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Myford ML7 mandrel speeds

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choochoo_baloo20/11/2014 17:55:00
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Hello all,

I am a newcomer to model engineering. I have good condition ML7 with all the standard tooling and have been playing and practising lately.

One issue has arisen which I would really appreciate some help with please. I am getting lacklustre performance from turning and facing, and even drilling from the taitstock. This is most noticeable on brass. I am convinced that this is due to the rotational speed of the mandrel being far too low, even at the highest belt settings. I have the original Metropolitan Vickers 1450 rpm motor with 1-7/8" dia pulley fitted.

According to Bradley's Myford Series 7 manual, thus all gives a highest mandrel speed of 640 rpm. Now from checking a basic spindle rate reference table:

http://www.southbaymachine.com/setups/cuttingspeeds.htm#Quick%20RPM%20Table

for the 1/4" dis brass that I was practising with, 3200 rpm is the approx. rpm I require.... !!

Hence this is what I'm planning to do to increase the mandrel speeds available:

swap the 1-7/8' motor pulley for a Super 7 4-3/8" and 1-3/4" dual puller. This bigger pulley brings me up c 1500 rpm. Possibly buy a new faster motor?

What have other ML7 owners done to tackle this issue?

Thanks in advance for any help.

Matthew

Robbo20/11/2014 19:54:00
1504 forum posts
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With an ML7 with the normal white metal spindle bearings,and I would guess if you have a Metropolitan Vickers motor then this is the case, this sort of speed is not recommended.

Myford supplied an alternative motor pulley of 2½" dia, which gave a top speed of 870 rpm with a 1450 rpm motor, higher than this was not recommended.

ML7s are used at higher speeds, but care needs to be taken to open the headstock oilers to ensure adequate lubrication.

choochoo_baloo20/11/2014 20:08:57
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Thanks for the info. I am keen to up the mandrel speed. I have original Myford reservoir headstock oilers fitted - do they need to be set to fully open?

Would it be feasible to replace the mandrel bearings with something capable of higher speeds, say graphite or phosphor bronze (though I am not sure of the ideal material!) ?

Andrew Johnston20/11/2014 20:27:51
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Matthew: What do you mean by lacklustre performance? Poor finish, or just takes an inordinate amount of time?

Andrew

choochoo_baloo20/11/2014 21:02:36
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Posted by Andrew Johnston on 20/11/2014 20:27:51:

Matthew: What do you mean by lacklustre performance? Poor finish, or just takes an inordinate amount of time?

Andrew

Finish definitely. I have sharp new lathe tools and am very careful to set the tools at centre height with a height gauge.

Andrew Johnston20/11/2014 22:09:21
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Matthew: I suspect that spindle speed may not be your problem. Here's a picture of a bit of 3/8" diameter brass out of the scrap bin that I've just done a quick trial on:

brass_turning.jpg

I was using the tool that just happened to be in the toolpost, with a feedrate of 2 thou/rev. The finish feels smooth and measures about 2µm Ra. The larger diameter, to the left, was turned at 370rpm, the smaller diameter, to the right, at 180rpm.

While brass can be machined at high speeds, it is pretty forgiving of lower speeds. So I would suggest an issue elsewhere. The two most common issues are unknown material and incorrect tooling. What are the characteristics of the poor finish?

Andrew

choochoo_baloo20/11/2014 22:30:10
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Well I cannot get the brass (like your example picture) to have as shiny/mirror - like finish as yours. It always seems quite dull and can see fainting 'banding'. It's definitely brass - bought it from a metal suppliers.

Could you please elaborate on the common "incorrect tooling" issues that could be my problem.

Andrew Johnston21/11/2014 11:52:47
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It would be useful to see a picture of the finish obtained. If the finish is uniform, but poor, then it is likely to be a metal/tooling issue. However, if the finish is inconsistent, like the curates egg, then it is more likely to be the machine. Is the banding circular, or helical like a screw thread? Some time ago I had issues with very faint multi-start sets of helical bands, which was cured by fixing a broken hold down bolt for the top slide.

The most common brass, in rod form, is CZ121, which should turn very well. If your supplier states the grade that is good, if not who knows what you have. smile o Experience has shown that not all metal is the same, even from commercial metal stockists.

The potential tooling issues depend upon the type of tooling. For HSS then the most likely problem is grinding of the tool. Ideally the top of tool for brass is almost flat. A small radius (anything from a few thou to tens of thou) on the nose of the tool can help give a good finish. What feedrate per rev of the spindle are you using? If you are using carbide, or insert, tooling then the most common problem is the purchase of the cheap multi-tool sets. They are often poorly, and incorrectly, ground. What tooling are you using?

It may help if we knew roughly where in the UK, or world, you are based.

Regards,

Andrew

Russell Eberhardt21/11/2014 14:32:38
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Posted by choochoo_baloo on 20/11/2014 21:02:36:

I have sharp new lathe tools and am very careful to set the tools at centre height with a height gauge.

That is almost certainly your problem. Rather than general purpose tools you need tools ground for brass as Andrew suggests. Really sharp HSS is best with no rake (ie., flat top) and 5 to 10 degrees clearance angles. Small radius or flat at the tip. I doubt if your tool is sharp enough as bought. Assuming it is HSS and the right shape, give it a few strokes on an oilstone. An old trick is to make sure that it will scrape the back of your thumnail.

Additionally, make sure that there is no shake in any of the slides to prevent any vibration being set up.

The swarf should fly off as a coarse dust with a hissing sound, much like the sound made by a Rolls Royce passing on a wet road.

Russell.

Edited By Russell Eberhardt on 21/11/2014 14:35:04

choochoo_baloo21/11/2014 15:17:20
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Thanks very much chaps. It probably is the tools then. I've just found a handy "table of tool angles" in my Bradley manual. I shall consult that. Please can you both/either recommend good quality HSS tool blanks - I assume that the quality varies wildly between suppliers?

Secondly, I have a bench top grinding wheel, it has available white, grey and green wheels (interchangeable - not all fitted at once!). Are are of these used for tool grinding, or is an oilstone better. What is an oilstone? Finally with these various angled faces all being formed (rake, top etc.) how do engineers accurately measure the angle of these faces to guide the process - a simple protractor won't work! Sorry about my nobody questions!

Thanks again.

Gordon W21/11/2014 16:09:36
2011 forum posts

Good quality blanks- I buy from the dealers advertised on this site, never had any problems. you probably only need super stuff for super material. Grinding wheels- usually green is for carbide, I would use the finest wheel you have, without knowing more about them. I rough grind a new blank with an angle grinder and then a smoothish wheel and then a diamond file or oilstone. Oilstone is a natural stone used for tool and knife sharpening, many varieties. Angles on tools not really important, an educated guess will usually be near enough. Keep to the basics, flat top for brass, sharp angles for ally etc. and a small radius/ flat for a finish.

DMR21/11/2014 18:30:57
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Just for the record and since I have never seen it said:

That fine man Malcolm Townsend once told me about ten years ago (at proper Myford of course) that ML7 mandrels had started to come in for repair as people had fitted variable speed systems and changed pulleys to get more speed and were wrecking the mandrels/bearings. I was always an S7 person, so I suppose he told me without come-back that "800rpm was about the limit for the ML7, and don't get the bearings hot".

Just so it assists anyone tempted with a supercharge on the model.

Dennis

Lordedmond21/11/2014 19:01:46
18 forum posts

For brass zero top rake ie flat just a smidge front and side clearance

Honed to a fine finish note the work finish cannot be better than the tool finish

And not least iif the tool has touched steel it will be NBG for brass unless you re hone it

Stuart

JasonB21/11/2014 20:24:44
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Matthew

Are you using the lathes feed to move the tool along or just trying to wind by hand? The autofeed should give a better finish but you will need to ensure the right gears are driving it otherwise you will end up with what is in effect a very fine thread.

Hand feeding can be irratic and if you tend to use just one hand on the handwheel can tend to lift the tool as your hand is moving in an upward arc then push it down as you move downwards.

Neil Wyatt21/11/2014 20:35:39
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Slower speeds just mean things take longer, much more of a pain when your lathe won't slow enough.

The exception is some carbide tools that need to be worked hot to do their best.

Neil

Russell Eberhardt21/11/2014 20:43:04
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Posted by choochoo_baloo on 21/11/2014 15:17:20econdly, I have a bench top grinding wheel, it has available white, grey and green wheels (interchangeable - not all fitted at once!). Are are of these used for tool grinding, or is an oilstone better. What is an oilstone?

White or grey wheels should do for initial grinding. The oilstone is for polishing the cutting edges to get a really sharp smooth edge. A modern alternative to an oilstone is a Diamond Hone. Those are best used with a little water.

Russell

Russell Eberhardt21/11/2014 20:46:48
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Posted by JasonB on 21/11/2014 20:24:44:The autofeed should give a better finish but you will need to ensure the right gears are driving it otherwise you will end up with what is in effect a very fine thread.

True, but very difficult if you're turning up to a shoulder.

Russell.

JasonB21/11/2014 21:03:38
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Don't usually have a problem going upto a shoulder, just flick it out of drive before the shoulder and complete the cut by hand

imag2855.jpg

imag2771.jpg

Edited By JasonB on 21/11/2014 21:04:28

Andrew Johnston21/11/2014 22:12:38
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On my main lathe I use HSS toolbits that I've collected over the years, mostly for specials. However, my repetition lathe uses only HSS toolbits, which I buy from Drill Services. I would guess from the marking that they are far eastern.

White/grey grinding wheels are aluminium oxide - both are fine for grinding HSS toolbits. Both my pedestal grinders have grey wheels. Green wheels are silicon carbide, for grinding tungsten carbide. For grinding normal HSS toolbits I judge angles by eye, none of the angles are that critical. I use a cheap set of diamond hones to remove the grinding wheel marks from HSS toolbits before using them.

Let us know how you get on.

Regards,

Andrew

Russell Eberhardt22/11/2014 10:30:38
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Posted by JasonB on 21/11/2014 21:03:38:

Don't usually have a problem going upto a shoulder, just flick it out of drive before the shoulder and complete the cut by hand

Nor do I Jason but we're advising a beginner here and I would hate for him to have a disaster.

Russell.

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