How To Get A Perfect Fit For 1/4" Mills?

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dontrinko

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I am making a tool holder to hold 1/4" mills. This is my second try. On the 1st try I used a "1/4" reamer to make the hole for the milling bit. It ended up about .002" over size. The mill fits but with enough slop to produce a little runout.
I measured the reamer and sure enough it is a little oversize. The mill bits are .250" and not under or over size.
How do I make a perfect fit for the mill bit? Thanks; Don
 
I think you just need to buy the appropriate size reamer diameter. Depending on the supplier, they will come in 0.0005" increments & typically have a +/- tolerance on them, for example +0.0002" and -0.0000". Measure your shanks & then choose appropriate fit. Also examine the shanks for wear or marring, particularly raised features from set screw impressions. This can easily alter the fit. Avoid bottoming out reamers in the pilot hole, I've heard it can enlarge the hole from spec size.

reamer dimension ~0.25" example from Travers online catalog

10-14-2016 0000.jpg
 
Or make a D Bit that is exactly the size you want. Drill the hole slightly undersize, then run the D Bit in. Google is your friend for making these bits. If you use O1 or W1 drill rod then easy to harden but would probably do fine for a hole or two without hardening.
 
kd4gij said:
What is the purpose for a 1/4" end mill holder. A collet will run more true.
Click to expand...
Why would it? A purpose made holder, made on the machine it is used on, will run truer than any collet. It all depends on how he makes it.
The purpose of the holder is to keep tools preset and ready to use. The tool length will be known, for use in CNC equipment. Much like QCTP tool holders.
 
gerritv said:
Why would it? A purpose made holder, made on the machine it is used on, will run truer than any collet. It all depends on how he makes it.
The purpose of the holder is to keep tools preset and ready to use. The tool length will be known, for use in CNC equipment. Much like QCTP tool holders.
Click to expand...
What he said.
If you do 50 tool changes per day you will see the wisdom of tool holders in an appropriate spindle, I would not trust an R8 spindle tool holder to repeat in the Z axis between changes. This is what cat and other design spindles are for, the holders may be grasped by an automatic tool changer as well
 
A standard 1/4" reamer is 0.002" over? You said the reamer was a little over - that seems like an awful lot. Now, if it is just cutting oversize - that's another matter.

Are you following good preparation? I generally drill, bore then ream. Drilling is a roughing operation, the hole won't be round, straight or in quite the right location. Bore the hold to get it at least straight and in the right location (if there are issues, make at least 2 passes). I get it that this is a small, relatively deep hole - it does not need to be bored perfectly. Just use a small end mill (say 3/16") held in the tool post, position it so one flute is the cutting edge and go in as deep as you can. Then the reamer will start well, if it starts well, it is likely to finish well.

Some materials don't cut very well. I had some 3/8" holes to ream, and was working with some sort of mystery plate, seemed sort of hard - after 3 tries making oversize holes I quit. I tried again with some regular mild steel plate, worked great on the first try.
 
The cnc shop I worked used ER collets with cat 40 And tools where set in a pre setter. A high-end Weldon tool holder has the hole off centered by half the clearance so when you tighten the set screw the tool is on center. Most hobbyist aren't setup to do that. I asked what is intend use is because I do agree that r8 collet don't repeat.
 
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