Is heat causing runout on shrink fit tooling? 1045 vs H13

[Alan12345]

I made some heat shrink end mill holders to use on a small CNC machine. It appears that some randomly have too much runout after they are heated to insert the end mill. I'm using them to hold 1/4 and 5/16" end mills. Diameter of holder is about 0.59" with about 3/4" grip length on end mill.

I used 1045 rod. I know most shrink tools use H13 but for my application it is a one time only use tool so didn't think H13 was needed and I had 1045 laying around. Wondering if it's possible that the when the tool holder is heated (with induction heater) that it distressed the steel and it doesn't shrink back to original dimension causing the run out?

Appreciate any guidance. Thanks!

 

[3DDave]

Was the material stress relieved after rough machining and before finish machining? It's possible there were residual stresses in the raw material and the heating was enough to relieve them.

 

[Compositepro]

Temperature uniformity when heating is important, and I doubt you will get that with induction heating. Even holding the bar horizontal rather than vertical will cause top to bottom differences due to convective cooling in air.

 

[mfgenggear]

Alan

did not say at what temp & time.
if the temp is to high over the tempering temperature , a it draw the steel soft, and it will distort. it will turn into pretzel.

 

[Alan12345]

@3DDave - No there was no stress relieving processes done. Can the material be stressed relived before any machining?

@Compositepro / mfgenggear - I'm using a small coil about 2" ID. It heats the tool up pretty quickly and looks even. I have not checked the temp is is reaching when I insert the end mills. The machine is powerful enough to start melting the steel in about a minute. At "mid power" setting I typically hold the power on for about 5-6 seconds and when it just starts to get a slight red glow insert the end mill.

Sounds like maybe I should use lower power setting where it will take a little longer but doesn't exceed the tempering temp of the steel? Looked temp for 1045 and is is between 200-500C tempering temp. Hopefully enough expansion at or bellow 200C to insert the end mills...

I just made a few more with left over 303 SS to play around with.

 

[EdStainless]

5-6 sec is so short that nearly assures non-uniformity.
And why heat to 1500F, you don't need them nearly that hot.
Use lower power, or better yet put them in an oven at 450F for 30min and then put them together.
The hotter you go the more likely that you will see more distortion.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Use oven. 100°C temperature differential is more than enough to expand interference fit parts.

One trick I use, especially for large parts that you don't get second chances on, is to measure the shaft with an outside mic, open the mic and additional 0.005", set a telescoping gauge to the gap on the mic, proceed to heat the part until the gauge passes freely, and assemble.

For small stuff most guys just spit on it. If the spit ball bounces around on the surface you're well above 100°C and you're ready to mount.

 

[BrianE22]

Heat it up and then let it cool without trying to insert the end mill. Check the bore for run-out after that. If the end mill is slightly cocked and the holder is weak then maybe the hole is getting distorted during assembly.

 

[TugboatEng]

One thing to add is that if you do need more than 100°C differential to open the bore enough to clear the interference, the outer part will likely plastically deform or crack.

 

[Alan12345]

Ok, sounds like I'm using way too much heat, and too fast. I will try the recommended oven technique first and determine what is the min temperature needed to get the end mills to insert. If I still get distortion, then look into possible stress relief issues. Thanks for all the feedback!

 

[Alan12345]

Tried using oven. Didn't work even at 550F and end mills taken from freezer. The 1/4" made it in about 50% and the 5/16 barely went it. Looks like I need to experiment with the induction machine so temp is not too high but can keep the heat on during insertion of the end mill.

 

[TugboatEng]

What is the interference? This is a sign, if it takes that much heat the interference is putting your coupling in plastic deformation and you're going to get deflection. Especially if you experimented with 303 stainless as the coefficient of expansion is higher and you still couldn't make it big enough. Have a vise or arbor press ready to finish the push if necessary. It also helps to hold it together as straight fits tend to walk off the shaft during cooling.

 

[mfgenggear]

Alan

couple of suggestions, calculate the thermal expansion of the end mill holder at maximum temper, 1000 Deg For the temper as an example.
then calculate the max value for the end mill contraction. using dry ice. -100 deg F
this will be the the clearance that will be required. if the interference fit exceeds the clearance then it has to be press into the end mill holder.
as said before small diameters have very little change in size, and the process has to be very fast in assembling.
I recommend to use a laser thermal thermometer to obtain the temperature of the end mill holder. as a guide. an set the temperature of induction harden coils.
there are calculator on line (google) to calculate thermal expansion. make sure to use manufacture recommended press required.
I would do this to assemble small shafts into a small gear with a hole, it is a real pita, and it is common for the shaft to seize partially.
then has to be press in and hope not to damage the shaft.

 

[EdStainless]

Get out your machinist or mechanical engineering handbook and look up shrink or press fit.
You don't want to exceed the yield strength of the 1045, but get very close to it.
Engineering yield is 0.2% strain, so if the tool shank is 0.2500" your hole needs to be no less than 0.2495".
My guess is that you have way too much interference.
And as the the 1045 yields it is not happening evenly so you get more distortion.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed