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S&S_ShovelHead
03-10-2011, 01:17 AM
I just finished my 1st pay job (thanks a ton for all that helped with the Q's I had) and the same person gave me another. I need to turn a peice of aluminum 1" long with a hole drilled and tapped in the center. It needs to press fit into a aluminum tube with a I.D. of 0.8785". I was thinking of turning it to 0.8785" and heating the tube it will be going to & pressing it into place, but thats just a guess. So...

How hot can I safely heat the tube up without any chance of damaging it? At that temp how much will it expand? And finally what size should I turn the cylinder so it will fit into the heated tube and then when it cools it will stay in place & what tolerances would I need to hold?

Thanks a ton.

RussZHC
03-10-2011, 02:10 AM
This certainly does not answer your question directly but the thread discussion and one of several links may help.

http://www.eng-tips.com/viewthread.cfm?qid=167880&page=6

partial answer seems to be you need more info as to loads and temp ranges
[from what I have seen in a quick search, there is often a concern about aluminum galling or splitting with inserts of a press fit (granted almost all of those discussions are about steel and aluminum) but also the possibilities of heat during use that may cause a press fit to loosen significantly enough for the inserted bit to either move or fall out]
by the way I did find one reference to Machinerys Handbook w a comment that "his" version did not have tolerances for aluminum inserts/pins/hubs though which version that was it did not say

Walter
03-10-2011, 03:54 AM
S&S,

Pressfits in aluminum can be a tricky business. turning on size as you suggest
It needs to press fit into a aluminum tube with a I.D. of 0.8785". I was thinking of turning it to 0.8785" and is not the way to go. Here's why...

Your talking an on size fit, not really an accpetable "pressfit", it would be a light interference fit. Going the same size as you suggested will cause the holding action to fail quickly.

Look first at what the purpose of the item you are making is, and the stresses it will face, then work from there. You also need to consider the thickness of the tube your pressing into as the thinner walled it is, the less holding power it will have. You also need to consider the depth at which the rod will be fitted into the tube. these variables will weigh heavily on the steps to be taken. Must it be pressfit? Can it be crossdrilled and pinned? is Loctite an acceptable alternative to a full on press fit?

If you can tell us more on whats your working with you will get some more concrete info.

ikdor
03-10-2011, 06:57 AM
I second the Loctite idea. If you take any of the 600 series from Loctite it'll be solid as a house and take a few tonnes to dislodge. Way more solid than you'll get from a pressfit.
Depending on the type it'll also hold up to 230 degrees C.

Igor

SGW
03-10-2011, 07:15 AM
The expansion rate for aluminum is about 13 millionths of an inch per inch per degree F. So, if you could achieve a 400 degree temperature difference you'd have about 0.005" expansion to play with.

But, I agree with the Loctite idea. You've got a huge amount of surface area, relatively speaking, and with the correct grade of Loctite you would never pull it out. I use 609 as a general-purpose retaining Loctite, since my requirements are not too particular, but there are probably other grades of the 600-series that would be even better. Read the specs on the Loctite website and see what kind of clearances are recommended for which grades, and their recommended applications.

S&S_ShovelHead
03-10-2011, 11:27 AM
I didnt even think of loctite, thats a great idea. Thanks for the tip. It looks like 603 or 680 would be perfect.

Since Ill be going this route should I make the part undersized to give the loctite room to work? If so y how much?

JCD
03-10-2011, 12:54 PM
I believe the use of Loctite will require some clearance between the mating surfaces. Without room for the "glue" between the mating surfaces, you will "scrape off" the Loctite when you try to assemble.
Use Manufactures recommendations.
Machineries Handbook will give you the answer on fits, all sizes and classes.

Rich Carlstedt
03-10-2011, 06:10 PM
For a .8785" hole, you want about a .0005" to .001 "interference fit, so your plug should be .879 to .8795".
Heat the tube up to 400 degrees and you will get ( .875 x .000011 x 320 =) a .003" growth in the tube. This will give you .002 clearence ( WORK VERY FAST !) the 320 is 400 minus ambiant temperature of 80 degrees.

Mark the ring with a felt tip pen. when you heat up the tube, the mark will fad at 400 to 450 degress, and you will have your temp . For thin rings, stay near .001" per inch diameter beacuse of material yield.

Rich
Do a sample first to see how much time you have. Especially is you will use his part

Leadfootin
03-10-2011, 06:20 PM
Give it a few tenths interference, wet both surfaces with loctite and it will gently push together never to part under reasonable conditions.
Fit can be made easier by chilling inner and slightly warming outer. Loctite on inner before chilling.

Peter

Ed P
03-11-2011, 08:29 AM
Just a reminder, bonding aluminum to aluminum requires Loctite primer since it is an "inactive" material.

Ed P

S&S_ShovelHead
03-11-2011, 11:26 AM
Thanks for all the help, I tapped & turned the part to 0.872" (about 0.003 gap per side) and just ordered some loctite 680 and 7649 primer. Ill let you know how it works out.

D_Harris
03-12-2011, 08:22 PM
I'm confused.

Can someone explain why the inner wall of a tube wouldn't expand into the center making the I.D. smaller if the tube is heated?

I would have thought that in order to increase the I.D. one would have to lower the temperature of the metal.

Thanks.

Darren Harris
Staten Island, New York.

precisionmetal
03-12-2011, 10:31 PM
Because all the molecules are trying to get further apart from each other. The circumference of the tube gets longer, which makes the diameter get larger.

darryl
03-12-2011, 11:32 PM
The walls of the tube do get thicker as it expands from heat, and you'd expect that the ID would shrink- except that the circumference grows as well, increasing the ID a lot more than the increased wall thickness shrinks it.

S&S_ShovelHead
03-13-2011, 05:50 PM
Tried out the 680.

Primed the hole (aluminum) and the shaft of a bolt (steel) with the head cut off. The hole is .010 larger than the shaft. Applied 680, waited 24hrs (temp was about 14 celcius) then put a bolt on the shaft and digital torque wrench to see where it would fail. (This was a test). Didnt even register at 10 ft lbs before comming loose. Any ideas?

Toolguy
03-13-2011, 06:18 PM
That's too much space. You want maybe a .001 slip fit.

S&S_ShovelHead
03-13-2011, 07:04 PM
The specs say it will fill up to 0.015, this gap is o.010/2 = 0.005.