Pete.
Member
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- 14,976
- Location
- Kent, UK
I've been making a tapered gib for a guy who had a mis-hap on his Warco lathe and needed a new gib. The specs are: 320 long, 1:100 taper, 55 degree bevel, 1mm tall and thickness from about 5.5mm to 8.7mm.
Now because of the 1:100 taper you have to be very careful to make sure that you leave plenty on the gib for fitting. to take up 1 thou of adjustment you'll slide the gib in 100 thou or 2.5mm so it takes very little to make you run out of adjustment. I had a piece of very nice cast iron at about 450mm long so I was able to make the gib vastly over-length and give the guy the best chance to fit it up.
I started by setting up my mill for horizontal cutting. i used a 3/32" slitting saw to cut a long slice from the cast iron flat bar. Because my mill has only 300mm travel in X I set two hardened blocks against the part so I could un-clamp and slide it along to cut the whole length. this was cut in a single pass at 40RPM and on the mill's slowest feed. The bar is about 7/8" thick.
Once it was cut I was quite pleased with the finish. The F clamp you can see was for clamping the thin end with a spacer (just in view under the clamp) in the gap as I finished cutting the thick end, to stop it vibrating and possibly breaking the gib. It might have been unneccessary but I wasn't taking a chance.
After that I set up my surface grinder for grinding 1:100 taper by placing a piece of 2.5mm ally welding rod under one end of the chuck and setting the taper with a dial gauge and recording 2mm of rise for 200mm travel (marker pen on the top). Then I ground both sides of the gib.
For cutting the top and bottom bevel I started out by carefully clocking in my 3-way tilting table on my mill in all three directions. Very glad I did as I really needed it later on. The table allows for 30 degrees of tilt forwards and backwards, but I needed 55 degrees. i also have a 12" tilting table which goes 45 degrees each way, so I had to use both of these together.
I put 40 degrees of tilt on the small sub-table and 15 degrees on the mill main table to give me a total of 55 degrees:
To hold the gib for milling I clamped a length of gauge plate to the sub-table and clocked it in parallel to the X travel. Then I set the gib against that and used another smaller piece of gauge plate on top for the clamps to hold it down. tHis allowed me to slide the gib along and mill the whole 450mm length with my 300mm of travel.
This all went without event so I figured I would just flip over the gib and cut the opposite side. So I did, and it was wrong. The gib was about 11mm on the fat end and getting on for 14mm on the thin end. After scratching my head I realised that after cutting the first bevel and flipping the gib over, the gap under the thick end was now bigger than the gap under the thin end, so when I flipped it over and cut the upper bevel (thankfully starting from the thick end) the bottom bevel was not parallel to the table travel but the cut I made on top WAS parallel to the table travel so I effectively milled a wedge shape in the height. You can see the small gap above compared to the big gap at the fat end below.
All I could do from this point is rotate the table in the x axis to compensate for this error by re-cutting one side which is not ideal but all I could work with. To do this I removed the gib, put 2 marks on it at 200mm apart and measured the height at each mark which was 1.3mm different. Then I tilted the table back up flat and used my dial gauge to rotate the table anticlockwise1.3mm on the dial for 200mm travel on the DRO, to effectively cancel out the discrepancy. Then I re-set the table, re-mounted the gib and cut it to a nice even 11mm height all the way along.
Here's the gib now finished for fitting and scraping.
So yeah, my first tapered gib made from scratch. Not nearly as straightforward a job as I had presumed, but a good learning experience. I hope this helps someone in the future.
Now because of the 1:100 taper you have to be very careful to make sure that you leave plenty on the gib for fitting. to take up 1 thou of adjustment you'll slide the gib in 100 thou or 2.5mm so it takes very little to make you run out of adjustment. I had a piece of very nice cast iron at about 450mm long so I was able to make the gib vastly over-length and give the guy the best chance to fit it up.
I started by setting up my mill for horizontal cutting. i used a 3/32" slitting saw to cut a long slice from the cast iron flat bar. Because my mill has only 300mm travel in X I set two hardened blocks against the part so I could un-clamp and slide it along to cut the whole length. this was cut in a single pass at 40RPM and on the mill's slowest feed. The bar is about 7/8" thick.
Once it was cut I was quite pleased with the finish. The F clamp you can see was for clamping the thin end with a spacer (just in view under the clamp) in the gap as I finished cutting the thick end, to stop it vibrating and possibly breaking the gib. It might have been unneccessary but I wasn't taking a chance.
After that I set up my surface grinder for grinding 1:100 taper by placing a piece of 2.5mm ally welding rod under one end of the chuck and setting the taper with a dial gauge and recording 2mm of rise for 200mm travel (marker pen on the top). Then I ground both sides of the gib.
For cutting the top and bottom bevel I started out by carefully clocking in my 3-way tilting table on my mill in all three directions. Very glad I did as I really needed it later on. The table allows for 30 degrees of tilt forwards and backwards, but I needed 55 degrees. i also have a 12" tilting table which goes 45 degrees each way, so I had to use both of these together.
I put 40 degrees of tilt on the small sub-table and 15 degrees on the mill main table to give me a total of 55 degrees:
To hold the gib for milling I clamped a length of gauge plate to the sub-table and clocked it in parallel to the X travel. Then I set the gib against that and used another smaller piece of gauge plate on top for the clamps to hold it down. tHis allowed me to slide the gib along and mill the whole 450mm length with my 300mm of travel.
This all went without event so I figured I would just flip over the gib and cut the opposite side. So I did, and it was wrong. The gib was about 11mm on the fat end and getting on for 14mm on the thin end. After scratching my head I realised that after cutting the first bevel and flipping the gib over, the gap under the thick end was now bigger than the gap under the thin end, so when I flipped it over and cut the upper bevel (thankfully starting from the thick end) the bottom bevel was not parallel to the table travel but the cut I made on top WAS parallel to the table travel so I effectively milled a wedge shape in the height. You can see the small gap above compared to the big gap at the fat end below.
All I could do from this point is rotate the table in the x axis to compensate for this error by re-cutting one side which is not ideal but all I could work with. To do this I removed the gib, put 2 marks on it at 200mm apart and measured the height at each mark which was 1.3mm different. Then I tilted the table back up flat and used my dial gauge to rotate the table anticlockwise1.3mm on the dial for 200mm travel on the DRO, to effectively cancel out the discrepancy. Then I re-set the table, re-mounted the gib and cut it to a nice even 11mm height all the way along.
Here's the gib now finished for fitting and scraping.
So yeah, my first tapered gib made from scratch. Not nearly as straightforward a job as I had presumed, but a good learning experience. I hope this helps someone in the future.
