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New (Old) mill R8 problems.

Got a second hand mill and canít get the collets in

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Jim Beagley05/07/2020 14:22:03
77 forum posts
41 photos

Hi all

I just got myself a Chester 626 mill. Second hand but in good condition.

Got some R8 collets thrown in and just had a chance to try it out.
Immediate problem, the collets are really tight to wind up into the spindle enough the the point of compressing them.

Im not familiar with collets, so what could be the problem?

I see some dings in the pin but there’s no marks being left in the keyway on the collets.
I do see some marks on the upper end of the collets after trying to wind them in but I know the tolerance is close.
Any tips?

cheers, Jim

Andrew Johnston05/07/2020 15:26:29
5553 forum posts
650 photos
Posted by Jim Beagley on 05/07/2020 14:22:03:

Any tips?

1. Get rid of the spigot part way up the spindle. It's a waste of space and import collets often have a poorly dimensioned keyways. Some of the spigots aren't wonderful either. I've just bought an import ISO40 to R8 converter and the "spigot" was hand ground on the end of a socket head grub screw. sad

2. Check the dimension of the parallel section, especially at the top. The tolerance is tight - 0.949" to 0.9495". On the aforementioned adaptor a collet with a parallel section of 0.9492" went in, one at 0.9495" wouldn't go in. I stoned the top of the parallel section until it fitted. The top section was slightly bulged; possibly due to repeated tightening of the drawbar, although it fitted my Bridgeport mill with no problem.


old mart05/07/2020 16:18:37
1825 forum posts
148 photos

When we got our first mill at the museum, a large drill mill with R8, the spindle had to come out to replace the lower taper roller bearing. Thic particular spindle had two screwed in pins and I kept them, not knowing any better. The mill still has them in the spindle. Some of the bought tooling was reluctant to fit, and I found that the cause was the milled slot was too narrow for the pin diameters. Only widening the slot slightly enabled the tooling to fit easily. When I did the Tom Senior light vertical MT2 TO R8 conversion, I did without the pins. The pins only use is to stop something like a collet from spinning when the drawbar is being tightened and after the taper is broken, but before the drawbar is finally unscrewed. Collets are the most likely to spin as there is nothing to get hold of. To minimise this problem, I have run a 7/16 UNF tap up the tooling and lubricated the threads with moly grease. If the drawbar is free running into the tooling, there is very little likelyhood of having problems with tooling.

Try all your tooling in the spindle, and check whether the tight ones have a narrower slot. Also check the parallel diameter at the top end for variations in size. Use a light and mirror to inspect the inside of the spindle.

Should you decide to remove the pin, then a Dremel would be best to take the pin down flush without dismantling the whole spindle.

Jim Beagley05/07/2020 17:23:17
77 forum posts
41 photos

This this cold be it? Seems to be two big scores up the spindle.

Chris Evans 605/07/2020 17:32:08
1699 forum posts

Plus 1 for getting rid of the pesky location pin.

Baldric05/07/2020 17:46:49
154 forum posts
11 photos

Just incase you were not aware, I believe R8 collects are designed to grip one size, therefor do not compress much, so don't expect to use a 1/2" collect to grip a 12mm tool, where as with the ER system you could do this if needed.

My mill still has the pin and I have not had a problem with any tooling, but as most of it came with the mill I guess the previous owner may have solved any issues and I may have been lucky with the couple of tools I bought.


old mart05/07/2020 19:18:59
1825 forum posts
148 photos

You might just be able to reach the scores with your middle finger. How far up do the collets go before getting tight? Normally, when the arbor is almost right in, it should be possible to rotate it a tiny bit which proves the pin is not tight in the milled keyway. Removing the key should not be top of your list of priorities, but an option when you have got used to the mill. A needle file with a safe edge could be used to make the pin narrower, and a 3/4" stick, or bar with a bit of 400 paper stuck on the end with double sided tape could smooth any burrs inside the top of the spindle. Just a few strokes after carefully aligning it would do.

Better than the collets would be an arbor with er 25 or 32 and collets in 6, 8, 10, 12 and 16mm for the commonest sizes of end mills. We have er25, because going above 16mm in end mills gets expensive, especially solid carbide and replaceable insert and shell mills are much better value. The only time that I find collets are useful is if there is a shortage of Z axis height. We have a full set of collets from 2 to 20mm and some imperial ones as well, they rarely get used.

Edited By old mart on 05/07/2020 19:21:47

Jim Beagley07/07/2020 23:16:55
77 forum posts
41 photos

Thanks for all the advice. That pin sure does cause some argument on the web

For now I’m leaving my pin in.
Ive discovered that some of the (brand new) qty 8 collets supplied with the machine fit better than others, which can only be the collets I guess. Strange.
I have remedied the spindle bore with some careful honing, and now the collets do all pull up into place without having to use a scaff bar on the drawbar (not really).

So having experienced how fragile the spindle/collet interface is, does anyone have any advice on general good practice to protect this clearly vulnerable area?

And, are machine-DRO recommended

Cheers all, Jim

Tony Pratt 108/07/2020 08:54:38
1147 forum posts
5 photos

I would leave the pin in atm ,decent tooling will not cause an issue with it & it does help to stop the collet turning whilst you are tightening it.


John Haine08/07/2020 09:11:23
3172 forum posts
171 photos

I'm a big R8 fan. When I bought the collets for my VMB I did find that some of them were tight and tracked this down to the keyways. They needed some attention with files and stones but have been fine ever since.

I bought an M-DRO 3 axis kit earlier this year for the VMB. I'm very pleased, it is transforming how I use the machine. I only used one of the supplied L-brackets and had to make a couple of mounting blocks for the sensors, but pretty easy. There's some photos in my "DRO" album.

old mart08/07/2020 12:53:39
1825 forum posts
148 photos

As I mentioned earlier, removing the pin should not be considered until you have had much more experiance with the mill. I also said that we have one mill at the museum that has two pins in the spindle bore, and I'm in ne rush to remove them. One advantage of not having pins is being able to use an ISO30 spindle wiper to clean the taper part of the spindle. I did notice the step between the drawbar hole and the upper part of the spindle bore in your mill. I took advantage of that and the space between the end of the tool shank and the step, to make a self extracting drawbar for the TS.


Jim Beagley09/07/2020 23:07:49
77 forum posts
41 photos

Hi old Mart.
thanks for taking the time to make the sketch.
i understand the parts but I’ll have to do some research to see how the self extracting system works.
I know have all my collets working satisfactorily after some very careful honing if those marks in the spindle so I’m halo with that.
I have aDRO in order so that’s the next order of business.

Can someone recommend a good speeds and feeds chart for milling novices? I’ve already broken a 2mm end mill and don’t want to waste any more time and cash doing it again.

Cheers all.

old mart10/07/2020 13:41:52
1825 forum posts
148 photos

To get the best out of a tiny cutter like 2mm, would take about four times the speed your mill is capable of. So you will have to treat them with great care, 0.5mm depth of cut and very slow feeds in steel. HSS must be lubricated in steel and aluminium and carbide in aluminium.

The Tom Senior R8 conversion has room in the spindle bore to accept a self extracting drawbar, which works well, but when I was making the spindle, I put flats on the spindle to make using the drawbar easier. An advantage is not having to strike the loosened drawbar to break the taper.

Another thing to take into consideration is that not all R8 tooling has a 7/16 UNF thread in it. I have a few with 12mm threads, and there is also a MT2 arbor for slitting saw blades which fits into a MT2-R8 adaptor which uses 10mm threads.


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