View Full Version : Press fit
08-17-2001, 04:26 AM
I agree with coles-webb. The component cooler is so much safer to use. The Co2 may be the next safest. I can also buy a little piece of dry ice if I need to cool a larger part. Keep in mind that dry ice lets off Co2 witch will displace oxygen. You need to do all of this in a ventelated area or it will sneak up on you. The cooler spray has tetraflouroethane in the can, even if you can pronounce it don't breath it either.
04-09-2004, 08:17 PM
Am looking for input for press fitting premanufactured hole drill guide bushings. What size is proper undesize etc. Assume that type of metal being used makes a difference. Ie alum, brass, steel. Any input would be greatly appreciated. Also am using 3 ton press.
04-09-2004, 10:32 PM
If you’re using pre-manufactured hole drill guide bushings they are precision ground on the outside diameter. They aren't always press fit in place they are sometimes held in place with a setscrew or screw from the top. If you do wish to press fit them you can, usually by reaming (not drilling, as it is inaccurate) the hole .0005 to .001 undersize. This can be accomplished by using an undersize reamer (see links below for "over & under” reamers). It is best to spray some Zinc Chromate Primer in the hole before pressing the bushing in to provide lubrication and to help against dissimilar metal corrosion. The type of metal it is going into doesn't make much difference except that the softer metals (copper, some brasses and some aluminums) will expand easier as you press the bushing in as steel will not. If you have too much of an interference fit and the bushing is not hardened you can actually compress the bushing and may have to ream it out after.
Links for undersize reamers:
KBC, page 82 - http://www.kbctools.com/can/main.cfm
Enco, page 31 - http://www.use-enco.com/CGI/INPDFF?PMPAGE=31
Travers Tool, page 71 - https://www.travers.com/index.asp
I think a normal press fit allowance is about a thousandth of an inch per inch of diameter...but that's just what I've heard.
04-16-2004, 12:45 AM
SGW that sounds extreme. You would have a hell of a time pressing a 6.0 inch diameter part into a 5.994 hole. You have a huge amount of surface area on a 6 inch dia as compared to 1 inch dia. Makes the pushing exponetialy more difficult. I am not an engineer nor interference fit expert just my thoughts on subject.
Well, I have virtually no experience doing press fits (I'm a Loctite fan) so it may well be extreme.
Or it may be the allowance for a shrink fit...that would probably make more sense.....
04-16-2004, 12:51 PM
Most of the Jig and Fixture guys I have worked around use a trick of shrinking the bushing in Liquid Nitrogen before pressing it in. This is not something that I would want to do at home but you could leave the bushings in the freezer overnight and kind of get the same result with less danger. It has worked for me. I also heat up the fixture in an old toaster oven first just enough to get it warm, makes the hole just big enough that the frozen bushing almost slips in.
04-16-2004, 03:51 PM
Press-fit drill bushings made to ANSI Standards (types P and H) have outside diameters LARGER than their fractionally-expressed nominal diameters, and are in fact intended to be pressed into slightly-over-nominal holes. See Section 4 of the American Drill Bushing webpage of the attached link.
04-17-2004, 04:14 PM
The use of liquid nitrogen for shrink fitting is usually limited to industrial work as they are the only ones that have access due to storage and cost. It is dangerous as it can cause instant frostbite on the skin and even more damage if splashed in your eyes. There are some industrial sprays that are used in the electronic industry for cooling components (one I have used is called "Freeze Mist")which will work for instant cooling for shrink fits as they cool to about minus 65 F. If using an item cooled in a freezer pack it in a bag in ice (but keep the moisture off of it to avoid corrosion later) and bring the part to the piece that you have pre-heated as most bushings are thin walled and warm up quickly(even if using liquid nitrogen. I will usually have a nylon plug with a shoulder on it to be used to adjust the bushing in place if it warms up prematurely (the nylon will not conduct heat or cold well and therefore not increase the warming up of the part or cause damage when adjusting). As for heating the part prior this is good but be careful not to overheat and affect the temper of the material (most aluminum's can only be heated to 212 F before affecting the temper) and some materials must be heated evenly such as castings. Also unsure there are no flammables or other materials that can be affected by the heat. I usually spray a coat of zinc chromate on the warmed up part first before pressing the part in to avoid dissimilar metal corrosion later. Depending on the tolerance used you can have parts just drop into place and then mate for a great fit (I have hade 4 inch diameter bearing races fall into the holes on aircraft wheels and I have had 1 inch dial bushing that warm up to quickly and require a quick adjustment with a nylon plug and a hammer) Good luck.
LNo2 is not especially dangerous, no more so than welding. I get it from the local welding supply that brings it in every week for the doctors. After they fill thier dewars there is usually some left over which the welding supply will give away for free. You just have to know what day. Use a stainless steel thermos bottle to hold it and it will last overnight. Wear welders gloves and eye protection. It is about the same hazard as hot metal.
04-17-2004, 11:47 PM
Just don't screw the lid on tite on the thermos. Or )or (and I have not tried this "workaround") let the bottle cool by boiling the nitrogen and pray for a leak to keep the pressure down.
Some of the men used to use liquid nitrogen to "burn " off warts and skin cancers. One man decide to take some home. According to him, the cork makes a big noise when the pressure gets high, and a floor board with a quart of liquid nitrogen gets smokey in a hurry- made it worth while pulling over where his problem was noted by the rest of the shop (and discussed next day and at a safety meeting). I understand the vapor looks very much like a fire smoke- at least here in Florida where humidity is high.
[This message has been edited by docsteve66 (edited 04-17-2004).]
Yeah Steve, I failed to mention not to screw the lid on tight.
04-18-2004, 12:58 PM
I get fussed at for not worrying about the safety stuff where the accident MIGHT happen. Sealing a container of liquid nitrogen and staying close comes under the heading of those things you only do once. Speaking of things you might only do once:
I use liquid propane or carbon dioxide to chill things.
Propane boils at -40 to -45 degrees (F or C), leaves no residue and is flammable (explosively so) at air fuel ratios of 1 to 10. I boil it in well ventilated space, no sparks etc. I DO NOT consider it dangerous, but I am careful with it. I get lectures EVERY time I do it, so no need to warn me here. I just turn a cylinder of propane upside down and let it collect in a can in the quantity I need. Add only a little at a time until the part is covered then remove it and install. I do not advise anyone trying this unless really in a jam where the risks are such that the dangers are considered safe enough to justify proceeding. Same problem faced by a bomb disposal man.
For CO2, I use a fire extinguisher nozzle pointing up, work part inside and hit it couple times. part come out in a chunk of dry ice which sublimates slowly as you get it ready to install. very noisy, easy to freeze a hand even with gloves so I use only pliers or the like to handle the part. CO2 boils at -78, does not exist as a liquid (its freezing point is above boiling point at 1 ATM) for my purposes.Usual cautions about keeping fire extinguishers filled apply. I figure my chances of injury with the CO2 exceed those of the propane but the chances of death are higher with the propane.
Either method will chill a beer (freeze it) quickly but I guess beer and explosives are a bad combo- so use the fire extinguisher.
04-18-2004, 03:34 PM
Ya,Doc,on the farm we used to do it with propane all the time.Had one old John Deere tractor with a liquid lock before the vaporiser,we had a short length of gas hose hooked to a barb on side of the lock,we would pull the tractor over to were we needed the chill shut off the engine and ground out the mag unroll the hose and let it drip on the offending part till done.Always worked,just had to be careful thou,propane tended to settle out near the ground were you couldn't smell it,had to make sure nobody came around smoking and lit a match and tossed it http://bbs.homeshopmachinist.net//biggrin.gif
04-18-2004, 07:39 PM
Usually for shrink fits we go with the old emergency beer cooler. CO2 fire extingusher