# Press Fit Tolerances?



## Dwight Ennis (Jan 2, 2008)

I want to make a grate for my 1.5" scale 4-4-0. It's made up of 11 1/8 x 1/2 stainless bars held together by two 3/16 stainless rods with 1/2 OD x 3/16 long stainless spacers between the bars. Since I'm not a welder, I thought about using a press fit on the two end bars to hold the 3/16 rod in place and hold everything together. If I drill the two end bars 5/32, what kind of tolerance do I need on the rod for such a press fit (remember... it's 304 stainless)?


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## Larry Green (Jan 2, 2008)

Dwight, I don't think that is a good idea. With the rough handling and repeated expansion/contraction that grates experience, a press fit probably will come loose. If you cannot have the rod ends welded, consider countersinking the end bars 1/16" or so on one side; then, leaving the rods a bit long, peen them over to make the joint. Some grates have been made with threaded rod and nuts, the rods peened to keep everything tight. 

Larry


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## steveciambrone (Jan 2, 2008)

Dwight, 
I had bought a set of over and under reams a long time ago and never used them. Until recently I wanted to press bearings in and a friend recommended i use them to ream the hole then press the bearing in. For the .25 or 1/4 inch bearing I used the "Under" ream at .249, it worked perfectly, the materials were bronze and steel. You could always try a test sample before commiting to the whole job. 

Another alternitive could be drilling and pining the bars in place with spring pins, available in stainless also. 

Steve


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## Dwight Ennis (Jan 2, 2008)

Thanks for the advice gents. I'm curious... it would seem to me that if temperature fluctuations are enough to loosen a press fit, would not the same happen to peened ends? I realize that the peening would prevent it from coming apart, but wouldn't the assembly loosen up? 

Sounds like maybe I might need to buy a welder and learn to weld stainless. Probably a good tool to have and a good skill to learn anyway. Would I use a wire welder? Any recommendations?


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## Gary Armitstead (Jan 2, 2008)

Dwight,

We put inconel grates in our engines. Had the slots EDM'd in solid inconel, on draft tapers so the coal wouldn't "stick" and believe me these grates just do not wear out or warp! Let me talk to my buddy down here who has them and I can get a "price". All I need is the finished grate size. Ours are about .375 to .437 thick-I forgot the size. If you are interested, just e-mail me and I can talk to the guy and get an answer this weekend. Quite a few of the guys at LALS have them.


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## RimfireJim (Mar 25, 2009)

Fluctuations in temperature won't cause the diametral fit of the press-fit to come loose, both pieces being made of the same material, but if there are longitudinal forces generated by expansion at high temperatures, the joint could "walk" loose (assuming no counteractive force when it cools). That should be a one-time thing, though, since the force would come from the expansion, which should not be any greater the 2nd time than it was the 1st for a given temp.

Re. tolerances involved in a press-fit: Factors involved are 1) material (elasticity), 2) nominal size of hole, 3) retention force or torque required 4) compatibility of the two materials. Too tight and they won't go together or you'll break one of the pieces; too loose and it won't stay tight. OK, that's pretty obvious. What isn't as obvious is that, for all other factors being equal, as the size of the hole gets smaller, the tolerances get tighter. Sort of a percentage thing. Down in the 3/16" & 5/32" sizes ( I say "down" because in non-model engineering, that's at the lower end of the range typically encountered), tolerances are dang tight. For a fit that can withstand some force (not just a locational fit), you're looking at keeping the diametral interference in the range of 0.0002"-0.0010" (2-10 "tenths"). Definitely requires a reamer for the hole, then shoot for 5 tenths over the reamer size when you turn the ends of the rod.


Are you going to turn down the rod ends just to control diameter? If they are precise enough as is, just buy a reamer 5 tenths under and skip the turning step. I don't think you need a shoulder for the the outer bars to go up against - if the press-fit is good, the shoulder doesn't serve any purpose after initial assembly.

Use a little oil or grease when pressing together to help prevent galling, making it go together more easily.


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## Les (Feb 11, 2008)

Dwight,

As has already been mentioned, 0.001" is the usual 'undersize' for the hole. You chill the insert in the freezer and gently heat the hole in the oven to 100 deg or so, everything _should_ slide right together, but you can't dally around.

Peening would work to hold everything together, provided it was built so the peen acted as a sort of permanent nut. And that would depend on how malleable the stainless you're using is.

The only welding on stainless I ever did was with an arc welder and stainless rod. That's been a long time ago.

There's a fairly sharp learning curve with stick welding, and it helps if you stay in practice, so I'd think about that before I laid down a pile of money for a welder. If you're a tool nut like me, however, go for it.


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## RimfireJim (Mar 25, 2009)

Posted By Dwight Ennis on 07/02/2009 1:28 PM
Thanks for the advice gents. I'm curious... it would seem to me that if temperature fluctuations are enough to loosen a press fit, would not the same happen to peened ends? I realize that the peening would prevent it from coming apart, but wouldn't the assembly loosen up? 

Sounds like maybe I might need to buy a welder and learn to weld stainless. Probably a good tool to have and a good skill to learn anyway. Would I use a wire welder? Any recommendations?

Stainless can be brazed or welded. Welded can be stick, TIG, or MIG. You already have an oxy-acetylene torch - look into getting the correct brazing rod you need for 304 stainless.


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## RimfireJim (Mar 25, 2009)

Posted By Les on 07/02/2009 4:03 PM
Dwight,

As has already been mentioned, 0.001" is the usual 'undersize' for the hole. You chill the insert in the freezer and gently heat the hole in the oven to 100 deg or so, everything _should_ slide right together, but you can't dally around.



Those temperature differences aren't going to do diddly on a 5/32" diameter fit. 100°F delta T will only cause 0.0002" delta D. It's help in the right direction, I'll admit. If you're going to bother heating it, heat it up to something like 500°F (oven) or even higher (torch) - it's not going to hurt the 304.


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## SalM (Jan 2, 2008)

Heres a chart on loose and tight fit tolerances......I would drill a clearance hole to assemble the grate and then braze it .......

http://www.stanford.edu/~jwodin/holes.html


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## Kovacjr (Jan 2, 2008)

I would just thread the ends of the rods. Seem to hold up just fine around here.


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## Dwight Ennis (Jan 2, 2008)

Thanks for all the advice guys. A guy at work is willing to wire weld the joints.  I still might also do the press fit just to get the experience (in addition to welding). Thanks again!


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## Les (Feb 11, 2008)

Posted By RimfireJim on 07/02/2009 4:14 PM
Posted By Les on 07/02/2009 4:03 PM
Dwight,

As has already been mentioned, 0.001" is the usual 'undersize' for the hole. You chill the insert in the freezer and gently heat the hole in the oven to 100 deg or so, everything _should_ slide right together, but you can't dally around.



Those temperature differences aren't going to do diddly on a 5/32" diameter fit. 100°F delta T will only cause 0.0002" delta D. It's help in the right direction, I'll admit. If you're going to bother heating it, heat it up to something like 500°F (oven) or even higher (torch) - it's not going to hurt the 304.








While the temp deltas might/might not do diddly, there's a lot of other factors that have to be guessed at, given the lack of some important info. First, the tolerances the machine that Dwight's using, the skill in using it, skill in taking the measurements--not to mention the quality and resolution of the instruments used, the nature of the assy itself, jump to mind. I picked those numbers as much to be conservative as to be the last word. It's unclear, as stated (I have no idea what the assy looks like)--is it part of a prefabbed assy, like a cab, with soldered joints, and if so, what temp solder was used? But with simple guidelines, Dwight can investigate the topic to his heart's content. In the meantime, he won't scorch his fingertips.

When tolerance in the tenths & hundredth thousandths of an inch are under discussion, the ball game moves to a whole new level. At dims of 0.15625" attained on lathes that would do well holding a thousanth, the whole discussion is pretty much moot. I've read many posts to the effect that sub- thou dimensions can be attained and have no doubt these are true and sincere, but they're not repeatable easily. 

My intention was to point Dwight at a concept worth considering, not to define the work and practice to the last nubbin'. To publish tolerance that are so very tight is commendable, informative, but probably not attainable by very many without tools outside a certain price range. That too needs to be pointed out.

Lest I come off negative, I want to point out the quality of the pictures you've posted on tooling jigs. I think they are of great value to the average lathe owner on this board. Since setup tooling (jig/fixtures) are so basic to success, those pictures are very welcome. As I know you're aware, making a jig is often the most tedious part of a job--because if it isn't right, neither will be the product.


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## Larry Green (Jan 2, 2008)

I am a bit late getting back here. Perhaps the words "press fit" and "grates" don't belong in the same sentence. Grates will warp, stick, get dropped on the ground, be hammered back into shape and lead a generally rough life. Take Gary up on his high-tech offer, if you can, and they will outlast the rest of the locomotive, and probably you, too. If you choose to make your own, make sure they are a very free fit on the supports when cold, and just put them together any which way works for you. 

Larry


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## Old Boy (Feb 9, 2009)

Larry Green has muttered words to this effect "Grates and press fits don't belong in the same sentence." and he is absolutely correct, yet this press fit thread, relative to grates anyway, slogs on. The information on press fits is excellent, save it . . . but not for grates. Use any of the alternative mentioned, threading, welding, silver soldering, peening, but forget about press fitting. If the alternative you choose doesn't work out all that well (ALL of them will work to some extent) then try another one the next time you have to make grates . . . and you will have to make more grates during the life of your loco, they burn through surprisingly quickly, even stainless. 

Something everyone who uses coal and anticipates making grates might want to look into . . . wedge-wire. Look it up and see if you can see what the possibilities might be.


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