for a dedicated machine. Had to put new brass bushings in the bottom, replacing the worn out "locator plugs", ended up putting the whole thing in the oven at 140*c, the bushings in the shop refridgerator, dropped right in.

And I belive they were 30mm OD with .010mm interference.

[mc-motorsports]

In a situation like that, I would probably make the holes what I call a "push fit", which means you wouldn't have to press them, or shrink fit them, but it would require effort to fit them, and put some permatex or loctite cylindrical locker on them. Good stuff! Trust me! You just can't get it hot, which shouldn't be a problem.

[Geof]

Just for fun I looked up the thermal expansion of aluminum; 24 x 10^-6/degree C.

Eight feet is about 100 inches and taking the table up to 140 C is a change of 120 degres C; it will expand almost 0.3".

Freezing the bushings is more practical.