This is a little project I did some time ago and I have found the
modification to be quite useful. It is based on a series of articles
in Engineering in Miniature from July 2007 written by Anthony Mount. I
have made a couple of changes to the original and later changed the
design of the clamp. Both the original and altered clamping methods are
shown below.
As supplied the WM250 along with many of it′s "Chiwanese" stable mates has a rudimentary saddle clamp operated by an allen key. The original clamp is part of the saddle guide block and it′s design is not well thought out, or to put it another way - it doesn′t work!
1
Photo (1) is a view of the top of the saddle as the clamp and
guide block is being removed. The two screws at the front hold the
apron
in place and the line of three holes nearer the bed are for the cap
screws that hold the clamp and guide in place. The leftmost screw is M8
and the other two screws are M5. The M8 screw for the saddle clamp is
very close to the cross-slide and at certain settings the cross-slide
gib adjustment screws cover the saddle clamp so that it is impossible
to insert an allen key. Removing the clamp and guide block is
straightforward, just undo the three screws but put something under the
block to support it because it can fall behind the apron and get
wedged in amongst the half nuts and lead screw!
The clamp and guide block removed from a similar lathe (2) this is much better machined than the block from my lathe and the saw cut is centred between the M8 and M5 threaded holes. The saw cut is intended to let the clamp block flex upwards but the remaining web is much too solid to allow any bending. To overcome this shortfall you may find that the two smaller screws are left loose so that the M8 screw can pull the block up under the lathe bed to provide the clamping force, not an altogether satisfactory arrangement.
3
The modification is to split this block into 2 pieces by continuing
the saw cut so that each part can perform it′s own function.
Fortunately the three holes are at equal centres so the re-machining
needed is minimal. The sizes of the two parts are
dependant on the position of the original saw cut which appears to have
been put in, somewhat arbitrarily, by hand.
The cut in the block from my lathe (3) was much closer to the
5mm threaded hole meaning that the smaller clamping block required
more machining to fit neatly under the saddle. The M8 hole will also
need to be filled with a length of studding or a bolt loctited in as the
original M8 cap screw will no longer be used. In photo (4) you
can see the poor quality of the machining on the block. The
right hand part is the original shape which is now the guide block the
remains of the saw cut can still be seen and the left hand part
I have squared up to form the clamp block. Because the original saw cut
was close to the M5 hole this has had the effect of pushing the guide
block to the right (as you stand in front of the lathe) and the clamp
block consequently sticks out to the right of the saddle by about 5mm. I
have machined this 5mm off so that the clamp block is level with the
side of the saddle which is why so much of the stud filling the M8 hole
can be seen.
The saddle guide section can be fitted back to the lathe now using the two leftmost holes in the saddle 8mm on the left and a 5mm in the centre. The choice in refitting the guide block is either to drill out the block and tap 8mm for the leftmost screw or to make a small collar to take a 5mm cap screw. I followed the original article and made a small collar for a 5mm cap screw.
The re-machined clamp is inserted and held in place under the saddle tight against the guide block and the front of the bed. The centre for the M5 clamp screw can now be marked. The new hole can then be drilled and tapped to take the clamp screw. Depending on the position of the saw cut this may be nearer the centre than mine was. The original article had the new M5 clamp thread going almost down the centre of the glued in stud.
5
The next job is to make the clamping screw and lever, this is fairly straightforward turning and the drawing (5)
I hope is self explanatory
(Click on the image for a readable version). The original article shows
the clamping screw with a head similar to mine but with a large washer
under
it bearing on the top of the saddle. I didn′t like this as it overhung
the edge of the saddle somewhat. I made mine similar in shape to a cap
screw with the head added on top, this means that screw tightens in the
original counterbore rather than on the top of the saddle which saves
the
paint. There is a space between the head of the clamping screw and the
top of the saddle and it may be worthwhile checking the depth of the
counterbore to ensure that this happens. Photo (6) shows the
clamp screw, handle and the spacing collar to reduce the M8 counterbore
to
M5 (Note there is no drawing for the spacer, just measure and make to
fit). If you do not want to make the handle a ready made bristol lever
could probably be found to suit.
7
Photo (7) shows the finished article fitted to the lathe. You
can see the spacing collar on the left hand side. I could have made the
handle a little longer
but it would have hit the cross slide, as it is, it can spin all the
way around. Once fitted I adjusted the angle of the handle when tight by
shaving a little
off the shoulder of the cap screw part of the clamping bolt. Photo (8)
Is a wider view of the finished clamping bolt and the ′L′ shaped
clamping block can be seen now painted yellow to match the saddle. It
certainly works but doesn′t really lock the saddle as tight as it could
be this is
because the clamping block tends to tilt (front to back) as it
tightens. Also it is possible for the tailstock to collide with the
handle when the
clamp is unlocked.
9
After using this for some time I decided to try and make the
clamping a bit more solid. I noticed that there was a small ledge inline
with the bed
where the saddle and apron meet. I measured up and made the new clamp
shown in photo (9) alongside the old clamp. The only difficult
bit is to get the
clamping face to fit level and without gaps to the underside of the
bed. I used a feeler gauge and took shavings off of the step that hooks
under the
saddle until it was a good fit. The clamping is now much better and
fairly light pressure on the clamping handle will stop the saddle
moving.
Photo (10) shows the block in place, I have removed the screwcutting dial to enable a clear view.
Whilst rumaging about under the saddle with feeler gauges it is a good idea to check the clearance between the guide block and the underside of the bed. I found that there was too much clearance for my liking and reduced this to give a better fit and stop the saddle lifting. Mine was a small enough amount to remove by rubbing on wet & dry paper on a flat surface. There is another guide block at the left side of the saddle and this can be treated in a similar fashion. Don′t take too much off though or the saddle will become difficult to move along the bed! The guide at the rear of the saddle has an adjustable gib so this is easily tightened if necessary.
As supplied the WM250 along with many of it′s "Chiwanese" stable mates has a rudimentary saddle clamp operated by an allen key. The original clamp is part of the saddle guide block and it′s design is not well thought out, or to put it another way - it doesn′t work!
1
2 
The clamp and guide block removed from a similar lathe (2) this is much better machined than the block from my lathe and the saw cut is centred between the M8 and M5 threaded holes. The saw cut is intended to let the clamp block flex upwards but the remaining web is much too solid to allow any bending. To overcome this shortfall you may find that the two smaller screws are left loose so that the M8 screw can pull the block up under the lathe bed to provide the clamping force, not an altogether satisfactory arrangement.
3
4 
The saddle guide section can be fitted back to the lathe now using the two leftmost holes in the saddle 8mm on the left and a 5mm in the centre. The choice in refitting the guide block is either to drill out the block and tap 8mm for the leftmost screw or to make a small collar to take a 5mm cap screw. I followed the original article and made a small collar for a 5mm cap screw.
The re-machined clamp is inserted and held in place under the saddle tight against the guide block and the front of the bed. The centre for the M5 clamp screw can now be marked. The new hole can then be drilled and tapped to take the clamp screw. Depending on the position of the saw cut this may be nearer the centre than mine was. The original article had the new M5 clamp thread going almost down the centre of the glued in stud.
5
6 
7
8 
9
10 
Whilst rumaging about under the saddle with feeler gauges it is a good idea to check the clearance between the guide block and the underside of the bed. I found that there was too much clearance for my liking and reduced this to give a better fit and stop the saddle lifting. Mine was a small enough amount to remove by rubbing on wet & dry paper on a flat surface. There is another guide block at the left side of the saddle and this can be treated in a similar fashion. Don′t take too much off though or the saddle will become difficult to move along the bed! The guide at the rear of the saddle has an adjustable gib so this is easily tightened if necessary.
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