Spindle/bearings fit query for X2 type mill

[fingerscrossedParticipant @fingerscrossed]

The old bearings and spindle are out of the housing of my X2 type mill. I’ve now decided to fit angular contact bearings (7206).

I have read that (with angular contact bearings) the top bearing should be a relatively loose fit on the spindle, and the bottom bearing should be a tight fit. This is apparently to be able to apply preload.

I have also read that both bearings should be a slide fit on the spindle, and an example of someone putting the spindle on their lathe and (gently) sanding it to achieve this.

So this is a bit confusing. Quantifying what is a ‘slide’ or ‘tight’ fit is part of the problem. The old bearings obviously slid off (albeit not all that easily).

The spindle itself doesn’t look like a particularly fine example of precision machining, and seems to be the same diameter all the way down. I’m not sure why this is when just the portions where the bearings locate need to be the 30mm diameter? I’ll be installing the bearings into the housing first, so will then have to draw the spindle through both bearings. Anything that makes this easier would be great (sanding the ‘unused’ portion, freezing the spindle?).

I don’t have a press, I’ll be using the threaded rod method to gently pull things.

Any advice would be most welcome !

[David George 1Participant @davidgeorge1]

Have a look on here at SKF web site.  Selecting fits | SKF

 

https://www.skf.com/uk/products/rolling-bearings/principles-of-rolling-bearing-selection/bearing-selection-process/bearing-interfaces/selecting-fit

David

[fingerscrossedParticipant @fingerscrossed]

Many thanks David, useful stuff. (Your link is missing an ‘s’ from the end – ‘fits’ rather than ‘fit’ – but I got there!)

The SKF site is full of great info, although I suspect few mere mortals have the resources to analyse their situation in the same way – the effect of temperature differences on bearing rings for instance.

Having slept on it, I think that (using angular contact bearings) the spindle must be able to slide relatively easily through the top bearing. This so that preload can be applied by tightening the top nut down – this would push the top inner race down, and draw the spindle up, in turn pushing the bottom inner race up (in both cases compressing against the outer race).

So I think the lower bearing can be a tight fit on the spindle (but doesn’t have to be), whereas the top bearing should be an easier fit (but of course with no ‘slop’!).

Am I correct in thinking that too tight a fit on top bearing could make reversal (easing) of any applied preload impossible?

 

 

[howardbParticipant @howardb]

“Am I correct in thinking that too tight a fit on top bearing could make reversal (easing) of any applied preload impossible?”

Not a problem, if the preload you generate by tightening the top nut is excessive, just slack the top nut a tad and give the top of the spindle a tap with a copper hammer.

[fingerscrossed]

On 11 April 2026 at 19:50 fingerscrossed Said:

[…]

So this is a bit confusing. Quantifying what is a ‘slide’ or ‘tight’ fit is part of the problem. The old bearings obviously slid off (albeit not all that easily).

[…]

Whilst I would always be happy to defer to David’s wisdom on such matters … I will just elaborate on that ‘sliding’ point ^^^

I recall having a similar situation a few years ago, with a Record wood-turning lathe … and I spent a satisfying afternoon in the sunshine, ‘fitting’ the poorly finished spindle by hand.

Having removed the bearings from the spindle, there were clearly visible witness marks, which I dressed-down using a fine Eze-Lap diamond hone

… Rinse and repeat until you have a genuine sliding fit: You will recognise one when you feel it !

MichaelG.

.

https://www.mscdirect.co.uk/eze-lap-34-x-6in-600-grit-diamond-hand-lap-eze-14100b

Other sources are available

[Graham MeekParticipant @grahammeek88282]

I would not advise drawing the spindle through the bearings, especially at the spindle nose end. There is a risk of Brinelling the bearing doing this. Also pulling the spindle through the top bearing could make the inner track “pop-out” of the outer track, or probably just pull this bearing out of the housing.

Having an angular contact bearing “tight” at the spindle nose end could make disassembly in the future very difficult as any pressure applied to the outer track will only make the outer track come off. Leaving the inner track tightly up against the shoulder and very little options to get this off.

It is far better if all the “fits” are “sliding fits”. This will not affect the spindle accuracy.

Regards

Gray,

[fingerscrossedParticipant @fingerscrossed]

Thanks everyone for your replies. It is good to have ‘sliding fits’ confirmed as preferential.

I’ve been using Neil Wyatt’s ‘Improvements to an X2 Mill (2)’ pdf doc as a basis for the operation, very useful. This forum is a mine of information – my X2 type already has belt drive but I will change the belt type and turn new pulleys, and will be using your PJ Poly-V drawing Graham.

I already have a couple of sets of (cheap) diamond hones (a marketing man once said to me ‘Don’t say cheap, say cost effective‘). I only have a small lathe (a Unimat 3, which is great fun) and this is – quite literally – at it’s limit. As with all things U3 I will take it slowly. Bearing renewal is already on the cards for the lathe so don’t worry about that.

As has been pointed out, the races in the angular contact bearings are prone to separate, so wouldn’t really be suitable to test out the ongoing fit. I don’t trust the sizing of the chinese bearings, so have bought an additional quality deep groove bearing to use as a gauge.

(Note – although this is an R8 spindle it is 30mm in both bearing positions. The central keyway doesn’t serve any purpose).

[Howard LewisParticipant @howardlewis46836]

If you have to pull the bearings along the spindle, make sure that the plate against the bearing bears on the inner race (This is where the resistance to bearing movement will be.  In this way you do not load the balls, and risk brinelling the races.

When you come to fit the spindle and bearings into the head, the load should be applied to the outer race, again to avoid putting pressure on the balls

The preload is only going to be of the order of 0.002″ (0.05mm) or so, best applied whilst rotating the spindle, to allow everything to settle into place.

Howard

[southernchapParticipant @southernchap]

Such a shame that Arc Euro Trade is no longer going.  They had good PDF guides to disassembly and bearing changes on the SEIG machines (both lathes and mills).

Sadly with their website gone, their guides are no longer available.

[JasonBModerator @jasonb]

The article on the mill that was on the ARC site is the one the OP is using.

They can all still be found on the net

https://web.archive.org/web/20220308232853/https://www.arceurotrade.co.uk/Catalogue/Projects-Articles

The central keyway was used to drive the two speed plastic gear but is not needed on the later brushless motor machines that drive the spindle directly.

[HOWARDTParticipant @howardt]

When fitting any bearing only apply force to the ring you are fitting, press on the inner race to fit on the spindle and the outer race to fit in the housing.  As I am sure you can see in the SKF design requirements, a thing I used a lot in my work days, a sliding fit is not that loose on the shaft.  Too loose a fit will result in fretting on the shaft.  The ideal assembly would use a pair of angular contacts and a roller or ball bearing, with the front pair being mounted back to back.  I don’t think you gain anything with angular contacts at the speeds available on the machine, but we all like to play.

[John MCParticipant @johnmc39344]

Why not fit tapered roller bearings rather than angular contact ball bearings?  The advantage would be in the far greater load capacity of the tapered rollers, more than double.  Speed my be compromised if you plan to run over 9-10k RPM!

There is a tapered roller the same size in diameters, but slightly longer, 1.25mm, could that be accommodated?

One problem might be lubricant sealing, a compact “Nilos” type seal would sort that out.

As for fits; the fits needed for the job will be an interference for the lower or non-adjustable bearing, both inner and outer race.  The upper or adjustable race needs to be a transition fit on the spindle, there can be no clearance here.  Its outer race an interference fit.  The adjuster will overcome the fit easily.  “Sliding” fits are no good here, even with a suitable pre-load the sliding fit, which will have a clearance, this may well allow fretting and be a source of vibration and inaccuracy.  If the trouble is taken to fit bearings that can and should run, in this application, with pre-load, why spoil the job with the incorrect fits?

Unless there is a catastrophic failure, these bearing are unlikely to need removing, ever.  If they do, then grinding the race to split them is easy enough, if a little time consuming.

A properly designed adjuster would help, a split threaded ring with a screw to squeeze onto the spindle thread rather than, from what I can see, is a grub screw bodging into the spindle thread.

As has been stated, the loads being applied to assemble the bearings should be applied to the element of the bearing being assembled, not something that seems to be done with the ARC article.  Will that cause a “brinelling” type failure?  Extremely unlike but best not take the chance.

 

 

[Graham MeekParticipant @grahammeek88282]

Given the loadings these machines are likely to experience Taper roller bearings are in my opinion an overkill. There seems to be an obsession with replacing the manufacturers bearings with something “Superior”.

The Emco FB 2 uses a Needle roller bearing for the bearing nearest the cutter. Deckel also used this type of bearing on their Die Sinking mills.

There were many in these pages who criticised the Emco arrangement and reckoned Taper or Angular Contact bearings would be “Superior”. Just out of curiosity I made a new spindle and quill assembly for one of my FB 2 Mills. Using a Taper Roller bearing as the Main spindle bearing and an Angular Contact bearing to take out the play.

I can report there was no significant improvement in the spindle during usage. The pre-load had to be carefully monitored or the Motor could not start on top RPM. Due to the over-gearing and any excessive preload. It would seem the Emco arrangement could not be bettered.

Regards

Gray,

[JasonBModerator @jasonb]

I seem to remember Ketan on sevaral occasions warning of the problems of overloading the motor on startup due to the preload but I’m sure he did well selling replacement motors and boards.

The article was not by ARC, just hosted there.

[fingerscrossedParticipant @fingerscrossed]

Yes, I’ve been studying (at some length!) this thread – https://www.model-engineer.co.uk/forums/topic/pre-load-of-new-bearings/ – where (amongst other’s posts, on page 2) he says to ‘hand turn chuck and feel the resistance’, and emphasises doing that after every slight adjustment of the preload. The spindle on my X2 clone was very stiff (it did run, and the motor turns freely), which made me decide to strip it down and replace the bearings.

It looks like Neil was supporting the inner race of the top bearing when the spindle passed through it, that is the direction that could come apart. The lower bearing can’t come apart when the spindle passes through it but I guess it should be supported somehow if there is a risk of the balls marking the race.

Again it is all down to the force required, and the ‘fit’. There do seem to be differing opinions here, it would be useful to hear from someone who has done this (and is happy with the result!), and what ‘fit’ they used.

[HOWARDTParticipant @howardt]

As a first step having stripped the assembly down you need to measure both the housing bores and the spindle diameters, as well as circularity and alignment. All measurements taken to 0.001mm accuracy. Them you can decide wether all the change is worth it.  As most of us don’t have access to the sort of measuring kit required to asses the the existing assembly, is it worth the worry of doing it different.

[JasonB]

On JasonB Said:

I seem to remember Ketan on sevaral occasions warning of the problems of overloading the motor on startup due to the preload but I’m sure he did well selling replacement motors and boards.

The article was not by ARC, just hosted there.

I’m wondering just how much preload it would take to do that?  The answer is, too much.  Without experience of such things, its a tricky thing to do, hence significant discussion on the subject of preloading bearings, or not, in some applications.

The OP wants to replace and improve his mill by using angular contact ball bearings, a good idea.  I’ve suggested a further improvement would be tapered rollers.  May require a little more work but the benefits will be there.  To suggest that using tapered rollers is “overkill” is not good advice.  These lightweight, low cost “hobby” type machines need all the help they can get in improving rigidity and stiffness.  Doing the best job on what ever part of the machine is being worked on is always worth doing.

Cost, at one time, could be a reason to not use tapered rollers, not so now.  The pair of tapered rollers I used on my latest mill cost less than the Nilos seal that keeps the oil in the bearings!

Quoting Deckel’s use of needle rollers on their diesinkers and engravers is misleading.  It was done for specific reasons, probably speed capacity and compactness.  No matter how good the needle roller bearing is there will still be clearance in the bearing.  Set up properly, that clearance can be taken out with angular contact and tapered roller bearings with great benefit.

 

 

 

 

[IdlerParticipant @idle1]

Using bearings with two rubber seals can create excessive load on start up too. That’s why many machine tools use steel shielded bearings instead. No seal drag due to the gap between steel shield and race

[JasonBModerator @jasonb]

Its a balancing act, yes rigidity and stiffnes is good to get but is it worth it at the cost of a cooked motor/board?

[Graham MeekParticipant @grahammeek88282]

There is clearly a misconception concerning needle roller bearings. The Emco and the Deckel designs are extremely difficult to assemble due to the very tight limits. In fact the Emco FB 2 has 3 different spindle sizes to achieve the optimum setting.

Also these bearings are running in oil, in the case of the Emco FB2 Hydraulic oil. The needle roller bearing when it becomes dynamic acts as a pump. Any play that there may have been disappears. Mr JCB built an earthmoving empire around the principle of incompressible liquids.

Upgrade the original bearing with a better quality item by all means. There is nothing to be gained by using a different type of bearing. The loading which most hobby machines impose on the bearings is nowhere near their maximum capacity. Run-out accuracy of a standard radial ball bearing will far exceed that of the Collet, or Collet Holder.

Regards

Gray,

 

[John MCParticipant @johnmc39344]

So, Emco and Deckel used selective assembly to achieve the required fit?  So do many other manufactures, nothing unusual or special there but to be avoided or reduced if at all possible.

The argument that the oil removes any (extremely small) clearance in the bearing has some merit.  But, what happens when the bearing and lubricating fluid warm up?

I do not understand the resistance to doing the best possible job.  In this instance, angular contact vs tapered rollers, if a large amount of work is required to fit the best, over the second best arrangement, then I can see why.  But not here.  Cost might influence the work, again, not here.

Moving on to motor problems due to excess friction of seals and or excess preload, anyone care to elaborate on that?

If friction is a problem, then ball bearings have the lowest friction, then tapered rollers, then angular contact.  Stay with ball bearings!

An aside regarding preload.  Steel shafts supported on angular contact or tapered rollers in aluminium housings need considerable preload to cope with differences in thermal expansion.  Thankfully, because aluminium has no place in machine tools, other than belt guards and the like, not a problem for us!

 

[JasonBModerator @jasonb]

Not unless you have a Sherline, Taig, etc with aluminium headstock, etc.

Have a look at the thread the OP linked to earlier, Ketan talks about possible damage to boards.

[Graham MeekParticipant @grahammeek88282]

The point I was trying to make earlier, but the point seems to have been lost on some. Was concerning the use of a Taper Roller bearing over the original Emco/Deckel Needle Roller design, and that there was “no significant improvement“. This was despite the voices on the Forum saying this bearing would be superior, (this sounds familiar).

There was a significant down side to using a Taper Roller bearing in the Emco set-up and that was the additional loading on the spindle to take out the “play” with this bearing set-up. Making the pre-loading of the assembly super critical and temperature dependent. Something which Jason and Ketan have already pointed out, and adds to the argument to “leave well enough alone”.

Thus if there is no real advantage to fitting Taper Roller bearings why go to all the trouble? This is generally not a straight forward substitution. Making machining of the housing and, or the spindle necessary to accommodate the increased overall length of the bearing. They need additional protection to keep the swarf out and the lubrication in. These bearings are not always available with built in seals.

As far as I can make out from the OP the existing bearings have not failed. It is merely a desire to “improve” the bearings.

What therefore is an improvement? If it does not offer any significant improvement. One might as well stay with the original design.

Or is it just a case of the bearings are “better”, so there must be an improvement?

Regards

Gray,

 

 

 

[Graham MeekParticipant @grahammeek88282]

There seemed to be some doubt, possibly even sarcasm, as to whether Emco used selective assembly on the FB 2 Quill assembly. The enclosed works drawing will dispel any doubts I think.

Please note the tolerances on the “Spindel” diameter X, (+ & – 0.000*. mm, 0.00000* inches)

I cannot say whether Deckel used the same system or not. I only mentioned them originally as using the same bearing arrangement. My comment about the tight limits was based on having dismantled a Deckel Milling Spindle and reassembling it afterwards. It is not for the faint hearted.

Regards

Gray,

[John MCParticipant @johnmc39344]

Jason, I forgot about the Sherline and Taig lathes, I would have thought that they would have been priced out of the market by now by the availability of small machine tools of reasonable quality from the far east.  Still doesn’t make me think aluminium is a suitable material for the structure of any machine tool.  A discussion for another time.

I did read the thread regarding pre load the OP linked to earlier.  I was delighted to see a couple of posters actually seemed to understand the correct mounting of bearings.  (An unusual quality rarely reflected in a lot of ME designs).

I note that Ketan from ARC says ” Over the years I have seen a small amount of bearing/motor/control board failures resulting from ‘overdoing’ things.”  I can see excessive preload could be the problem but not lubricant sealing.   I think the idea that “rubber” oil seals can cause an overload of the electrics is a red herring.

looking at the Emco drawing, my first thought was “why use two bearings when three will do?”.   (For the avoidance of doubt, I am being sarcastic).

I can see plenty of scope for improvement, its not a good example of quill design.  Again, a discussion for another time.

One thing about the tolerancing on that drawing that, to me, stands out like a sore thumb.  The geometrical tolerancing, 0.01mm (~0.0004″), that’s quite poor.   Also, what taper is that?

 

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