Hercus Model VBC Lathe
Parting on my Chinese lathe was always a stressful disaster because the top slide isn't rigid enough and the blade holder was held in a 4-way toolpost. So when the load increased the blade would twist slightly and also get dragged into the job as the top slide flexed forward. If I was lucky the lathe would just stall. Otherwise it would bend the job and/or break a chunk off the expensive parting blade. I eventually made a blade holder that bolted to the chuck side of the cross slide. The base of this holder extended back past the blade cutting point. This means the downward force on the blade never forces the blade into the work. See this page for details.
My holder for the Hercus is different because the chuck side of the cross slide isn't a machined vertical surface. I considered a holder that replaced the top slide but the effort of removing the top slide and installing and aligning such a holder made this idea less appealing. So I made a holder that fits into the T-slot on the top slide. It is automatically aligned when the top slide is parallel to the spindle and the blade top is always at centre height.
The holder is made from a piece of 10mm thick angle (gal lintel) and machined to slide into the top slide T-slot. The blade is held in its slot by two bolts. The original plan was to remove the first 25mm of the vertical part of the holder near the front of the blade. Then the blade could be positioned at or behind the front of the holder and then the downward force when parting would not try to rock the top slide and push the blade into the job. This would restrict the maximum size I could part to 50mm. The Hercus top slide is rigid and having the blade extended past the support point on the base hasn't caused a problem so far. If it does then I'll cut away the front of the vertical part and position the blade behind the front of the base.
The second photo shows the holder assembled and with a P1 blade from LittleMachineShop. The top of the blade is about .004" wider than the bottom and I made the top of the slot .002" deeper to allow for this. The Warner P1 blade from LittleMachineShop is also T-shaped like this one. The Rimet blades in this size are bevelled from top to bottom. Most of the slot here is .064" deep but the bottom 2mm is .062" deep.
Here is the holder installed in the top slide. Installation takes about 10 seconds provided the T-slot is clean. The middle hole above the blade can be used if the blade is too short to reach the back hole. The second photo shows the blade actually doing some work. It works well with a slow speed and soluble lube. The little blade loses its edge far too quickly. Luckily all I have to do is polish the front of the blade with a stone to restore the sharp corner. I am using a cheap Chinese blade so perhaps the steel isn't as good as it could be.
How well does it work ? Well, it is better than previous parting holders that I tried but it's still not as good as the the holder for my CQ lathe unfortunately. The holder here still has a small overhang which doesn't help. The CQ does have a bigger spindle with an integral backplate for the chuck and roller bearings with preload and I'm sure this explains most of the difference. Recently I was trying to part some 70mm round bar and it was heavy going on the Hercus. I switched to the CQ and got the job done but it still took ages. Of course this is an extreme case and for more sensible jobs the Hercus holder is fine and definitely a convenient addition.
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Last modified 2010-11-02