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Member postings for blowlamp

Here is a list of all the postings blowlamp has made in our forums. Click on a thread name to jump to the thread.

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Thread: Parting off on Myford lathes
21/01/2011 13:58:56
If anyone needs further evidence of the effect that chip crowding has, you only have to drill deep holes in certain materials such as aluminium to see how easily it can jam the bit.
 
Another example is if you try to deepen an already threaded hole at core size. In this instance the swarf is forced back down the hole as it rotates with the bit and is grabbed by the thread section of the hole.
 
Both are easy to verify for oneself.
 
Martin.
 

Edited By blowlamp on 21/01/2011 14:00:54

21/01/2011 12:26:58
Right, second bite...
So what we've established so far is that chatter is always present, but with care in the design of the machine, tooling and by using appropriate speeds etc, we can tune it out of harms way and so produce good work.
 
This is my take on what happens and is what I believe causes parting-off problems with the commonly available HSS blades when used in the front toolpost:
 
1/ The cut commences and is initially fine.
My View Is That:-
Depite the tool being quite wide and so removing more material than in normal turning, the basic Machine/Tool/Speed setup is adequate for the job.
 
2/ Once a certain depth is reached, sensations can be felt through the lathe and the sound of the cut can change. At this point I would say a lot of people instinctively withdraw the tool, clean the groove and carry on.
My View Is That:-
This is the point that the system is beginning to become unstable.
 
3/ A short while after the cut is recommenced, the cram-up happens - but why?
My View Is That:-
The parting-off blade is now working in a confined space with very, very little clearance.
As various sized chips are generatedby the cutting action, most of the larger ones are ejected by other swarf as it is being produced. However, small, gritty size pieces can remain and some of these will become lodged between the tool and the workpiece and might even 'pick-up' or 'gaul' the job.
From there, it can be seen that this unstable system is working in a situation where a log-jam can now occur as other swarf is produced.
When the log-jam does happen, the system experiences large forces and from these forces deflections can be seen about the tool and it's mounting, as well as the visible signs of the work moving too.
 
So to work around the problem:-
Some folks will widen the groove, before carrying on with the first cut as it gives more room for the swarf to clear.
Some will part-off from the rear, which allows gravity to help keep the groove clear of swarf.
Others will use a specially designed 'tipped' tool which curls the chip to make it narrower and also has plenty of clearance at the sides.
Etc.
 
Martin.

Edited By blowlamp on 21/01/2011 12:30:41

Edited By blowlamp on 21/01/2011 12:31:35

Thread: C A D for Mac
20/01/2011 11:29:21
ViaCAD is the software I've chosen for use on my PC and is also available for the Mac.
Note that demo downloads aren't available on the website at the moment, since it was revamped and apparently were left out by mistake. They should be back on again soon, but if anyone is desparate I can forward an email address to you by PM, where you can ask to be sent a link.
Martin.
Thread: Parting off on Myford lathes
19/01/2011 20:51:15
Michael.
 
Thanks for this very interesting reply, but in it, you seem to be concentrating on chatter, rather than the topic of parting-off.
 
When I've had parting-off cram-ups in the past, I don't particularly remember chatter being part of the problem. It was more a sense of the lathe having a 'rough' feeling coming through the feed handle, accompanied by an unpleasant sound before the inevitable crack of the tool breaking.
 
Another comment I might make, is that if springiness in the drive system is the problem with these lathes, then I can't see how using a rear mounted toolpost can cure it, as we will still be using that same drive system along with all it's supposed inadequacies.
 
Did you find that using backgear eliminates the problem?
 
 
Martin.

Edited By blowlamp on 19/01/2011 20:52:17

19/01/2011 15:08:16
"I was not talking about wear or about belts slackening in use due to permanent deformation. I was talking about elasticity."
 
As I tried to point out, you are talking about the elasticity (acting as a spring) of the belt at a minute level, which in comparison to belt slip in this context, is inconsequential. You also stated that belts stretch, which I took you to mean as a permanent deformation.
 
"Like it or not, when you drive a spindle via a drive train, there is a measurable amount of elasticity in the drive train which, when coupled to the various spinning masses involved (motor armature, spindle, chuck, workpiece, plus any intervening shafts/pulleys/gears) forms a mechanical system that can be made to oscillate at one or more resonant frequencies..."
 
It's not a matter of if I like it - it's just fact.
If you take a look at my previous posts on this, you'll see I asked for clarification on this very point, by posing the question about how a geared-head lathe with a motor belt drive in the system, differs from the original poster's example (Myford).
As almost all lathes have a belt drive somewhere in the train and therefore have an 'elastic' transmission setup, it's strange that apparently these aren't susceptible in the same way as a Myford.
 
Martin.

Edited By blowlamp on 19/01/2011 15:11:30

19/01/2011 13:34:56
Quote:
"Firstly, ALL belts, Myford or otherwise, stretch. Some stretch more than others, but they do stretch. And yes, that is what I mean when I say that they are elastic - if you want it in mechanical terms, they have a Young's modulus that is finite. So, if you apply a tension to the belt, it will stretch. Maybe not by much, but it WILL stretch. End of.
Secondly, given the above, and given that the gear train itself is also elastic (although not so much as the belt), yes, it does all apply to a greared train with or without a belt for primary drive. As I said earlier, the principle is the same; the variable is the degree of elasticity - a gear train is (generally) less elastic than a belt drive.
Regards,
Tony"
End Quote:
 
 
I was trying to keep the discussion reasonable, by speaking in practical terms of this particular topic, but if we are now at the microscopic level, yes drive belts are elastic. They are elastic in the same sort of way that a lump of granite or wood is, but not in the same way that a rubber band is.
 
If we're talking about belts slackening whilst in use, it will be seen as stretch by some, but I think it is more likely to be down to wear on the vee surfaces, which will allow the belt to sink deeper into the pulleys.
 
Any manufacturer worth his salt will choose materials that are appropriate for the job and a belt that progressively stretches, wont hold it's tension and will start to slip.
 
Martin.

Edited By blowlamp on 19/01/2011 13:35:50

Edited By blowlamp on 19/01/2011 13:37:02

19/01/2011 10:52:26
Myford drive belts are flexible, but they aren't stretchy - if that is what people are meaning when they say they are elastic.
 
I would still like an explanation as to why this theory doesn't apply to lathes that have geared heads and primary drive belts from the motor.
 
Taking a second cut creates a wider slot and simply gives more room for chip clearance.
 
Martin.

Edited By blowlamp on 19/01/2011 11:02:34

17/01/2011 14:40:13
Those drawings are put together in such a way as to reinforce the argument of the author.
 
To my way of thinking - and this is assuming that both the tool and toolpost are up to the job - then the pivot point should actually be at the front foot of the toolpost on the left, which is almost directly under the tip of the cutting tool.
In that position though, there is so little extension of the tool beyond it's support footprint, that there is practically no opportunity for a rocking movement to happen due to a downward force on the tool.
 
Non of the above helps the assertion of the rear toolpost being stiffer, as the strength of the tee-slot mounts must also called be called into question when evaluating it's merits.
 
Anyone with a Gibralter type toolpost should be able to verify that deflection is minimal when parting-off and thus do so with few problems, provided the groove can be reliably cleared of swarf. If a jam-up still occurs with a Gibralter - but not with the same tool mounted at the rear - then I'm pretty sure it's chip crowding.
 
All the above assumes a lathe in good condition.
 
Martin.
17/01/2011 10:39:19
I'm struggling with some of the logic of the 'elastic' drive belt theory in this thread.
Anyone that's ever tried to stretch one of these belts will know that they've got virtually no give in them at all, let alone be elastic.
They are after all reinforced with various types of cord to prevent this.
 
It also would be interesting to know if this theory for a Myford holds true whilst utilising it's back-gear.
 
I have a geared-head head lathe that is belt driven from the motor and am intrigued to know if the theory predicts that it also will be subject to the problem, particularly when geared at a similar motor speed / headstock speed ratio to that of the Myford when parting off.
 
As has been pointed out in the other thread on this, it's either deflections in the tool/workpiece interface and/or chip crowding in the groove being cut.
My view is that chip crowding is the main culprit as it's quite unusual to get a lock-up in the early stages of the cut, while the groove is still shallow.
 
Martin.
Thread: Rear mounting parting-off tools
12/01/2011 10:46:42
Provided that the workpiece itself is mounted rigidly, there can only be two causes for a cram-up.
Either the tool digs in somehow, due to something like backlash or bending of the support system, or there is chip crowding of the groove being machined.
 
In any particular situation, it might be worth experimenting to find which is the cause of this bother, by cutting a groove just a few thou' in from the end of a bar, such that the full width of the tool is utilised, but (apart from the thickness of this flimsy shim) one side remains open for swarf clearance.
 
If a cram-up still happens, then it must be that the tool is moving into the workpiece for some reason. If the problem disappears, it must be chip crowding.
 
Martin.
Thread: Rulers - my pet peeve
08/01/2011 11:58:56
Posted by Dave Jones 1 on 08/01/2011 11:36:49:
...I had the luxury of missing out of capital punishment in school! ...
 
Dave
 
 
 
 
Good job you did, or we wouldn't have the pleasure of your company
 
 
Martin.
Thread: New Year's Resolutions
01/01/2011 18:47:22
David.
I've submitted a couple of articles within the last six months, but I don't have any idea if they were suitable for publication.
How long should we leave it before either chasing it up, or giving up any hope of becoming published, as I'm forever mindful of how busy you must be? 
Also, do you tend to get in touch with the author before sending it to press?
 
Martin.
Thread: Which slideway oil is best?
31/12/2010 13:18:06
I can see no real downside to using any typical low viscosity modern day engine oil. It's designed for use in hot and cold extremes, where condensation exists and under other very arduous working conditions.

The talk of its unsuitabiliy is a little overstated in my opinion and seems to be based on the theoretical problems of it containing detergents which will hold particles in suspension - but why is that a problem and why would it be better for the swarf to remain in contact with the ways? It could be seen as academic anyway, given the likely film thickness.

By the same token, if you've got that much swarf floating around the headstock, then surely you'd be better off cleaning it out before filling with oil. I also doubt that much dirt will stay 'on the bottom' - detergent oil or not, with all the rotating components in there. After all, my wifes juicer/blender doesn't leave much down there once it's running at full pelt.
 
Martin.
Thread: Compression Ratio required
15/12/2010 17:29:04
Compression ratio is Swept volume + Unswept volume divided by Unswept volume. So a cylinder of swept capacity of 100cc plus combution chamber space of 15cc will have a compression ratio of about 7.66:1 which is low by modern standards, but might be a reasonable figure for you to start from.
If you still don't have good compression once the theoretical is right, then it's time to check the basics such as valves and clearances and the fit of piston and rings etc.
 
Martin.
Thread: lathe belt
07/12/2010 16:30:15
Posted by Dinosaur Engineer on 07/12/2010 16:00:42:
Maybe the toothed belt is used due to a small pulley being used  ( high torque at small radius) and the pulley wheel centres are close so that it's difficult to provide enough "wrap around" for a 'V' belt. A friction belt would be better than a "geared" belt as it's more likely to slip rather than cause mechanical damage. The equivalent ' v ' belt drive would probably need more space and/or a tensioning roller to increase the small pulley "wrap around" 
 
 
 I agree with all of that and would say that if a small 'toothed belt' drive pulley is used, then that is probably a bad thing in this instance, as the belt will be prone to shedding its teeth.
Poly-V belts are much better under these circumstances, due to their slim construction and will wrap around smaller pulleys than other V-belts, but do need to be properly tensioned.
If the lathe in question has inadequate tensioning arrangements then that might be why the maker went with a toothed belt design, as it is more forgiving if left loose.
Almost all other manufacturers use V-belts of one style or another.
 
Martin.
06/12/2010 16:56:04
Looks like it could be a good candidate for a Poly-V belt conversion, if you can get the right length.
 
Martin.
05/12/2010 12:16:18
Gordon.
Would it be a big job to change the pulleys and belt for something more current?
 
Martin.
Thread: Myford Squareness.
04/12/2010 23:11:41
Myford have a fixture - much like a long angle plate, which uses the relevent vertical shears to reference it at an accurate 90 degrees to the lathe bed, ie. nominaly parallel to cross slide movement. The saddle is then scraped on it's lower vertical guiding surface, so that a dial gauge when affixed to the cross slide, shows a very small positive reading when moved from the front towards the rear of the lathe. So in effect, the whole saddle/cross slide assembly is turned very slightly anti-clockwise when viewed from above, with everything within that assembly being machined at (nominally) 90 degrees and no special allowance is machined into the tee slots or other faces as this in built 'error' is so small.
 
Martin.
Thread: Endmill, Slot Drill
01/12/2010 12:58:44
The whole parting-off on a Myford thing has always intrigued me too.
My thoughts are that the problem is largely down to chip crowding in the groove as well as a slightly flimsy compound assembly.
So using coolant can help flush the chips, but as the chip is at least as wide as the slot, (but probably wider due to plastic flow) it can still get jammed and cause a cram-up. Turning the tool upside down and mounting at the rear allows gravity to help remove the swarf, but I find that the modern insert style tooling can be used conventionaly in the front position with confidence as the tips are designed to shrink the width of the swarf and thus enable easy ejection.
 
Martin.
Thread: levelling
29/11/2010 23:25:08
Terry.
There is no fundamental reason to have a lathe bed level in the sense that one would ensure a shelf or kitchen worktop is level.
The engineers precision level is used across the bed as a means of comparing readings between the headstock and tailstock ends of the machine until the difference either disappears, or becomes so small as to become negligible.
At that point, the bed is without significant twist, and can be relied upon to be accurate in use.
However, other benefits may become apparent from having a machine bed level, such as the correct drainage of coolant and being able to setup surfaces of castings etc with a level, secure in the knowledge it will be parallel the the machine bed.
 
Martin.
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