MINI-LATHES

Some observations and suggestions for using and improving
one of the cheapest and neatest machine tools you can buy

 

 

A great deal of information is available about the many versions of the mini-lathe, and I strongly recommend www.mini-lathe.com and www.LittleMachineShop.com as excellent sources of advice. Most of the information in these sites is biased towards machines available in the United States, which may be similar but not necessarily identical to those sold in the UK and Europe. Here I am focusing on mini-lathes in the UK, although most of the information is equally relevant to similar machines in other countries.

The mini-lathes have two features which are unusual in machines of this size and price: they have electronic variable speed with a high/low ratio gearbox, and the rotation of the leadscrew can be reversed for machining away from the headstock or for cutting left-hand threads. They are also inexpensive and relatively small and light, so they will fit into the budgets and workshops of most small-scale or model engineers.

Just a note on terminology for US readers… We call 0.001" a ‘thou’ (with a hard ‘th’). I believe 0.001" is known as a ‘mil’ in the US, but in metricated Europe a ‘mil’ means a millimetre! Also, what you call electrical ‘ground’ we call ‘earth’. ‘Wrenches’ are ‘spanners’, and ‘aluminium’ has an extra ‘i’. ‘Bush’ is a thick washer or a small tree, and Washington is in Tyne & Wear.

“One nation divided by a common language…”

 

Versions

Internationally, mini-lathes are generally referred to as 7x10, 7x12 or 7x14. The figure 7 is the swing over the bed of the lathe in inches – i.e. the maximum diameter of workpiece that can be turned. The second figure is (theoretically) the maximum distance between headstock and tailstock centres in inches – i.e. the longest length that can be turned. In fact, the 7x10 versions of the mini-lathe are not as long as their name suggests, and have only about 8 inches between centres. As far as I know, the only size of mini-lathe readily available in the UK is the 7x12, and this does actually have approximately 12 inches between centres. However, if you need to turn items 12 inches long, please read the details below about the actual dimensions when a chuck and a revolving tailstock centre are fitted, because these reduce the capacity significantly.

Although mini-lathes from different UK suppliers may look almost identical apart from their colours and labels, it is important to realise that they are not. All mini- lathes are made in China, but not in the same factories. There appears to be a standard pattern or design of mini-lathe which various Chinese manufacturers work to, but the details, features and quality can vary quite considerably. So can the prices.

There are at least three suppliers of mini-lathes in the UKWarco (www.warco.co.uk), Chester Machine Tools (www.chesteruk.net) and Machine Mart (www.machinemart.co.uk). The Warco version is known simply as the Mini Lathe, Chester’s is called (somewhat ambitiously) the Conquest Lathe, and Machine Mart’s is the CL300M Variable Speed Metal Lathe. As of December 2003, the prices (including 17.5% VAT) were £400, £380 and £470 respectively. Both Warco and Chester were offering delivery and some accessories in with this price, but please check their websites for the latest details.

Both Warco and Chester say that their versions have US-made printed circuit boards, instead of the original Chinese ones which could be troublesome. Warco also say that their Mini Lathe has hardened and ground bedways. Chester’s Conquest Lathe has heat-treated bedways, but this may not be quite the same thing. Machine Mart makes no mention of the origin of the PCB or hardening of the bed.

 

 

There are other differences too. For example, the Warco and Chester machines have a retaining nut and concentric grub screw on the right-hand end of the main leadscrew, whereas (from the photo in their catalogue) the Machine Mart version does not. The Warco and Chester machines also have an interlocked chuck guard, so the lathe cannot be started with the guard raised. This prevents you from accidentally starting the lathe with the chuck key still in the chuck, the consequences of which can be unpleasant.

I am not in the business of recommending or denigrating individual suppliers, but there does not seem to be any good reason to pay substantially more for the Machine Mart product.

The mini-lathes are available in either imperial (inch) or metric versions, but there is a caveat here. Most (if not all) the imperial versions still have metric leadscrews for the cross-slide and compound slide, with a thread pitch of 1 mm. So one 360º turn of the handle moves the slide 1 mm, even if the dial says 40 thou. One millimetre is actually 0.03937", so there is an error in the dial reading of roughly 1.6%. You can adapt to this, but it’s worth bearing in mind.

The main leadscrew for the carriage is correct, being 16 tpi in the imperial versions. Inexpensive leadscrew conversion kits are available for turning metric threads on an imperial lathe and vice versa, but remember that the cross-slide and compound slide dials are not calibrated in both sets of units.

As delivered, the changegears are set up for normal turning operations, and on the imperial version give a feedrate of 0.00385"/rev.

UK and European mini-lathes run off 220-240 VAC 50 Hz, as opposed to 110 VAC 60 Hz US versions. The fuse in the top panel of the control box is a 30 mm 5 amp cartridge fuse (not the same as a UK mains plug fuse), and from what I’ve heard it may be a good idea to have a spare (note that this should be rated for mains voltage and not 12V).

 

Capabilities

Please look at www.mini-lathe.com for comprehensive information on what these mini-lathes can and cannot do – there is no point in saying the same things again here. Essentially, if you are working with reasonably small diameters and lengths of mainly soft metals or plastics, and don’t expect to machine too much off at one pass, the mini-lathes have the potential to do the job perfectly well. For machining hard materials, larger diameters, or lengths over about 250 mm (10"), the mini-lathe is either unsuitable or far from ideal.

Fitted with the standard 80 mm three-jaw chuck and a revolving tailstock centre, and with the tailstock flush with the end of the bed and the quill wound fully in, my Chester Conquest measures 250 mm between the face of the chuck jaws and the point of the tailstock centre. Using a dead centre in the tailstock would increase the distance to about 300 mm.

It’s common sense really. My other lathe is a 13" swing gear-head machine with 800 mm between centres and weighing about 600 kg, compared with 35 kg for the mini-lathe. You wouldn’t expect the wee beastie to be able to compete with something 17 times as big, although for certain operations it is more convenient and easier to use, which is why I like it.

 

Adjustments

Many early versions of the mini-lathe were of dubious quality, and needed considerable modification and improvement to perform acceptably. Most of the current versions are much better, and it has been suggested that modern mini-lathes will work fine straight out of the box. However, this has not been my experience. Although the overall quality may be OK, there are numerous adjustments, and a couple of modifications, which I would strongly recommend before using the machine.

Information on how to prepare and adjust the mini-lathe is on www.mini-lathe.com. Mine needed most of these adjustments, especially gibs, saddle retaining strips, apron alignment, cross-slide backlash and tailstock alignment. I’ll deal with the last of these in the section on Modifications, but it’s worth saying a little more about the others.

 

Gib Adjustment

This is a fairly standard procedure which will be familiar to anyone who has used a lathe before. The set-screws on the cross-slide and compound slide are adjusted so the gib-strips just make contact with the surface they slide against, and are then held in position with the lock-nuts. The idea is to eliminate play without introducing any more friction than is absolutely necessary. See www.mini-lathe.com for full instructions.

 

Saddle Retaining Strips

When I got my lathe, you could press down on the saddle and see the rear edge move down closer to the bed! Again, www.mini-lathe.com has instructions for adjusting the (rather fragile) retaining strips. I managed to adjust the rear strip with the saddle in place, using a set of feeler gauges and a little patience, so the clearance is less than 0.001". You’ll need to remove the lathe’s rear splash guard to do this, and, in my opinion, leave it off because it’s a waste of time and just gets in the way.

Adjustment of the front saddle retaining strip is less critical, because the forces acting on the saddle tend to push the front down onto the bed rather than trying to lift it off.

 

Apron Alignment

This was way out of adjustment on mine, so the leadscrew was being pushed away from the lathe when the half-nut was engaged.

The apron is fixed to the saddle by two cap-head bolts on the front edge of the saddle. Adjustment is simple. Loosen the two bolts, then close the half-nut with the lever on the front of the apron, ensuring that the half-nut has engaged fully with the leadscrew threads. The effect of this is to move the apron relative to the saddle, so it aligns itself with the leadscrew. Then tighten the two cap-head bolts again, and everything should be fine.

Having done this, check that the two pillow-blocks in which the leadscrew runs are also aligned correctly. This is also straightforward – see www.mini-lathe.com.

If everything is adjusted correctly, you should be able to flick the half-nut lever down somewhere it doesn’t quite mate with the leadscrew thread, move the carriage a fraction, and see the lever drop into place by itself as soon as the nut and leadscrew threads coincide. The lever should feel like a switch, with no force required to fully engage the half-nut.

 

Leadscrew Retaining Nut

The retaining nut at the right hand end of the leadscrew should be adjusted so it allows the leadscrew to turn freely but prevents longitudinal movement. The nut has a concentric setscrew which locks it in position. First back off the setscrew, and then tighten the nut with your fingers so it just makes contact with the pillow block (the bearing block in which the leadscrew runs). Do not tighten it with a spanner! Then back off the nut about a quarter turn, and hold it in this position while tightening the setscrew.

Cross-slide Backlash

It is almost impossible to eliminate backlash completely from lathes or milling machines fitted with conventional leadscrews. Expensive CNC machines usually have ballscrews instead of leadscrews, but for the rest of us backlash is something we have to live with. If you try to eliminate it completely, you risk over-tightening something, and the resultant wear will soon cause the backlash to return.

Unless you are using a cutter with a negative approach angle (which can grab the workpiece and pull the cross-slide inwards), backlash on a lathe is not very important provided you always introduce the cutter to the workpiece in the direction you intend to cut.

Having said that, excessive backlash can be a nuisance, so ideally you should keep it to within a few thou. There is advice on www.mini-lathe.com on how to adjust the cross-slide nut to remove backlash, but I honestly cannot recommend this procedure. It involves tilting the nut, so the screw threads will have only a small area of contact with the nut, and this is likely to significantly increase the rate of wear on both components.

As delivered, the cross-slide on my mini-lathe had a backlash of 13 thou, which I thought was a bit much. On examination, it was clear that about 10 thou of this play was in the cross-slide leadscrew bearing on the front of the saddle, rather than on the cross-slide nut – so adjusting the nut would not have helped much.

If you remove the cross-slide handle and the rotating dial, you’ll see two cap-head screws which fasten the remaining part of the dial/handle assembly to the edge of the saddle. Undo these and remove this component. The cross-slide leadscrew has a flange which acts as a bearing, sandwiched between the saddle and the dial/handle mount. Both are bored to accommodate this flange, and in the case of my machine they had been bored about 10 thou too deep, so the flange could move backwards and forwards within its housing. The solution is simple but a little fiddly. You need to make a steel or brass washer of the correct thickness to take up the play, an o.d. small enough to fit into the bored housing in the dial/handle mount, and an i.d. large enough for the leadscrew spindle. Lubricate and reassemble everything with the thin washer in place, and Hey Presto! the backlash is now 3 thou. I can cope with that.

 

Lubrication

The www.mini-lathe.com website recommends two basic lubricants – Mobil 1 and white lithium grease. For those of us in the UK, Mobil 1 is no problem (although it costs about £34 for 4 litres), but white lithium grease is more elusive. Tubes of the stuff may be available in every hardware store in the US, but over here it is a rare breed.

I am not a big fan of grease. Swarf, dust and grit stick to it, turning it into grinding paste. Unless the gears, screws or whatever are well protected from debris, I recommend oil rather than grease.

There are two areas on the mini-lathe where grease may be appropriate. One is the changegears (easy to get at), and the other is the headstock gearbox (difficult). For the plastic changegears, I use silicone grease which is available from pneumatic or hydraulic equipment suppliers, or by mail order from RS Components. Silicone grease does not harm plastics, and is colourless so you can see if any dirt has got into it.

There are instructions on www.mini-lathe.com on how to remove and lubricate the headstock, but it is a fairly lengthy process and some people may be concerned about messing up the alignment of something that used to be OK (although not much seems to fall into the ‘OK’ category as delivered!). There is an alternative method of headstock lubrication which (so far) seems to have worked on my machine.

First disconnect the lathe from the mains, remove the changegear cover, and then unscrew the control box from the front of the machine. Remove the screws fastening the two earth wires to the headstock casing, but do not disconnect any other wires. You will see that the holes for these earthing screws go right through the casing, so they provide (small) points of access to the gearbox within.

You now need an aerosol spray-can of grease or oil, with a thin nozzle extension that fits through the two holes. Unless you have access to an American hardware store for spray lithium grease, a silicone grease or oil spray is likely to suffice. (I don’t know about WD40 – you can use it on plastic components under the bonnet of cars, so I guess it shouldn’t harm the plastic gears within the headstock, but I can’t give any guarantees.) Spray the grease through each of the holes while manually rotating the chuck slowly. Do this in both high and low ratios. Try to move the nozzle tube so it sprays in different directions and everything inside the headstock casing gets its fair share.

While the control box is detached, check that the printed circuit board is well-protected from swarf and debris – the leadscrew can act as a swarf-conveyor, which plays havoc with the electronics. If in doubt, stick some PVC or duct tape between the PCB shield and the inside of the box so that nothing can get to the components. Then replace the two earth leads, the control box and the changegear cover.

Finally, lubricate around the high-low gear change lever on the back of the headstock with Mobil 1. Do this fairly regularly, so the oil works its way onto the shaft and bearing of the lever. Hopefully this will stop it seizing up, as has been known to happen.

If you do decide to remove the headstock to give better access for lubrication, it may be worth drilling or milling a decent-sized hole in the top or side of the casing, which can be covered with a plate or grommet. This should make internal lubrication much easier next time.

 

Modifications

Most of the following modifications are entirely optional, and can be done if and when you think they’re a good idea. The only exception, depending on the state of your machine when it arrives, is the tailstock adjustment screws. I couldn’t align the tailstock properly without carrying out this modification, but maybe you’ll be better or luckier than me.

 

Spindle End

This is the simplest possible modification, but may involve drinking a bottle of wine! The left hand end of the spindle ends just above the changegears, so any swarf or debris carried through the spindle is deposited on the gears. Clever, eh? The solution is to leave a plug in the end of the spindle unless you are through-feeding a long length of stock. A wine cork is just the right size!

Mine had a lot of loose rust and dirt inside the spindle when it arrived, so it’s a good idea to clean this out before it gets onto everything. Don’t forget to put a cloth over the gears before you brush through the spindle!

I’ve also made some small knobs to replace the cap-head screws that hold the changegear cover in place, which makes it quicker to remove and replace the cover. There is probably some health and safety law against this.

 

Speed Dial Calibration

If you have (or can borrow) a tachometer, it’s useful to calibrate the speed dial in rpm rather than the 0-100 scale.

I’ve also replaced the rather gaudy front panel with one that is equally gaudy but more informative.

 

Leadscrew and Bed Protectors

Prevention is better than cure, so I made a protector for the leadscrew which keeps most of the debris off it. I also fitted a polycarbonate swarf tray which stops debris dropping immediately in front of the saddle. This is held on by two screws which fit into the holes for the travelling steady. I strongly recommend polycarbonate for things like this, rather than acrylic sheet which is brittle and cracks if you look at it hard.

Tailstock Alignment

Strangely, in all the information about mini-lathes on the Web, there is hardly a mention of how to adjust the tailstock. Even mini-lathe.com is silent on this, although there is a section on how to convert the tailstock to cam-lock.

I’m not surprised. It’s an absolute cow. In my opinion, you need to modify the tailstock to be able to adjust it properly.

A  The tailstock must be aligned parallel to the bed, so the quill runs true to the longitudinal axis of the machine. As delivered, mine was 50 thou out of alignment from one end of the quill to the other, which is totally useless.

B  The tailstock must also be in alignment with the headstock. If you are using the tailstock to turn something, a misalignment of 1 thou between headstock and tailstock will cause a difference in diameter of 2 thou between the two ends of the workpiece. I aim for alignment within 0.5 thou, which is acceptable for most purposes.

Out of the box, the mini-lathe tailstock has two setscrews on the right hand end, which are supposed to permit axial adjustment by allowing the flange on the lower part of the assembly to rock against a slightly curved surface on the upper part. This is really abysmal, especially since nothing seems to be machined quite true. Even if you can get the tailstock parallel (A above), you have to loosen the screws to alter the lateral alignment (B), and everything else then goes out of adjustment again.

To make it worse, the upper and lower parts of the tailstock are secured together by a cap-head screw underneath the tailstock, so even if (by good luck) you manage to get everything right, you’re almost bound to upset it while trying to tighten the locking screw.

There are two modifications to ease the problem. The first is to replace the locking screw with a bolt which can be tightened from above. This involves drilling out the threaded hole in the upper section of the tailstock, and using a bolt which goes right through from the top, and is fitted with a square or oblong nut that cannot rotate in the slot in the underside of the tailstock. Tightening the bolt from the top locks the two parts together. Why they don’t do that to start with is anybody’s guess.

The second modification involves drilling and tapping two more holes in the left hand end of the upper section, similar to those that are already in the right hand end. The flange in the lower section is thereby sandwiched between four adjustment screws. To alter the angle of the tailstock (A) back off one of the screws very slightly, and tighten the corresponding one on the other side. When you’ve finished, make sure that all four screws are equally tight.

Once this is correct, the lateral alignment (B) can be adjusted by backing off both screws on either the left or the right hand end, tapping the headstock lightly to move it, and then retightening the same two screws.

I’ve seen a further modification where a plate is fixed to the back of the tailstock, and lateral alignment is achieved by turning a screw which acts against the top section and moves it quite precisely. I’m sure this would be an excellent idea if you intended to alter the tailstock alignment regularly to turn long tapers, and needed a grief-free method of setting it back again.

In order to check the alignment, I suggest using an MT2 test-bar and a decent lever-type dial test indicator (DTI). Using a normal plunger-type dial-gauge on the mini-lathe can be tricky because of lack of space.

 

Tailstock Lock

As mentioned earlier, www.mini-lathe.com has instructions for converting the tailstock to cam-lock. www.LittleMachineShop.com sells a kit for doing this, although you still have to drill a hole through the tailstock. After doing all the other adjustments and modifications, I thought I’d put this on the back-burner for a while.

The main problem with the standard locking-nut arrangement is that the nut is difficult to get to when the carriage is close to the tailstock – the compound slide is in the way. A simple solution is to raise the height of the nut.

In practice it’s not quite so simple, because the M10 threaded bar isn’t long enough, and is welded to the plate that presses onto underside of the bed. You’ll see on www.mini-lathe.com that some modifications to this plate are recommended – such as fitting a spring to prevent it jamming, and chamfering the leading edges to make it easier to refit the tailstock after removal.

It’s probably better to make a whole new plate out of 3/16" or 4 mm steel plate (or something close to this) – which can be rather longer than the original one. Drill and tap this M10 (or 3/8" BSW/UNC) and use a length of M10/BSW/UNC threaded bar together with a ground- or cut-down nut to lock the bar to the plate. If you decide at some stage to convert to cam-lock, you’ll be able to use the same components and simply cut the threaded bar to the right length.

Then all you need is a long spacer cut from steel, brass or aluminium tubing of about the right diameter, plus a couple of washers, and you can position the locking nut right at the top of the aperture in the tailstock casting. This enables you to reach it easily with a normal spanner, even with the carriage and compound slide right up against the tailstock. It helps even more if you heat up the spanner and put a slight bend in it just before the head.

I’ve used this arrangement for a while now, and, although I accept that a cam-lock system would probably be even better, I’m not yet motivated to go the extra mile (or kilometre).

 

Tailstock Arbors

The MT2 socket in the mini-lathe’s tailstock quill is shorter than a standard MT2 arbor, so it is impossible to wind the quill fully in before the arbor is ejected. This restricts the range of quill movement, and also means that the end of the centre or chuck is further from the tailstock body than would be ideal. If this bothers you, the only solution is to shorten the arbors of chucks and centres you intend to use in the mini-lathe. Generally you need to shorten them by between 10 and 20 mm. If the arbor is hardened, it may be necessary to use a Dremel or cut-off saw, and then clean up the end on a grinding wheel.

 

Thanks…

to Frank J Hoose Jr, the author of www.mini-lathe.com, for compiling more advice and information about mini-lathes than exists on any other piece of machinery in the world…

… and to www.LittleMachineShop.com for plenty of useful info and ideas, even if they don’t sell to the UK.

 

Disclaimer

I’m a Chartered Civil Engineer who knows quite a bit about underground pipelines but is entirely self-taught when it comes to using machine tools. I went to an old-fashioned Grammar School where they made me do Latin instead of metalwork. So you can regard my ideas as the voice of experience or the ramblings of an idiot, as you choose.

If you don’t know what you’re doing, don’t do it, and certainly don’t blame me if you hurt yourself.

All the opinions herein are just that, and you are entitled to disagree.

If any of the facts are wrong, please let me know because I’d like to put them right.

 

Contact

You can e-mail me, Paul Hayward, by clicking here

 

© 2003 Paul Hayward