Interference fit

[Esmith41]

I have .080 of backlash in my cross slide on my SB 13 lathe and turned new pieces to replace the screw and handle end of the cross slide screw. I am reusing the spline gear. I drilled and reamed the hole at .500 and turned the male end to .501. I heated the female piece and put the male side in the freezer. When I went to mate them together it went about 1/2” of the 1 1/2” I needed to go. My question is, is .001 too much? I cannot pin it as it is in the gear and I don’t want to mess with the teeth. I don’t want to do this again so I would like to get it right this time. Any help will be appreciated.
Thanks,
Eric

 

[JimDawson]

A 0.001 press fit seems about right. Normal is 0.001/inch of diameter, so by that formula 0.0005 would have been better.

This is the point that I normally use brute force to assemble.

 

[rgray]

How long did you heat and freeze. Make sure you give enough time for temp change. I would probably freeze it over night if convenient.
Heating in a toaster oven or some such beats trying to guess with flame.
I'm guessing your heated part is small. So a couple of hours in the oven would get it heated completely. 300-400 degrees, but if it's a hardened part and you don't want to remove hardness stay at 300.
Don't have any oils on the part and apply nothing when trying to assemble. Don't even get a fingerprint on the surfaces.
Take the bigger part out first, then retrieve the smaller part and assemble.
They will slide together very quickly and be movable for a very short time, like 1 second maybe.

If you get it stuck halfway leave it alone and press apart when temps even out, or if set up and can do it quickly press it home immediately.
I usually do my assembly on the press with it set up to press the piece if needed. My press is electric so can do it quickly.
A hammer and a driver would be quick as an install helper also.

 

[RJSakowski]

steel has a linear expansion coefficient of around 11 x 10^-6 inch/inch- ºC. For a 1/2" diameter shaft, and an interference of .001", you would need a minimal temperature difference of 180ºC (325ºF) between the two parts. This would be for a tight slip fit. Because once mating of the the two parts begins, heat is being transfered to the colder part and the temperature is equalizing, the mating has to be done quickly and decisively. For that reason, you will want to have additional clearance of the parts. You will probably want to heat the gear to close to the point where you would start to draw the temper and ideally chill the shaft in an alcohol o dry ice bath. The dry ice bath will get you down to -57ºC (-70ºF). 250ºC is a fairly safe temperature to avoid drawing any temper. That would give a temperature difference of about 300ºC (540ºF) and should give you a clearance fit of slightly less than .001".
With that minimal clearance, the assembly will have to be done quickly. Some sort of assembly jig would help. If it were me, I would machine a cup to hold the gear and heat the gear and cup. This will help retain the heat. For the shaft, I would use a socket. A drill press should do the job job of mating the two parts. The chuck can serve as the socketYou will wnt to do a dry run to get the choreography down pat. Use some means of locating the cup so you can pull it from the oven and position it quickly. Pull the shaft from the dry ice bath and place it in its socket and immediately press the shaft into the gear. You only get one shot at it

 

[RJSakowski]

In the absence of a dry ice bath, an ice/calcium chloride bath will get you down -40ºC (-40ºF). Calcium chloride is very corrosive to steel so you will want to wash the assembly thoroughly after assembling.

 

[mikey]

I've done similar leadscrew repairs where you're press fitting old to new. I bored to a 0.0002" press fit and pinned it, though. No heating or freezing involved.

One suggestion is to measure the reamed part to be sure of the exact ID. When press fitting parts, tolerances are tight and they matter.