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slidway lapping

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Philip Burley16/10/2019 08:56:12
136 forum posts
3 photos

I have just been reading in an old copy of ME about how to lap in a saddle that is tight at tailstock end due to years of work at the headstock end using something called Timesaver compound on the bed edge under the gib strips . Has anyone tried it ? Might end up being sloppy fit every where !!

regards

Hopper16/10/2019 09:05:10
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3744 forum posts
76 photos

IT's not a recommended practice because you have no control over where and how much metal is removed. And equal parts will be removed from both saddle and bed. Various self-appointed YouTube gurus show how to wreck your brand new Chinese minilathe using the same technique. I would not go there.

But if one were to go there, Timesaver abrasive compound would be the one to use. It supposedly does not embed in the parent metal and keep on grinding away forever more. Rather, it breaks down in use to a finer powder that eventually is not abrasive.

If the lathe in question is Myford you are probably far better off to make the Wide Guide Conversion (see a soon upcoming MEW for how to do it.).

One a new minilathe etc, if saddle travel is a bit rough etc, probably better off to smooth the bed ways with a few judicious strokes of a dead smooth file or a fine rubbing stone, just enough to take off burrs and machining roughness.

Pete Rimmer16/10/2019 10:53:21
446 forum posts
18 photos

Don't do it.

Slideways need anything OTHER than a lapped surface. Just as you can wring two gauges together by the exclusion of any matter (including air) between them so can you wring two sliding surfaces by the same method and that includes the necessary lubricating oil. You end up with a high-stiction poorly lubricated movement joint that is prone to very high rates of wear.

Pete

IanT16/10/2019 13:56:21
1345 forum posts
137 photos

I agree with other posters here Philip - I think in the case of slide-ways a better approach is to measure the bed and matching surface(s) and see where correction is needed, most likely by scraping in. With lapping, I suspect it might be possible to actually make matters worse - as any 'twist' in the parts as you lap will also tend to lap already worn areas. One of those things where a 'shortcut' might work but might also make things worse - so best to do it the 'hard' way and scrape the parts together - or modify the machine and reference an unworn surface.

However, lapping is certainly useful sometimes. Recently I've been making an adaptor to fit my Taig milling head to my MF (horizontal) mill overarm. I managed to find a suitable lump of mild steel and bored it to 1.5" (the overarm diameter) - a 2" deep hole. I was pretty happy with the bore dimension (as measured) but the overarm when offered up (had to remove the tailstock) would barely enter. I'd already taken several spring passes, so managed to avoid the temptation to take another cutting pass. I wanted a really nice tight sliding fit. Part of the problem was that the overarm is old and slightly corroded - although it felt reasonably smooth to the touch.

MF Overarm

I removed the work from the 4-Jaw and managed to 'work' the part onto the overarm - but I then had to use a puller to get it off again. I had some fine valve grinding paste (purchased from Halfords many years ago) and wiped this over the overarm and then worked the adaptor for a few minutes until it was a good sliding fit. I then cleaned both parts with Paraffin.

Taig adaptor plate

Here is the overarm shaft after the lapping process and you can see the change in the surface - although very little metal has been removed.

MF Overarm

I just need to cut keyways in both parts now and hopefully I'll then be able to use the Taig (ER16) head on my MF.

So lapping can be very useful in some circumstances, especially for close fits in round components. Frankly, I can't always get parts to the exact sizes others here seem to manage but lapping can be a great help sometimes. Incidentally, these parts will not be 'rotating' - I have a cast-iron bearing & shaft to machine shortly and for that work I will be using green Timesaver, as I don't want problems with any embedded grinding paste.

Sorry if this was too far off topic - but lapping can be very useful in some circumstance (but not others)

Regards,

IanT

Philip Burley16/10/2019 14:41:33
136 forum posts
3 photos

hello , the answers are just about what I thought . My old S7 saddle is ok where most of my work is done but tight at the tail end ., But I never need to go there anyway !!!!

regards

Hopper17/10/2019 11:21:13
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3744 forum posts
76 photos
Posted by Philip Burley on 16/10/2019 14:41:33:

hello , the answers are just about what I thought . My old S7 saddle is ok where most of my work is done but tight at the tail end ., But I never need to go there anyway !!!!

regards

If your old Super 7 is a pre-1972 model you are in luck. You can give it a new lease of life by performing the relatively simple "Wide Guide Conversion". This consists of adding a strip of thin gauge plate to the rear of the saddle to bear on the unworn vertical surface of the rear bed shear instead of the worn out front shear. I believe Model Engineers Workshop will have a how-to article on this in the near future.

If you get hold of last month's MEW mag, it has the preliminary article on how to measure up your Myford bed to determine the exact amount of wear and how much is tolerable, per factory spec, before a bed regrind is absolutely necessary. Counter-intuitively most of the wear is usually not on the horizontal bed way surfaces but on the one vertical surface that takes the cutting loads. The wide guide conversion shifts this load on to the unused, unworn other vertical surface. The horizontal surface is less critical to the lathe's performance and is often less worn and is within tolerance for further use.

Much better than attacking the old girl with grinding paste.

Philip Burley17/10/2019 12:33:40
136 forum posts
3 photos

Hello , I have taken out a sub on MEW , so will be able to read " how to do it " , But I wonder if any kind person would send me a copy of the measuring article in last months mag please

regards

KWIL17/10/2019 14:33:12
3127 forum posts
57 photos

You do not really need the detail.

Measure the tailstock end of the bed as accurately as you can. Overall front to back, each of the slideway widths and depths. You now have all the dimensions you need for the original factory dimensions. The rear vertical face will not have been worn at all (narrow guide carriage). Wear on the carriage will be predominately on the short narrow guide and it will probably be tapered from side to side.You can check this by reference to the other vertical faces under the carriage.

Alan Vos17/10/2019 19:33:10
136 forum posts
7 photos

I am going to present an alternate view. I have a mini lathe. Around 15 years old. I read that newer models are better.

There was no good setting of the compound slide gib. The compound either flopped about, or bound at high spots. Lappinng with PEEK polish made it usable. Compared to a proper scraping job, the geometry may not be quite right and lubrication will be compromised. However, it is now useful rather than decorative.

DiogenesII17/10/2019 20:27:25
13 forum posts

Philip,

If you have a subscription, I think you should be able to access the digital archive of all MEW back issues - click the "Digital" heading in the black bar at the top of this page to find out how to do it.

regards

D

Hopper18/10/2019 12:15:12
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3744 forum posts
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Or Magzter might sell you one digital issue. Hardcopies usually for sale on eBay etc. Copying articles on the forum is a bit un-PC re copyright breaches, particularly on recent issues.

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