just to be clear, this is meant as a conversation starter and to get some good ideas for improvements.

its an alternative to an $800 clamprite

this is a fun beginners hydraulic project, and a useful tool, and there are lots of different ways to skin this cat so i think most people would enjoy making it. its also cheap and is a good way to get into making proper bore seals with orings.

DaOne came up with this design in another thread. I just machined it before he did hehe

Im not absolutely sure it works correctly. i cant really calibrate it with my drawbar because my drawbar has unknown springs in it at this time. and just putting a heavy weight on it doesnt really test it as it might work in a spindle..any ideas?

Materials:

toolholder that is "hollow" (coolant pass through). no modifications needed.
pullstud (gets modified)
threaded rod (cut to length)
"cylinder" (block of metal, custom made)
"piston" (block of metal, custom made)
"bridge" (block of metal, custom made)
various common fasteners
oring (standard sizes..buna n is fine)
pressure gauge* (a high pressure gauge from a welding regulator might work and people usually have one or two lying around unused)

*figure out how big you will be making the piston (probably driven by what size oring you will be using). then once you figure out the cylinder bore ID (driven by oring size, again), you will be able to calculate the cross sectional surface area involved. use the cross sectional area of the cylinder, not the oring or piston. then figure out what kinds of forces you will be likely to measure on your drawbar, and get a gauge that will measure that much pressure, corrected for the cross section of the cylinder. you could just use a 5000 psi gauge and be done with it, but that might be an expensive gauge and not provide much resolution where you will be measuring. or maybe you have a few gauges lying around and want to make use of them (i.e. high pressure gauges from a welding gas regulator..or what have you..) so just take all this into account when designing your particular setup.

use one of the many online calculators or tables to design your oring bore seal. this is very straightforward to do and its all based on tables. brainless! this type of seal is basically a static bore seal since it moves so little. its high pressure though so you should probably not just throw it together and cross your fingers. calculate the correct compression and gland dimensions for your oring!

you should put a little grease on your oring so it doesnt stick to the piston or cylinder. i used spark plug dielectric grease. that may or may not be a good idea, i just wanted to get it over with. but i suppose you should make sure its compatible with your oring material lol

for instance: a 1 square inch cylinder would be 1.128" diameter. so for every pound of drawbar force you would measure 1 psi on the attached gauge. if your going to be measuring 1500lbs of drawbar force, you should probably get a gauge with some headroom ...how about an 1800psi gauge in this instance?

Heres how it works:

The "bridge", the pullstud, the threaded rod, and the cylinder, are all bolted together during use and become one solid piece, not designed to flex.

In the picture, the bridge is the C shaped aluminum thing with two screws in it, the cylinder is the large block with the blind hole bored in it, and the pull stud and threaded rod are..well..pictured.

The pullstud threads get cut off with a lathe. Then the pullstud gets welded to the threaded rod. Now its free to move in the toolholder.

So when the drawbar pulls up on the pullstud, it ends up transmitting that force to the cylinder block, which squishes the piston and cylinder together (between the tool holder and cylinder block). That force is transmitted hydraulically to the gauge. Fill the cylinder block with water or something. Bleed out as much air as possible somehow.

You need to machine a passage from the cylinder to the gauge, obviously.

Ill be putting in springs of known constant into my drawbar soon so hopefully ill get to try it out and get numbers that make more sense.

This design needs to be improved!! Its very easy to make, but id make the following changes:

-make the "bridge" out of steel, and make it thicker. It ends up taking the full force of the drawbar, and its lame when it flexes.
-figure out a clever way to make the system air-free or easily bleedable.
-mount the bridge to the cylinder in a way where the screws will definitely not hit the tool holder! mine come very very close.
-an overall guideline should be to make it so the movement of the piston during use is as little as possible. i.e. you dont want the pullstud to be coming out of the toolholder because that makes the measurement inaccurate. so bleed stuff of air and tighten the pullstud into the bridge to the point where there is no play between it and the toolholder
-DaOne has suggested he would have done it using a lip seal. that may be a better way! although this oring way seems to work.
-remember you need some volume below the piston when its compressed! you dont want it bottoming out! although i think technically that may be okay, but i left about 1/16" fluid underneath the piston when its in working position.

Sorry i know ive done a terrible job explaining it but if you look at it youll figure it out.