[PSUBS-MAILIST] Press Fit

Fri Mar 30 19:19:58 EDT 2018

Hank, Alan, 

It is generally a combination of both.

The quality of lathes varies hugely.  Generally people doing handyman jobs buy second hand lathes and also small Chinese lathes.  The bearing tolerances and bearing configurations in the headstocks is a big issue and on a reasonable lathe can be adjusted but it is fiddly work.  Then there is the saddle  and cross slides.  The better lathes have good length saddles and the cross slides are good widths which is missing on most small Chinese lathes.  Old lathes tend to have worn slides, worn cross slide leadscrews and nuts and will have movement in them.  The effect of all that is that it has an effect on accuracy and tool pressure. i.e. if you take a heavy cut and then a light cut using the dial or even a digital readout it will not take the intended amount off but will undercut on a heavy cut and you will have to take a spring cut (2nd cut on the same setting) to get your diameter.  Your finishing cuts and trial cuts should be the same depth of cut but generally not less than 0.005” or 0 .10 mm.  You need to try and have the same tool pressure on your trial finishing cut to your actual finishing cut.  Trying to take .001” cut off generally will over cut.  You need a fresh tip for final cut otherwise a worn tip edge  will tend to push off and then to get it to cut it will then overcut.  Cutting speed, tip selection, feed are all really important.  As well as good quality CNC machines we also have a couple of old dunga lathes, one Chinese small and one Colchester 2000. Both I bought just to do the odd thing and ended up trying to do more.  Both challenging.  I often curse myself for getting them.

Hank, the things you achieve are pretty amazing and finishing off with emery tape, well I guess you have to do what you have to do.  How is that vertical lathe going?    

From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org] On Behalf Of hank pronk via Personal_Submersibles
Sent: Friday, 30 March 2018 10:42 PM
To: Alan via Personal_Submersibles
Subject: Re: [PSUBS-MAILIST] Press Fit

Alan, I rarely can machine to the tolerance your talking about.  I cheat and get very close then finish with a real good flat file then polish the part with progressively finer wet and dry sand paper.  I will go right down to 2000 grit some times..  I am not sure if it is me or the lathe.

Hank

On Thursday, March 29, 2018, 8:43:07 PM MDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote: 

Thanks Hugh,
I'll have a look in the shops after easter to get a handle on whats available.
On the net there seems to be quite a range in prices from about $30- to thousands.
Always enjoy visiting your premises & seeing the Q-sub and would appreciate
any advice with this precision machining. 
How is the Q-sub going?
Cheers Alan

Sent from my iPad

> On 30/03/2018, at 2:03 PM, Hugh Fulton via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> Alan,  You are really brave to try and measure that way.  Even tele gauges
> are hard unless you have a lot of practice with them but not with calipers.
> You have to use them with an outside mic to give you a fighting chance as
> you have just found out.  You should be using a bore Micrometer or at least
> inside mic for bearing fits.  I think you should pay a visit out to our
> workshop if you are planning on doing that sort of thing in future. So much
> is in the feel using instruments.  CRC do a freeze spray for that sort of
> thing as well.  As the guys say get the manufacturers recommendations.  The
> bearings are not universal as they come in different clearances such as C3
> or C4.  Hugh  
> 
> -----Original Message-----
> From: Personal_Submersibles [mailto:personal_submersibles-bounces at psubs.org]
> On Behalf Of Alan via Personal_Submersibles
> Sent: Friday, 30 March 2018 9:49 AM
> To: Personal Submersibles General Discussion
> Subject: Re: [PSUBS-MAILIST] Press Fit
> 
> Thanks for the advice guys.
> The bearing diameters are 35 & 24mm.
> I went to a lot of fuss last time I did this & ended up having to use
> loctite as the last pass on the lathe took too much off.
> Emile, I will shoot for .02-.03mm & see how that goes. Have found other
> advice saying .03. But found a lot of confusing information googling.
> I was intending to heat & freeze parts prior to pressing.
> Hugh, I have been using inside callipers & measuring  off these with the
> digital callipers.
> The digital callipers are on the cheap side so may upgrade them & as you
> say, purchase telescopic gauges the right size.
> Also on my radar is a 10 ton hydraulic hand operated press, as I am also
> press fitting the two end sections of the thruster in to a tubular middle
> section. I am figuring that with epoxy in the press fit, screws & an epoxy
> paint finish it should be water tight without o-rings.
> Cheers Alan
> 
> 
> 
> 
> Sent from my iPad
> 
>> On 30/03/2018, at 8:42 AM, Hugh Fulton via Personal_Submersibles
> <personal_submersibles at psubs.org> wrote:
>> 
>> Alan,
>> What diameter is the bearing?
>> As the guys are saying it is a real test of your machining accuracy.  
>> Do you have a micrometer and telescopic gauges the right size?  If you 
>> are relying on digital calipers you are playing with fire.  The answer 
>> to your question is yes you can push the bearing in with the tailstock 
>> but they don't have much push.  You can use a vice as well if it is in 
>> good condition.  I suspect you are talking about something that is 
>> possibly only 30-40mm.  It is very easy to over machine but you can use
> Loctite if you want to.
>> Suggest you  try a bit of aluminium bar as a test piece first.  Easy 
>> to machine it off after or push it right through.
>> Hugh
>> 
>> -----Original Message-----
>> From: Personal_Submersibles 
>> [mailto:personal_submersibles-bounces at psubs.org]
>> On Behalf Of Alan via Personal_Submersibles
>> Sent: Thursday, 29 March 2018 10:33 PM
>> To: personal_submersibles at psubs.org
>> Subject: [PSUBS-MAILIST] Press Fit
>> 
>> Hi,
>> I'm wanting to press fit a bearing in to my aluminium thruster housing 
>> & wondering if I could make a tool slightly less in diameter than my 
>> bearing, fit it in the lathes tail stock, & wind it in to force the 
>> bearing in to the press fit.
>> I have heard figures like 500lb force being used for press fitting, 
>> which I won't get winding the tail stock, & probably don't require as 
>> there is not a lot of dynamic force on the bearing.
>> I imagine there will be a lot of trial & error involved getting the 
>> bore to the right diameter, & this method will allow me to keep the 
>> work in the chuck & keep doing fine passes until I get it to the right
> diameter.
>> Any thoughts or experience with this thanks.
>> Alan
>> 
>> Sent from my iPad
>> 
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