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
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
“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
Although
mini-lathes from different
There are at
least three suppliers of mini-lathes in the
Both Warco and
There are
other differences too. For example, the Warco and
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.
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
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