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endmill holder trouble

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fast204

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#1
I recently purchased a sherline 12" mini mill to go along with my atlas 12"x36" lathe. It only came with a mt1 drill chuck so I figured I would make an endmill holder to hold 3/8" shanked endmills. Not a big deal I figured? I have tried everything I can think of but all 5 of the ones I have turned (in materials from aluminum to mild steel) have been a no go. I am turning the ID, shank holder, OD, and single pointing the threads without removing the piece from the lathe. The final operation tonight was to take the part with finished threads, mount it to the mill, rotate the head of the mill 90 deg and bring the shank holder section to .370" with a boring bar mounted to the mill bed. I then removed the part and hand reamed the final ID.

The OD when mounted to the mill will spin within .001" or under every time. The problem is that the endmill is spinning within .001" at the base of the shank but appears to be crooked at the end of the cutter flutes??? This is verified when taking a 5/16" endmill and making a pass, then measuring the width of the slot. it is usually .325" or slightly more.

I am baffled by this and could use a 2nd set of eyes or ideas on what I am doing wrong when making this part. It is visually crokked when spinning by hand. The OD of the part is dead nuts from one end to the other.
 

ddickey

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#2
Your Sherline mill has an MT1 taper?
What are you single point threading?
Grub screw possibly shifting the end mill.
 

mikey

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#3
Not sure what you mean by "shank holder". Is this the bore that holds the end mill?

Have you also machined the shoulder that is required to fit the Sherline spindle register? You didn't mention this but it has to be there because that is what determines the accuracy of the end mill holder. Not only does it need to be there but it also has to be accurately machined to a very close fit.

There are several options open to you:
  • Try again, this time paying close attention to the register. I would bore the hole for the 3/4-16 threads and turn the area for the register carefully. I would then carefully bore the hole for the end mill down the center and screw cut the threads. Then turn the OD to suit your tastes. I would then flip it over, do whatever contouring you want to the nose and then part it off. Then drill for the set screw that will hold the end mill and tap it. I would not try to bore the center end mill shank hole by turning the headstock of the mill; not accurate enough.
  • Buy a ready made Sherline end mill holder. Cheap, accurate, less work: https://www.sherline.com/product/3079-end-mill-holders/
  • Best option: buy a Beall ER-32 collet chuck and a decent set of collets. This is a very good chuck that will definitely fit the Sherline spindle. Just tell them its for a Sherline and they will send the right one. All standard ER-32 collets will fit this chuck. Run out on mine is somewhere around 0.0001-0.0002" TIR. ER chucks will hold an end mill with much greater accuracy than an end mill holder will, it dampens vibration better than just about any tool holding system that fits a Sherline mill and you can hold any end mill or cutter with shanks up to 3/4". I don't go that big on this mill but you could. Finally, unlike a Sherline end mill holder that is limited to 3/8" double end mills, the ER chuck will allow you to use any double ended end mill that you can fit in a collet with no fitment issues.
The one downside to the Beall chuck is that it chews up space in Z. For that reason, I chose to use the 15" column extension and the riser block to raise the column up. Adds some much needed space and
it works fine.

I've run a Sherline 5400 mill for over 25 years now and with the column extension and riser, it is a fine precision light duty mill. I measured my spindle concentricity when I got it and again multiple times and it was and is still 0.0001" TIR. There is no change in run out with the Beall chuck and Techniks collets so that is the way I chose to go. I've run a 3/8" roughing end mill in a slot 3/8" deep in aluminum many times with no problems with this chuck. A woodruff keyseat cutter held in a Techniks ER collet will cut a precise interference fit in 1144 steel as it should; more than anything else, this tells me how good this chuck is.

Sorry, I know I'm not addressing your machining concerns but I've been down this road before. If I had to do it all over again, I would not even bother with an end mill holder on this machine. I would go straight to the Beall ER-32 chuck. Just an opinion, of course, but one based on long term use.
 

higgite

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#4
I don’t have a Sherline, but will expose my ignorance anyway. If I’m picturing this right, the spindle axis and the boring bar are both parallel to the table for the boring operation, at least in theory. Do you tram them both in to verify that? It kinda sounds like you’re boring an ever so slightly “tilted” hole in the end mill holder.

Tom

Edit: I just re-read Mikey’s post and saw that he hinted at the same thing. I agree with him that the best way to ensure that everything is plum and concentric is to through bore it while it is still chucked up on the lathe.
 
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fast204

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#5
I'm unsure what you mean by "register". Yes, the shank bore is being referred to as the hole the endmill is mounted in. No I did not tram the mill head to 90.
The first couple attempts where out of aluminum and although I drilled and bored the smaller diameter hole for the endmill it still appeared and measured crooked. This was without using the mill at all.
Even if the headstock were to be out of square to my lathe bed wouldn't that result in a slight taper?
I do however understand how having the mill head not trammed in would cause a crooked hole.
 

JimDawson

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#6
Are you slotting with a 4 flute end mill? They tend to walk sideways, a 2 flute is a better choice. What is the actual size of the end mill? I normally start out with a 1/4 inch then finish with 5/16 for a 5/16 slot.
 

fast204

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#7
Yes. But I can see it wobble when turning it in the mill by hand.
 

mikey

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#8
I'm unsure what you mean by "register".
Look at the spindle. See that clear area behind the threads that looks like a shoulder? That is the spindle register. It is what determines the accuracy of anything that is mounted on the spindle. The threads only provide a way to lock things in place and provide no accuracy whatsoever. Your end mill holder has to have a matching recess that is a close slip fit to allow the register to align the holder to the spindle when it is locked down. This recess must be machined in the same set up that includes the bore for the end mill holder so as to minimize stacking tolerances from producing the wobbling you are seeing.
 

fast204

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#9
This might be a large chunk of what I am seeing. I have been producing this part to turn directly to the shoulder of the mill and not go over it. There is a hole that runs through the rwgister I assume is for aiding in removing tooling?
 

mikey

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#10
I am embarrassed to admit that I misspoke. I was sure that the spindle had a register shoulder but I just checked and Sherline does not use a shoulder. Instead, the accessory must register on the face of the spindle shoulder. That is, on the face of that shoulder with the holes in it. My apologies for the confusion. The same reasoning applies, though. The accessory must register accurately or run out will be excessive.
 

fast204

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#11
No worries. I was looking at it and thought it would be an awful small counter bore to improve accuracy.
 

JimDawson

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#12
Yes. But I can see it wobble when turning it in the mill by hand.

In that case, it may be that the hand reaming operation is causing a tapered hole. Is it possible to ream in the lathe after boring? Or to bore to size in the mill?
 

fast204

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#13
I have the start of another one in the lathe now. I'm going to get the threading done first then work on the 3/8" through hole. I purchased a reamer to go in my tailstock and will be doing this all while still chucked in the lathe. Hopefully this one will turn out well.
 
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